Novel Polymer Systems and Surface Modifications of Planar and

Transcription

Novel Polymer Systems and Surface Modifications of Planar
and Porous Substrates for Advanced Applications
Dissertation
zur Erlangung des Grades
“Doktor der Naturwissenschaften”
im Promotionsfach Chemie
am Fachbereich Chemie, Pharmazie und Geowissenschaften
der Johannes Gutenberg-Universität Mainz
Basit Yameen
geb. in
Lahore, Pakistan
Mainz, 2008
1
Die vorliegende Arbeit wurde in der Zeit von April 2005 bis November 2008 am Max
Planck Institut für Polymerforschung unter Betreuung von Prof. Dr. Wolfgang Knoll
und Dr. Ulrich Jonas angefertigt.
Dekan: Prof. Dr. Wolfgang Hofmeister
1.
2.
3.
4.
Berichterstatter : Prof. Dr. Wofgang Knoll
Berichterstatter : Prof. Dr. Holger Frey
Prüfer: Prof. Dr. Karl W. Klinkhammer
Prüfer: Dr. habil. Heiner Detert
Tag der mündlichen Prüfung : 8-12-2008
2
Für Lubna und Shiza
3
Table of contents
List of Abbreviations....................................9
Chapter 1..................................................... 10
1. General Introduction ................................... 10
1.1. Methods of surface modification .............................................................. 11
1.1.1.
Plasma treatment and plasma polymerization ................................... 12
1.1.2.
Wet chemical surface modification................................................... 14
1.1.3.
Polymer brush for surface modification: “Grafting-to” approach ...... 15
1.1.4.
Polymer brush for surface modification: “Grafting-from” approach.. 18
1.2. Aim and motivation of the present work................................................... 22
1.3. Materials of relevance.............................................................................. 23
1.3.1.
Polyether ether ketone (PEEK)......................................................... 23
1.3.2.
Cyanate ester resins.......................................................................... 24
1.3.3.
Solid state nano/macro-pores............................................................ 26
1.3.3.1.
Polymer membranes with Track-Etched Nanopores.................. 26
1.3.3.2.
Ordered Macroporous Si/SiO2 .................................................. 29
1.4.
Reference ................................................................................................... 31
Chapter 2..................................................... 40
2. Materials and methods................................. 40
2.1. Materials.................................................................................................. 40
2.1.1.
Polymer Substrates........................................................................... 40
2.1.2.
Porous substrates.............................................................................. 40
2.1.2.1.
Fabrication of conical nanopore in PET and PI membranes....... 40
2.1.2.2.
Fabrication of ordered macroporous Si membrane .................... 41
2.1.3.
Substances purified prior to use........................................................ 41
2.1.4.
Substances used as received ............................................................. 42
2.2. Methods................................................................................................... 43
2.2.1.
Thermal and mechanical analysis ..................................................... 43
2.2.2.
Fourier transform infrared spectroscopy (FTIR) ............................... 44
2.2.3.
X-ray photoelectron spectroscopy (XPS), ......................................... 45
2.2.4.
Current-voltage (I-V) measurements ................................................ 46
2.2.5.
Atomic force microscopy (AFM) and scanning electron microscopy
(SEM) 46
2.2.6.
Water contact angle measurement and surface roughness ................. 47
2.2.7.
Atomic absorption spectroscopy (AAS)............................................ 47
2.2.8.
Dielectric spectroscopy .................................................................... 48
2.3.
References .................................................................................................. 49
Chapter 3..................................................... 50
3.
Cyanate Ester Resins: Thermally Stable Adhesives
for PEEK ...................................................... 50
3.1. Choice of PT-30 CEM as adhesive for PEEK........................................... 52
3.2. Surface activation of PEEK for adhesion improvements........................... 57
3.2.1.
Wet chemical surface activation ....................................................... 58
3.2.2.
Plasma-assisted surface activation .................................................... 62
3.3. Conclusions ............................................................................................. 64
4
3.4.
Experimental ...............................................................................................
3.4.1. Sample preparation ...................................................................................
3.4.2. Wet chemical surface activation: Reduction of surface carbonyl groups to hydroxy
groups (PEEK-OH)...............................................................................................
3.4.3. Wet chemical surface activation - Transformation of surface hydroxy groups into
cyanate groups (PEEK-OCN) ..................................................................................
3.4.4. Plasma-assisted surface activation .................................................................
3.4.5. Gluing and adhesion tests ...........................................................................
3.5.
References: .................................................................................................
65
65
66
66
66
66
67
Chapter 4..................................................... 69
4. Effect of structural variations on thermal properties
of aryl ether ketone based cyanate ester resins with linear
and tri-arm molecular architectures........................ 69
4.1. Synthesis and characterization.................................................................. 70
4.1.1.
Synthesis of CEMs with linear molecular architecture...................... 71
4.1.2.
Synthesis of CEMs with symmetric triarm molecular architecture .... 73
4.2. Thermal properties of linear bifunctional CE resins.................................. 77
4.2.1.
Differential scanning calorimetry ..................................................... 77
4.2.2.
Thermogravimetric analysis ............................................................. 78
4.3. Thermal properties of linear multifunctional CE resin .............................. 79
4.3.1.
4.3.2.
Thermogravimetric analysis ............................................................. 79
4.4. Thermal properties of symmetric triarm CE resins .................................. 80
4.4.1.
4.5. Electronic effect of the substituents on the reactivity of the cyanate groups
82
4.6. Conclusions ............................................................................................. 85
4.7.
Experimental ............................................................................................... 87
4.7.1. Synthesis of oligomeric aryl ether ketone with hydroxy end groups ....................... 87
4.7.2. Synthesis of linear aryl ether ketone oligomeric mixture with pendant and terminal
hydroxy groups (9) ............................................................................................... 88
4.7.3. Synthesis of symmetrically triarmed aryl ether ketone with hydroxy end groups (17) 89
4.7.4. Synthesis of symmetrically triarmed aryl ether ketone with hydroxy end groups 1,3,5-(4(4-hydroxyphenoxy)-4'-oxy)benzene (17), deprotection of methoxy groups ........................ 89
4.7.5. A general procedure for the synthesis of linear and symmetrically triarmed aryl ether
ketone cyanate ester monomers (4, 5, 6, 10, 13, 18, 19, 20) ............................................. 90
4.8.
Reference ................................................................................................... 91
Chapter 5..................................................... 92
5.
Polycyanurate Thermoset Networks with High
Thermal, Mechanical, and Hydrolytic Stability Based on
Liquid Multifunctional Cyanate Ester Monomers with
Bisphenol A and AF Units ................................... 92
5.1. Synthesis and characterization of bisphenol AF and bisphenol A based room
temperature processable CEMs............................................................................ 94
5.1.1.
FTIR monitoring of Cure kinetics of bisphenol AF and bisphenol A
based CEMs .................................................................................................... 97
5.1.2.
Thermal properties of bisphenol AF and bisphenol A based CERs ..100
5
5.1.3.
Rheological measurements of PCs derived from bisphenol AF and
bisphenol A based CEMs................................................................................103
5.1.4.
Young’s moduli and coeffecients of thermal expansion for PCs
derived from bisphenol AF and bisphenol A based CEMs...............................104
5.1.5.
Dielectric measurements PCs derived from bisphenol AF and
bisphenol A based CEMs................................................................................105
5.1.6.
Template assisted fabrication of polycyanurate nanorods and their
hydrolytic stability..........................................................................................106
5.2. Conclusions ............................................................................................108
5.3.
Experimental ..............................................................................................109
5.3.1. Synthesis of oligomeric aromatic ether with pendant methoxy and terminal hydroxy
groups 1a and 1b: ................................................................................................109
5.3.2. Deprotection of pendant methoxy groups to yield 2a and 2b: ..............................109
5.3.3. Synthesis of oligomeric aromatic ether with pendant and terminal cyanate groups 3a:
110
5.3.4. Curing of CEMs to PC thermosets: ..............................................................110
5.3.5. Monitoring the kinetics of thermal curing of CEMs 3: .......................................110
5.3.6. Template assisted fabrication of polycyanurate nanorods (PCNs) and determination of
their hydrolytic stability: .......................................................................................111
5.4.
References .................................................................................................111
Chapter 6.................................................... 113
6. Polyether ether ketone (PEEK) Surface
Functionalization via Surface Initiated Atom Transfer
Radical Polymerization ...................................... 113
6.1. Immobilization of ATRP initiator and subsequent polymer brush growth by
SI-ATRP ............................................................................................................115
6.2. Characterization of PEEK surface modified by SI-ATRP ........................115
6.2.1.
Surface topography by AFM and SEM............................................115
6.2.2.
Surface chemical characterization by FTIR, XPS and Contact angle
goniometry .....................................................................................................116
6.3. Demonstration of properties imparted to PEEK surface by polymer brushes
120
6.4. Conclusions ............................................................................................122
6.5.
Experimental ..............................................................................................123
6.5.1. Reduction of PEEK surface carbonyl groups to the hydroxy groups (PEEK-OH) .....123
6.5.2. Immobilization of ATRP Initiator on the PEEK-OH Membrane ..........................123
6.5.3. SI-ATRP on the surface of PEEK-Br membrane ..............................................123
6.5.4. Exploiting the surface charge: Electrostatic interaction of PEEK-PolySPM and
Rhodamine 6G ...................................................................................................123
6.5.5. Bio-repellency evaluation: Growth E. coli bacteria on the surface of pristine PEEK and
PEEK-MeOEGMA ..............................................................................................124
6.5.6. Thermally responsive switching between hydrophilicity and hydrophobicity of PEEKPolyNIPAAm: Measurement of static water contact angles above and below the LCST: .......124
6.6.
Reference ..................................................................................................124
Chapter 7.................................................... 126
7.
Plasma Polymerised Polyallylamine as ad-layer for
Anchoring Surface initiated polymerization initiator on
6
polymeric (PEEK, PI, PET) Surfaces: A General Route to
Polymer Surface Functionalization via SI-ATRP .......... 126
7.1. Substrate independent anchoring of ATRP initiator and subsequent SIATRP 128
7.1.1.
XPS analysis ...................................................................................128
7.2. Conclusions ............................................................................................133
7.3.
Experimental ..............................................................................................134
7.3.1. Deposition of plasma polymerised PAAm on polymer surface ............................134
7.3.2. Immobilization of ATRP Initiator on the PEEK-OH Membrane ..........................134
7.3.3. PolyMeOEGMA brush25............................................................................134
7.4.
Reference ..................................................................................................134
Chapter 8.................................................... 136
8.
A facile route for the preparation of azideterminated polymers. “Clicking” macromolecular building
blocks on planar surfaces and nanochannels............... 136
8.1. Synthesis of the azide-terminated azo initiator. .......................................138
8.2. Synthesis of azide-terminated polyelectrolytes ........................................140
8.3. “Clicking” polyelectrolyte chains on planar Si surfaces...........................143
8.4. Functionalization of single conical polymer nanochannels via a “click”
chemistry approach.............................................................................................146
8.5. Conclusions ............................................................................................149
8.6.
Experimental Section ....................................................................................149
8.6.1. Synthesis of acyl chloride-terminated azo initiator (2) .......................................149
8.6.2. Synthesis of 3-Amino-1-azide propane (4) .....................................................149
8.6.3. Synthesis of azide functionalized azo initiator (5) ............................................150
8.6.4. Synthesis of azide-terminated polyelectrolytes ................................................150
8.6.5. Click chemistry on the silicon surface ...........................................................150
8.6.5.1.
Synthesis of ethynyldimethylchlorosilane (EDMS) ...................................150
8.6.5.2.
Functionalization of silicon wafer with 10 and subsequent click chemistry ...151
8.6.6. Functionalization of the conical nanochannel inner surface with alkyne groups and
subsequent click chemistry ....................................................................................151
8.6.7. Gel permeation chromatography (GPC) .........................................................151
8.7.
References .................................................................................................151
Chapter 9.................................................... 154
9.
Facile Large-Scale Fabrication of Proton
Conducting Channels......................................... 154
9.1. Grafting of polyelectrolyte brush on the surface of ordered macroporous
membranes via SI-ATRP ....................................................................................156
9.1.1.
Characterization ..............................................................................157
9.1.2.
Proton conductivity .........................................................................160
9.2. Development of self-humidifying proton conducting channels generated by
scaffolded polyelectrolyte brushes “doped” with hygroscopic monomer units.....162
9.2.1.
Characterization and proton conductivity measurements..................164
9.3. SI-ATRP of 2-acrylamino-2-methylpropane sulfonate in macroporous
silicon 169
9.4. Conclusions ............................................................................................171
9.5.
Experimental ..............................................................................................171
7
9.5.1. Synthesis of initiator (1) for SI-ATRP ...........................................................171
9.5.2. Anchoring 1 onto the surface of macroporous silica and subsequent PEB growth by SIATRP 171
9.5.3. Effective loading by weighing the macroporous silica membrane after each step of
functionalization .................................................................................................172
9.5.4. Ion exchange capacity ...............................................................................172
9.5.5. Water uptake study...................................................................................172
9.5.6. Proton Conductivity measurements ..............................................................172
9.6.
References .................................................................................................172
10.
Stimuli responsive artificially fabricated solidstate nanopores ............................................. 175
10.1.
Ionic transport through single solid state nanopore controlled with
thermally nanoactuated macromolecular gate......................................................176
10.1.1. Nature: The Inspiration I .................................................................176
10.1.2. Grafting of polyNIPAAm brush at the surface of PI membrane bearing
a single conical nanopore via SI-ATRP...........................................................178
10.1.3. Pore dimensions ..............................................................................179
10.1.4. Thermally induced nano-actuation read by current-voltage (I-V) curve
measurements .................................................................................................179
10.2.
Single conical nanopores displaying pH-tunable rectifying characteristics.
Manipulating ionic transport with zwitterionic polymer brushes .........................183
10.2.1. Nature-The Inspiration II.................................................................183
10.2.2. Grafting of poly-L-lysine brush at the surface of PI membrane bearing
a single conical nanopore via surface initiated conventional radical
polymerization and current-voltage (I-V) curves measurement .......................185
10.3.
Experimental Section ....................................................................................187
10.3.1.
Functionalization of nanopore bearing PI surface with ethylenediamine ............187
10.3.2.
Immobilization of ATRP initiator on the membrane: ....................................187
10.3.3.
PolyNIPAAm brush growth ....................................................................188
10.3.4.
Anchoring 4,4′-azobis(4-cyanopentanoic acid) on the surface of PI membrane with
single conical nanopore .........................................................................................188
10.3.5.
Poly-L-Lysine brush growth on initiator functionalized single nanopore-containing PI
membrane 188
10.4.
References .................................................................................................188
Summary and outlook ....................................... 191
11. Summary and outlook ................................. 191
11.1.
Polyether ether ketone-Cyanate ester resins an innate combination: A
proposal 191
11.2.
Surface initiated polymerization from initiator functionalized flat
polymeric surfaces..............................................................................................192
11.3.
Click chemistry as a new grafting-to methodology boosted up by cRP 195
11.4.
Functional porous materials achieved by a combination of elements form
macromolecular and materials research fields .....................................................195
11.5.
Facile Large-Scale Fabrication of Proton Conducting Channels ..........195
11.6.
Mimicing the ever-versatile ever-inspiring nature: Stimuli responsive
artificially fabricated solid-state nanopores .........................................................196
Acknowledgement............................................ 197
List of Publications ............................................. 199
8
List of Abbreviations
PEEK
CEM
CER
PC
PCN
PET
PI
PAEK
AEK
AEKOM
PAA
AMPS
MPS
NIPAAm
MeOEGMA
BIBr
PEL
PEG
EDA
EDC
Tg
Tm
T5%
LCST
CA
RH
WU
IEC
DSC
TGA
FTIR
ATR-IR
AFM
XPS
SEM
AAS
GPC
SIP
SI-ATRP
ATRP
cRP
Polyether ether ketone
Cyanate ester monomer
Cyanate ester resin
Polycyanurate
Polycyanurate nanorods
Polyethylene terephthalate
Polyimide
Polyaromatic ether ketone
Aryl or aromatic ether ketone
Aromatic ether ketone Oligomeric mixture
Polyallylamine
2-Acrylamino-2-methylpropane sulfonate
3-(Methacryloyloxy)propane-1-sulfonate
N-Isopropylacrylamide
Mono-methoxy terminated oligo(ethylene glycol)methacrylate
2-Bromoisobutyryl bromide.
Polyelectrolyte
Polyethylene glycol
Ethylene diamine
N-(3-dimethylaminopropyl)-N'-ethylcarbodiimide
hydrochloride
Glass transition temperature
Melting temperature
Temperature at 5% weight loss
Lower critical solution temperature
Contact angle
Relative humidity
Water uptake
Ion exchange capacity
Differential scanning calorimetry
Thermogravimetric analysis
Fourier transform infrared spectroscopy
Attenuated total reflection infrared spectroscopy
Atomic force microscopy
X-ray photoelectron spectroscopy
Scanning electron microscopy
Atomic absorption spectroscopy
Gel permeation chromatography
Surface initiated polymerization
Surface initiated atom transfer radical polymerization
Atom transfer radical polymerization
Conventional radical polymerization
9
Chapter 1
1. General Introduction
A polymer is a so-called macromolecule composed of repeating structural units
connected by covalent chemical bonds. Polymeric materials can be subdivided into
two classes, thermoset and thermoplastics. The two types differ primarily in their
response to heat. Thermoset is a polymer that, once formed, cannot be melted whereas
thermoplastic polymer melts upon heating and can be reshaped. Synthetic polymers
today find application in nearly every industry and area of life. They are widely used
as adhesives and lubricants, as well as structural components for products ranging
from children's toys to aircraft. They have also been employed in a variety of
biomedical applications ranging from implantable devices to controlled drug
delivery.1-4 In the present work various aryl ether ketone-based cyanate ester
oligomers were synthesized and investigated for their thermal and mechanical
properties for application as polycyanurate thermosets.
Polymeric materials are exceptional with respect to their diverse property profiles
and application ranges. No other class of materials is capable of matching similarly
broad property profiles. Polymer science continually evolves. A few decades ago the
focus of polymer science was very much on developing new high-performance
materials and understanding the associated structure–property relationships to achieve
ever improved thermal and mechanical properties. Nowadays the focus is on
developing polymeric materials to achieve well-defined structure coupled with
functionality.5 The majority of polymers possess interesting bulk physical, chemical
and mechanical properties combined with processing flexibility hence are sought after
for numerous applications.
Besides bulk properties, surface chemical & physical nature of a material
(polymeric or non-polymeric including ceramics and metals) has fundamental
implications on its success for a certain application. Surfaces act as phase boundaries
residing between bulk and the outer environment. Often the materials chosen for a
particular application (like iron as construction material) possess the appropriate bulk
properties (like mechanical stability), while their surface properties (like chemical
resistance against corrosion) are insufficient for the target application. This can
consequently have adverse problems in adhesion, coating, painting, colouring,
10
biocompatibility etc. To overcome these problems the regulation of a material’s
surface interaction with its environment and other substances is of prime importance.6
In this work, a number of different strategies have been employed to control the
material surface properties. In the following section the techniques relevant to present
work will be introduced briefly. The materials utilized will be introduced later in this
chapter.
1.1. Methods of surface modification
In recent years, alongside the development of new polymeric materials, many
advances have been made in developing surface treatments/modifications to alter the
surface chemical and physical properties of existing polymers without affecting their
bulk properties. Modification of a polymer surface can be achieved by means of
various physical or chemical processes. The physical processes of surface
modification chiefly involve the use of high-energy species like flame, corona,
plasmas, ozone, photons, UV light, electrons, and γ-rays. The surface treatment with
high-energy species mainly results uncontrolled surface chemical changes by
introduction of polar groups, ablation and crosslinking at the surface. These effects
may in turn enhance surface wettability and change surface topography. The wetchemical treatments of polymeric surfaces have always been of special interest
because they present rational surface chemical property control through organic
surface chemistry and are considered the methods of choice to produce well-defined
and reproducible products.7, 8
Recently, the modification of surfaces in general, polymeric or non-polymeric,
with thin polymer films is also widely used to tailor the surface properties. Surface
modifications employing polymer films can be achieved in a number of ways. A
rather general way of applying such a thin film, presented by Du Yeol Ryu et al.,9
involves depositing or spraying a polymeric coating from solution followed by
thermal cross-linking. In the case of surface modification with polyelectrolytes, layerby-layer (LBL) method has emerged as the most versatile tool. The method typically
involves the alternative adsorption of polycations and polyanions and can be simply
realized on essentially any substrate that will support the adsorption of an initial layer
of polyelectrolyte.10
Alternatively, polymer chains can be covalently grafted to the surfaces either by
“grafting-to” or by “grafting-from” approach. Both the approaches result in so-called
11
“polymer brushes”,11, 12 which are assemblies of long-chain polymers attached by one
end to a support and extended from the surface. The key advantage of polymer
brushes over the other thin polymer film based surface modification methods is their
mechanical and chemical robustness. In addition, a provision of high degree of
synthetic flexibility towards the introduction of a variety of functional groups have
made polymer brushes attractive for fabrication of surfaces with a controlled surface
properties like adhesion, wettability, biocompatibility, stimuli responsiveness, etc.13
Following surface modification methods that are relevant in context of present
work will be introduced briefly here.
•
Plasma treatment and plasma polymerization
•
Wet chemical surface modification
•
Polymer brush for Surface modification 1: “Grafting-to” approach
•
Polymer brush for Surface modification 2: “Grafting-from” approach
1.1.1.
Plasma treatment and plasma polymerization
A plasma has been described as electrically conducting ionized gas state, which is
a mixture of negatively, and positively charged particles, neutral atoms and
molecules. It is also referred as fourth state of matter distinct from solid, liquid and
gaseous states.
Plasmas can be generated by supplying energy to a neutral gas causing the
formation of charge carriers. There are various ways to supply the necessary energy
for plasma generation from a neutral gas. One possibility is to supply thermal energy,
as in flames, where exothermic chemical reactions of the molecules are used as the
prime energy source. The most commonly used method of generating and sustaining a
low-temperature plasma for technological and technical application is by applying an
electric field to a neutral gas. Any volume of a neutral gas always contains a few
electrons and ions that are formed, for example, as the result of the interaction of
cosmic rays or radioactive radiation with the gas. These free charge carriers are
accelerated by the electric field and new charged particles may be created when these
charge carriers collide with atoms and molecules in the gas or with the surfaces of the
electrodes. This leads to an avalanche of charged particles that is eventually balanced
by charge carrier losses, so that a steady-state plasma develops.14
In general the reactions of gas plasmas with the polymeric surfaces can be
classified as follows:
12
1) “Surface reactions during plasma modification processes” are those processes
in which the surface structure of a substrate is modified by the plasma-activated
species of a non-polymerizable atom or molecule, which includes gases such as
oxygen, nitrogen, carbon dioxide, ammonia and argon. During plasma modification
processes, the substrate surface is bombarded by the activated gaseous species in the
plasma. As a result, the reactions between gas-phase species and surface species and
reactions between surface species produce functional groups and cross-links at the
surface. It is probably the most versatile surface treatment technique and has become
an important industrial process in modification of polymer surfaces to improve their
adhesion, printability etc.
2) “Plasma polymerization processes” involve a vaporizable organic precursor or
monomer (CH4, C2H6, C2F4 or C2F6, allylamine etc.) fed into the reaction chamber,
either alone or in combination with an activator gas, which itself does not necessarily
participate in the polymerisation reaction (e.g., argon, helium etc.). The monomer
molecules are activated in the plasma phase and bombard the substrate surface leading
to the formation of a thin cross-linked polymeric film on the surface. It involves
reactions between gas-phase species, reactions between gas-phase species and surface
species, and reactions between surface species. The structure of plasma-deposited
films is highly complex and depends on many factors, including reactor design, power
level, substrate temperature, electric field frequency, monomer structure, monomer
pressure, and monomer flow. It is well known that conventional continuous wave
plasma polymerization produces a variety of functional groups at the surface due to
severe fragmentation of the precursor molecule within the electrical discharge.15
However, a considerable degree of control and reproducibility regarding the physical
and chemical characteristics of the plasma-polymerized films can be achieved by
employing pulse plasma polymerization method. The inherent limitations of plasma
polymerization in continuous wave mode can be overcome by pulsing the electric
discharge, which yields high levels of functional retention. In this regard, the
application of pulse plasma-polymerized films for biosensing, adhesion improvement
and stimuli responsive surfaces has been recently reported.16-19
3) “Surface etching by plasma” is a process in which materials are removed from a
surface by physical etching and chemical reactions to form volatile products.20, 21 An
increased surface roughness is the chief consequence of this process.
13
1.1.2.
Wet chemical surface modification
In wet chemical surface modification, a material is treated with liquid reagents to
generate reactive functional groups on the surface. This classical approach to surface
modification does not require specialized equipment and thus can be conducted in
most laboratories. It is also more capable of penetrating porous three-dimensional
substrates than plasma and other energy source surface modification techniques.22
Chemical etchants are generally used to wet chemically convert smooth hydrophobic
polymer surfaces to rough hydrophilic surfaces by surface oxidation. For instance,
chromic acid and potassium permanganate in sulfuric acid have been used to
introduce reactive oxygen-containing moieties (like –COOH, –OH, and –SO3H) on
the surface of polyolefins, namely, polyethylene (PE) and polypropylene (PP).23-27
The oxidative etching methodology is, however, highly non-specific as it produces a
broad variety of surface chemical properties and results in considerable changes in
surface morphology. McCarthy and coworkers, on the other hand, have developed
modification reactions that transform only the a specific chemical structure of
functional groups at the polymer surfaces.28-31 This methodology takes advantage of
the high reactivity and selectivity of the reaction solution towards chemical functions
at the polymer surface, producing a very diffuse interface and very specific functional
groups are introduced on the surface. For example, the carbonyl group at poly(ether
ether ketone) (PEEK) surface was reacted with different reagents, producing various
specific functional groups (Figure 1.1).32
Figure 1.1: Wet chemical surface modification of PEEK leading to specific surface functional groups.
14
The surface modifications resulting in controlled surface chemical structure are
especially important for immobilization of specific bioactive molecules where a
covalent bonding is preferred between the compound and the functionalised polymer
surface.33 In the biomedical field, a covalent immobilization can be used to extend the
life cycle of a biomolecule, prevent its metabolism (as with compounds which provide
anti-tumor activity when applied locally, but may be toxic if metabolized), or allow
for continued bioactivity of implanted devices (as in vascular devices, shunts, or
catheters). In the case of active food packaging applications, a covalent linkage
ensures that the bioactive compound will not migrate to the food and thus may offer
the regulatory advantage of not requiring approval as a food additive.34
1.1.3.
Polymer
brush
for
surface
modification:
“Grafting-to”
approach
The “grafting-to” approach involves covalent tethering of preformed and endfunctionalised polymer chains to an appropriately functionalised surface under
suitable conditions to form a tethered polymer brush (Figure 1.2).35
Figure 1.2: Schematic illustration of “grafting-to” approach for polymer brush fabrication.
The end-functionalised polymer chains can be synthesized by living anionic-,
cationic-, radical-, group transfer- and ring opening metathesis polymerizations. These
polymerization techniques allow for the facile synthesis of polymer chains end
functionalized with a range of desired functionalities (hydroxy, carboxyl, amino, thiol,
etc.). Lo Verso and Likos have very recently reviewed the subject of end
functionalised polymer chains synthesis.36
The second component of this approach is an appropriately functionalised surface.
In case of a silica (SiO2) surface, a polymer chain can be either directly grafted to the
15
surface silanol groups (Si-OH) or a chloro/alkoxy silane bearing head group suitable
for subsequent coupling can be anchored onto the surface through silanizaiton. For
instance, Mansky et al.37 synthesized a series of hydroxy-terminated random
copolymers of styrene and methyl methacrylate with different ratios by a nitroxide
mediated living radical polymerization employing a hydroxy functionaized 2,2,6,6tetramethylpiperidinyloxy (TEMPO) initiator. These end-functionalized polymers
were reacted with silanol groups on the silicon wafer to form tethered random
copolymer brushes. Prucker et al.38 modified the silica surface with 4-(3’chlorodimethylsilyl)propyloxybenzophenone followed by deposition of a polystryrene
or poly(ethyloxazoline) film. Illumination with UV light could produce a covalently
bound film via a photochemical attachment. Typically, several nanometers of
polymeric overcoat could be attached.
In case of gold surfaces the thiol-gold affinity has been particularly explored.
Koutsos et al.39,
40
synthesized a series of thiol-terminated polystyrenes with a low
polydispersity by anionic polymerization and prepared chemically end-grafted
polystyrene chains on a gold surface by exposing the gold substrate to a toluene
solution of these polymers. For polymeric surfaces a variety of wet chemical and
plasma assisted surface modification methods can be employed (sections 1.1.1 and
1.1.2). For example, Bergbreiter et al.41 tethered terminally functionalized poly(tertbutyl acrylate) onto the wet chemically oxidized polyethylene films. Their approach
was quite similar to that of Mansky et al.37 as described earlier.
Another rather new but very promising and effiecient coupling approach is “click
chemistry” originally developed by Sharpless et al.42,
43
It comprises a number of
organic heteroatom coupling procedures. Nowadays the most popular click reaction is
the copper(I) catalyzed 1,3-dipolar azide–alkyne cycloaddition (Scheme 1.1).
Scheme 1.1: 1,3-Dipolar cycloadditions between alkynes and azides resulting in the formation of a
1,2,3-triazole. A mixture of regioisomers forms when only heat is applied and only the 1,4-regioisomer
forms when copper(I) is used as catalyst.
16
Originally, the azide–alkyne (Huisgen) cycloaddition suffered from the problem of
the reaction being performed at high temperatures with no stereospecificity (Scheme
1.1).44,
45
These major drawbacks of the azide–alkyne cycloaddition were only
successfully overcome in 2002, when two independent research teams led by
Sharpless46 and Meldal47 reported the use of copper(I) catalysts for the 1,3- dipolar
cycloaddition of azides and alkynes. The use of a copper(I) catalytic system results in
the exclusive formation of the 1,4-substituted 1,2,3-triazole and it accelerates the
reaction tremendously allowing room-temperature cycloadditions (Scheme 1.1).48, 49
In practice, the copper(I) catalyst can be generated in situ using copper(II) sulfate and
sodium ascorbate as reducing agent or a copper(I) halide is used together with a
stabilizing ligand. The mechanism of the copper catalyzed azide–alkyne cycloaddition
reaction has been studied in detail by Finn and co-workers.50 The versatility and
robustness, i.e. excellent functional group tolerance, that eliminates the need for
protecting groups, of the copper(I) catalyzed 1,3-dipolar cycloaddition between azides
and alkynes has led to its rapid evolution into a common tool in various research areas
including, e.g., organic synthesis,51 biochemistry,52-55 sugar derivatization56 and drug
discovery43. Additionally, employing azide or alkyne functionalised small molecules,
the 1,3-dipolar cycloaddition has been used to tune the surface functionality of
different polymeric57 or non-polymeric materials such as electrode surfaces,58 gold,59
magnetic metal oxides,60 silica particles61 and porous beads.62,
63
Moreover,
microcontact printing64, 65 has been applied to modify azide- or alkyne-functionalized
self-assembled monolayers (SAMs).
In recent years, polymer chemists have also discovered the numerous advantages
of using the copper(I) catalyzed 1,3- dipolar cycloaddition as an easy tool for the
preparation of novel polymeric structures. Since the first report of click chemistry in
polymer science by Wu et al.,66 the construction of well-defined and complex
macromolecular architectures via click chemistry is a strongly growing field of
research throughout the world. This has resulted in the development of an enormous
number of well-defined polymeric building blocks with one or multiple azide67-69
and/or alkyne70-72 functionalities at the chain-end or as pendant groups. All the known
and well established polymerization techniques like living anionic73 and cationic74
polymerizations, atom transfer radical polymerization (ATRP),75, 76 reverse addition
fragmentation transfer polymerization (RAFT),77-79 nitroxide mediated polymerization
(NMP),80,
81
ring opening metathesis polymerization (ROMP),82-84 and ring opening
17
polymerization (ROP),85 have been combined with the azide/alkyne processes, mostly
relating to the chemical possibilities and the chemical realization of this endeavor.
Still at its infancy the subject has already been extensively reviewed.86-92
Despite of the enormous work published on the subject, so far there has not been
much attention paid on using alkyne-azide click chemistry as a potential “grafting-to”
approach for polymer brush fabrication. There is only one article published, in this
context,
from
R-V
Ostaci
et
al.93
They
reported
on
polystyrene,
polymethylmethacrylate and poly(ethylene glycol) brushes fabricated by the “graftedto” approach on a silica substrates “passivated” with ethynyldimethylchlorosilane, an
alkyne group containing silane using “click chemistry”.
The “grafting-to” approach of tethering polymer chains to the surface is, from
chemical point of view, rather simple and versatile. However, in general, only a small
amount of polymer can be immobilized onto the surface. Macromolecular chains must
diffuse through the existing polymer film to reach the reactive sites on the surface.
This barrier becomes more pronounced as the tethered polymer film thickness
increases. Thus the polymer brush obtained has a low grafting density and low film
thickness. To circumvent this problem, investigators have used the “grafting-from”
approach, which has become more attractive in preparing thick, covalently tethered
polymer brushes with a high grafting density.
1.1.4.
Polymer brush for surface modification: “Grafting-from”
approach
The “grafting-from” approach, on contrary to “grafting-to” approach, involves a
particular polymerization starting from a substrate surface previously modified with a
suitable initiator moiety (Figure 1.3).35
Figure 1.3: Schematic illustration of the “grafting-from” approach for polymer brush fabrication.
18
As the chains are growing from the surface, the rate controlling step for
propagation is the diffusion of monomers to the chain ends, thus resulting in thick
tethered polymer brushes with high grafting density. This so-called surface initiated
polymerization (SIP) thus overcomes the problems associated with the “grafting-to”
approach. The immobilization of appropriate initiator groups onto the surface prior to
SIP, is typically achieved by preparing a SAM with initiator terminated adsorbates.
These adsorbates are often synthesized to contain thiol94-96 or disulfide97, 98 groups to
enable chemisorption at gold and chlorosilane99-101 or alkoxy silane102-104 groups for
modification of silica surfaces. In some cases the simplest route is to chemically
couple the initiator to an existing SAM, for example, by exposing hydroxy105 or
amino106 terminated monolayers to a reactive initiator functionalized molecule. In a
similar manner the initiator groups can also be anchored to polymeric surfaces that
directly present -COOH and -OH groups, or are plasma/wet chemically modified to
present carboxylic acid-107 or hydroxy- 108, 109 rich surfaces.
Over the last few years, the “grafting-from” approach has rapidly evolved and all
the major polymerization strategies (anionic,110 cationic,111 radical, ROP112-116 and
ROMP117, 118) have been used to grow polymer brushes. In the context of this thesis
only the radical polymerization methodologies are utilized here. T. Sugawara and T.
Matsuda119 were the first to report on SIP method via the following steps:
photochemical fixation of an aminated polymer on a substrate, coupling reaction of
conventional radical polymerization initiators, and subsequent graft copolymerization
of vinyl monomers. However, it was Rühe and coworkers120-124 who did the seminal
work in this regard. Their strategy was superior to the predecessors as they, for the
first time, synthesized a silane bearing a conventional radical polymerization initiator
that was attached to the silica surface in one step by the SAM technique (Scheme 1.2).
19
Scheme 1.2: Schematic description of the concept for the preparation of polymer brushes by surface
initiated conventional radical polymerization as “grafting from” approach and the initiator
synthesized by Rühe et al.
The self-assembly of the initiator was followed by in situ thermally initiated
radical polymerization of styrene or other monomers. The SIP employing
conventional radical polymerization represents the most versatile approach, as it is
compatible with a wide variety of monomers; however, its uncontrolled/non-living
nature and initiation of polymerization in solution besides from the substrates are the
major drawbacks. In order to achieve a better control of molecular weight and
molecular weight distribution, film thickness, and synthesis of novel polymer brushes
of block copolymer nature, controlled radical polymerizations including ATRP125,
RAFT100, 126 and TEMPO-mediated
127, 128
radical polymerizations have been used to
synthesize tethered polymer brushes on solid substrate surfaces. Under ideal
conditions the SIP involving controlled radical polymerization methods is not only
surface initiated but also surface confined i.e. no polymerization in solution. Among
these controlled radical polymerization techniques, the grafting-from approach
involving surface-initiated atom-transfer radical polymerization (SI-ATRP) is more
versatile, allowing the preparation of well-defined polymer brushes on various types
of substrates, practically in the majority of solvents, and minimizes polymerization in
solution, providing polymer brushes with controlled growth rates and low
polydispersities. In addition, SI-ATRP is experimentally more accessible than for
example, the living anionic and cationic polymerizations, which require rigorously
dry conditions. The first report on exploiting SI-ATRP as a “grafting-from” approach
for tethered polymer brushes came from Ejaz and coworkers129 in 1998. They
20
immobilized the ATRP initiator 2-(4-chlorosulfonylphenyl)ethyltrimethoxy silane
onto
a
silica
substrate
by
Langmuir–Blodgett
technique
followed
by
polymethylmethacrylate (PMMA) brushes with high grafting density. Since then the
technique has been equally well explored on both silica130 and gold131-133 substrates.
ATRP134 is based on the reversible reaction of a low oxidation state metal
complex, Mtn-Y/Ligand (superscript n represents the metal ion oxidation state and Y is
a counterion), with an alkyl halide (RX). This reaction yields radicals and the
corresponding high oxidation state metal complex, XMtn+1-Y/Ligand, with a
concomitant abstraction of a halogen atom, X, from a dormant species, RX.
Mechanistically, ATRP is closely related to the radical addition of alkyl halides across
an unsaturated carbon-carbon bond, termed atom transfer radical addition (ATRA)135.
ATRP can be viewed as a special case of ATRA, which involves the reactivation of
the alkyl halide adduct of the unsaturated compound (monomer) and the further
reaction of the formed radical with the monomer (propagation). The process occurs
with a rate constant of activation, kact, and deactivation kdeact. Polymer chains grow by
the addition of the intermediate radicals to monomers in a manner similar to a
conventional radical polymerization, with the rate constant of propagation kp.
Termination reactions also occur in ATRP, mainly through radical coupling and
disproportionation, however, in a well-controlled ATRP, no more than a few percent
of the polymer chains undergo termination.76, 136
Scheme 1.3: Mechanism of metal complex-mediated ATRA and ATRP
Despite of its enormous potential, very little research has been conducted on
subjecting macroscale polymeric substrates to SI-ATRP. The most notable exceptions
are
quoted
here.
Genzer
and
coworkers137
investigated
SI-ATRP
from
21
polydimethylsiloxane (PDMS) substrates, Xu and coworkers138 explored SI-ATRP on
Nylon surface, Friebe and coworkers107 and Farhan and coworkers108 separately
investigated the SI-ATRP on polyethylene terephthalate surface (PET). The huge
potential of SI-ATRP for controlling surface related properties promises more active
future research involving fabrication of flexible polymeric functional surfaces.
1.2. Aim and motivation of the present work
The aim of this work, in general, was to build up a competence around the control
of polymeric material surface properties by employing the state of the art as well as
new surface modification strategies. This in turn may provide a basis for the
systematic development of new functionalized surfaces for a variety of different
applications. The work encompasses the surface modification of flat polymeric
surfaces as well as of ceramic and polymeric macro/nano-porous membranes for
various applications like adhesion improvement, thermo-responsiveness, antifouling,
and ion channeling. The work presented here is divided into two parts, first part
dealing with the surfaces in planar substrates and the second part dealing with
surfaces in the porous materials.
In the first part, wet chemical and plasma treatment/polymerization have been
employed to tune the surface related properties of technologically relevant materials.
The material in focus was mainly polyether ether ketone (PEEK). In combination with
PEEK, the potential of cyanate ester resins (CERs) as thermally stable adhesive was
explored. In the same framework, several new CER systems were developed. In this
context, the investigation of “structure-property relationship” for CERs led us to
develop easy-to-use resin systems with excellent thermal properties. The
technological relevance of these materials will be introduced later in this chapter.
The control over the surface properties of the PEEK was further extended by
grafting different polymer brushes onto the PEEK surface via SI-ATRP. Furthermore,
a high relevance of SI-ATRP to control the surface related properties of polymeric
materials and limited work published in this context, motivated the development of a
general route to functionalize the polymeric surfaces with an ATRP initiator for
subsequent SI-ATRP. Plasma polymerized polyallylamine was used as an adlayer for
this purpose. In addition to PEEK the success and the verstility of this approach was
also demonstrated on polyethylene terephthalate (PET) and polyimide (PI) surfaces.
22
The second part of the work focuses on the fabrication of functional materials by
tuning the surface chemistry of porous substrates. Ordered macroporous silicon
membranes and polymer membranes (PI and PET) containing a single track etched
nanopore are the two investigated substrate classes. Polymeric macromolecules were
tethered to the surface of these substrates leading to artificial proton conducting
channels, thermally nanoactuated macromolecular gates, and ionic gates displaying
pH tunable rectification.
In the following section the materials of relevance will be briefly introduced.
1.3. Materials of relevance
1.3.1.
Polyether ether ketone (PEEK)
PEEK is a high temperature resistant semicrystalline thermoplastic polymer
patented by Bonner in 1962,139 and first appeared in the literature in the early
1980s.140 It is a derivative of aromatic polyether ketone and is manufactured by the
nucleophilic polycondensation of hydroquinone and 4,4´-difluorobenzophenone using
diphenyl sulphone as solvent in the presence of alkali metal carbonates (Scheme
1.4).140, 141
Scheme 1.4: Synthesis of polyether ether ketone (PEEK).
Despite of the problems with high processing costs, the commercialization of the
nucleophilic route was successfully achieved and the PEEK, commercially known as
Victrex PEEK, was developed and manufactured by ICI Ltd. from 1980 until 1993.
PEEK possesses an excellent combination of thermo-mechanical properties. Its
thermal stability is such that it has a continuous working temperature of about 250 °C
with retention of the excellent mechanical properties and solvent and chemical
resistance. It exhibits a Tg of 143 °C and a melting temperature (Tm) of 342 °C.
Excellent bulk thermo-mechanical properties, chemical stability and low flammability
with low toxic gas emission have made PEEK an attractive material for applications
in a wide variety of fields such as automotive, electrical engineering, home
appliances, aerospace, and microfiltration membranes. Furthermore absence of
23
toxicity and biological inertness have attracted a lot interest from medical industries
like prosthetic and medical instruments.142, 143
Like most of the polymeric materials, PEEK exhibits a hydrophobic, chemically
inert surface nature, which leads to problems in structural adhesion, coating, painting,
coloring, biocompatibility, etc. To date, mainly two different strategies to tune the
surface related properties of PEEK have been explored. These include the use of highenergy species (plasmas, ozone, UV light, electrons, and γ-rays)144-149 and wet
chemical methods.32, 150-152 The use of high-energy species has been applied mainly to
improve the adhesion while employing epoxies as adhesives. On the other hand, the
wet chemical methods have been applied to tune surface properties through selective
organic surface chemistry. For example, the PEEK surface was fluorinated to make it
protein- repelling and blood compatible material.150 The reduction of surface carbonyl
groups to hydroxyl groups resulted in an improved cultivation of epithelial cells as the
presence
of
surface
hydroxyl
groups
moderately
improves
the
polymer
biocompatibility.151 The subsequent derivatization of surface hydroxy groups to
surface
amino
biocompatibility.
and
152
carboxylic
acid
functionalities
further
improved
the
These literature quotes represent a facile control of surface
chemistry and foresee plenty of promise to convert this technologically important
polymer into a functional material merely by simple surface modification methods
introduced earlier in the section 1.1 of this chapter.
1.3.2.
Cyanate ester resins
Cyanate ester resins (CERs), present a relatively new class of thermosetting
polymers that have been under intense investigation and development in recent years.
CER systems are being designated as the next generation of thermosetting polymers
following the widely used epoxy resins. Like PEEK, they attract increasing attention
due to their outstanding performance regarding resistance to fire and moisture, good
mechanical strength and electric stability at cryogenic and elevated temperatures, high
glass transition temperature (Tg), low dielectric constant, radiation resistance, radar
transparency, excellent metal adhesion, and compatibility with carbon fiber
reinforcements. These unique properties of CE resins make them preferential
candidates as structural materials for high-temperature applications in air-, and
spacecraft technologies, insulation, microelectronics, and adhesive industries.153-155
24
The first successful synthesis of aromatic cyanate ester monomers (CEMs) was
developed by Grigat et al. in the 1960s at Bayer AG, which involved the reaction of
phenolic compounds with a cyanogen halide in the presence of a base (Scheme 1.5).
Scheme 1.5: General scheme for the synthesis of CEM.
The remarkable aspect of CEMs is their polymerisation via a cyclotrimerization
reaction of multiple cyanate groups (-OCN) to form a polycyanurate (PC) thermoset
with high yield (Scheme 1.6). 155, 156, 157
Scheme 1.6: A generalized scheme for thermal curing by cyclotrimerization of the cyanate groups in
cyanate ester monomers (CEMs) to give a thermosetting polycyanurate. For simplicity only three
triazine rings are shown while the real crosslinked polycyanurate network (marked with *) consists of
many triazine rings.
The versatility of the synthetic method developed by Grigat et al. made it possible
to incorporate different aromatic structural entities into CEMs, thus offering a control
over the chemical, physical, and thermal properties of CEMs and PCs by careful
selection of the precursor phenols. The development of ambient temperature
processable CEMs, which could produce PC with good thermal and mechanical
properties, is an active area of current CE resin research. The superior thermal
properties of CER systems manifest a near future transition from epoxy based
adhesive systems to CER based ones, especially in combination with thermally stable
material like PEEK.
25
1.3.3.
Solid state nano/macro-pores
Since long, mimicking the processes of nature has been an inspiration for many
researchers. In this context the importance of biological channels in many
physiological processes of a living organism has stimulated the interest in artificially
fabricated functional nanopores and nanochannes.158 Although biological channels
have proved to be very useful for a range of interesting translocation experiments,
which include gating and selective ion permeation or even the translocation of
polymeric substances.159-162 However, they do exhibit a number of disadvantages
including the limited stability. For example, in biological membranes, the pores and
their embedding lipid bilayer are susceptible to changes in external parameters such as
pH, salt concentration, temperature, mechanical stress, etc. Fabrication of nanopores
from solid-state materials (polymeric or ceramic membranes) presents advantages
over their biological counterpart such as high stability, control of diameter and
channel length and the potential for integration into devices and arrays. Various routes
have been explored to meet the challenge of fabricating pores with true nanometer
dimensions.163 Two types of artificially fabricated solid-state pores investigated in the
present work are introduced here.
1.3.3.1.
Polymer membranes with Track-Etched Nanopores
The nanopores can be produced in polymer foils using the ion track-etching
technique. This method, which has become well established to create very uniform
pores in insulators, is based on the following effect: When a swift heavy ion e.g Pb, U
or Au), generated at a synchrotron or nuclear accelerator, passes through matter, it
deposits its energy along its trajectory, thus creating a cylindrical damaged zone
called “the latent track”. By a suitable wet chemical etching the damaged material
along the track can be removed quicker than the bulk material, thus developing the
tracks into pores (Scheme 1.7).164 The number of pores formed in this way is equal to
the number of ions penetrating the sample. By reducing the number of impinging ions
to one, it is possible to prepare a membrane with one single pore.
26
Scheme 1.7: A Schematic illustration of nanopore fabrication with different shapes.
The track of a heavy ion is cylindrical in shape, whereas the geometry of the
developed pore can be controlled by the etching conditions. To obtain cylindrical
pore, it is necessary to apply a procedure that assures fast penetration of the etchant
along the track with minimum non-specific etching of the material.165 Tapered-cone
(conical) pores can be obtained by asymmetric etching of the irradiated foils.166-168
Etching is performed in a conductivity cell from one side only, while the other side of
the foil is protected by a so-called stopping medium, which neutralizes the action of
the etchant. Double-conical nanopores can be prepared in a two-step process: i) short
pre-etching of an irradiated foil from both sides; and ii) subsequent etching from one
side, while protecting the other side of the membrane with stopping solution.
The most widely used polymeric materials for solid-state track etched nanopore
fabrication are PET (Hostaphan RN 12) and PI (Kapton 50 HN). The chemical
etching (Etchants: NaOH for PET and sodium hypochlorite with 13% active chlorine
content for PI) of both the polymeric materials results in the formation of carboxylate
groups on the pore walls and on the surface of the membrane. A direct consequence of
the presence of carboxylate groups is the possibility of regulating the surface charge
by immersing the membranes in electrolyte solutions buffered at various pH values.
At neutral and basic conditions, the carboxylate groups are deprotonated and the net
surface charge is negative. Lowering the pH to values close to the isoelectric point of
the track-etched surface neutralizes the surface charge.166-168
The single conical nanopores are of special interest as they are able to rectify the
ion transport flowing through them. The behavior is called “rectifying”, because the
27
nanopore transports more ions in one direction than in the other. It has been
demonstrated, both theoretically169-171 and experimentally,172,
173
that the rectifying
characteristics of the nanopores emerge due to a synergy of the entropic driving force
caused by the channel asymmetry and the electrostatic effects due to the fixed charges
on the pore wall. Experimentally, by applying a transmembrane potential across the
membrane and measuring Current–Voltage (I-V) curve under symmetrical electrolyte
conditions, an electric diode like current rectification is obtained.174-176 This indicates
a preferential direction for ion flow. This behaviour is in close resemblance to
voltage-gated biological ion channels.172, 173
The surface negative charge renders the PET and Kapton nanopores cation
selective and it was found by Siwy et al. that 90% of the ion current signal is due to
potassium ions, while using KCl as electrolyte.177 Neutral pores do not rectify the ion
current, while positively and negatively charged pores rectify the current in opposite
directions.175 An example of this pH dependence rectification phenomenon is shown
in Figure 1.4 for a conical track-etched pore, whose pore walls are negatively charged
at neutral pH (-COO−) and become neutral at low pH, where the carboxylate groups
are protonated (-COOH). It can be clearly seen that the negatively charged pore
rectifies the ionic current while the neutral one does not.
Figure 1.4: I-V curve measurement of track etched conical nanopore in PI membrane at different pH
values.
Ion channels with asymmetric I−V characteristics are also found in nature.178 Their
behavior is also called ‘rectifying’. This behavior is seen in the inwardly rectifying
potassium channels,179, 180 which transport potassium effectively into, but not out of
the cell. Such analogies of artificially fabricated solid-state conical nanopores with
naturally occurring systems have triggered the interest of the scientific community
28
related to diverse research fields, including life sciences, chemistry, and applied
biophysics.172, 173, 181, 182
Although the surface physical and chemical properties of these nanopores are yet
to be fully exploited, separation techniques, biological sensing, and fabrication of
nano-actuators are the research areas actively considering these materials.183-187
1.3.3.2.
Ordered Macroporous Si/SiO2
Ordered macroporous silicon is another class of artificially fabricated solid-state
pores that was developed by V. Lehmann188 in 1993 at Siemens AG. Macroporous
silicon develops if silicon is anodically biased and photoelectrochemically etched in
hydrofluoric acid (HF) (Figure 1.5).
Figure 1.5: a) Sketch of an electrochemical etching setup for the growth of macroporous silicon. The
silicon is anodically biased, the front side is exposed to HF and the back side is illuminated to generate
electron-hole pairs. b) An enlarged view of the situation at the bottom of the pores: The SCR extends to
the red line in the silicon. Hence, this region is depleted and an electric field forms that focuses the
minority charge carriers to the pore tips. (Taken from Ref.189).
This is a well known process especially for n-type silicon and is briefly described
here. Prestructured nucleation spots called etch pits are first defined at the surface of
an (100)-oriented n-type silicon wafer. The pore formation initiates at these etch pits.
The prestructuring is necessary for fabrication of ordered macroporous structure
(Figure 1.6), otherwise applying this procedure to an unpatterned, polished wafer
surface results in the random distribution of pores instead.190 An ohmic contact at the
edge of the sample and a platinum wire in the electrolyte is used to apply a bias.
During electrochemical etching, the front side of the sample (lithographically
prestructured) is in contact with the HF electrolyte and kept in the dark while the
backside is illuminated to create electronic holes via light absorption in the silicon.
The electronic holes diffuse from the backside to the etch front and are consumed for
the electrochemical etching process almost exclusively at the pore tips. The pore walls
29
are protected against electrochemical dissolution by a space charge region (SCR)
originating from the silicon/electrolyte contact. This process results in a periodic array
of straight air pores in silicon. Due to electrochemical passivation of the pore walls
very high aspect ratios (ratio between pore length and pore diameter) of 100-500 are
obtained.
Figure 1.6: Scanning electron microscope (SEM) images of macroporous silicon: a) Bird’s eye view of
KOH etch pits formed at the silicon surface prior to the etching process in a square arrangement with
a lattice constant of a=2µm (taken from Ref.189 b) Straight pores with 50 µm depth etched into silicon
with a predefined square lattice of 0.6 µm pore-to-pore distance.
Since the discovery of ordered macropore arrays in 1993, most of the research is
devoted to the applications intrinsically originating from material inherent properties.
These include photonics applications,191, 192, lithographically defined waveguides and
microresonators193. First application of these materials resulting from the
manipulation of surface chemistry came form Sonia E. Létant et al.194 where they
presented the first functionalized silicon membranes and demonstrate their ability to
selectively capture streptavidine coated polystyrene microbeads termed as simulated
bio-organisms. They could functionalize the hydride terminated silicon membrane
employing a Lewis acid catalyzed hydrosilylation reaction which results in a Si-C
linkage.195 Alternatively oxidation of the silicon surface can be performed to generate
surface silanol groups for functionalization with a chloro- or alkoxysilane.196 This
later strategy has been employed by Keiki-Pua S. Dancil et al.197 while investigating
the use of porous Si as an immobilization and transducing matrix for monitoring
protein-protein binding between protein A with IgG. Biosensing was achieved by
monitoring the change in effective optical thickness (EOT) of a functionalized porous
Si Fabry-Perot film upon analyte binding. These examples highlight the potential of
this system for applications other than those inherent with the material physical bulk
properties. A control over surface chemical properties will be a key element for the
exploration to the full potential.
30
1.4. Reference
1.
Angelova, N.; Hunkeler, D., Rationalizing the Design of Polymeric Biomaterials. Trends in
Biotechnology 1999, 17, (10), 409-421.
2.
Pillai, O.; Panchagnula, R., Polymers in Drug Delivery. Current Opinion in Chemical Biology
2001, 5, (4), 447-451.
3.
Wang, Y. D.; Ameer, G. A.; Sheppard, B. J.; Langer, R., A Tough Biodegradable Elastomer.
Nature Biotechnology 2002, 20, (6), 602-606.
4.
Weinberg, B. D.; Blanc, E.; Ga, J. M., Polymer Implants for Intratumoral Drug Delivery and
Cancer Therapy. Journal of Pharmaceutical Sciences 2008, 97, (5), 1681-1702.
5.
MacDonald, W. A., Structural Materials Underpinning Functional Materials: Teaching Old
Dogs New Tricks. Polymer International 2008, 57, (5), 672-677.
6.
Kato, K.; Uchida, E.; Kang, E. T.; Uyama, Y.; Ikada, Y., Polymer Surface with Graft Chains.
Progress in Polymer Science 2003, 28, (2), 209-259.
7.
Jagur-Grodzinski, J., Modification of Polymers under Heterogeneous Conditions. Progress in
Polymer Science 1992, 17, (3), 361-415.
8.
Chen, C. M., Polymer Surface Modification and Characterization. Hanser Publishers:
Munich, 1993.
9.
Ryu, D. Y.; Shin, K.; Drockenmuller, E.; Hawker, C. J.; Russell, T. P., A Generalized
Approach to the Modification of Solid Surfaces. Science 2005, 308, (5719), 236-239.
10.
Decher, G., Fuzzy Nanoassemblies: Toward Layered Polymeric Multicomposites. Science
1997, 277, (5330), 1232-1237.
11.
Halperin, A.; Tirrell, M.; Lodge, T. P., Tethered Chains in Polymer Microstructures. Advances
in Polymer Science 1992, 100, 31-71.
12.
Milner, S. T., Polymer Brushes. Science 1991, 251, (4996), 905-914.
13.
Edmondson, S.; Osborne, V. L.; Huck, W. T. S., Polymer Brushes Via Surface-Initiated
Polymerizations. Chemical Society Reviews 2004, 33, (1), 14-22.
14.
Conrads, H.; Schmidt, M., Plasma Generation and Plasma Sources. Plasma Sources Science &
Technology 2000, 9, (4), 441-454.
15.
Lopez, G. P.; Ratner, B. D.; Rapoza, R. J.; Horbett, T. A., Plasma Deposition of Ultrathin
Films of Poly(2-Hydroxyethyl Methacrylate) - Surface-Analysis and Protein Adsorption
Measurements. Macromolecules 1993, 26, (13), 3247-3253.
16.
Chifen, A. N.; Jenkins, A. T. A.; Knoll, W.; Forch, R., Adhesion Improvement of PlasmaPolymerized Maleic Anhydride Films on Gold Using Hmdso/O-2 Adhesion Layers. Plasma Processes
and Polymers 2007, 4, (9), 815-822.
17.
Chu, L. Q.; Forch, R.; Knoll, W., Surface-Plasmon-Enhanced Fluorescence Spectroscopy for
DNA Detection Using Fluorescently Labeled Pna As "DNA Indicator". Angewandte ChemieInternational Edition 2007, 46, (26), 4944-4947.
18.
Chu, L. Q.; Knoll, W.; Forch, R., Plasma Polymerized Epoxide Functional Surfaces for DNA
Probe Immobilization. Biosensors & Bioelectronics 2008, 24, (1), 118-122.
19.
Chu, L. Q.; Zou, X. N.; Knoll, W.; Forch, R., Thermosensitive Surfaces Fabricated by Plasma
Polymerization of N,N-Diethylacrylamide. Surface & Coatings Technology 2008, 202, (10), 20472051.
20.
Forch, R.; Zhang, Z. H.; Knoll, W., Soft Plasma Treated Surfaces: Tailoring of Structure and
Properties for Biomaterial Applications. Plasma Processes and Polymers 2005, 2, (5), 351-372.
21.
Yasuda, H.; Yasuda, T., The Competitive Ablation and Polymerization (Cap) Principle and
the Plasma Sensitivity of Elements in Plasma Polymerization and Treatment. Journal of Polymer
Science Part A-Polymer Chemistry 2000, 38, (6), 943-953.
22.
Liu, X. H.; Ma, P. X., Polymeric Scaffolds for Bone Tissue Engineering. Annals of
Biomedical Engineering 2004, 32, (3), 477-486.
23.
Bag, D. S.; Kumar, V. P.; Maiti, S., Chemical Modification of Ldpe Film. Journal of Applied
Polymer Science 1999, 71, (7), 1041-1048.
24.
Goddard, J. M.; Talbert, J. N.; Hotchkiss, J. H., Covalent Attachment of Lactase to LowDensity Polyethylene Films. Journal of Food Science 2007, 72, (1), E36-E41.
25.
Kong, J. S.; Lee, D. J.; Kim, H. D., Surface Modification of Low-Density Polyethylene (Ldpe)
Film and Improvement of Adhesion between Evaporated Copper Metal Film and Ldpe. Journal of
Applied Polymer Science 2001, 82, (7), 1677-1690.
26.
Rasmussen, J. R.; Stedronsky, E. R.; Whitesides, G. M., Introduction, Modification, and
Characterization of Functional-Groups on Surface of Low-Density Polyethylene Film. Journal of the
American Chemical Society 1977, 99, (14), 4736-4745.
31
27.
Tao, G. L.; Gong, A. J.; Lu, J. J.; Sue, H. J.; Bergbreiter, D. E., Surface Functionalized
Polypropylene: Synthesis, Characterization, and Adhesion Properties. Macromolecules 2001, 34, (22),
7672-7679.
28.
Bening, R. C.; McCarthy, T. J., Surface Modification of Poly(Tetrafluoroethylene-CoHexafluoropropylene) - Introduction of Alcohol Functionality. Macromolecules 1990, 23, (10), 26482655.
29.
Lee,
K.
W.;
McCarthy,
T.
J.,
Chemistry
of
Surface-Hydroxylated
Poly(Chlorotrifluoroethylene). Macromolecules 1988, 21, (8), 2318-2330.
30.
Lee, K. W.; McCarthy, T. J., Synthesis of a Polymer Surface Containing Covalently Attached
Triethoxysilane Functionality - Adhesion to Glass. Macromolecules 1988, 21, (12), 3353-3356.
31.
Shoichet, M. S.; McCarthy, T. J., Convenient Syntheses of Carboxylic-Acid Functionalized
Fluoropolymer Surfaces. Macromolecules 1991, 24, (5), 982-986.
32.
Franchina, N. L.; McCarthy, T. J., Surface Modifications of Poly(Ether Ether Ketone).
Macromolecules 1991, 24, (11), 3045-3049.
33.
Goddard, J. M.; Hotchkiss, J. H., Polymer Surface Modification for the Attachment of
Bioactive Compounds. Progress in Polymer Science 2007, 32, (7), 698-725.
34.
Alferiev, I. S.; Connolly, J. M.; Stachelek, S. J.; Ottey, A.; Rauova, L.; Levy, R. J., Surface
Heparinization of Polyurethane Via Bromoalkylation of Hard Segment Nitrogens. Biomacromolecules
2006, 7, (1), 317-322.
35.
Advincula, R. C. B., William J.; Baster, Kenneth C.; Ruhe, Jurgen, Polymer Brushes:
Synthesis, Characterization, Applications. Publisher: (Wiley-VCH Verlag GmbH & Co. KGaA,
Germany) Weinheim, 2004.
36.
Lo Verso, F.; Likos, C. N., End-Functionalized Polymers: Versatile Building Blocks for Soft
Materials. Polymer 2008, 49, (6), 1425-1434.
37.
Mansky, P.; Liu, Y.; Huang, E.; Russell, T. P.; Hawker, C. J., Controlling Polymer-Surface
Interactions with Random Copolymer Brushes. Science 1997, 275, (5305), 1458-1460.
38.
Prucker, O.; Naumann, C. A.; Ruhe, J.; Knoll, W.; Frank, C. W., Photochemical Attachment
of Polymer Films to Solid Surfaces Via Monolayers of Benzophenone Derivatives. Journal of the
39.
Koutsos, V.; van der Vegte, E. W.; Hadziioannou, G., Direct View of Structural Regimes of
End-Grafted Polymer Monolayers: A Scanning Force Microscopy Study. Macromolecules 1999, 32,
(4), 1233-1236.
40.
Koutsos, V.; vanderVegte, E. W.; Pelletier, E.; Stamouli, A.; Hadziioannou, G., Structure of
Chemically End-Grafted Polymer Chains Studied by Scanning Force Microscopy in Bad-Solvent
Conditions. Macromolecules 1997, 30, (16), 4719-4726.
41.
Bergbreiter, D. E.; Franchina, J. G.; Kabza, K., Hyperbranched Grafting on Oxidized
Polyethylene Surfaces. Macromolecules 1999, 32, (15), 4993-4998.
42.
Kolb, H. C.; Finn, M. G.; Sharpless, K. B., Click Chemistry: Diverse Chemical Function from
a Few Good Reactions. Angewandte Chemie-International Edition 2001, 40, (11), 2004-+.
43.
Kolb, H. C.; Sharpless, K. B., The Growing Impact of Click Chemistry on Drug Discovery.
Drug Discovery Today 2003, 8, (24), 1128-1137.
44.
Gothelf, K. V.; Jorgensen, K. A., Asymmetric 1,3-Dipolar Cycloaddition Reactions. Chemical
Reviews 1998, 98, (2), 863-909.
45.
Huisgen, R., Cycloadditions - Definition Classification and Characterization. Angewandte
Chemie-International Edition 1968, 7, (5), 321-&.
46.
Rostovtsev, V. V.; Green, L. G.; Fokin, V. V.; Sharpless, K. B., A Stepwise Huisgen
Cycloaddition Process: Copper(I)-Catalyzed Regioselective "Ligation" Of Azides and Terminal
Alkynes. Angewandte Chemie-International Edition 2002, 41, (14), 2596-+.
47.
Tornoe, C. W.; Christensen, C.; Meldal, M., Peptidotriazoles on Solid Phase: [1,2,3]-Triazoles
by Regiospecific Copper(I)-Catalyzed 1,3-Dipolar Cycloadditions of Terminal Alkynes to Azides.
Journal of Organic Chemistry 2002, 67, (9), 3057-3064.
48.
Appukkuttan, P.; Dehaen, W.; Fokin, V. V.; Van der Eycken, E., A Microwave-Assisted Click
Chemistry Synthesis of 1,4-Disubstituted 1,2,3-Triazoles Via a Copper(I)-Catalyzed Three-Component
Reaction. Organic Letters 2004, 6, (23), 4223-4225.
49.
Lewis, W. G.; Magallon, F. G.; Fokin, V. V.; Finn, M. G., Discovery and Characterization of
Catalysts for Azide-Alkyne Cycloaddition by Fluorescence Quenching. Journal of the American
Chemical Society 2004, 126, (30), 9152-9153.
50.
Rodionov, V. O.; Fokin, V. V.; Finn, M. G., Mechanism of the Ligand-Free Cu-I-Catalyzed
Azide-Alkyne Cycloaddition Reaction. Angewandte Chemie-International Edition 2005, 44, (15),
2210-2215.
32
51.
Bock, V. D.; Hiemstra, H.; van Maarseveen, J. H., Cu-I-Catalyzed Alkyne-Azide "Click"
Cycloadditions from a Mechanistic and Synthetic Perspective. European Journal of Organic Chemistry
2006, (1), 51-68.
52.
Alvarez, R.; Velazquez, S.; Sanfelix, A.; Aquaro, S.; Declercq, E.; Perno, C. F.; Karlsson, A.;
Balzarini, J.; Camarasa, M. J., 1,2,3-Triazole-[2',5'-Bis-O-(Tert-Butyldimethylsilyl)-Beta-DRibofuranos Yl]-3'-Spiro-5''-(4''-Amino-1'',2''-Oxathiol 2'',2''-Dioxide) (Tsao) Analogs - Synthesis and
Anti-Hiv-1 Activity. Journal of Medicinal Chemistry 1994, 37, (24), 4185-4194.
53.
Genin, M. J.; Allwine, D. A.; Anderson, D. J.; Barbachyn, M. R.; Emmert, D. E.; Garmon, S.
A.; Graber, D. R.; Grega, K. C.; Hester, J. B.; Hutchinson, D. K.; Morris, J.; Reischer, R. J.; Ford, C.
W.; Zurenko, G. E.; Hamel, J. C.; Schaadt, R. D.; Stapert, D.; Yagi, B. H., Substituent Effects on the
Antibacterial Activity of Nitrogen-Carbon-Linked (Azolylphenyl)Oxazolidinones with Expanded
Activity against the Fastidious Gram-Negative Organisms Haemophilus Influenzae and Moraxella
Catarrhalis. Journal of Medicinal Chemistry 2000, 43, (5), 953-970.
54.
Krasinski, A.; Radic, Z.; Manetsch, R.; Raushel, J.; Taylor, P.; Sharpless, K. B.; Kolb, H. C.,
In Situ Selection of Lead Compounds by Click Chemistry: Target-Guided Optimization of
Acetylcholinesterase Inhibitors. Journal of the American Chemical Society 2005, 127, (18), 6686-6692.
55.
Lee, L. V.; Mitchell, M. L.; Huang, S. J.; Fokin, V. V.; Sharpless, K. B.; Wong, C. H., A
Potent and Highly Selective Inhibitor of Human Alpha-1,3-Fucosyltransferase Via Click Chemistry.
Journal of the American Chemical Society 2003, 125, (32), 9588-9589.
56.
Fazio, F.; Bryan, M. C.; Blixt, O.; Paulson, J. C.; Wong, C. H., Synthesis of Sugar Arrays in
Microtiter Plate. Journal of the American Chemical Society 2002, 124, (48), 14397-14402.
57.
Fleischmann, S.; Hinrichs, K.; Oertel, U.; Reichelt, S.; Eichhorn, K. J.; Voit, B., Modification
of Polymer Surfaces by Click Chemistry. Macromolecular Rapid Communications 2008, 29, (12-13),
1177-1185.
58.
Collman, J. P.; Devaraj, N. K.; Chidsey, C. E. D., "Clicking" Functionality onto Electrode
Surfaces. Langmuir 2004, 20, (4), 1051-1053.
59.
Lee, J. K.; Chi, Y. S.; Choi, I. S., Reactivity of Acetylenyl-Terminated Self-Assembled
Monolayers on Gold: Triazole Formation. Langmuir 2004, 20, (10), 3844-3847.
60.
White, M. A.; Johnson, J. A.; Koberstein, J. T.; Turro, N. J., Toward the Syntheses of
Universal Ligands for Metal Oxide Surfaces: Controlling Surface Functionality through Click
Chemistry. Journal of the American Chemical Society 2006, 128, (35), 11356-11357.
61.
Lummerstorfer, T.; Hoffmann, H., Click Chemistry on Surfaces: 1,3-Dipolar Cycloaddition
Reactions of Azide-Terminated Monolayers on Silica. Journal of Physical Chemistry B 2004, 108,
(13), 3963-3966.
62.
Punna, S.; Kaltgrad, E.; Finn, M. G., "Clickable" Agarose for Affinity Chromatography.
Bioconjugate Chemistry 2005, 16, (6), 1536-1541.
63.
Slater, M.; Snauko, M.; Svec, F.; Frechet, J. M. J., "Click Chemistry" In the Preparation of
Porous Polymer-Based Particulate Stationary Phases for Mu-Hplc Separation of Peptides and Proteins.
Analytical Chemistry 2006, 78, (14), 4969-4975.
64.
Nandivada, H.; Chen, H. Y.; Bondarenko, L.; Lahann, J., Reactive Polymer Coatings That
"Click"". Angewandte Chemie-International Edition 2006, 45, (20), 3360-3363.
65.
Rozkiewicz, D. I.; Janczewski, D.; Verboom, W.; Ravoo, B. J.; Reinhoudt, D. N., "Click"
Chemistry by Microcontact Printing. Angewandte Chemie-International Edition 2006, 45, (32), 52925296.
66.
Wu, P.; Feldman, A. K.; Nugent, A. K.; Hawker, C. J.; Scheel, A.; Voit, B.; Pyun, J.; Frechet,
J. M. J.; Sharpless, K. B.; Fokin, V. V., Efficiency and Fidelity in a Click-Chemistry Route to Triazole
Dendrimers by the Copper(I)-Catalyzed Ligation of Azides and Alkynes. Angewandte ChemieInternational Edition 2004, 43, (30), 3928-3932.
67.
Lutz, J. F.; Borner, H. G.; Weichenhan, K., Combining Atom Transfer Radical Polymerization
and Click Chemistry: A Versatile Method for the Preparation of End-Functional Polymers.
Macromolecular Rapid Communications 2005, 26, (7), 514-518.
68.
Lutz, J. F.; Borner, H. G.; Weichenhan, K., Combining ATRP And "Click" Chemistry: A
Promising Platform toward Functional Biocompatible Polymers and Polymer Bioconjugates.
Macromolecules 2006, 39, (19), 6376-6383.
69.
Sumerlin, B. S.; Tsarevsky, N. V.; Louche, G.; Lee, R. Y.; Matyjaszewski, K., Highly
Efficient "Click" Functionalization of Poly(3-Azidopropyl Methacrylate) Prepared by ATRP.
Macromolecules 2005, 38, (18), 7540-7545.
70.
Ladmiral, V.; Mantovani, G.; Clarkson, G. J.; Cauet, S.; Irwin, J. L.; Haddleton, D. M.,
Synthesis of Neoglycopolymers by a Combination Of "Click Chemistry" And Living Radical
Polymerization. Journal of the American Chemical Society 2006, 128, (14), 4823-4830.
33
71.
Tsarevsky, N. V.; Sumerlin, B. S.; Matyjaszewski, K., Step-Growth "Click" Coupling of
Telechelic Polymers Prepared by Atom Transfer Radical Polymerization. Macromolecules 2005, 38,
(9), 3558-3561.
72.
Van Camp, W.; Germonpre, V.; Mespouille, L.; Dubois, P.; Goethals, E. J.; Du Prez, F. E. In
New Poly(Acrylic Acid) Containing Segmented Copolymer Structures by Combination Of "Click"
Chemistry and Atom Transfer Radical Polymerization, 2007; Elsevier Science Bv: 2007; pp 11681180.
73.
Thomsen, A. D.; Malmstrom, E.; Hvilsted, S., Novel Polymers with a High Carboxylic Acid
Loading. Journal of Polymer Science Part A-Polymer Chemistry 2006, 44, (21), 6360-6377.
74.
Luxenhofer, R.; Jordan, R., Click Chemistry with Poly(2-Oxazoline)S. Macromolecules 2006,
39, (10), 3509-3516.
75.
Gao, H. F.; Matyjaszewski, K., Synthesis of Star Polymers by a Combination of ATRP and
The "Click" Coupling Method. Macromolecules 2006, 39, (15), 4960-4965.
76.
Tsarevsky, N. V.; Matyjaszewski, K., "Green" Atom Transfer Radical Polymerization: From
Process Design to Preparation of Well-Defined Environmentally Friendly Polymeric Materials.
Chemical Reviews 2007, 107, (6), 2270-2299.
77.
Gondi, S. R.; Vogt, A. P.; Sumerlin, B. S., Versatile Pathway to Functional Telechelics Via
RAFT Polymerization and Click Chemistry. Macromolecules 2007, 40, (3), 474-481.
78.
Quemener, D.; Le Hellaye, M.; Bissett, C.; Davis, T. P.; Barner-Kowollik, C.; Stenzel, M. H.,
Graft Block Copolymers of Propargyl Methacrylate and Vinyl Acetate Via a Combination of RAFT
/Madix and Click Chemistry: Reaction Analysis. Journal of Polymer Science Part A-Polymer
Chemistry 2008, 46, (1), 155-173.
79.
Ranjan, R.; Brittain, W. J., Combination of Living Radical Polymerization and Click
Chemistry for Surface Modification. Macromolecules 2007, 40, (17), 6217-6223.
80.
Gromadzki, D.; Lokaj, J.; Cernoch, P.; Diat, O.; Nallet, F.; Stepanek, P., Morphology of
Polystyrene-Block-Poly(Styrene-Co-Acrylonitrile)
and
Polystyrene-Block-Poly(Styrene-CoAcrylonitrile-Co-5-Vinyltetrazole) Diblock Copolymers Prepared by Nitroxide-Mediated Radical
Polymerization And "Click" Chemistry. European Polymer Journal 2008, 44, (1), 189-199.
81.
Malkoch, M.; Thibault, R. J.; Drockenmuller, E.; Messerschmidt, M.; Voit, B.; Russell, T. P.;
Hawker, C. J., Orthogonal Approaches to the Simultaneous and Cascade Functionalization of
Macromolecules Using Click Chemistry. Journal of the American Chemical Society 2005, 127, (42),
14942-14949.
82.
Binder, W. H.; Kluger, C., Combining Ring-Opening Metathesis Polymerization (Romp) with
Sharpless-Type "Click" Reactions: An Easy Method for the Preparation of Side Chain Functionalized
Poly(Oxynorbornenes). Macromolecules 2004, 37, (25), 9321-9330.
83.
Binder, W. H.; Kluger, C.; Josipovic, M.; Straif, C. J.; Friedbacher, G., Directing
Supramolecular Nanoparticle Binding onto Polymer Films: Film Formation and Influence of Receptor
Density on Binding Densities. Macromolecules 2006, 39, (23), 8092-8101.
84.
Binder, W. H.; Kluger, C.; Straif, C. J.; Friedbacher, G., Directed Nanoparticle Binding onto
Microphase-Separated Block Copolymer Thin Films. Macromolecules 2005, 38, (23), 9405-9410.
85.
Shi, Q.; Chen, X. S.; Lu, T. C.; Jing, X. B., The Immobilization of Proteins on Biodegradable
Polymer Fibers Via Click Chemistry. Biomaterials 2008, 29, (8), 1118-1126.
86.
Binder, W. H.; Sachsenhofer, R., 'Click' Chemistry in Polymer and Materials Science.
87.
Binder, W. H.; Sachsenhofer, R., 'Click' Chemistry in Polymer and Material Science: An
Update. Macromolecular Rapid Communications 2008, 29, (12-13), 952-981.
88.
Johnson, J. A.; Finn, M. G.; Koberstein, J. T.; Turro, N. J., Construction of Linear Polymers,
Dendrimers, Networks, and Other Polymeric Architectures by Copper-Catalyzed Azide-Alkyne
Cycloaddition "Click" Chemistry. Macromolecular Rapid Communications 2008, 29, (12-13), 10521072.
89.
Le Drournaguet, B.; Velonia, K., Click Chemistry: A Powerful Tool to Create Polymer-Based
Macromolecular Chimeras. Macromolecular Rapid Communications 2008, 29, (12-13), 1073-1089.
90.
Lecomte, P.; Riva, R.; Jerome, C.; Jerome, R., Macromolecular Engineering of Biodegradable
Polyesters by Ring-Opening Polymerization and 'Click' Chemistry. Macromolecular Rapid
Communications 2008, 29, (12-13), 982-997.
91.
Lundberg, P.; Hawker, C. J.; Hult, A.; Malkoch, M., Click Assisted One-Pot Multi-Step
Reactions in Polymer Science: Accelerated Synthetic Protocols. Macromolecular Rapid
Communications 2008, 29, (12-13), 998-1015.
92.
Meldal, M., Polymer "Clicking" By Cuaac Reactions. Macromolecular Rapid
Communications 2008, 29, (12-13), 1016-1051.
34
93.
Ostaci, R. V.; Damiron, D.; Capponi, S.; Vignaud, G.; Leger, L.; Grohens, Y.; Drockenmuller,
E., Polymer Brushes Grafted To "Passivated" Silicon Substrates Using Click Chemistry. Langmuir
2008, 24, (6), 2732-2738.
94.
Advincula, R.; Zhou, Q. G.; Park, M.; Wang, S. G.; Mays, J.; Sakellariou, G.; Pispas, S.;
Hadjichristidis, N., Polymer Brushes by Living Anionic Surface Initiated Polymerization on Flat
Silicon (Siox) and Gold Surfaces: Homopolymers and Block Copolymers. Langmuir 2002, 18, (22),
8672-8684.
95.
Paul, R.; Schmidt, R.; Dyer, D. J., Synthesis of Ultrathin Films of Polyacrylonitrile by
Photoinitiated Polymerization from Self-Assembled Monolayers on Gold. Langmuir 2002, 18, (23),
8719-8723.
96.
Rutenberg, I. M.; Scherman, O. A.; Grubbs, R. H.; Jiang, W. R.; Garfunkel, E.; Bao, Z.,
Synthesis of Polymer Dielectric Layers for Organic Thin Film Transistors Via Surface-Initiated RingOpening Metathesis Polymerization. Journal of the American Chemical Society 2004, 126, (13), 40624063.
97.
Jones, D. M.; Huck, W. T. S., Controlled Surface-Initiated Polymerizations in Aqueous
Media. Advanced Materials 2001, 13, (16), 1256-1259.
98.
Shah, R. R.; Merreceyes, D.; Husemann, M.; Rees, I.; Abbott, N. L.; Hawker, C. J.; Hedrick,
J. L., Using Atom Transfer Radical Polymerization to Amplify Monolayers of Initiators Patterned by
Microcontact Printing into Polymer Brushes for Pattern Transfer. Macromolecules 2000, 33, (2), 597605.
99.
Matyjaszewski, K.; Miller, P. J.; Shukla, N.; Immaraporn, B.; Gelman, A.; Luokala, B. B.;
Siclovan, T. M.; Kickelbick, G.; Vallant, T.; Hoffmann, H.; Pakula, T., Polymers at Interfaces: Using
Atom Transfer Radical Polymerization in the Controlled Growth of Homopolymers and Block
Copolymers from Silicon Surfaces in the Absence of Untethered Sacrificial Initiator. Macromolecules
1999, 32, (26), 8716-8724.
100.
Tsujii, Y.; Ejaz, M.; Sato, K.; Goto, A.; Fukuda, T., Mechanism and Kinetics of RAFTMediated Graft Polymerization of Styrene on a Solid Surface. 1. Experimental Evidence of Surface
Radical Migration. Macromolecules 2001, 34, (26), 8872-8878.
101.
Zhao, B.; Brittain, W. J., Synthesis of Tethered Polystyrene-Block-Poly(Methyl Methacrylate)
Monolayer on a Silicate Substrate by Sequential Carbocationic Polymerization and Atom Transfer
Radical Polymerization. Journal of the American Chemical Society 1999, 121, (14), 3557-3558.
102.
Kong, X. X.; Kawai, T.; Abe, J.; Iyoda, T., Amphiphilic Polymer Brushes Grown from the
Silicon Surface by Atom Transfer Radical Polymerization. Macromolecules 2001, 34, (6), 1837-1844.
103.
Wang, H. J.; Zhou, W. H.; Yin, X. F.; Zhuang, Z. X.; Yang, H. H.; Wang, X. R., Template
Synthesized Molecularly Imprinted Polymer Nanotube Membranes for Chemical Separations. Journal
of the American Chemical Society 2006, 128, (50), 15954-15955.
104.
Zhang, Z.; Chao, T.; Chen, S. F.; Jiang, S. Y., Superlow Fouling Sulfobetaine and
Carboxybetaine Polymers on Glass Slides. Langmuir 2006, 22, (24), 10072-10077.
105.
Huang, W. X.; Skanth, G.; Baker, G. L.; Bruening, M. L., Surface-Initiated Thermal Radical
Polymerization on Gold. Langmuir 2001, 17, (5), 1731-1736.
106.
Brown, A. A.; Khan, N. S.; Steinbock, L.; Huck, W. T. S., Synthesis of Oligo(Ethylene
Glycol) Methacrylate Polymer Brushes. European Polymer Journal 2005, 41, (8), 1757-1765.
107.
Friebe, A.; Ulbricht, M., Controlled Pore Functionalization of Poly(Ethylene Terephthalate)
Track-Etched Membranes Via Surface-Initiated Atom Transfer Radical Polymerization. Langmuir
2007, 23, (20), 10316-10322.
108.
Farhan, T.; Huck, W. T. S., Synthesis of Patterned Polymer Brushes from Flexible Polymeric
Films. European Polymer Journal 2004, 40, (8), 1599-1604.
109.
Kim, D. J.; Heo, J. Y.; Kim, K. S.; Choi, I. S., Formation of Thermoresponsive
Poly(Nisopropylacrylamide)/Dextran Particles by Atom Transfer Radical Polymerization.
110.
Jordan, R.; Ulman, A.; Kang, J. F.; Rafailovich, M. H.; Sokolov, J., Surface-Initiated Anionic
Polymerization of Styrene by Means of Self-Assembled Monolayers. Journal of the American
Chemical Society 1999, 121, (5), 1016-1022.
111.
Zhao, B.; Brittain, W. J., Synthesis of Polystyrene Brushes on Silicate Substrates Via
Carbocationic Polymerization from Self-Assembled Monolayers. Macromolecules 2000, 33, (2), 342348.
112.
Choi, I. S.; Langer, R., Surface-Initiated Polymerization of L-Lactide: Coating of Solid
Substrates with a Biodegradable Polymer. Macromolecules 2001, 34, (16), 5361-5363.
35
113.
Husemann, M.; Mecerreyes, D.; Hawker, C. J.; Hedrick, J. L.; Shah, R.; Abbott, N. L.,
Surface-Initiated Polymerization for Amplification of Self-Assembled Monolayers Patterned by
Microcontact Printing. Angewandte Chemie-International Edition 1999, 38, (5), 647-649.
114.
Jaworek, T.; Neher, D.; Wegner, G.; Wieringa, R. H.; Schouten, A. J., Electromechanical
Properties of an Ultrathin Layer of Directionally Aligned Helical Polypeptides. Science 1998, 279,
(5347), 57-60.
115.
Jordan, R.; Ulman, A., Surface Initiated Living Cationic Polymerization of 2-Oxazolines.
116.
Wieringa, R. H.; Siesling, E. A.; Geurts, P. F. M.; Werkman, P. J.; Vorenkamp, E. J.; Erb, V.;
Stamm, M.; Schouten, A. J., Surface Grafting of Poly (L-Glutamates). 1. Synthesis and
Characterization. Langmuir 2001, 17, (21), 6477-6484.
117.
Juang, A.; Scherman, O. A.; Grubbs, R. H.; Lewis, N. S., Formation of Covalently Attached
Polymer Overlayers on Si(111) Surfaces Using Ring-Opening Metathesis Polymerization Methods.
Langmuir 2001, 17, (5), 1321-1323.
118.
Kim, N. Y.; Jeon, N. L.; Choi, I. S.; Takami, S.; Harada, Y.; Finnie, K. R.; Girolami, G. S.;
Nuzzo, R. G.; Whitesides, G. M.; Laibinis, P. E., Surface-Initiated Ring-Opening Metathesis
Polymerization on Si/Sio2. Macromolecules 2000, 33, (8), 2793-2795.
119.
Sugawara, T.; Matsuda, T., Novel Surface Graft-Copolymerization Method with MicronOrder Regional Precision. Macromolecules 1994, 27, (26), 7809-7814.
120.
Biesalski, M.; Ruhe, J., Preparation and Characterization of a Polyelectrolyte Monolayer
Covalently Attached to a Planar Solid Surface. Macromolecules 1999, 32, (7), 2309-2316.
121.
Peng, B.; Johannsmann, D.; Ruhe, J., Polymer Brushes with Liquid Crystalline Side Chains.
Macromolecules 1999, 32, (20), 6759-6766.
122.
Prucker, O.; Ruhe, J., Synthesis of Poly(Styrene) Monolayers Attached to High Surface Area
Silica Gels through Self-Assembled Monolayers of Azo Initiators. Macromolecules 1998, 31, (3), 592601.
123.
Prucker, O.; Ruhe, J., Polymer Layers through Self-Assembled Monolayers of Initiators.
Langmuir 1998, 14, (24), 6893-6898.
124.
Ruhe, J. In Polymers Grafted from Solid Surfaces, 1998; Wiley-V C H Verlag Gmbh: 1998;
pp 215-222.
125.
Pyun, J.; Kowalewski, T.; Matyjaszewski, K., Synthesis of Polymer Brushes Using Atom
Transfer Radical Polymerization. Macromolecular Rapid Communications 2003, 24, (18), 1043-1059.
126.
Baum, M.; Brittain, W. J., Synthesis of Polymer Brushes on Silicate Substrates Via Reversible
Addition Fragmentation Chain Transfer Technique. Macromolecules 2002, 35, (3), 610-615.
127.
Blomberg, S.; Ostberg, S.; Harth, E.; Bosman, A. W.; Van Horn, B.; Hawker, C. J.,
Production of Crosslinked, Hollow Nanoparticles by Surface-Initiated Living Free-Radical
Polymerization. Journal of Polymer Science Part A-Polymer Chemistry 2002, 40, (9), 1309-1320.
128.
Husseman, M.; Malmstrom, E. E.; McNamara, M.; Mate, M.; Mecerreyes, D.; Benoit, D. G.;
Hedrick, J. L.; Mansky, P.; Huang, E.; Russell, T. P.; Hawker, C. J., Controlled Synthesis of Polymer
Brushes By "Living" Free Radical Polymerization Techniques. Macromolecules 1999, 32, (5), 14241431.
129.
Ejaz, M.; Yamamoto, S.; Ohno, K.; Tsujii, Y.; Fukuda, T., Controlled Graft Polymerization of
Methyl Methacrylate on Silicon Substrate by the Combined Use of the Langmuir-Blodgett and Atom
Transfer Radical Polymerization Techniques. Macromolecules 1998, 31, (17), 5934-5936.
130.
Cheng, N.; Brown, A. A.; Azzaroni, O.; Huck, W. T. S., Thickness-Dependent Properties of
Polyzwitterionic Brushes. Macromolecules 2008, 41, (17), 6317-6321.
131.
Cheng, N.; Azzaroni, O.; Moya, S.; Huck, W. T. S., The Effect of [Cu-I]/[Cu-Ii] Ratio on the
Kinetics and Conformation of Polyelectrolyte Brushes by Atom Transfer Radical Polymerization.
132.
Choi, E. Y.; Azzaroni, O.; Cheng, N.; Zhou, F.; Kelby, T.; Huck, W. T. S., Electrochemical
Characteristics of Polyelectrolyte Brushes with Electroactive Counterions. Langmuir 2007, 23, (20),
10389-10394.
133.
Jones, D. M.; Brown, A. A.; Huck, W. T. S., Surface-Initiated Polymerizations in Aqueous
Media: Effect of Initiator Density. Langmuir 2002, 18, (4), 1265-1269.
134.
Patten, T. E.; Xia, J. H.; Abernathy, T.; Matyjaszewski, K., Polymers with Very Low
Polydispersities from Atom Transfer Radical Polymerization. Science 1996, 272, (5263), 866-868.
135.
Minisci, F., Free-Radical Additions to Olefins in Presence of Redox Systems. Accounts of
Chemical Research 1975, 8, (5), 165-171.
136.
Matyjaszewski, K.; Xia, J. H., Atom Transfer Radical Polymerization. Chemical Reviews
2001, 101, (9), 2921-2990.
36
137.
Wu, T.; Efimenko, K.; Vlcek, P.; Subr, V.; Genzer, J., Formation and Properties of Anchored
Polymers with a Gradual Variation of Grafting Densities on Flat Substrates. Macromolecules 2003, 36,
(7), 2448-2453.
138.
Xu, F. J.; Zhao, J. P.; Kang, E. T.; Neoh, K. G.; Li, J., Functionalization of Nylon Membranes
Via Surface-Initiated Atom-Transfer Radical Polymerization. Langmuir 2007, 23, (16), 8585-8592.
139.
Bonner, W. H., Jr Aromatic Polyketones. US 3065205
1962.
140.
Attwood, T. E.; Dawson, P. C.; Freeman, J. L.; Hoy, L. R. J.; Rose, J. B.; Staniland, P. A.,
Synthesis and Properties of Polyaryletherketones. Polymer 1981, 22, (8), 1096-1103.
141.
Kricheldorf, H. R., Aromatic Polyethers 2nd ed.; 2005.
142.
Petillo, O.; Peluso, G.; Ambrosio, L.; Nicolais, L.; Kao, W. J.; Anderson, J. M., In-Vivo
Induction of Macrophage Ia-Antigen (Mhc Class-Ii) Expression by Biomedical Polymers in the Cage
Implant System. Journal of Biomedical Materials Research 1994, 28, (5), 635-646.
143.
Sonnenschein, M. F., Micro- and Ultrafiltration Film Membranes from Poly(Ether Ether
Ketone) (Peek). Journal of Applied Polymer Science 1999, 74, (5), 1146-1155.
144.
Baalmann, A.; Vissing, K. D.; Born, E.; Gross, A. In Surface-Treatment of
Polyetheretherketone (Peek) Composites by Plasma Activation, 1994; Gordon Breach Sci Publ Ltd:
1994; pp 57-66.
145.
Inagaki, N.; Tasaka, S.; Horiuchi, T.; Suyama, R., Surface Modification of Poly(Aryl Ether
Ether Ketone) Film by Remote Oxygen Plasma. Journal of Applied Polymer Science 1998, 68, (2),
271-279.
146.
Jama, C.; Dessaux, O.; Goudmand, P.; Gengembre, L.; Grimblot, J., Treatment of Poly(Ether
Ether Ketone) (Peek) Surfaces by Remote Plasma Discharge - Xps Investigation of the Aging of
Plasma-Treated Peek. Surface and Interface Analysis 1992, 18, (11), 751-756.
147.
Kim, S.; Lee, K. J.; Seo, Y., Polyetheretherketone (Peek) Surface Functionalization by LowEnergy Ion-Beam Irradiation under a Reactive O-2 Environment and Its Effect on the Peek/Copper
Adhesives. Langmuir 2004, 20, (1), 157-163.
148.
Mathieson, I.; Bradley, R. H., Surface Oxidation of Polyether Ketone Films Using Ultraviolet
Ozone. Journal of Materials Chemistry 1994, 4, (7), 1157-1157.
149.
Pawson, D. J.; Ameen, A. P.; Short, R. D.; Denison, P.; Jones, F. R., An Investigation of the
Surface-Chemistry of Poly(Ether Etherketone) .1. The Effect of Oxygen Plasma Treatment on SurfaceStructure. Surface and Interface Analysis 1992, 18, (1), 13-22.
150.
MarchandBrynaert, J.; Pantano, G.; Noiset, O., Surface Fluorination of Peek Film by Selective
Wet-Chemistry. Polymer 1997, 38, (6), 1387-1394.
151.
Noiset, O.; Henneuse, C.; Schneider, Y. J.; MarchandBrynaert, J., Surface Reduction of
Poly(Aryl Ether Ether Ketone) Film: Uv Spectrophotometric, H-3 Radiochemical, and X-Ray
Photoelectron Spectroscopic Assays of the Hydroxyl Functions. Macromolecules 1997, 30, (3), 540548.
152.
Noiset, O.; Schneider, Y. J.; MarchandBrynaert, J., Surface Modification of Poly(Aryl Ether
Ether Ketone) (Peek) Film by Covalent Coupling of Amines and Amino Acids through a Spacer Arm.
Journal of Polymer Science Part A-Polymer Chemistry 1997, 35, (17), 3779-3790.
153.
Fang, T.; Shimp, D. A., Polycyanate Esters - Science and Applications. Progress in Polymer
Science 1995, 20, (1), 61-118.
154.
Feucht, S. W.; Fabian, P. E.; Munshi, N. A. In Development of Novel Radiation-Resistant
Insulation Systems for Fusion Magnets, 2003; Ieee-Inst Electrical Electronics Engineers Inc: 2003; pp
1516-1519.
155.
Nair, C. P. R.; Mathew, D.; Ninan, K. N., Cyanate Ester Resins, Recent Developments. In
New Polymerization Techniques and Synthetic Methodologies, Springer-Verlag Berlin: Berlin, 2001;
Vol. 155, pp 1-99.
156.
Hamerton, I., Chemistry and Technology of Cyanate Ester Resins. Blackie Academic:
Glasgow, 1994.
157.
Hamerton, I.; Hay, J. N., Recent Developments in the Chemistry of Cyanate Esters. Polymer
International 1998, 47, (4), 465-473.
158.
Bayley, H.; Martin, C. R., Resistive-Pulse Sensing - from Microbes to Molecules. Chemical
Reviews 2000, 100, (7), 2575-2594.
159.
Ivanov, I.; Cheng, X. L.; Sine, S. M.; McCammon, J. A., Barriers to Ion Translocation in
Cationic and Anionic Receptors from the Cys-Loop Family. Journal of the American Chemical Society
2007, 129, (26), 8217-8224.
37
160.
Muthukumar, M.; Kong, C. Y., Simulation of Polymer Translocation through Protein
Channels. Proceedings of the National Academy of Sciences of the United States of America 2006, 103,
(14), 5273-5278.
161.
Vockenroth, I. K.; Atanasova, P. P.; Jenkins, A. T. A.; Koper, I., Incorporation of AlphaHemolysin in Different Tethered Bilayer Lipid Membrane Architectures. Langmuir 2008, 24, (2), 496502.
162.
Ying-Cai, C.; Chao, W.; Meng-Bo, L., Simulation Study on the Translocation of Polymer
Chains through Nanopores. Journal of Chemical Physics 2007, 044904-1-6.
163.
Dekker, C., Solid-State Nanopores. Nature Nanotechnology 2007, 2, (4), 209-215.
164.
R. L. Fleischer, P. B. P., R. M. Walker, Univ. California Press: Berkeley, CA 1975.
165.
Trautmann, C.; Bruchle, W.; Spohr, R.; Vetter, J.; Angert, N., Pore Geometry of Etched Ion
Tracks in Polyimide. Nuclear Instruments & Methods in Physics Research Section B-Beam
Interactions with Materials and Atoms 1996, 111, (1-2), 70-74.
166.
Siwy, Z.; Apel, P.; Baur, D.; Dobrev, D. D.; Korchev, Y. E.; Neumann, R.; Spohr, R.;
Trautmann, C.; Voss, K. O. In Preparation of Synthetic Nanopores with Transport Properties
Analogous to Biological Channels, 2003; Elsevier Science Bv: 2003; pp 1061-1066.
167.
Siwy, Z.; Dobrev, D.; Neumann, R.; Trautmann, C.; Voss, K., Electro-Responsive
Asymmetric Nanopores in Polyimide with Stable Ion-Current Signal. Applied Physics a-Materials
Science & Processing 2003, 76, (5), 781-785.
168.
Siwy, Z.; Gu, Y.; Spohr, H. A.; Baur, D.; Wolf-Reber, A.; Spohr, R.; Apel, P.; Korchev, Y. E.,
Rectification and Voltage Gating of Ion Currents in a Nanofabricated Pore. Europhysics Letters 2002,
60, (3), 349-355.
169.
Cervera, J.; Schiedt, B.; Neumann, R.; Mafe, S.; Ramirez, P., Ionic Conduction, Rectification,
and Selectivity in Single Conical Nanopores. Journal of Chemical Physics 2006, 124, (10).
170.
Kosinska, I. D.; Goychuk, I.; Kostur, M.; Schmid, G.; Hanggi, P., Rectification in Synthetic
Conical Nanopores: A One-Dimensional Poisson-Nernst-Planck Model. Physical Review E 2008, 77,
(3).
171.
Liu, Q.; Wang, Y.; Guo, W.; Ji, H.; Xue, J.; Ouyang, Q., Asymmetric Properties of Ion
Transport in a Charged Conical Nanopore. Physical Review E 2007, 75, (5).
172.
Sexton, L. T.; Horne, L. P.; Martin, C. R., Developing Synthetic Conical Nanopores for
Biosensing Applications. Molecular Biosystems 2007, 3, (10), 667-685.
173.
Siwy, Z. S., Ion-Current Rectification in Nanopores and Nanotubes with Broken Symmetry.
Advanced Functional Materials 2006, 16, (6), 735-746.
174.
Ali, M.; Schiedt, B.; Healy, K.; Neumann, R.; Ensinger, A., Modifying the Surface Charge of
Single Track-Etched Conical Nanopores in Polyimide. Nanotechnology 2008, 19, (8).
175.
Siwy, Z.; Heins, E.; Harrell, C. C.; Kohli, P.; Martin, C. R., Conical-Nanotube Ion-Current
Rectifiers: The Role of Surface Charge. Journal of the American Chemical Society 2004, 126, (35),
10850-10851.
176.
Umehara, S.; Pourmand, N.; Webb, C. D.; Davis, R. W.; Yasuda, K.; Karhanek, M., Current
Rectification with Poly-L-Lysine-Coated Quartz Nanopipettes. Nano Letters 2006, 6, (11), 2486-2492.
177.
Siwy, Z.; Fulinski, A., Fabrication of a Synthetic Nanopore Ion Pump. Physical Review
Letters 2002, 89, (19), 4.
178.
Hille, B., Ionic Channels of Excitable Membranes. 2nd ed. ed.; Sinauer Associates Inc.: 1992.
179.
Doupnik, C. A.; Davidson, N.; Lester, H. A., The Inward Rectifier Potassium Channel Family.
Current Opinion in Neurobiology 1995, 5, (3), 268-277.
180.
Reimann, F.; Ashcroft, F. M., Inwardly Rectifying Potassium Channels. Current Opinion in
Cell Biology 1999, 11, (4), 503-508.
181.
Baker, L. A.; Bird, S. P., Nanopores - a Makeover for Membranes. Nature Nanotechnology
2008, 3, (2), 73-74.
182.
Martin, C. R.; Siwy, Z. S., Learning Nature's Way: Biosensing with Synthetic Nanopores.
Science 2007, 317, (5836), 331-332.
183.
Healy, K., Nanopore-Based Single-Molecule DNA Analysis. Nanomedicine 2007, 2, (4), 459481.
184.
Heins, E. A.; Siwy, Z. S.; Baker, L. A.; Martin, C. R., Detecting Single Porphyrin Molecules
in a Conically Shaped Synthetic Nanopore. Nano Letters 2005, 5, (9), 1824-1829.
185.
Savariar, E. N.; Krishnamoorthy, K.; Thayumanavan, S., Molecular Discrimination inside
Polymer Nanotubules. Nature Nanotechnology 2008, 3, (2), 112-117.
186.
Siwy, Z.; Trofin, L.; Kohli, P.; Baker, L. A.; Trautmann, C.; Martin, C. R., Protein Biosensors
Based on Biofunctionalized Conical Gold Nanotubes. Journal of the American Chemical Society 2005,
127, (14), 5000-5001.
38
187.
Xia, F.; Guo, W.; Mao, Y. D.; Hou, X.; Xue, J. M.; Xia, H. W.; Wang, L.; Song, Y. L.; Ji, H.;
Qi, O. Y.; Wang, Y. G.; Jiang, L., Gating of Single Synthetic Nanopores by Proton-Driven DNA
Molecular Motors. Journal of the American Chemical Society 2008, 130, (26), 8345-8350.
188.
Lehmann, V., The Physics of Macropore Formation in Low Doped N-Type Silicon. Journal of
the Electrochemical Society 1993, 140, (10), 2836-2843.
189.
Langner, A. Fabrication and Characterization of Macroporous Silicon. Martin-LutherUniversität, Halle-Wittenberg 2008.
190.
Lehmann, V.; Foll, H., Formation Mechanism and Properties of Electrochemically Etched
Trenches in N-Type Silicon. Journal of the Electrochemical Society 1990, 137, (2), 653-659.
191.
Birner, A.; Wehrspohn, R. B.; Gosele, U. M.; Busch, K., Silicon-Based Photonic Crystals.
Advanced Materials 2001, 13, (6), 377-388.
192.
Lehmann, V.; Stengl, R.; Reisinger, H.; Detemple, R.; Theiss, W., Optical Shortpass Filters
Based on Macroporous Silicon. Applied Physics Letters 2001, 78, (5), 589-591.
193.
Koenderink, A. F.; Wuest, R.; Buchler, B. C.; Richter, S.; Strasser, P.; Kafesaki, M.; Rogach,
A.; Wehrspohn, R. B.; Soukoulis, C. M.; Erni, D.; Robin, F.; Jackel, H.; Sandoghdar, V., Near-Field
Optics and Control of Photonic Crystals. Photonics and Nanostructures-Fundamentals and
Applications 2005, 3, (2-3), 63-74.
194.
Letant, S. E.; Hart, B. R.; Van Buuren, A. W.; Terminello, L. J., Functionalized Silicon
Membranes for Selective Bio-Organism Capture. Nature Materials 2003, 2, (6), 391-396.
195.
Hart, B. R.; Letant, S. E.; Kane, S. R.; Hadi, M. Z.; Shields, S. J.; Reynolds, J. G., New
Method for Attachment of Biomolecules to Porous Silicon. Chemical Communications 2003, (3), 322323.
196.
Buriak, J. M., Organometallic Chemistry on Silicon and Germanium Surfaces. Chemical
Reviews 2002, 102, (5), 1271-1308.
197.
Dancil, K. P. S.; Greiner, D. P.; Sailor, M. J., A Porous Silicon Optical Biosensor: Detection
of Reversible Binding of Igg to a Protein a-Modified Surface. Journal of the American Chemical
Society 1999, 121, (34), 7925-7930.
39
Chapter 2
2. Materials and methods
2.1. Materials
2.1.1.
Polymer Substrates
PEEK samples used in joint fabrication in combination to CER as adhesives were
purchased from Victrex Europa GmbH, Hofheim/Ts, Germany. PEEK membranes
used in SI-ATRP experiments were obtained from Victrex, Lancashire, England
(Grade 1000-050, with a thickness of 50 µm). Prior to use, PEEK samples were
immersed in refluxing acetone for 48h, rinsed twice with acetone and dried under
vacuum (1 mmHg) at 60°C for 3h. The thus obtained PEEK membrane is designated
as pristine PEEK. Polyethylene terephthalate, PET (Hostaphan RN 12) membranes
with a thickness of 12 µm were obtained from Hoechst, Germany, and polyimide, PI
(Kapton 50 HN) membranes with a thickness 12.3 µm were obtained from DuPont,
Germany.
2.1.2.
Porous substrates
2.1.2.1.
Fabrication of conical nanopore in PET and PI membranes
PET and PI membranes were irradiated at the linear accelerator UNILAC
(Gesellschaft für Schwerionenforschung, GSI, Darmstadt, Germany) with single swift
heavy ions (Pb, U or Au) of energy 11.4 MeV/nucleon. Etching, for the fabrication of
conical nanopores in PET and PI membranes, was carried out according to the
methods developed by Apel and Siwy et al.1, 2 The nanopores were fabricated by the
collaboratorat GSI.i Briefly, the heavy ion-irradiated membrane was placed between
the two halves of a conductivity cell in which it served as a dividing wall between the
two compartments. An etching solution (9M NaOH for PET and sodium hypochlorite
with 13% active chlorine content for PI) was added to one compartment of the cell
whereas the other compartment was filled with stopping solution (IM HCOOH+1M
KCl for PET and 1M potassium iodide for PI). For PET the etching process was
carried out at room temperature while for PI the process was carried out at 50°C.
i
M. Ali, from the group of Prof. W. Ensinger at Darmstadt University of Technology, Department of
Materials Science.
40
During the etching process, a potential of -1V was applied across the membrane in
order to observe the current flowing through it. The current remains zero as long as
the pore is not yet etched through. After the break through, the stopping solution on
the other side of the membrane neutralizes the etchant. The etching process was
stopped when the current was reached at a certain value and the pore was washed first
with stopping solution in order to quench the etchant, followed with deionised water.
The etched membrane was immersed in deionised water in order to remove the
residual salts. After etching, the diameter of the large opening (D) of the pore was
determined by scanning electron microscopy (SEM) using a PET or PI sample
containing 107 pores/cm2, which was etched simultaneously with the single pore
under the same conditions. The diameter of the small opening (d) was estimated from
its conductivity by the following relation,
d = 4LI ⁄ лDкV
where L is the length of the pore which is approximated to the thickness of the
membrane, d and D are the small and large opening diameters of the pore
respectively, к is the specific conductivity of the electrolyte (1 M KCl), V is the
voltage applied across the membrane and I is the measured current.
2.1.2.2.
Fabrication of ordered macroporous Si membrane
Ordered macroporous Si membranes were obtained from collaboratorii at MaxPlanck-Institut für Mikrostrukturphysik, Halle, Germany. The fabrication process has
been briefly introduced in chapter 1.
2.1.3.
Substances purified prior to use
Copper(I) chloride, CuCl, ≥97% (Fluka) and copper(I) bromide, CuBr, 98%
(Aldrich) used for ATRP were purified according to the procedure of Keller and
Wycoff.3 Monomethoxy oilgo(ethylene glycol) methacrylates (MeOEGMA, average
Mn ~300, Sigma-Aldrich), was passed through a short plug of basic alumina to
remove the stabilizer. Dimethylsulfoxide (DMSO) was distilled prior to use.
Potassium carbonate was dried overnight at 120°C prior to use. Acetone was refluxed
overnight with potassium carbonate and calcium oxide before distilling and stored
under argon. Triethylamine was refluxed overnight with calcium hydride, distilled and
ii
Dr. A. Langner from Prof. Ulrich Gösele.
41
stored under argon. N-Isopropylacrylamide (NIPAAm, Aldrich) was purified by
recrystallization from a mixture of toluene/hexane (1/4) and dried in vacuum.
2.1.4.
Dry
Substances used as received
tetrahydrofuran
(THF),
sodiumhydride,
sodiumborohydride,
iron
acetylacetonate, 4,4'-dihydroxybenzophenone 99%, 1,3-dibromobenzene ≥97.0%,
1,3,5-tribromobenzene 98%,
1,10-phenanthroline ≥99%,
phloroglucinol
≥99.0%
(Fluka), copper(I)iodide 98%, anhydrous N,N-dimethylformamide (DMF) 99.8%,
anhydrous toluene
99.8%, potassium methoxide 95%, pyridine ≥99.75%, 4-
bromoanisole 99%, phenol ~99%, 4-fluorobenzoyl chloride 98%, aluminum chloride
anhydrous
powder 99.99% , anhydrous 1-methyl-2-pyrrolidinone (NMP) 99.5%,
anhydrous methanol 99.8%, hydrobromic acid 37%, cyanogen bromide reagent grade
97%, 2-bromoisobutyryl bromide 98%, 2,2′-bipyridine 99%, copper(II) chloride,
CuCl2, ≥98% (Fluka), Rhodamine 6G 99%, 3-(methacryloyloxy)propane-1-sulfonate
(MPS) in the form of potassium salt 98%, (3-aminopropyl)triethoxysilane 99%,
sodium
4-vinylbenzenesulfonate
(technical,
≥90%,
Fluka),
[2-
(methacryloyloxy)ethyl]trimethylammonium chloride (METAC) as 75% aqueous
solution, 4,4′-azobis(4-cyanopentanoic acid) (purum, ≥98.0%, Fluka), phosphorus(V)
chloride
(purum,
≥98.0%,
hexamethylphosphoramide
(HMPA,
Fluka),
≥98.0%,
methyltrichlorosilane
Fluka),
(99%),
1,3-dichloro-1,1,3,3-
tetramethyldisiloxane ≥97.0% (Fluka), ethynylmagnesium chloride (0.5M in THF), N(3-dimethylaminopropyl)-N'-ethylcarbodiimide hydrochloride (EDC) 98% (Fluka)
and pentafluorophenol (PFP) +99%,
were used as received from Sigma-Aldrich,
Schnelldorf, Germany. Sodium chloride 99.99% was obtained from Merck,
Darmstadt, Germany. Sodiumborohydride 99%, propargylamine (99%), dry
dichloromethane was obtained from Acros Organics, Geel, Belgium. Sand paper of
600grit, ERSTA, was obtained from Dieter Schmid Fine Tools, Berlin, Germany.
E. coli – BL12 (DE3), Cat. No. 70235-4 and SOC medium, Cat. No. 69319, used
in antifouling studies, were obtained from Novagen, Merck Biosciences, Darmstadt,
Germany. LB-Agar (Luria/Miller) medium, Cat. No. X969.2, was obtained from Carl
Roth GmbH+Co. KG, Karlsruhe, Germany.iii
iii
Filipe Natálio from Department of Physiological Chemistry, University of Mainz is highly
acknowledged for antifouling experiments.
42
All the other materials were used as received without further purification unless
otherwise described.
2.2. Methods
2.2.1.
Thermal and mechanical analysis
Thermal properties of the materials were investigated using differential scanning
calorimetric (DSC) and thermogravimetric analysis (TGA).
DSC is a very well known method to accurately obtain the thermal properties of
materials. It measures the energy necessary to establish a nearly zero temperature
difference between a substance and an inert reference material, as the both substances
are subjected to identical temperature regimes in an environment heated or cooled at a
controlled rate. Two types of systems are commonly used, the heat flux DSC and
power compensation DSC. In the heat flux DSC the sample and the reference material
are enclosed in the same furnace. The enthalpy or heat capacity changes are registered
via a low heat resistance coupling below the samples, measuring temperature
differences. In the power compensation DSC, the temperatures in the sample and
reference furnace are kept identical by varying the power input, directly in relation to
the enthalpy or heat capacity differences between the sample and the reference. With
this method, one can precisely determine phase transitions or reactions occurring in
the sample material. In this study, a heat flux DSC model 822 (Mettler-Toledo,
Greifensee, Switzerland) under a nitrogen purge of 30 cm3 min-1.
TGA is generally performed to determine change in weight with respect to change
in temperature. It is commonly employed to determine polymer degradation
temperatures, absorbed moisture content of materials, the level of inorganic and
organic components in materials. In the present work TGA was performed on TGA
model 851 (Mettler-Toledo, Greifensee, Switzerland) at a heating rate of 10 K min-1
under a N2 or N2:O2 (80:20) purge of 30 cm3 min-1.
Rheology is the study of the flow of matter: mainly liquids but also soft solids or
solids under conditions in which they flow rather than deform elastically. We have
employed rheology to determine the viscosity of viscous liquids and Tg of the some
polymers where it was not evident form DSC analysis. Viscosity of the liquid
monomers (using plate plate geometry, diameter 13 mm and thickness 65 mm) and
rheological measurements of polymers were performed on an advanced rheometric
43
expansion system (ARES, Rheometric Scientific Inc., New Jersey NJ 08854, USA).
During rheological measurements of polymers, torsion deformation was applied on
rectangular samples (50mm × 10mm × 1mm) under conditions of controlled
deformation amplitude, which was always remaining in the range of the linear
viscoelastic response. A temperature ramp of 2°C min-1 was used to determine
temperature dependent storage (G') and loss (G") moduli and damping factors (tan δ)
of polymers at a frequency of 10 rad s-1.
The PEEK-CER-PEEK joint strength of half cut tensile bone joined together by
PT-30 CEM was tested on a universal material-testing machine (Instron 6022, Instron
Co., Buckinghamshire, UK) equipped with a 10 kN load cell and an oven (Brabender
Realtest, GmbH, Moers, Germany) to control the sample temperature.
Tensile tests were performed on neat cured thermosets were also performed using
the same universal material-testing machine (Instron 6022). Samples were drawn with
a rate of 0.5 mm min-1 at room temperature. A strain gauge extensometer with an
initial gauge length of 12.5 mm was used to follow the extension. The sample width
and thickness were about 5 mm and 1 mm respectively. The dependence of nominal
stress vs. drawing ratio was recorded. The Young’s moduli (E) were determined from
the linear slope of this dependence at small strain.
Pressure-volume-temperature (PVT) measurements were performed using a fully
automated high-pressure dilatometer (GNOMIX, Gnomix Inc., Boulder, Colorado,
USA). With this technique the specific volume as a function of pressure and
temperature can be determined. A detailed description of the apparatus and the
method can be found elsewhere.4 Each run was performed by varying the pressure
from 10 to 200 MPa in steps of 10 MPa at constant temperatures. The isothermal
measurements were performed in the range from 25 to 300°C in steps of 5K. Absolute
densities at room temperature used to derive the thermal expansion coefficients were
measured in ethanol using a Mettler-Toledo AG204 (Greifensee, Switzerland) balance
equipped with a Mettler-Toledo (Greifensee, Switzerland) density determination kit.iv
2.2.2.
Fourier transform infrared spectroscopy (FTIR)
FTIR spectroscopy is particularly attractive to identify the chemical functionalities
in a material. It depends on the interaction of infrared radiation with the chemical
iv
A. Best and A. Hanewald (MPI-P) are highly acknowledged for PVT, mechanical and rheological
analysis.
44
functionalities of a sample. When an infrared (IR) beam hits a sample, chemical
bonds stretch, contract and bend, causing it to absorb IR radiation in a defined wave
number (cm-1).
Attenuated total reflectance (ATR) spectroscopy is a type of IR spectroscopy in
which the IR spectra of the thin films/plates can be obtained from the near surface
region (~5 µm).5 The ATR-IR is based on the idea that when a beam of radiation
passes from a more dense medium to a less dense medium, reflection occurs. The
sample is simply pressed against an internal-reflection element, IRE (ZnSe, Ge, or
diamond crystal). The radiation is internally reflected from the interface between
sample and the IRE, but also penetrates a certain distance beyond the reflecting
interface. The extent of penetration of the radiation into sample practically depends on
IRE and the refractive index of the sample, and is on the order of microns. ATR-IR
spectra in the present study were recorded on a Nicolet FT-IR 730 spectrometer.
IR spectroscopy can also be implemented for the study of thin film on surfaces in a
number of ways. For instance, transmission IR spectroscopy for the characterization
of polymer brushes grown on the silicon (transparent to IR radiations) surfaces is
becoming increasingly popular. The technique employs the same basic experimental
geometry as that used for liquid samples and mulls. In the present study, FTIR
spectroscopy in transmission geometry was conducted on Nicolet Magna-IR 850,
Series II.v For all the FTIR spectroscopic analysis, Omnic series software was utilized
for data acquisition.
2.2.3.
X-ray photoelectron spectroscopy (XPS),
XPS or Electron Spectroscopy for Chemical Analysis (ESCA), determines the
atomic composition of a solid’s top several nanometers (sampling depth variability
from about 0.2 to 10 nm). Upon exposure to X-ray photons, a surface emits
photoelectrons whose binding energies can be compared to known values to identify
the element and its oxidation state.6 The resulting spectrum is a plot of intensity
(arbitrary units) versus binding energy (eV). The intensity of the ejected
photoelectrons relates directly to the material surface atomic distribution and can
therefore be used to quantify percent atomic composition and stoichiometric ratios.7, 8
X-ray photoelectron spectroscopy (XPS) measurements during this study were carried
out using a Physical Electronics 5600 A instrument. The Mg Kα (1253.6 eV) X-ray
v
W. Scholdei (MPI-P) is acknowledged for transmission IR.
45
source was operated at 300 W. A pass energy of 117.40 eV was used for the survey
spectra. The XPS scans were analysed using the MultiPak 5.0 software. The analyses
were performed at IMTEK, Albert-Ludwigs University, Frieburg. vi
2.2.4.
Current-voltage (I-V) measurements
The I-V curvesvii of the PET and PI membranes with single naopores were
determined while the membrane was mounted between the two halves of the
conductivity cell, and both half of the cell were filled with 0.1M KCl solution
prepared in phosphate buffer saline (pH = 7.6). A Ag/AgCl electrode was placed into
each half-cell solution, and the Keithley 6487 picoammeter/voltage source (Keithley
Instruments Inc., Cleveland, OH) was used to apply the desired transmembrane
potential in order to measure the resulting ion current flowing through the nanopore
by applying a scanning voltage from -2V to +2V on the tip side while the base side of
the pore remain connected to the auxiliary electrode.
2.2.5.
Atomic force microscopy (AFM) and scanning electron
microscopy (SEM)
The AFM was initially developed to overcome the limitations of its ancestor, the
scanning tunnelling microscope (STM), in imaging non-conducting samples.9
Nowadays, it has developed into an invaluable multi-disciplinary technique for
advanced characterization of surfaces on an atomic scale.10 In AFM, a tip attached to
a flexible cantilever moves across the sample surface to measure surface morphology.
The change in surface height is then measured by the location of the reflected laser
beam in the quadrant photodetector, and a surface topographical map is generated
from which surface roughness values can be calculated.11 AFM can operate with a
lateral resolution of 30 Å and vertical resolution of less than 1 Å.12
In the present work, AFM images of the samples were taken in air at room
temperature with a commercial AFM Dimension 3000 (Veeco) controlled with a
Nanoscope V, in tapping mode. Silicon cantilevers (Olympus) 160 µm long, 50 µm
wide and 4.6 µm thick, with an integrated tip (tip radius < 10 nm) of a nominal spring
constant of 42 N/m and a resonance frequency of 300 kHz were used. Typically the
vi
vii
D. Mössner from Prof. Dr. Jürgen Rühe’s group.
M. Ali from GSI, Darmstadt is highly acknowledged for I-V cure measurements of the samples.
46
tip was scanned at velocity of 0.5 Hz and minimal applied forces were used when
imaging. Topography and phase images were used to record the structures. viii
SEM is widely used to study surfaces of solid objects, it utilizes a beam of focused
electrons as an electron probe that is scanned in a regular manner over the specimen.13
When electron beam impinges on the sample, back scattered electrons, secondary
electrons, and X-rays are produced. A scintillation detector detects these secondary
electrons. This technique is essentially employed when high-resolution threedimensional images of the surface morphology are desired. SEM imaging in the
present work was performed with a LEO Gemini 1530 SEM.ix
2.2.6.
Water contact angle measurement and surface roughness
Water contact angle measurement is the fundamental, accurate and most sensitive
tool for determining the surface wettability. Since it is very difficult to obtain
exquisite information of the outermost few angstroms of a solid surface by any other
technique, solid/liquid/vapour (S/L/V) contact angle measurement emerges as one of
the most surface-sensitive methods.12 Contact angle goniometry was performed on a
DROP SHAPE ANALYSIS SYSTEM (DSA 10-Mk2, Krüss GmbH Germany),
equipped with a thermostat chamber (TC3010/3410, Krüss GmbH Germany), which
in turn is connected to a Thermo Haake K 10/ThermoHaake DC 10 circulation system
for controlling the temperature inside the chamber. The water CAs reported are
average of at least three individual measurements.
The average roughness (Ra) of surfaces was determined using a TENCOR P-10
surface profiler (Tencor Instruments, San Jose, CA, USA).
2.2.7.
Atomic absorption spectroscopy (AAS)
AAS is very useful technique for determining the concentration of a particular
metal element in a sample.14 The technique makes use of absorption spectroscopy to
assess the concentration of an analyte. It relies therefore heavily on Beer-Lambert
law. In short, the electrons of the atoms in the atomizer can be promoted to higher
orbitals for an instant by absorbing a set quantity of energy (i.e. light of a given
wavelength). This amount of energy (or wavelength) is specific to a particular
viii
ix
Dr. M. Alvarez (MPI-P) is highly acknowledged for AFM imaging.
Gunnar Glasser (MPI-P) is highly acknowledged for SEM imaging.
47
electron transition in a particular element, and in general, each wavelength
corresponds to only one element. This gives the technique its elemental selectivity.
AAS was performed on a Perkin Elmer 5100 ZL spectrometer working in flame
emission mode.x
2.2.8.
Dielectric spectroscopy
Dielectric spectroscopy measures the interaction of the electromagnetic radiation in
the frequency range from 10-6 to 1012 Hz with molecular system. Dielectric dispersion
and absorption phenomena occur in this vast frequency range due to (i) dipole
relaxation arising from the reorientational motions of molecular dipoles and (ii)
electric conduction arising from the translation motions of electric charges (ions,
electrons).15 This technique has become increasing popular for measuring specific
conductivity of a material.
In the present study, dielectric spectroscopy was used to determine the dielectric
constants of cured resins and proton conductivity of polyelectrolyte-porous silicon
hybrid membranes.
The dielectric constants were determined by temperature dependent dielectric
measurements which were performed with an experimental setup of Novocontrol
GmbH (Hundsangen, Germany). The system was equipped with an Alpha highresolution dielectric analyzer and temperature controller Quatro version 4.0. The
samples were milled down to a thickness of about 200 µm and sandwiched between
two brass discs with diameters of 10 mm, forming a flat parallel plate capacitor. An
AC voltage of 1V was applied to the capacitor. The temperature was controlled using
a nitrogen gas cryostat and the temperature stability at the sample was better than 0.1
K. The dielectric constant ε*(ω) = ε′(ω) - iε′′(ω) was measured at a frequency of 1
MHz between -100 to 150°C.xi
The ionic conductivities were measured by dielectric spectroscopy in a twoelectrode geometry. The combination of high conductivity and thin sample can lead to
distortions of impedance plots above 1 MHz. Initially an Alpha high-resolution
dielectric analyzer and a Novocontrol active sample cell was used to expand the
frequency range to ~ 10 MHz. After proving that the resonance is below 1 MHz the
x
Dr. B. Mathiasch (Department of Chemistry, University of Mainz) is highly acknowledged for AAS
analysis.
xi
Dr. K. Koynov (MPI-P) is highly acknowledged for dielectric constant measurements.
48
spectra were recorded using an SI 1260 impedance/gain-phase analyzer and a
Novocontrol broadband dielectric converter. For humidity dependent measurements
an atmosphere of saturated humidity was generated by using a closed sample cell with
a water reservoir on the bottom that was not in contact with the sample. Saturation
was controlled by a Sensirion SHT75 humidity sensor and found to be 100% (within
the error bar of the sensor (2%). Humidity between 18 and 95% was created using a
temperature controlled climate chamber (Binder KBF 240). From the Cole-Cole and
Bode plots, the resistance of the membrane was estimated, and then the specific
conductivity of the composite membrane was calculated using the apparent thickness
and electrode area.xii
2.3. References
1.
Apel, P. Y.; Korchev, Y. E.; Siwy, Z.; Spohr, R.; Yoshida, M., Diode-Like Single-Ion Track
Membrane Prepared by Electro-Stopping. Nuclear Instruments & Methods in Physics Research Section
B-Beam Interactions with Materials and Atoms 2001, 184, (3), 337-346.
2.
3.
Keller, R. N.; Wycoff, H. D., Copper(I) Chloride. Inorganic Syntheses 1946, 2, 1-4.
4.
P. Zoller, D. J. W., Standard Pressure-Volume-Temperature Data for Polymers. Technomic:
Lancaster, P.A, 1995.
5.
Harrick, N. J., Surface Chemistry from Spectral Analysis of Totally Internally Reflected
Radiation. Journal of Physical Chemistry 1960, 64, (9), 1110-1114.
6.
Briggs, D., Surface Analysis of Polymers by Xps and Static Sims. Cambridge University press:
Cambridge UK, 1998.
7.
Crombez, M.; Chevallier, P.; Gaudreault, R. C.; Petitclerc, E.; Mantovani, D.; Laroche, G.,
Improving Arterial Prosthesis Neo-Endothelialization: Application of a Proactive Vegf Construct onto
Ptfe Surfaces. Biomaterials 2005, 26, (35), 7402-7409.
8.
Kingshott, P.; Wei, J.; Bagge-Ravn, D.; Gadegaard, N.; Gram, L., Covalent Attachment of
Poly(Ethylene Glycol) to Surfaces, Critical for Reducing Bacterial Adhesion. Langmuir 2003, 19, (17),
6912-6921.
9.
Binnig, G.; Rohrer, H.; Gerber, C.; Weibel, E., Surface Studies by Scanning Tunneling
Microscopy. Physical Review Letters 1982, 49, (1), 57.
10.
Binnig, G.; Quate, C. F.; Gerber, C., Atomic Force Microscope. Physical Review Letters 1986,
56, (9), 930.
11.
Alessandrini A, F. P., Afm: A Versatile Tool in Biophysics. Meas Sci Technol 2005, 16,
R65–92.
12.
Shrojal M. Desai, R. P. S., Surface Modification of Polyethylene. Advance Polymer Science
2004, 169, 231–293.
13.
Rochow, T. G. R., E. G., An Introduction to Microscopy by Means of Light, Electrons, XRays, or Ultrasound Plenum Press: New York.
14.
Sperling, M. B. W., Bernhard, Atomic Absorption Spectrometry. Wiley-VCH: Weinheim,
1999.
15.
Friedrich Kremer, A. S., Broad Band Dielectric Spectroscopy. Springer-Verlag: 2002.
xii
Dr. A. Kaltbeitzel (MPI-P), from Prof. G. Wegner’s group, is highly acknowledged for proton
conductivity measurements.
49
Chapter 3
3. Cyanate Ester Resins: Thermally Stable Adhesives
for PEEK
Cyanate ester resins (CERs) have been proposed as thermally stable adhesives for PEEK
in combination with appropriate surface pretreatment. A commercial novolac based PT-30
cyanate ester monomer (CEM) is presented as an example. The choice for CERs as adhesives
is based on their superior thermal properties compared to the more commonly used epoxy
resins. Thermal properties of PT-30 CEM obtained from DSC and TGA were evaluated in
context of its suitability as an adhesive for PEEK for high temperature applications. In order
to further improve PEEK-CER-PEEK joint strengths wet chemical and plasma assisted
surface activations were applied in addition to PT-30 CEM as the adhesive.
50
Due to the increased need for PEEK with complex shapes and geometries, the
joining of separate components in an assembly by adhesive bonding is often
inevitable. Epoxies have been the most widely used adhesives in this context1-5, which
are generally processable at ambient temperatures. They form a thermoset resin at a
relatively low curing temperature (<180°C) and are therefore suitable for industrial
processing. The major drawback, however, is their poor performance at elevated
temperatures. With a general recommended application temperature below <180°C,
epoxies loose their adhesive properties as a result of their poor thermal stability above
this temperature.6 Conversely, PEEK exhibits a continuous working temperature of
250°C with retention of its excellent mechanical properties, solvent-, and chemical
resistance.7 Thus, in order to benefit from the superior thermal properties of PEEK,
more sophisticated adhesives with high thermal stability are needed.
Cyanate ester resin (CER) is a relatively new class of thermosetting resins. Beside
their thermal stability, CERs possess other interesting properties such as low dielectric
constants, radar transparency and low water absorption, which, like PEEK, make
them attractive materials for microelectronic-, air-, and spacecraft technologies.8 In
the light of superior thermo-mechanical properties we hereby propose cyanate ester
monomers (CEMs) as thermally stable adhesives for PEEK. For the proof-of-concept,
a range of experiments using a commercial novolac based PT-30 CEM are presented
here (Figure 3.1).
Figure 3.1: Chemical structures of PEEK and PT-30 CEM.
To improve the PEEK-PEEK joint strength while using PT-30 CEM as adhesive
wet chemical and plasma assisted surface activations were applied. The wet chemical
surface activation involved the functionalisation of the PEEK surface with -OCN
groups, which are reactive towards the CEM adhesive. This will result in higher joint
strengths due to interfacial covalent bonding between the PEEK surface and the PT30 adhesive. Surface treatment of polymers by plasma techniques has proved to be
one of the possible methods for improving the adhesion properties without affecting
the bulk characteristics (see also Chapter 1.4).9 Plasma treatments using different
51
mixtures of N2 and O2 were investigated here to activate the PEEK surface. The
PEEK-CER-PEEK joint strength after wet chemical and plasma activations were
tested at room temperature and at 200°C.
3.1. Choice of PT-30 CEM as adhesive for PEEK
In comparison to the widely used epoxies, CEMs generally suffer from poor precuing processability due to their usually solid physical state at room temperature.
Besides recent efforts to develop CEMs10-12 that are processable at ambient
temperature, some highly processable CEMs have already been commercialised. The
PT-30 CEM used in the present study is a commercially available CEM with a room
temperature viscosity of 2047 Pa.s, which decreases substantially to 9 Pa.s at 50°C.
This reflects a high epoxy like ambient temperature processability of the PT-30 CEM.
Thermal
curing
of
CEMs
generally
yields
polycyanurate
(PC)
by
cyclotrimerization of the –OCN groups to triazine rings. This cyclotrimerization is an
exothermic process, which can be monitored by DSC. The DSC thermogram of PT30 CEM showed the exothermic transition corresponding to a curing reaction at
287°C (Figure 3.2a). This exothermic curing transition of PT-30 CEM occurs at a
temperature, which is 37 K higher than the recommended continuous working
temperature of PEEK, quoted at 250°C. In order to use PT-30 as an adhesive for
PEEK it is, however, necessary to carry out curing below 250°C.
It is known; that the curing temperature of CEM can be lowered by catalysis of the
cyanate groups cyclotrimerization by a transition metal acetylacetonate.13 The inset in
Figure 3.2a shows the concentration dependence (in wt-%) for iron acetylacetonate as
catalyst on the curing of PT-30 CEM. With increasing concentration of the catalyst
the exothermic curing transition shifted to lower temperatures. Thus, for 0.1 wt-%
catalyst the exothermic transition was substantially lowered from 287°C to 135°C,
which is already 152 K below the continuous working temperature of PEEK. The
exothermic transition shifted further down to 112°C if 0.4 wt-% of the catalyst was
used. Increasing the catalyst concentration from 0.4 wt-% to 0.8 wt-% had no
apparent effect in the DSC, but concentrations beyond 0.8 wt-% resulted in
spontaneous curing at room temperature.
While using a catalyst has the advantage of lowering the curing temperatures for
CEMs, several disadvantages have to be considered: 1) when mixed with the CEM the
52
shelf live of the adhesive may be substantially lowered. 2) Any residual solvent
(acetone in present case) used to homogenize the CEM-catalyst mixture can adversely
affect the mechanical strength of the resulting thermoset. 3) Another major
disadvantage of the catalyst, which will remain in the thermoset PC, is its activation
of hydrolysis reactions of the PC network and hence accelerated ageing.14-19 The
included metal ions may also negatively influence the absorption behavior of the PC
with respect to radiation (radar, light, or x-rays). These combined effects will
ultimately reduce the joint strength when employed as adhesive. Therefore the use of
a curing catalyst for CEM is not encouraged for practical applications.
Figure 3.2: (a) DSC thermogram of neat PT-30 CEM. The exothermic transition peaking at 287ºC
corresponds to the thermal curing by cyclotrimerization of cyanate groups (-OCN). The inset shows the
catalytic effect of iron acetylacetonate (in wt-%) on thermal curing. (b) Inset: DSC thermogram of PT30 following the heating program: heated dynamically at 10 K/min to 200°C where it was annealed
isothermally for 12 h (represented by the very sharp exothermic transition at 200°C) followed by
heating to 400°C at 10 K/min. Beside the exothermic transition at 200ºC, a 100 K wide shoulder
related to a diffusion controlled cure occurs around 340ºC. Figure b is the isothermal part of the same
heating scan depicting heat flowing (W/g) as a function of time (minutes) at 200°C.
These observations and problems motivated the study of neat PT-30 CEM curing
at lower temperatures. As the DSC thermogram of neat PT-30 CEM (Figure 3.2a)
indicates a very first onset of the exothermic transition for curing via
cyclotrimerization already around 200°C, curing may be induced at this lower
temperature even without employing a catalyst. For this purpose isothermal DSC at
200°C was employed for PT-30 CEM curing. The temperature program followed in
DSC was as follows: PT-30 CEM was first heated to 200°C dynamically at 10 K/min,
where it was annealed isothermally for 12 h followed by heating up to 400°C at 10
K/min. The resulting DCS thermogram is depicted in Figure 3.2b as an inset. The heat
flow (W/g) during the 200°C isothermal annealing step of the whole DCS scan is
depicted as a function of time (minutes) in Figure 3.2b. It took about 20 min to reach
to 200°C, therefore, the annealing curve starts from 20min. An exothermic transition
53
related to curing can be observed that raises sharply at 20 min, peaks around 1 h and
ceases at about 180 min. The isothermal heating was continued for another 9 h but no
significant further change was observed. After isothermal heating at 200°C for 12h
the temperature was raised dynamically to 400°C at 10 K/min during the same DSC
scan. Apart from an exothermic curing transition at 200°C, a 100 K wide shoulder
related to diffusion-controlled curing centred at 340°C was also observed.20 No
exothermic transition at 287°C, as found in dynamic DSC of untreated PT-30 CEM
(Figure 3.2a), was observed, which reflects a reasonably high extent of curing by
chemical conversion of –OCN groups to the triazine ring at 200°C even without
adding a catalyst.
Beside DSC, FT-IR spectroscopy is another powerful tool for monitoring the
thermal curing of CEMs by cyclotrimerization of -OCN groups to a network of
triazine rings. -OCN groups have a characteristic stretching band in the IR spectrum,
which in case of PT-30 CEM appeared as a bifurcated band at 2260 cm-1 and 2240
cm-1 (Figure 3.3). The intensity of the band was observed to decrease substantially
when PT-30 CEM was subjected to curing at 200°C for 12h with the appearance of
new bands at 1556 cm-1 and 1360 cm-1, which are characteristic for triazine rings of
PC (Figure 3.3).
Figure 3.3: Normalized FT-IR spectra of uncured and cured PT-30 CEM. Two new bands for the
cured PT-30 CEM appeared at 1556 cm-1 and 1360 cm-1 for triazine ring at the expense of cyanate
group -OCN stretching band at 2260 cm-1. The consumption of cyanate groups while curing at 200ºC
for 12h was calculated from the normalized areas of cyanate absorbance before (AOCN)t=0h and after
(AOCN)t=12h the thermal treatment and was found to be 70%.
In order to determine the extent of curing the IR spectra presented in Figure 4 were
normalized to the out-of-plane deformation of CH groups in the substituted aromatic
54
ring at 754 cm-1. The extent of curing was quantified by calculating the percent
conversion factor α(t)12 for the residual -OCN groups from the normalized area of –
OCN group absorbance before (AOCN)t=0h and after (AOCN)t=12h the thermal treatment
according to the following equation:
α (t ) =
( AOCN ) t =12 h
× 100
( AOCN ) t 0
(Equation 3-1)
It was found that curing PT-30 CEM at 200°C for 12h resulted in 70% conversion
of -OCN groups. These experiments also highlight that the DSC and FT-IR are very
convenient tools for qualitative and quantitative analysis of the thermal
transformations during CEM curing.
Thermal properties such as the Tg and thermo-oxidative stability of a material are
critical for any potential high temperature application, therefore, PT-30 CEM cured
for 12h at 200°C was subjected to DSC, rheological analysis and TGA. It was found
that the thermal curing for 12h at 200°C was enough for the resulting PC to show a
rheological Tg of 187°C (Figure 3.4a inset), which is 44 K higher than the Tg (143°C)
of PEEK and 7 K higher than the maximum recommended use temperature of epoxies
i.e. 180°C. The Tg of 200°C/12h cured PT-30 CEM was deduced from its tan δ curve
because no transition was observed in its DSC thermogram. The DSC thermogram for
PEEK is also depicted in Figure 3.4a for comparison. Two endothermic transitions for
Tg and melting Tm can be observed at 143°C and 342°C. The thermal stability of
200°C/12h cured PT-30 CEM was evaluated from the TGA shown in Figure 5b. The
TGA thermograms of PEEK are also plotted in the same Figure for comparison. The
TGA revealed a high thermal stability of 200°C/12h cured PT-30 CEM as no weight
loss was observed below 424°C under N2 atmosphere and 392°C under a mixture of
N2:O2 in the ratio of 80:20. PEEK on the other hand showed a higher thermal stability
as no weight loss was observed before 550°C, irrespective of gaseous atmosphere
used in TGA. The comparison of TGA reflects a slightly higher thermal stability of
PEEK but the stability of 200°C/12h cured PT-30 is still ~150 K higher than the
continuous working temperature of PEEK. An interesting feature observed in the
TGA under N2 was a higher char yield (68%) for 200°C/12h cured PT-30 than PEEK
(52%) at 900°C, which suggests that 200°C/12h cured PT-30 possesses a better flame
retardancy and fire resistance than PEEK. It was also observed during oxidizing TGA
(N2:O2 80:20) that both 200°C/12h cured PT-30 and PEEK exhibited a two-step
55
degradation process with the main degradation occurring during the second step
where 75% weight loss was observed. The onset temperatures for the second step of
degradation of 200°C/12h cured PT-30 and PEEK were quite close i.e. 575°C and
589°C respectively, and differ only by 14 K. It can thus be inferred from the TGA that
200°C/12h cured PT-30 possesses thermal stability comparable to PEEK. A
comparison of thermal properties of PEEK and 200°C/12h cured PT-30 CEM is
presented in Table 3.1.
Figure 3.4: (a) DSC thermogram of PEEK. The endothermic transitions at 143ºC and 342ºC
correspond to Tg and Tm respectively. The inset is a tan δ plot of the thermoset of 200°C/12h cured PT30 wherein the maxima is the Tg (187ºC). (b) TGA thermogram of PEEK and of the 200°C/12h cured
PT-30 thermoset. No weight loss was observed below 400ºC.
The investigation of thermal properties of PT-30 CEM and their comparison with
the thermal properties of PEEK demonstrates that PT-30 CEM is a highly promising
material for application in combination with PEEK under high temperatures. In order
to investigate this experimentally, two halves of a half-cut bone (see materials and
methods Figure 3.9) were glued together using PT-30 CEM as adhesive and the
assembly was kept for 12h at 200°C without special surface treatment of the PEEK
material. The resulting PEEK-CER-PEEK joint was subjected to a pull-apart test. At
room temperature the average PEEK-CER-PEEK joint strength obtained was 13 MPa.
For comparison a commercial two-component epoxy was also applied as an adhesive
for the same joint geometry. PEEK-epoxy-PEEK joints showed an average room
temperature strength of 7 MPa. This indicates that under the same joint geometry,
PEEK-CER-PEEK joints using the PT-30 CEM adhesive exhibit a higher average
room temperature strength than the joints using the epoxy adhesive.
The PEEK- adhesive -PEEK joint strengths were also measured at 200°C for the
PT-30 CEM and epoxy adhesives. Interestingly, at 200°C the joints with PT-30 CEM
could retain an average strength of 6 MPa whereas at the same temperature epoxy
56
simply failed to hold together the two halves of the glued tensile bone and
consequently no joint strength could be measured. At this temperature PT-30 CEM
shows a room temperature epoxy-like adhesive performance while the epoxy
completely fails under these conditions. The reduced joint strength for PT-30 CEM at
200°C is partially due to a softening of the adhesive above its Tg of 187°C. These
experiments clearly demonstrate the suitability of CEMs as thermally stable adhesives
for PEEK. In order to further improve the PEEK-adhesive-PEEK joint strength, the
PEEK surface in contact with the adhesive was subjected to different activation
treatments and the result obtained are discussed below.
Table 3.1: Comparison of thermal properties of PEEK and 200°C /12h cured PT30 and joint strengths of unmodified PEEK using epoxy and PT-30 CEM
adhesives
Material
Tg
Tm
T0-N2
T0-N2/O2 (80:20)
Joint
(°C)
(°C)
(°C)
(°C)
strength
(MPa)
PEEK
200°C/12h cured PT30
PEEK-Epoxy-PEEK
PEEK-CER-PEEK
143*
187**
342*
No melting
550
424
550
392
-
-
-
-
-
7†
13 & 6††
*DSC, **Rheological, at room temperature,
†
††
†
at 200°C, T0 = onset of decomposition in TGA
3.2. Surface activation of PEEK for adhesion improvements
The chemical nature of polymer surfaces govern a wide range of interfacial
properties.21 The chemical inertness of PEEK is highly attractive for many
applications, however, this inertness also leads to immense difficulties with the
adhesion of PEEK to other materials. In order to glue PEEK, the adhesion must be
optimized without loosing the bulk properties that make PEEK such an attractive
material. This can be achieved by modifying the chemical nature of the surface either
in wet chemical processes or plasma assisted processes. Several methods of surface
modification have been reported for adhesion improvements of PEEK-PEEK joints
with epoxies as adhesives.1-5 The use of CEM as adhesive for PEEK has previously
not been reported. Herein two means of surface activation, namely wet chemical and
plasma treatment, applied to the PEEK surface in order to improve the PEEKadhesive-PEEK joint strength with CEMs as adhesives are discussed.
57
3.2.1.
Wet chemical surface activation
During wet chemical surface activation, the surface of PEEK was functionalised
with -OCN groups by exposure to reagents in organic solvents in a two-step process,
as outlined in Scheme 3.1. The surface carbonyl groups of PEEK were first reduced to
hydroxy groups by treating with NaBH4 in DMSO at 120ºC for 3h. The modified
PEEK with surface hydroxy groups was designated as PEEK-OH.
The surface hydroxy groups of PEEK-OH were subsequently transformed into OCN groups under the conditions slightly modified as compared to those generally
applied for CEM synthesis from phenols.15 The procedure involved the activation of
surface hydroxy groups of PEEK-OH by treatment with NaH. This activation step
was necessary because the oxygen in alcohols is not as nucleophilic as that of phenols
to give a reasonable yield during the nucleophilic attack on BrCN. The activated
surface alkoxide groups –ONa were transformed into –OCN groups by reacting with
BrCN in THF first at 0ºC for 1h and then at room temperature for 12h. The success of
these wet chemical transformations at the PEEK surface were monitored by XPS and
ATR-IR spectroscopy.
Scheme 3.1: Wet chemical functionalization of the PEEK surface with cyanate groups (-OCN).
58
Figure 3.5: ATR-IR monitoring of wet chemical surface modifications of PEEK. -OH stretching bands
at around 3400 cm-1 can be observed after reduction of carbonyl groups at the PEEK surface to
hydroxy groups. The -OCN stretching band appeared at 2245 cm-1 after transformation of surface -OH
groups to -OCN groups.
The reduction of surface carbonyl groups of PEEK to hydroxy groups was evident
from the ATR-IR spectra (Figure 3.5). The characteristic stretching band for hydroxy
groups was observed around 3400 cm-1 in the ATR-IR spectrum of PEEK-OH, while
the band was absent in the ATR-IR spectrum of untreated PEEK. The transformation
of surface hydroxy groups to –OCN groups was confirmed by the appearance of a
band at 2245 cm-1 in the ATR-IR spectrum of PEEK-OCN, which is characteristic for
-OCN groups. The presence of chemically bound nitrogen at the PEEK surface was
confirmed in subsequent XPS measurements. It must be noted, that the conversion of
surface -OH groups to -OCN groups was never 100% and an -OH stretching band was
always present along with the -OCN stretching band in the IR spectrum of PEEKOCN.
The detail C1s and N1s XPS spectra for PEEK and PEEK-OCN are depicted in
Figure 3.6. Unmodified PEEK (inset in Figure 3.6a) shows a main C1s peak centred
around 285 eV with a high energy tail that leads directly into a broad, low intensity
peak around 294 eV. The main peak can be associated with the aromatic rings of
PEEK, while the high energy tail reflects carbon bonded to oxygen, presumably as the
ether and ketone groups present in unmodified PEEK. The peak at 294 eV represents
the π−π∗ shake up transition of the aromatic rings in PEEK. After wet chemical
modification the high energy tail became more prominent, indicating the formation of
59
the O C N groups. The successful functionalization of the PEEK surface with OCN groups was more evident from the comparison of the N1s spectral region of
PEEK and PEEK-OCN (Figure 3.6b). While the unmodified material revealed no
evidence for chemically bound nitrogen, the PEEK-OCN showed a signal at 401 eV,
very characteristic for nitrogen. The XPS analysis fully supports the FTIR data
showing the chemical incorporation of nitrogen into the PEEK surface and the
formation of -OCN surface groups.
Figure 3.6: (a) C1s detail XPS spectrum of PEEK-OCN showing the change in the spectrum after wet
chemically functionalisation of the PEEK surface. For comparison a C1s detail spectrum of
unmodified PEEK is shown in the inset. (b) N1s detail spectrum of PEEK-OCN with a N1s spectrum of
unmodified PEEK as an inset.
While XPS spectroscopy is sensitive down to a surface layer depth of ~5 nm,
ATR-IR spectroscopy can sample a depth of about ~5.0µm.22,
23
Since the present
samples possessed a high surface roughness (Ra) of around 9 µm on average, no
attempts were made to quantify the surface transformations by XPS or ATR-IR
spectroscopy. Probably because of the high surface roughness it was possible to
monitor the chemical changes by ATR-IR, which has previous been reported as rather
difficult.24
After wet chemical surface activation, the half-cut PEEK tensile bones with
surface -OCN groups were glued together with PT-30 CEM, yielding an average
room temperature PEEK-CER-PEEK joint strength of 28 MPa. This joint strength is
almost double compared to untreated PEEK (13 MPa). Apparently, the increased joint
strengths with PEEK-OCN is a result of the participation of surface -OCN groups of
PEEK in the cyclotrimerization of PT-30 CEM -OCN groups during thermal curing.
This results in strong covalent bonding between the PEEK surface and the PT-30
CEM adhesive through triazine ring formation at the PEEK/PT-30 CEM interface
(Figure 3.7). The results show that the combined effect of surface functionalisation
60
with -OCN groups and using CEMs as adhesives allows for a significant increase in
PEEK-to-PEEK bonding strength.
Figure 3.7: A schematic illustration of covalent bonding at the PEEK-adhesive interface after
introduction of cyanate functionalities on PEEK surface.
PEEK-CER-PEEK joints involving PEEK-OH were also fabricated. Interestingly,
these joints exhibited an average joint strength of 26 MPa, which is not much
different from PEEK-OCN. The high joint strength shown by PEEK-OH is probably
due to the chemical bonding between surface -OH groups and –OCN groups of PT-30
CEM. This chemical bonding involves the nucleophilic attack of the -OH group
oxygen on the electrophilic carbon of the -OCN group resulting in the formation of a
triazine ring passing through an imidate intermediate. A probable mechanism25 is
depicted in Scheme 3.2. These results show that surfaces with –OH groups can also be
used with CEMs as adhesives to substantially improve the joint strength.
Scheme 3.2: Proposed mechanism for the reaction of surface hydroxy groups of PEEK-OH with PT-30
CEM resulting in adhesion improvement.
The high temperature joints strength of PEEK-OH and PEEK-OCN were measured
at 200 ºC, where epoxies fail, and were found to range between 8-10 MPa. The
average values were always slightly higher than PEEK-CEM-PEEK joints of
untreated PEEK. In summary it was found that wet chemical surface activation
significantly improved the PEEK-CEM-PEEK joint strength at room temperature and
at 200ºC.
61
3.2.2.
Plasma-assisted surface activation
The modification of surfaces using plasma-assisted processes has been intensively
studied since the 1960s and a large number of today’s industrial processes are based
and rely on plasma assisted modification and deposition processes.26 Cleaning,
ablation, and surface activation are the major effects of plasma treatment. One or
more of these effects can dominate and can be controlled by tuning the nature of gas
and plasma operating parameters.27, 28 To date the available literature demonstrates the
effect of plasma treatment on PEEK-PEEK adhesion improvement using epoxy
adhesives, but only few references report on the functionalities introduced at the
PEEK surface after plasma treatment using different gases.1-5 In the present
framework, half-bones of PEEK were subjected to different N2/O2 plasma conditions
in order to optimise the PEEK-CER-PEEK joint strength using PT-30 CEM as
adhesive. Even though PEEK is known to be chemically rather inert towards many
reagents, its surface chemistry can be modified using an oxygen plasma.29 The extent
to which the surface is oxidized with oxygen as the reactive gas depends on the input
power, Ppeak (or the plasma density) and leads to the dissociation of bonds at the
material surface with the formation of new oxygen-based functional groups. This
process involves for PEEK the dissociation of the aromatic rings and the C-O-C
bonds at the polymer surface (Figure 3.1). The breaking of aromatic rings in oxygen
or nitrogen plasmas has previously been demonstrated and results in a number of
different free radical sites that spontaneously react with oxygen to form alcohols,
ketones, aldehydes and carbonyl groups.30 The quantification of the new functional
groups on the surface is difficult and has not yet been achieved satisfactorily. If the
gas used during plasma treatment is nitrogen the radical sites will also react with the
nitrogen species in the plasma. It is thus also possible to form various amines, amides,
and imines on the surface, that are available for subsequent reaction.31
The different plasma conditions applied in this work for PEEK treatment are
summarized in Table 3.2. From the data in Table 3.2 it can be concluded that
increasing input power during oxygen plasma treatment leads to an increase in the
PEEK-CER-PEEK joint strength. Similar values as those obtained by wet chemical
surface modification are achieved (i.e. about 26MPa). When using pure nitrogen at a
high input power of 180 W a similarly high PEEK-CER-PEEK joint strength was also
62
observed. For N2:O2 mixtures the adhesion strength improved with increasing input
power, treatment time, and nitrogen flow rate.
The highest room temperature adhesion strength of 48 MPa was obtained using a
N2:O2 mixture of 150sccm:50sccm flow rate at an input power of 210 W for 10
minutes and a pressure of 0.898 bar. It can be assumed that under these conditions the
PEEK structure at the uppermost surface is largely modified by the plasma and that
the surface is marked by a high density of different oxygen and nitrogen functional
groups, which undergo chemical bonding with the CEM adhesive as shown
schematically in Figure 3.8. At the same time the polymer molecules of the modified
PEEK are shortened due to extensive bond scission, such that the uppermost layer of
the polymer no longer resembles the original material, but consists of a multitude of
short and medium length molecules that are (i) highly reactive and (b) are mobile and
can diffuse partially into the CEM adhesive. It is possible that the combined effect of
a high reactive group density and the diffusion of molecular chain ends from the
PEEK surface into the adhesive leads to the high observed joint strengths.
Figure 3.8: Proposed scheme showing possible interfacial bonding occurring between plasma treated
PEEK and the CEM adhesive.
The strength of the PEEK-CER-PEEK joints obtained under these optimised
plasma conditions was also determined at 200ºC and was found to be 13 MPa. This
high temperature joint strength is slightly higher than that for the wet chemically
modified PEEK-CER-PEEK joint (8-10 MPa, see above) and represents a significant
improvement compared to epoxies.
63
Table 3.2: PEEK-CEM-PEEK joint strengths for different surface activations
Nature Surface activation
Without any treatment:
Wet Chemical Surface activation:
PEEK-OH
PEEK-OCN
Joint
strength at
room
temperature
(MPa)
13
26
28
Joint
strength at
200ºC
(MPa)
6
10
8
Plasma assisted surface activation:
O2 10 sccm* Power: 10 W 2 minutes
O2 10 sccm Power: 100 W 7 minutes
O2 10 sccm Power: 180 W 7 minutes
N2 10sccm Power: 180 W 7 minutes
N2:O2 MIXTURES
Flow 10:10 sccm, Ppeak: 10 W, 2 minutes, P: 0.898 mbar
Flow 10:10 sccm, Ppeak: 100 W, 7 minutes P: 0.898 mbar
16
26
25
28
Flow 20:10 sccm, Ppeak: 100 W 7 minutes P: 0.898 mbar
Flow 20:10 sccm, Ppeak: 160 W 7 minutes P: 0.898 mbar
23
21
38
Flow 150:50 sccm, Ppeak: 210 W, 10 minutes, P: 0.898 bar
25
36
48
21
27
33
13
*sccm = standard cubic centimetre, given in each case as the flow of N2:O2
P is the process pressure
3.3. Conclusions
The potential of CEMs as thermally stable adhesives for PEEK was demonstrated.
Commercially available PT-30 CEM was used as a model CEM adhesive. Even
though the maximum of the exothermic transition for the curing reaction was
observed at 287°C in the DSC of PT-30 CEM, curing could be induced at a
substantially lower temperature (200°C), as inferred from the onset of curing
exothermic transition. The thermal properties, like high Tg (187°C) and thermal
stability, of the PC obtained after curing PT-30 CEM at 200°C for 12h are comparable
to PEEK. The obtained PC exhibited high Tg (187°C) and a thermal stability
comparable to PEEK.
64
The potential of PT-30 as an adhesive was evaluated by fabricating PEEK-CERPEEK joints and subjecting them to a tensile test by applying a pull force
perpendicular to the plane of the joint (tensile stress geometry). Compared to a twocomponent epoxy PT-30 CEM exhibited better adhesive properties at room
temperature and at 200°C, where epoxies generally fail. The PEEK-CER-PEEK joint
strength could be improved substantially by wet chemical- and plasma-assisted
activation of the PEEK surface. During wet chemical activation the PEEK surface
was successfully functionalised with –OH and –OCN groups. These surface
transformations could be monitored with XPS and ATR-IR spectroscopy. The room
temperature joint strengths involving PEEK with surface –OH and –OCN groups (26
MPa and 28 MPa) were double compared to joints involving untreated PEEK surfaces
(13 MPa).
Furthermore, plasma treatment was applied to activate the PEEK surface, varying
the gas composition, input power, and treatment time. The highest room temperature
joint strength of 48 MPa was achieved for optimized plasma treatment conditions of
N2:O2 150sccm:50sccm, 210 W, 10 minutes, and a pressure of 0.898 bar. The joint
strength of 8 to 13 MPa at 200°C involving PEEK surface activation showed a slight
improvement over the untreated surfaces (6 MPa). In summary it can be concluded
that CEMs possess a favorable combination of thermal and mechanical properties that
make them a very attractive material class as a structural adhesives for PEEK at high
temperatures.
3.4.
3.4.1.
Experimental
Sample preparation
The as received PEEK was cut into bone-shaped structures (Standard: Din 52455 No. 4) with
the dimensions as illustrated in Figure 3.9. For tensile testing after surface modification and gluing,
these bones were cut again into two equal halves. The surfaces to be glued were roughened with sand
paper, thoroughly rinsed with THF, and dried in a stream of N2 gas before gluing. For ATR-IR and
XPS measurements, small slices of 1×1×0.5 cm3 and 0.5×0.5×0.1 cm3 were cut out of the commercial
PEEK blocks and treated in the same manner as the bone-shaped samples.
65
Figure 3.9: Bone-shaped samples for mechanical testing.
3.4.2.
Wet chemical surface activation: Reduction of surface carbonyl groups to
hydroxy groups (PEEK-OH)
A 1L three neck round bottom flask was equipped with a reflux condenser and argon inlet and outlet.
500 mL of freshly distilled DMSO and 1.2 g (0.032 mole) of sodium borohydride were added to the
flask. The reaction mixture was heated at 120°C under stirring until dissolution occurred. Five half cut
PEEK tensile bones along with samples for ATR-IR and XPS measurements were immersed in the
stirred reaction mixture and heated at 120°C for 3h under argon. After removing from the reaction
mixture the PEEK samples were successively immersed in stirred methanol for 15 minutes, in distilled
water for 10 minutes, in 0.5 N HCl for 10 minutes, in water for 10 minutes and in ethanol for 10
minutes. The samples were dried at 60°C under vacuum for 2h and stored under N2. These samples will
hereafter be referred to as PEEK-OH.
3.4.3.
Wet chemical surface activation - Transformation of surface hydroxy
groups into cyanate groups (PEEK-OCN)
To a 500 mL three neck round bottom flask, equipped with condenser and argon inlet and outlet, 360
mL of THF, and 3.2 g (0.13 mole) of sodium hydride were added. The mixture was stirred for 30
minutes at room temperature. Five half cut PEEK-OH tensile bones along with the samples for ATR-IR
and XPS measurements were immersed in the reaction mixture and refluxed for 5 h. After cooling to
room temperature all the samples were thoroughly rinsed with dry THF and transferred to another 500
mL round bottom Schlenk flask under argon. The Schlenk flask was cooled in a salt-ice bath and the
temperature was kept below 0°C. 13.2 g (0.128 mole) of bromocyan dissolved in 350 mL of dry THF
was added into the Schlenk flask. The reaction mixture was stirred below 0°C for 1h and at room
temperature for 12h. The thus obtained PEEK-OCN samples were rinsed twice with dry THF. The
residual solvent was removed under vacuum at room temperature for 3h and stored under N2.
3.4.4.
Plasma-assisted surface activation
The plasma reactor consisted of a 30 cm long and 10 cm diameter cylindrical flow reactor equipped
with two concentric metal braid rings (separated by about 10 cm), which delivered the 13.56 MHz
radio-frequency to the chamber. The reactor is described in detail elsewhere.19 The typical base
pressure achieved in the system using a Leybold Trivac 16BCS rotary vane pump was 10-4 mbar. Half
cut tensile bones were stacked (in packs of 3-4) on the glass substrate holder placed half way between
the electrodes with the surfaces to be treated and pointing in the direction of the incoming gases.
Treatment was carried out using different mixtures of O2 and N2, both with a purity of 99.9%. The
different gas ratios and plasma conditions used during the PEEK treatment are given in Table 2.
3.4.5.
Gluing and adhesion tests
PT-30 was applied to the surface of one half of the tensile bone of PEEK and two halves were then
glued at 200°C for 12h. A similar procedure was used for the epoxy glue with a curing temperature of
70°C. The joint strengths were determined by applying a pull force acting perpendicular to the plane of
the joint (Figure 2). The nature of stress that develops in such a joint geometry is tensile stress.
Structural adhesives perform worse under tension, therefore, any improvement of joint strength under
tension can be translated into higher strengths under shear, cleavage, or peel stress. During gluing, the
66
two halves of the tensile bone were held together with the help of a steel clip that induces a physical
restraint to ensure constant axial loading and avoids any undesirable cleavage or peel stresses. For the
sake of simplicity, the values of tensile stress determined in the experiment will be termed “joint
strength” in this work. The strength of the joint was measured by pulling apart the glued halves of the
PEEK tensile bone perpendicular to the joint cleft at room temperature and at 200°C. The joint
strengths reported are the average of 5 to 10 individual tensile tests.
3.5. References:
1.
Comyn, J.; Mascia, L.; Xiao, G.; Parker, B. M., Plasma-Treatment of Polyetheretherketone
(Peek) for Adhesive Bonding. International Journal of Adhesion and Adhesives 1996, 16, (2), 97-104.
2.
Comyn, J.; Mascia, L.; Xiao, G.; Parker, B. M., Corona-Discharge Treatment of
Polyetheretherketone (Peek) for Adhesive Bonding. International Journal of Adhesion and Adhesives
1996, 16, (4), 301-304.
3.
Davies, P.; Courty, C.; Xanthopoulos, N.; Mathieu, H. J., Surface-Treatment for Adhesive
Bonding of Carbon-Fiber Poly(Etherether Ketone) Composites. Journal of Materials Science Letters
1991, 10, (6), 335-338.
4.
Laurens, P.; Sadras, B.; Decobert, F.; Arefi-Khonsari, F.; Amouroux, J., Enhancement of the
Adhesive Bonding Properties of Peek by Excimer Laser Treatment. International Journal of Adhesion
and Adhesives 1998, 18, (1), 19-27.
5.
Mathieson, I.; Bradley, R. H. In Improved Adhesion to Polymers by Uv/Ozone Surface
Oxidation, 1996; Butterworth-Heinemann Ltd: 1996; pp 29-31.
6.
Vol. 155, pp 1-99.
7.
McGrail, P. T., Polyaromatics. Polymer International 1996, 41, (2), 103-121.
8.
Science 1995, 20, (1), 61-118.
9.
10.
Laskoski, M.; Dominguez, D. D.; Keller, T. M., Development of an Oligomeric Cyanate Ester
Resin with Enhanced Processability. Journal of Materials Chemistry 2005, 15, (16), 1611-1613.
11.
Laskoski, M.; Dominguez, D. D.; Keller, T. M., Synthesis and Properties of a Liquid
Oligomeric Cyanate Ester Resin. Polymer 2006, 47, (11), 3727-3733.
12.
Yameen, B.; Duran, H.; Best, A.; Jonas, U.; Steinhart, M.; Knoll, W., Polycyanurate
Thermoset Networks with High Thermal, Mechanical, and Hydrolytic Stability Based on Liquid
Multifunctional Cyanate Ester Monomers with Bisphenol a and Af Units. Macromolecular Chemistry
and Physics 2008, 209, (16), 1673-1685.
13.
Mathew, D.; Nair, C. P. R.; Krishnan, K.; Ninan, K. N., Catalysis of the Cure Reaction of
Bisphenol a Dicyanate. A Dsc Study. Journal of Polymer Science Part A-Polymer Chemistry 1999, 37,
(8), 1103-1114.
14.
Fan, J.; Hu, X.; Yue, C. Y., Dielectric Properties of Self-Catalytic Interpenetrating Polymer
Network Based on Modified Bismaleimide and Cyanate Ester Resins. Journal of Polymer Science Part
B-Polymer Physics 2003, 41, (11), 1123-1134.
15.
Hamerton, I., Chemistry and Technology of Cyanate Ester Resins. Blackie Academic:
Glasgow, 1994.
16.
International 1998, 47, (4), 465-473.
17.
Hamerton, I.; Hay, J. N., Recent Technological Developments in Cyanate Ester Resins. High
Performance Polymers 1998, 10, (2), 163-174.
18.
Herr, D. E.; Nikolic, N. A.; Schultz, R. A., Chemistries for High Reliability in Electronics
Assemblies. High Performance Polymers 2001, 13, (3), 79-100.
19.
Lakshmi, M. S.; Reddy, B. S. R., Synthesis and Characterization of New Epoxy and Cyanate
Ester Resins. European Polymer Journal 2002, 38, (4), 795-801.
20.
Mondragon, I.; Solar, L.; Recalde, I. B.; Gomez, C. M., Cure Kinetics of a Cobalt Catalysed
Dicyanate Ester Monomer in Air and Argon Atmospheres from Dsc Data. Thermochimica Acta 2004,
417, (1), 19-26.
67
21.
Henneuse, C.; Goret, B.; Marchand-Brynaert, J., Surface Carboxylation of Peek Film by
Selective Wet-Chemistry. Polymer 1998, 39, (4), 835-844.
22.
Briggs, D., Surface Analysis of Polymers by Xps and Static Sims. Cambridge University press:
Cambridge UK, 1998.
23.
Harrick, N. J., Surface Chemistry from Spectral Analysis of Totally Internally Reflected
Radiation. Journal of Physical Chemistry 1960, 64, (9), 1110-1114.
24.
Mirabella, F. M., Quantitative-Analysis of Polymers by Attenuated Total Reflectance FourierTransform Infrared-Spectroscopy - Vinyl-Acetate and Methyl Content of Polyethylenes. Journal of
Polymer Science Part B-Polymer Physics 1982, 20, (12), 2309-2315.
25.
GrenierLoustalot, M. F.; Lartigau, C., Molten State Reactivity of Difunctional Cyanates:
Thermal and Spectroscopic Studies by Liquid and Solid Cp-Mas C-13-Nmr. Journal of Polymer
26.
Forch, R.; Zhang, Z. H.; Knoll, W., Soft Plasma Treated Surfaces: Tailoring of Structure and
Properties for Biomaterial Applications. Plasma Processes and Polymers 2005, 2, (5), 351-372.
27.
Friedrich, J.; Loeschcke, I.; Frommelt, H.; Reiner, H. D.; Zimmermann, H.; Lutgen, P., Aging
and Degradation of Poly(Ethylene-Terephthalate) in an Oxygen Plasma. Polymer Degradation and
Stability 1991, 31, (1), 97-114.
28.
Liston, E. M. In Plasma Treatment for Improved Bonding - a Review, 1989; 1989; pp 199218.
29.
Shard, A. G.; Badyal, J. P. S., Surface Oxidation of Polyethylene, Polystyrene, and Peek - the
Synthon Approach. Macromolecules 1992, 25, (7), 2053-2054.
30.
Shard, A. G.; Badyal, J. P. S., Plasma Oxidation Versus Photooxidation of Polystyrene.
Journal of Physical Chemistry 1991, 95, (23), 9436-9438.
31.
Foerch, R.; McIntyre, N. S.; Sodhi, R. N. S.; Hunter, D. H., Nitrogen Plasma Treatment of
Polyethylene and Polystyrene in a Remote Plasma Reactor. Journal of Applied Polymer Science 1990,
40, (11-12), 1903-1915.
68
Chapter 4
4. Effect of structural variations on thermal properties
of aryl ether ketone based cyanate ester resins with
linear and tri-arm molecular architectures
The effect of structural variations, like crosslinking functionality (fc), molecular weight
between the crosslinks (Mc) and chain architecture of the spacer between crosslinks, on
thermal properties of cyanate ester resins was demonstrated. Six aryl ether ketone based
model cyanate ester monomers (CEM), mainly characterized by linear and symmetrically
triarmed molecular architectures, with different structural variations were synthesized. The
thermal properties like processability of CEM and the Tg and thermal stability of
polycyanurates (PC) obtained after thermal curing were correlated with the chemical
structure of CEM. Finally the electronic effect of the substituents on the reactivity of CEM
was demonstrated by synthesizing two model monofunctional CEMs with electron donating
and electron withdrawing groups substituted at the para position.
69
The commercial polyaryl ether ketones (PAEKs) are semicrystalline polymers with
Tgs around 140-160°C, melting points above 300°C and crystallinity values of 2030%. Crystallinity confers to the polymer better mechanical properties and better
resistance to solvents, but high melting point inherent with the crystallinity
necessitates the use of very high temperatures for processing, in the range of 340400°C. In addition, as demonstrated in chapter 3, the chemical inertness of PAEKs
leads to immense difficulties while fabricating adhesive based joints. This necessitates
the use of surface pretreatments. The ultimate effect of these problems results in a
high material cost. In this context, A. Marcos-Fernández et al.1 were the first to coin
the idea of using low melting CEMs with aromatic ether ketone (AEK) like spacers in
order to overcome the high processing temperature of PAEKs. They managed to
synthesize bifunctional CEMs with melting points in the range of 76-182°C. Another
similar approach presented by M. Laskoski et al.2 produced aryl ether ketone (AEK)
based bifunctional oligomeric CEMs with melting point of 120°C. Although, the AEK
based CEMs developed to date exhibit melting temperatures lower than PEEK, yet,
they are still solids at room temperature. In the framework of PEEK-CERs project, the
part of the work presented in this chapter will highlight the efforts to develop room
temperature processable CEMs with PEEK like spacers between the crosslinks. In this
unprecedented effort, the effect of molecular level structural variations on thermal
properties of the PEEK like CEMs and PCs derived from them has been
systematically investigated. The structural variations like crosslinking functionality
(fc), molecular weight between the crosslinks (Mc), and chain architecture of the
spacer between crosslinks were manipulated and six model CEMs, characterized by
linear and symmetrically triarm molecular architectures were synthesized.
The electronic effect of the substituents on the reactivity of CEMs was also
investigated with two synthesized model monofunctional CEMs bearing either
electron donating or electron withdrawing groups substituted at the para position.
4.1. Synthesis and characterization
In the following section, the synthesis of model CEMs with linear and triarm
molecular architectures used in the present study is described.
70
4.1.1.
Synthesis of CEMs with linear molecular architecture
The CEMs with linear molecular architecture synthesized were either bifunctional
(4, 5, 6) or multifunctional (10) with AEK based spacers between the crosslinks. The
syntheses of linear bifunctional CEMs 4, 5, and 6 are depicted in Scheme 4.1 and
Scheme 4.2. In the first step a linear aromatic ether ketone oligomeric mixture
(AEKOM) 1 was synthesized by reacting 1,3-dibromobenzene with 4,4'dihydroxybenzophenone in a modified Ullmann condensation reaction catalyzed by a
soluble Cu(I) complex,3-5 generated in situ from copper (I) iodide and 1,10phenanthroline (Scheme 4.1). K2CO3 was used as base. The reaction was carried out
at 135-145°C in a mixture of DMF and toluene, with later being used to remove water
formed during the reaction by azeotropic distillation. The FD mass revealed that the
AEKOM 1 contained chain lengths of n ≈ 0-4. The AEKOM 1 was separated into the
pure oligomers by subjecting it twice to column chromatography on silica gel with
different eluent. In the first column hexane and ethylacetate (2:2.5) solvent mixture
was used as eluent and AEKOM 1 (Rf = 0.5) was separated from any by-product. The
separation of AEKOM 1 into oligomeric analogues 2 (n=1, Rf = 0.4) and 3 (n=2, Rf =
0.6) was accomplished in the second column in which a dichloromethaneethylacetate-acetone (6:1:0.5) mixture was used as eluent. The respective fraction of
the two oligomers 2 and 3 separated from AEKOM 1 was ~55% and 30% and
reasonable amounts of 2 and 3 for transformation into respective CEMs could be
separated. The pure oligomers with the chain lengths n>2 could not be separated due
to their very close Rf values and relatively small fraction in the AEKOM 1. However
the Rf (0.8) value of n>2 oligomers was higher enough to allow their facile separation
from lower fractions in AEKOM 1.
71
Scheme 4.1: Synthesis of bifunctional pure oligomeric analogues CEMs 4 and 5 and their thermal
curing to the polycyanurate thermosets PC-4 and PC-5.
Beside the pure oligomeric analogues 2 and 3, the AEKOM 1 was also directly
transformed into the CEM 6 as a bifunctional oligomeric mixture (Scheme 4.2).
Scheme 4.2: Synthesis of CEMs 6 as a bifunctional oligomeric mixture and its thermal curing to the
polycyanurate thermoset PC-6.
The synthesis of multifunctional CEM 10 represented in Scheme 4.1. 1,3Dibromo-5-methoxybenzene was synthesized by nucleophilic substitution reaction of
1,3,5-tribrobenzene with potassium methoxide in DMF according to a literature
procedure.6
The
reaction
of
1,3-dibromo-5-methoxybenzene
with
4,4'-
dihydroxybenzophenone, in the same modified Ullmann reaction conditions as for
72
AEKOM 1,3-5 gave a mixture of AEKOM 8 with terminal hydroxyl and pendant
methoxy groups. The pendant methoxy groups were cleaved by heating AEKOM 8
with pyridium hydrochloride at 220-240°C for 2h to give the AEKOM 9 (n ≈ 1-3,
from mass spectrometry) with terminal as well as pendant hydroxyl groups. No
separation of the oligomeric analogues was carried out and the AEKOM 9 was
directly transformed into CEM 10.
Scheme 4.3: Synthesis of CEMs 10 as a multifunctional oligomeric mixture (n≈1-3) and its thermal
curing to the polycyanurate thermoset PC-10.
4.1.2.
Synthesis
of
CEMs
with
symmetric
triarm
molecular
architecture
In addition to the linear architecture, aromatic ether (AE) and aromatic ether
ketone (AEK) based CEMs with the symmetric triarm molecular architecture were
also synthesized. The syntheses of the triarmed CEMs were achieved by exploiting a
convergent approach. The phenolic CEM precursor 12 for the triarmed AE CEM 13
having shorter arms (Scheme 4.4) was synthesized following a procedure from
literature.7 The synthesis involved first the reaction of 4-methoxyphenol with 1,3,5tribromobenzene under Ullmann reaction condition resulting in the methoxy protected
73
symmetric triarm derivative 11. The methoxy groups in 11 were subsequently cleaved
under acidic hydrolysis by refluxing in a 1:1 mixture of 48% HBr and acetic acid for
12h to give the desired phenolic CEM precursor 12, which was then transformed into
symmetric triarm AE CEM 13.
Scheme 4.4: Synthesis of symmetrically triarmed AE CEM 13 and its thermal curing to the
The phenolic CEM precursor 17 for symmetric triarm aryl ether ketone CEM 18
having longer arms relative to CEM 13 was synthesized by following the strategy
depicted in Scheme 4.5. The 4-(4-fluorobenzoylphenoxy)anisole 15, which constitutes
the extended arms, was synthesized by the reaction of 4-phenoxyanisole 14 with 4fluorobenzoylchloride under Friedel-Krafts acylation condition. The extended arm 15
bears a methoxy group on one end and a fluoro group on the other. The presence of a
para-ketone group in 15 activated the fluoro group for the nucleophilic substitution
reaction with a phenol to form a diaryl ether linkage. Derivative 15 was reacted with
phloroglucinol under the condition generally applied for the synthesis of PAEKs. The
thus obtained methoxy-terminated symmetric triarm AEK 16 was subjected to
pyridium hydrochloride assisted hydrolysis of methoxy groups to give the phenolic
CEM precursor 17 and was transformed into symmetrically triarmed AEK CEM 18.
74
Scheme 4.5: Synthesis of symmetrically triarmed AEK CEM 18 and its tehrmal curing to the
The CEMs 4, 5, 6, 10, 13 and 18 were obtained by reacting the corresponding
phenols with cyanogen bromide in the presence of triethylamine in dry acetone at a
temperature between –20 to –30°C in high yields. This demonstrates the high
efficiency of this reaction and ease of incorporating different aromatic moieties in the
CEM molecules, which is important to gain a control over the thermal properties of
the CE resins. The formation of CEMs 4, 5, 6, 10, 13 and 18 from 1, 2, 3, 9, 12 and 17
phenolic CEM precursors can be well observed by IR spectroscopy (Figure 4.1) with
the disappearance of the hydroxyl stretching band (between 3400-3200 cm-1 for
phenolic CEM precursors 1, 2, 3, 9, 12 and 17) and the appearance of the –OCN
bands (between 2350-2200 cm-1 for CEMs 4, 5, 6, 10, 13 and 18). Moreover, as
already observed for the CEMs with related chemical structures, the –OCN bands
appear as a peak split into two maxima but when there is a ketone group para to the –
OCN group the peak is split at least into three maxima (Figure 4.1b).1 It is worth
mentioning here that all the synthesized CEMs follow this trend except CEM 10,
which also contains pendant –OCN groups meta to diaryl ether linkage in addition to
the terminal –OCN groups para to ketone groups. The presence of –OCN groups with
two different chemical environments probably resulted in a complex IR signal where
the effect of the para-ketone group was not evident. In addition to the IR
spectroscopy,
13
C-NMR spectroscopy is a very helpful tool that further confirms the
transformation of –OH to –OCN groups. The
13
C-NMR spectra of the CEMs
75
synthesized in the present study showed the characteristic signal for the –OCN
carbon, between 107-110 ppm, incorporated in CEMs during the chemical
transformation from respective phenols. The synthesized CEMs were transformed into
their respective PCs by thermal curing following the temperature program as
described in experimental section. The –OCN bands in the IR spectra vanish while
curing the CEMs 4, 5, 6, 10, 13 and 18 to respective PCs, with the concurrent
appearance of a characteristic stretching band for the triazine ring at 1360 cm-1
(Figure 4.1c). As the curing reaction is sensitive to residual phenolic groups, it is
important to note that within the resolution of the characterization methods no free
phenolic groups were detectable in the CEMs 4, 5, 6, 10, 13 and 18.
76
Figure 4.1: Figure 4.1a: FTIR spectra of all the phenolic CEM precursors, where –OH stretching band
can be observed. Figure 4.1b: With the conversion of phenolic CEM precursors to CEMs the –OH
stretching band between 3200-3400 cm-1 disappears and the –OCN bands appear between 2350-2200
cm-1. (Figure 4.1c) FTIR spectra of –OCN group region showing the effect of ketone group substituted
para to the –OCN goups on the splitting of the –OCN band. (Figure 4.1d) During the thermal curing of
CEMs to PCs the characteristic stretching band for the triazine ring appears at around 1360 cm-1
while –OCN bands vanish.
After synthesis and structural characterization, thermal characterization of the
developed CERs was conducted the results obtained are discussed in the following
section.
4.2. Thermal properties of linear bifunctional CE resins
4.2.1.
The bifunctional oligomeric analogues CEM 4 and CEM 5 have same fc but
different Mc. They were obtained as solids and their DSC thermograms are depicted in
Figure 4.2a. A sharp endothermic transition for melting at 101°C can be observed in
DSC thermogram of CEM 4. The DSC thermogram of CEM 5 showed a slightly
different trend comparing to its oligomeric analogue CEM 4 as before melting a broad
endothermic transition was observed. This broad transition was followed by a rather
sharp but less pronounced endothermic melting transition centered at 130°C, which is
29K higher than CEM 4. The relatively higher melting point of the CEM 5 can be
attributed to its higher Mc. For both the CEMs 4 and 5, the melting transitions were
immediately followed by exothermic transitions (151°C/CEM 4, 145°C/CEM 5) due
to the curing reaction by cyclotrimerization of –OCN groups leading to the PC-4 and
PC-5. On contrary to the observations for oligomeric analogues CEM 4 and CEM 5,
CEM 6, which is an oligomeric mixture constituted by the spacers of different chain
lengths between –OCN groups was obtained as a viscous liquid at ambient
temperature. The mixed oligomeric nature of the CEM 6 hinders the crystallization
and results in a viscous liquid at room temperature. The CEM 6 consequently does not
show an endothermic melting transition in its DSC thermogram above room
temperature (Figure 4.1a). Only one exothermic transition centered at 163°C was
observed, which relates to the curing reaction leading to the PC-6 by the
cyclotrimerization of the cyanate groups.
DSC thermograms of the PC-4, PC-5 and PC-6 derived from the CEMs 4, 5 and 6
are depicted in Figure 4.2b. The Tg values taken as the temperature corresponding to
the mid point of the endothermic transition were 156°C, 132°C and 145°C for PC-4,
77
PC-5 and PC-6. Comparatively shorter and less flexible spacer between the crosslinks
of PC-4 resulted in a higher Tg value than PC-5. Interestingly, the Tg of PC-6 was
almost the mean of Tgs of PC-4 than PC-5, which nicely reflects that the mixed
oligomeric nature of CEM 6 resulted in a network constituted by a mixture of shorter
and longer spacers between the crosslinks ultimately averaging out the network
flexibility and the Tg of the PC-6. PCs generally show high Tgs (200-300°C) but a low
crosslink density originated from the longer spacers resulted in flexible PC thermosets
and the Tgs of PC-4, PC-5 and PC-6 are well below the general range. These
observations reflect that Mc and spacer length between the crosslinks are critical
aspects while designing a CE resin for a certain application.
Figure 4.2: (a) DSC thermograms of CEMs 4, 5 and 6 in a nitrogen atmosphere. The exothermic
transitions for the cyclotrimerization of the cyanate groups peak at 151°C, 145°C and 163°C for CEM
4, 5 and 6. (b) DSC thermograms of PC-4, PC-5 and PC-6 showing the Tgs of these thermoset at
156°C, 132°C and 145°C.
4.2.2.
The PC-4 and PC-5 were investigated for their thermooxidative stability by TGA
under N2 and air atmospheres (Figure 4.3). Both the PCs exhibited high
thermooxidative stability as no weight loss was observed for both the thermosets
before 407°C under inert and before 391°C under oxidative atmosphere. PC-4 and
PC-5 followed a two-step degradation process typical for PCs where main
degradation process occurs during the second step. The onset for the second
degradation step for PC-4 and PC-5 was above 500°C. Besides the high onset
temperatures, the PC-4 and PC-5 also showed higher char yields (55% and 51%)
under inert atmosphere.
78
Figure 4.3: TGA thermograms of PC-4 and PC-5 under inert and oxidative atmosphere, reflecting
thermal stability of PC-4 and PC-5 up to 400°C.
4.3. Thermal properties of linear multifunctional CE resin
4.3.1.
It was inferred from the thermal analysis of the bifunctional CE resins 4, 5 and 6
that an oligomeric CEM mixture tends to not crystallize at room temperature
compared to its individual components. This characteristic is desirable for easy
processing. The Tgs of the respective PCs were found close to that of PEEK (142 °C)
but are still lower than the general Tg range for PCs (200-300 °C). These observations
lead to design a CEM 10 which is an oligomeric mixture but with multiple cyanate
groups i.e higher fc. As intended the CEM 10 is a viscous liquid at room temperature.
The DSC thermogram of CEM 10 (Figure 4.4a) shows no endothermic melting
transition around the ambient temperature and the only transition observed was an
exothermic transition due to the curing by cyclotrimerization of –OCN groups. The
curing exothermic transition was centered at 243°C and a shoulder with this peak
reflects the present of –OCN groups with different chemical reactivity corresponding
to the terminal and pendant –OCN groups. The electronic effect of the substituents on
the reactivity of the –OCN groups is discussed separately later in this chapter. The
curing of multifunctional CEM 10 resulted in densely crosslinked PC-10 network.
While DSC thermogram of PC-10 (Figure 4.4a) does not reveal any endothermic
transition and rheological measurements are being carried out to determine the Tg.
4.3.2.
The PC-10 was evaluated for thermal stability by subjecting it to TGA (Figure
4.4b). No weight loss was observed up to 400°C irrespective of the gaseous
79
environment (air or nitrogen) being used during the TGA measurements. The thermal
degradation under oxidative environment followed the typical two-step degradation
pattern with the second step onset at 478°C. The thermal degradation under inert
atmosphere, however, apparently followed a one-step degradation process in which
only 35% weight loss was observed at 900°C. Compared to PC-4 and PC-5, a higher
fc increases the crosslink density, which in turn increases the nitrogen content due to
higher number of triazine and this ultimately leads to a higher overall char yield
(65%). A higher char yield is usually considered as a reflection of flame retardancy
and fire resistance.8
Figure 4.4: (a) DSC thermograms of CEM 10 in a nitrogen atmosphere. The exothermic transitions for
the cyclotrimerization of the cyanate groups centered at 243°C. The DSC thermogram for the cured
PC-10 is depicted as dashed line and showed no obvious transition for Tg. (b) TGA thermograms of
PC-10 under inert and oxidative atmosphere, reflecting thermal stability of PC-10 up to 400°C.
4.4. Thermal properties of symmetric triarm CE resins
4.4.1.
Two CEMs with symmetrical triarm molecular architecture CEM 13 and CEM 18
differ only in their arm lengths, hence same fc i.e 3, but different Mc. Both the CEMs
were obtained as solids. Due to the shorter arms with favorable packing, the CEM 13
showed a higher melting point than CEM 18. The endothermic melting transition in
DSC thermograms (Figure 4.5) appeared at 23K higher for CEM 13 (M.P = 90°C)
than for CEM 18 (M.P = 67°C). The melting transitions were followed by exothermic
transitions, peaking at 277°C for CEM 13 and 212°C for CEM 18, related to the
curing by cyclotrimerization of cyanate groups. In case of CEM 18, a shoulder before
the curing exothermic peaks can be seen. A probable reason behind it is discussed
later in the section dealing with electronic effect of the substituents. The curing of the
CEM 13 and 18 produced thermosets PC-13 and PC-18. Like PC-10, the DSC
80
thermogram of PC-13 revealed no transition that could be attributed to Tg of the
thermoset. The PC-18 with its longer spacer arms between the crosslinks, i.e. higher
Mc, exhibits an endothermic transition for Tg at 212°C.
Figure 4.5: (a) DSC thermograms of CEM 13 in a nitrogen atmosphere. The endothermic transition for
melting appeared at 90°C followed by the exothermic transitions for the cyclotrimerization of the
cyanate groups centered at 277°C. The DSC thermogram for the cured PC-13 is depicted as dashed
line and showed no obvious transition for Tg. (b) DSC thermograms of CEM 18 in a nitrogen
atmosphere. The endothermic transition for melting appeared at 67°C followed by the exothermic
transitions for the cyclotrimerization of the cyanate groups centered at 212°C. The DSC thermogram
for the cured PC-18 is depicted as dashed line reflecting its Tg at 212°C.
The difference in the Mc, however, does not have any effect on the thermal stability
of the PC-13 and PC-18. Both exhibited a high thermal stability as only 5% weight
loss was observed at 420°C during TGA under inert and oxidative environments
(Figure 4.6). As observed for the other PCs in this study the degradation followed a
typical two-step degradation process. PC-13 was equally stable up to 412°C
irrespective of gaseous environment employed during TGA. However above 412°C
the first step degradation was rather sharp as compared to other PCs. PC-13 exhibited
a higher thermal stability under inert environment with a char yield of 48% at 900°C.
PC-18, compared to PC-13, was thermally more with an overall char yield of 55% at
900°C stable under inert condition.
The TGA thermograms of PC-18 (Figure 4.1b) look very much similar to the TGA
thermograms of PC-10 (Figure 4.4b). An analogous chemical nature of the aryl ether
ketone spacers in PC-10 and PC-18 resulted in analogous thermal stability.
81
Figure 4.6: TGA thermograms of PC-13 (a) and PC-18 (b) under inert and oxidative atmosphere,
reflecting thermal stability of PC up to 400°C. For both the PCs a 5% weight loss was observed at
420°C during TGA under both inert and oxidative environments.
The CE resins developed in the present work are either low melting with M.Ps
between 57-101°C (CEM 4, 5, 13 and 18) or even viscous liquids (CEM 6 and 10)
hence offering a wide range of processing temperatures. Depending on the length of
the spacers between the crosslinks, the PCs obtained from these CEMs showed a
range of Tgs, which are either higher or comparable to the Tg of PAEKs (140-160°C).
This comparison highlights that in addition to the elaboration of fundamental aspects,
the model CEMs synthesized in the present study are also of technological
importance. The thermal properties discussed so far are tabulated in the Table 4.1.
Table 4.1: Thermal properties of AE and AEK based CEMs and PCs
CEM
CEM-4
CEM-5
CEM-6
CEM-10
CEM-13
CEM-18
Melting
endothermic
transition/°C
(DSC)
101
130
Liquid*
Liquid*
90
67
Curing
exothermic
transition/°C
(DSC)
151
145
163
243
277
212
PC
Tg/°C
(DSC)
T5%
N2/°C
(TGA)
T5%
Air/°C
(TGA)
PC-4
PC-5
PC -6
PC -10
PC -13
PC -18
156
132
145
N/D**
N/D**
212
417
436
***
424
422
430
414
441
***
422
422
431
*CEM liquid at room temperature, no melting exothermic transition
**Could not be determined by DSC
*** Under investigation
T5%= Temperature at 5% weight loss
4.5. Electronic effect of the substituents on the reactivity of the cyanate
groups
The curing of a CEM is the result of a poly-cyclotrimerization reaction of –OCN
groups leading to an extended network, which is generally depicted as in Scheme 1.6
(chapter 1). However, there is an agreement on the reaction path involving catalysis
82
by compounds with mobile hydrogen (water or phenol residues used in the cyanate
synthesis), with the formation of a reaction intermediate called iminocarbonate
(Scheme 4.6).9-11
Scheme 4.6: (a) Possible electronic factors initiating and facilitating the cyclotrimerization of cyanate
groups. (b) A generalized scheme for cyclotrimerization mechanism involving catalysis by compounds
with mobile hydrogen and the formation of iminocarbonate. 10
The possible electronic factors initiating and facilitating the cyclotrimerization of cyanate
groups involve:
(I)
The attack of one cyanate nitrogen (free electron pair) at the electrophilic carbon
center of a second cyanate group.
(II)
Activation of the electrophilic carbon center in the cyanate group by protonation
of the nitrogen lone pair by mobile hydrogen.
(III)
Activation of electrophilic carbon by electron withdrawing R-groups
In the light of the mechanistics presented in an electron withdrawing group would
increase the electrophilicity of the –OCN group carbon in CEM hence favoring the
nucleophilic attack by water or residual phenol resulting in the formation of the
intermediates that autocatalyse the reaction, ultimately leading to higher reactivity. In
the present study, an electronic effect of the substituents was demonstrated by
comparing the reactivity of two aromatic mono-functional model CEMs (Scheme 4.7)
with electron donating (19) and electron withdrawing (20) groups substituted at para
position to the –OCN groups. The electronic features of the substructures in the CEMs
developed in this work are also empirically compared with the model CEMs 19 and
20 in Scheme 4.7. The position of exothermic transition peak, related to the
83
cyclotrimerization of –OCN groups, in the DSC thermograms was used to compare
the CEMs reactivity.
Scheme 4.7: Chemical structures of the CEM used to illustrate the electronic effect of the substituents
on the reactivity of –OCN group. The substructures in the developed CEMs are also empirically
represented.
The DSC thermograms of the CEM 19, which was a solid at room temperature,
with electron donating –OCH3 groups para to the –OCN groups showed first an
endothermic transition for melting at 32°C. The exothermic transition for –OCN
cyclotrimerization appeared at 277°C (Figure 4.7). By replacing the electron donating
–OCH3 group with electron withdrawing acetyloxy (CH3COO-) resulted in a higher
reactivity of CEM 20 (liquid at room temperature, hence no melting endothermic
transition in DSC analysis) and shifted the exothermic cyclotrimerization to 200°C.
Figure 4.7: DSC thermograms of CEM 19 and 20 showing the electronic effect of the substituents on
reactivity of CEMs. CEM 20 with electron withdrawing acetyloxy group substituted para to –OCN
group showed higher reactivity than CEM 19 having electron donating methoxy group instead.
Using this principle the reactivity order of the other model CEMs studied here can
be derived. Among the linear CEMs 4, 5 and 6 the electronic effect of electron
withdrawing ketone group substituted at para position to the –OCN groups is very
similar. This resulted in similar reactivity as reflected by very close exothermic peak
temperatures (151°C/CEM 4, 145°C/CEM 5, 163°C/CEM 6). CEM 10, on the other
hand, possesses –OCN groups with two different electronic environments i.e terminal
–OCN groups, which are para to the activating ketone groups and pendant –OCN
84
groups with weakly activating meta phenoxy groups. The overall effect is a
comparatively lower reactivity of CEM 10 as exothermic transitions appeared at
higher temperature (243°C) than the CEMs 4, 5 and 6. Based on the temperature
corresponding to the exothermic peak maximum the reactivity order of the linear
CEMs was found to be CEM 5 ≈ CEM 4 ≈ CEM 6 > CEM 10.
Among symmetrically triamred CEMs 13 and 18, the CEM 18 has an additional
electron withdrawing benzophenone unit between the core 1,3,5-substituted benzene
ring and terminal –OCN groups bearing benzene rings. This made the –OCN groups
carbon of CEM 18 more electrophilic and increased the reactivity. The higher
reactivity of CEM 18 is evident from the appearance of an exothermic peak in the
DSC thermogram (Figure 4.5) at relatively lower temperature (277°C/CEM 13,
212°C/CEM 18).
Due to high reactivity of CEMs with stronger electron withdrawing effect of the
substitutents para to the –OCN group as in case of CEMs 4, 5 and 18, the peaks for
melting and curing reaction tend to merge in DSC thermograms (Figure 4.2a and
Figure 4.5b).1 This can have severe consequences as the CEM will start to cure even
at the melting stage of the processing.
However, a careful design of the CEM
molecular architecture can overcome this issue. For instance, CEM 6 exhibited have
high reactivity like CEM 4 and CEM 5, but its liquid like physical state excludes the
melting stage of processing. The processing temperature range (room temperature to
50°C) offered by the CEM 6 is still narrow and it already started to cure when an
attempt was made to measure its viscosity at 50 °C. A further tuning of the chemical
structure by introducing less reactive pendant –OCN groups in CEM 10 resulted in a
relatively lower reactivity and wide range of working temperature (room temperature
to 175°C) as CEM 10 also turned out to be a liquid. The overall electronic effect of
the substituents has pronounced consequences on the chemical and thermal properties
of the CEMs and hence is critical parameters to be considered while designing a
CEM.
4.6. Conclusions
The effect of structural variations i.e. fc, Mc and spacer chain architecture, on the
thermal properties of the CEMs was investigated on six model CEMs. The
85
investigation of pure bifunctional oligomeric analogue CEMs 4 and 5 with the linear
molecular architecture revealed following conclusions:
1)
CEM 4 with shorter spacer exhibited a sharp melting endothermic transition
before the curing exotherm in the DSC analysis whereas the CEM 5 with the
longer spacer showed a less pronounced melting that immediately followed the
curing exotherm in DSC thermogram.
2)
The PC derived from CEM 4 with a shorter spacer between the crosslinks
possesses a higher Tg than the PC derived from the CEM with longer spacer
between the crosslinks. But this has no effect on the thermal stability.
3)
Instead of pure oligomeric CEMs, the CEM 6 constituted by a mixture of
oligomers was a viscous liquid in which the crystallization is hindered and only
vitrification below room temperature occurs. The Tg of PC-6 was almost the mean
of Tgs of PC-4 than PC-5, which nicely reflects that the mixed oligomeric nature
of CEM 6 resulted in a network constituted by a mixture of shorter and longer
spacers between the crosslinks ultimately averaging out the network flexibility
and the Tg of the PC-6.
In the light of above stated conclusions linear multifunctional mixed oligomeric
CEM 10 was designed. The pendant –OCN groups in addition to the terminal ones
resulted in higher crosslink density in the cured PC. The investigation of cyanate ester
resin 10 led to the following conclions:
1)
Like the bifunctional mixed oligomeric CEM 6, the multifunctional mixed
oligomeric CEM 10 is also a viscous oil with improved processability.
2)
The higher crosslink density after curing CEM 10 improves the thermal
stability of the PC derived from the multifunctional CEM 10 resulted in a high
thermal stability.
3)
As a result of higher crosslink density no endothermic transition for a Tg was
evident in the DSC analysis of PC-10.
In order to study the influence of monomer topology the CEMs with symmetric
triarm architecture of shorter and longer arm lengths were synthesized and
investigated, which led to the following conclusions:
1) CEM 18 with longer arms exhibited a lower melting transition in the DSC
compared to CEM 13.
2) The PC derived from CEM 13 with shorter arm length exhibited no transition
related to Tg in the DSC whereas the PC derived from CEM 18 with longer
86
arms exhibited a Tg of 212°C. This reflects a flexible nature of PC-18 network
due to longer spacers.
3) PCs derived from both CEMs were found thermally stable up to 420°C in
TGA irrespective of the gaseous environment used.
The
exothermic
transition
temperature
corresponding
to
curing
by
cyclotrimerization of the –OCN groups in mono-functional model compounds and the
multifunctional CEMs was used to investigate and compare the electronic effect of the
substituents on reactivity of –OCN groups. For CEMs with electron withdrawing
groups in para position to the –OCN groups the curing exothermic transition shifts to
lower temperature.
The conclusions derived from the resent work are significant for the design of new
CEMs with control over the thermal properties of the CEMs and their corresponding
PCs.
4.7.
4.7.1.
Experimental
Synthesis of oligomeric aryl ether ketone with hydroxy end groups
Aryl ether ketone oligomeric mixture with terminal hydroxyl groups (1)
4,4'-Dihydroxybenzophenone (3g, 14 mmole), 1,3-dibromobenzene (1.66g, 7 mmole), 1,10phenanthroline (0.110g, 0.61 mmole), toluene (3.2mL) and DMF (25mL) were added to a 100mL
three-necked round bottom flask fitted with a thermometer, a Dean–Stark trap with condenser, and an
argon inlet. The resulting mixture was degassed thoroughly with argon for ten minutes followed by the
addition of copper (I) iodide (0.104g, 0.54 mmole). After filling the Dean-Stark trap with toluene, the
mixture was heated for 30min to 1h at 135-145oC in order to completely dissolve all the starting
materials. The mixture was cooled to 100°C and potassium carbonate (1.57g, 11.36 mmole) was added
in one portion. The resulting mixture was again heated at 135-145°C for 3h and the water formed in the
reaction was removed by azeotropic distillation. After this time the reaction mixture was cooled again
to 100°C and another portion of potassium carbonate (1.57g, 11.36 mmole) was added. The reaction
mixture was again heated to 135-145°C for 12-14h until no further water deposited in the Dean-Stark
trap. The remaining toluene was then removed by distillation and the reaction mixture was cooled to
ambient temperature. Water was added (80mL) to the reaction mixture, which was made acidic by the
addition of 2 M HCl (80 mL) and extracted with ether (3 × 50mL). The combined ether extracts were
washed with water until neutral and dried over anhydrous MgSO4. The solvent was evaporated after
passing through a short silica plug to yield off-white solid, which was vacuum dried at 80°C overnight
to yield aryl ether ketone oligomers mixture (AEKOM) with terminal hydroxyl groups 1 (2.7g, 76%).
IR (film): 3216 (O-H), 3067 (C=CH), 1686 (diaryl ketoneC=O), 1582, 1500, 1472, 1439 (aromatic
C=C), 1219, 1159 (C-O), 960, 924 (C-OH), 848 cm-1 (aromatic). FDMS: m/z = 214 (n=0), 502 (n=1),
790 (n=2), 1078 (n=3), 1366 (n=4).
Separation of the mixture of aryl ether ketone oligomers with terminal hydroxyl groups 1 into pure aryl
ether ketone oligomeric analogues with n=1, and n=2 (2, 3)
The AEKOM 1 with terminal hydroxyl groups was subjected to two successive columns (silica).
In the first hexane and ethylacetate (2:2.5) solvent mixture was used as eluent and 1 was separated
from any byproduct (Rf = 0.5). The material from the first column was subjected to the second column
with dichloromethane, ethylacetate and acetone (6:1:0.5) solvent mixture as eluent and separated into
oligomeric analogues. The Rf value of aryl ether ketone oligomer (AEKO) with n=1, 2, was 0.4 and for
n=2, 3, was 0.6. The %age of oligomer 2 separated was 55% and of oligomer 3 was 30% when
compared to the initial amount of the AEKOM 1 fed into the columns.
87
Oligomer 2: IR (film): 3228 (O-H), 3064 (C=CH), 1698 (diaryl ketone C=O), 1579, 1472 (aromatic
C=C), 1219, 1155 (C-O), 960, 924 (C-OH), 848 cm-1 (aromatic). FDMS: 502 (n=1). 1H NMR (d6Acetone): δ [ppm] = 9.0 (2H of terminal –OH groups, br-s), 7.56-7.67 (8 aromatic H flanking C=O
groups of benzophenone, br), 7.35-7.41 (1 aromatic H of central ring meta to diary ether linkage, br),
7.00-7.05 (4 aromatic H of benzophenone next to the diaryl ether linkage, br), 6.76-6.86 (7 aromatic H,
4H of benzophenone next to –OH groups, 3H of 1,3-meta substituted central ring, H ortho to diary
ether linkage, br). 13C NMR (d6-Acetone): δ [ppm] 193.9, 162.3, 161.0, 158.4, 134.4, 133.2, 132.7,
132.2, 130.2, 118.5, 116.2, 115.9, 112.0.
Oligomer 3: IR (film): 3225 (O-H), 3061 (C=CH), 1695 (diaryl ketone C=O), 1582, 1476 (aromatic
C=C), 1219, 1159 (C-O), 960, 924 (C-OH), 848 cm-1 (aromatic). FDMS: 790 (n=2). 1H NMR (d6Acetone): δ [ppm] = 9.0 (2H of terminal –OH groups, br-s), 7.55-7.70 (12 aromatic H flanking C=O
groups of benzophenone, br), 7.34-7.40 (2 aromatic H of two 1,3-meta disubstituted rings rings, H meta
to diaryl ether linkage, br), 6.99-7.04 (8 aromatic H of benzophenone next to the diaryl ether linkage,
br), 6.74-6.86 (10 aromatic H, 4H of benzophenone next to –OH groups, 6H of two 1,3-meta
disubstituted rings, H ortho to diary ether linkage, br). 13C NMR (d6-Acetone): δ [ppm] 193.9, 162.3,
161.5, 161.0, 158.4, 158.2, 134.7, 133.7, 133.2, 132.7, 132.3, 130.2, 118.5, 116.3, 115.9, 112.1.
4.7.2.
Synthesis of linear aryl ether ketone oligomeric mixture with pendant and
terminal hydroxy groups (9)
Synthesis of linear aryl ether ketone oligomeric mixture with pendant methoxy and terminal hydroxy
groups (8)
The reaction was carried out in the same manner as for the synthesis of AEKOM 1 except 1,3dibromo-5-methoxybenzene was used instead of 1,3-dibromobenzene. After the completion of the
reaction water was added to the reaction mixture, which was made acidic by the addition of 2 M HCl
and extracted with three times with diethyl ether and twice with dichloromethane. The combined
organic phase was washed with water until neutral and dried over anhydrous MgSO4. The solvent was
evaporated after passing through a short silica plug to yield light brown solid, which was vacuum dried
at 80°C over night to yield an AEKOM with pendant methoxy and terminal hydroxyl groups 8 (83%).
IR (film): 3237 (O-H), 3067 (C=CH), 1686 (diaryl ketone C=O), 1579, 1485 (aromatic C=C), 1219,
1159 (C-O), 1049 (–OCH3) 997, 924 (C-OH), 845 cm-1 (aromatic). 1H NMR (d6-Acetone): δ [ppm]
= 9.0 (2H of terminal –OH groups, br-s), 7.52-7.70 (8 aromatic H flanking C=O groups of
benzophenone, br), 7.01-7.06 (4 aromatic H of benzophenone next to the diaryl ether linkage, br), 6.806.84 (4 aromatic H ofbenzophenone next to –OH groups, br), 6.42-6.43 (2 aromatic H ortho to the –
OCH3 groups, br), 6.29-6.31 (1 aromatic H, para to the –OCH3 groups, br), 3.6 (3H of –OCH3, s). 13C
NMR (d6-Acetone): δ [ppm] 193.9, 163.3, 162.3, 160.91, 159.1, 134.4, 133.2, 132.7, 132.2, 130.2,
118.6, 115.9, 104.0, 102.5, 56.1.
Synthesis of linear aryl ether ketone oligomeric mixture with pendant and terminal hydroxy groups (9),
deprotection of pendant methoxy groups
43.2mL of conc. HCl was added to rapidly stirred pyridine (40mL) in a 100mL one-neck round
bottom flask, which was subsequently equipped with a distillation condenser to a collecting flask. The
pyridine HCl mixture was heated to distil off the water. The temperature was raised to 220-240°C to
ensure the complete removal of water and unreacted pyridine. The pyridine-HCl salt formed was
cooled to 140°C and poured into a 250mL Schlenk containing 3.28g of 8. The reaction mixture was
heated at 220°C under argon for 2h and left at room temperature overnight. After cooling down to
room temperature, distilled water was added to the solidified reaction mixture and precipitates were
washed with warm dilute HCl and then with water till neutral. The AEKOM 9 (2.37g, yield=74%) was
dried in an oven at 80°C for 4h.
IR (film): 3234 (O-H), 3067 (C=CH), 1686 (diaryl ketone C=O), 1579, 1485 (aromatic C=C), 1219,
1159 (C-O), 997, 924 (C-OH), 842 cm-1 (aromatic). FDMS: m/z = 519 (n=1), 822 (n=2), 1127 (n=3).
1
H NMR (d6-Acetone): δ [ppm] = 9.0 (2H of terminal –OH groups, br-s), 7.52-7.70 (8 aromatic H
flanking C=O groups of benzophenone, br), 7.01-7.06 (4 aromatic H of benzophenone next to the
diaryl ether linkage, br), 6.80-6.84 (4 aromatic H ofbenzophenone next to –OH groups, br), 6.42-6.43
(2 aromatic H ortho to the –OCH3 groups, br), 6.29-6.31 (1 aromatic H, para to the –OCH3 groups, s).
13
C NMR (d6-Acetone): δ [ppm] 194.0, 162.4, 160.9, 159.2, 134.4, 133.2, 132.7, 132.2, 130.2, 118.7,
115.9, 103.6, 102.9.
88
4.7.3.
Synthesis of symmetrically triarmed aryl ether ketone with hydroxy end
groups (17)
Synthesis of 4-(4-fluorobenzoylphenoxy)anisole (15)
9.82g (50 mmole) of 4-phenoxyanisole 14 was added as a solution in 130mL of dry
dichloromethane to an oven dried 250mL round bottom Schlenk flask under argon. 8.56g (54 mmole)
of 4-fluorobenzoylchloride was added with the help of a syringe. The reaction mixture was cooled to
<0°C and 15.73g of AlCl3 (117.8 mmole) was added in portions keeping the temperature below 0°C.
After the complete addition of AlCl3 the reaction mixture was allowed to warm and stirred at room
temperature for 4h followed by a 4h reflux. After cooling to room temperature the reaction mixture was
poured on to 800mL of ice cooled 1N aq. HCl and stirred vigorously until most of the dichloromethane
was evaporated. The solid mass obtained was washed with distilled water until neutral and purified by
column chromatography on silica gel using dichloromethane:hexane (6:0.5) as eluent. The 4-(4fluorobenzoylphenoxy)anisole 15 was obtained in 96% yield (Rf = 0.64).
IR (film): 3067 (C=CH), 2844 (aliphatic C-H), 1640 (C=O), 1594, 1497 (aromatic C=C), 1226, 1148
(C-O), 1031 (–OCH3), 857 (Ar-F), 844, 836 cm-1 (aromatic). FDMS: m/z = 322.7. 1H NMR (CD2Cl2):
δ [ppm] = 7.64-7.75 (4 aromatic H flanking C=O group of benzophenone, m), 6.83-7.12 (8 aromatic
H, 4H flanking diaryl ether linkage, 2H ortho to fluoro group and 2H ortho –OCH3 group, m), 3.72 (3H
of –OCH3 group, s). 13C NMR (CD2Cl2): δ [ppm] 194.0, 167.1, 163.8, 163.0, 157.2, 148.8, 134.7,
132.5, 131.6, 122.0, 116.6, 115.7, 115.4, 55.9.
Synthesis of symmetrically triarmed aryl ether ketone with methoxy end groups, 1,3,5-(4-(4methoxyphenoxy)-4'-oxy)benzene (16)
20mL of dry NMP, 10mL of dry toluene, 2.68g (8.3 mmole) of 15, 1.98g (14.3 mmole, 6
equivalent to phloroglucinol) of K2CO3 and 0.3g (2.4 mmole) of phloroglucinol were added under
argon to an oven dried 50mL Schlenk flask, equipped with magnetic bar and Dean-Stark trap
connected to a reflux condenser. The reaction mixture was heated under argon at 180°C for 20h. After
cooling down to room temperature the reaction mixture was poured into the 50mL of distilled water
and was acidified with 0.05N HCl. The aqueous phase was extracted with CHCl3 (3 × 50mL). The
combined organic phase was washed with water until neutral and dried over MgSO4. The crude product
obtained, after evaporating CHCl3 on rotary evaporator, was purified by column chromatography on
silica gel using dichloromethane:acetone (10:0.1) as eluent. The 1,3,5-(4-(4-methoxyphenoxy)-4'oxy)benzene 16 was obtained in 50-60% yield (Rf = 0.25).
IR (film): 3043 (C=CH), 2836 (aliphatic C-H), 1636 (C=O), 1587, 1495 (aromatic C=C), 1220, 1148
(C-O), 1030 (–OCH3), 837 cm-1 (aromatic). FDMS: m/z = 1133.9. 1H NMR (CDCl3): δ [ppm] = 7.677.75 (12 aromatic H flanking C=O groups of benzophenone, br), 6.84-7.04 (24 aromatic H flanking the
diaryl ether linkage and aryl methyl ether linkage, br), 6.51 (3 aromatic H of core ring, s), 3.75 (9H of –
OCH3 group, s). 13C NMR (CDCl3): δ [ppm] 194.1, 162.5, 159.6, 158.6, 156.7, 148.5, 133.5, 132.2,
131.3, 121.6, 118.1, 116.2, 115.1, 106.1, 55.6.
4.7.4.
Synthesis of symmetrically triarmed aryl ether ketone with hydroxy end
groups 1,3,5-(4-(4-hydroxyphenoxy)-4'-oxy)benzene (17), deprotection of
methoxy groups
The deprotection was carried out in the same manner as for the synthesis of AEKOM 9. Distilled
water was added to the solidified reaction mixture and aqueous phase was extracted three times with
diethyl ether. The combined organic phase was washed twice with dilute HCl and then with water till
neutral. Diethyl ether was evaporated on rotary evaporator after drying over MgSO4 and the crude
product obtained was purified by column chromatography on silica gel using hexane:acetone (3:2.5) as
eluent. The 1,3,5-(4-(4-hydroxyphenoxy)-4'-oxy)benzene 17 was obtained in 90% yield (Rf = 0.8).
IR (film): 3331 (O-H), 2951 (C=CH), 1635 (C=O), 1585, 1493 (aromatic C=C), 1214, 1158 (C-O),
1004 (–OH), 837 cm-1 (aromatic). FDMS: m/z = 993.5. 1H NMR (d6-DMSO): δ [ppm] = 9.5 (3H of
terminal –OH groups, br-s), 7.70-7.77 (12 aromatic H flanking C=O groups of benzophenone, br),
7.21-7.24 (6 aromatic H meta to the hydroxy groups of the 3 outer most rings, br) 6.96-6.99 (12
aromatic H of benzophenone next to the diaryl ether linkage, br) 6.81-6.84 (6 aromatic H ortho to the –
OH groups on the outer ring, br), 6.73 (3 aromatic H of core ring, s). 13C NMR (d6-DMSO): δ [ppm]
193.0, 162.2, 159.3, 157.8, 154.6, 146.3, 132.7, 132.0, 130.7, 121.7, 118.0, 116.4, 115.8, 106.5.
89
4.7.5.
A general procedure for the synthesis of linear and symmetrically triarmed
aryl ether ketone cyanate ester monomers (4, 5, 6, 10, 13, 18, 19, 20)
To a 0.3M solution of phenol derivative (1, 2, 3, 9, 12, or 17) in dry acetone cyanogen bromide (2
equivalent of each phenolic group) was added and solution was transferred under argon to an oven
dried three neck round bottom flask equipped with a magnetic stirrer, dropping funnel, argon inlet and
outlet. The solution was stirred and cooled to –20 to –30oC. 8.6M solution of dry triethylamine (2.4
equivalent of each phenolic group) in dry acetone was added dropwise over a period of 1h while
maintaining the temperature of the reaction mixture below –20oC. After the addition was complete the
reaction mixture was further stirred for 1h below –20oC and 1h at room temperature while Et3N+Br–
salt precipitated. Et3N+Br– salt was removed by filtration and acetone was removed on rotary
evaporator in vacuo. The residue was dissolved in dichloromethane and washed several time with
water until the NMR showed the complete removal of Et3N+Br– and residual triethylamine. The organic
phase was dried over MgSO4 and solvent was removed in vacuo to yield CEM.
CEM 4: IR (film): 3067 (C=CH), 2266 (–CN) 1643 (diaryl ketone C=O), 1582, 1476 (aromatic C=C),
1210, 1157 (C-O), 1011, 968, 927 (C-OCN), 855 cm-1 (aromatic). 1H NMR (CDCl3): δ [ppm] = 7.717.85 (10 aromatic H, 8H flanking C=O groups of benzophenone and 2 H of central ring meta to diary
ether linkage, br), 7.30-7.37 (4 aromatic H of benzophenone next to the diaryl ether linkage, br), 7.017.04 (4 aromatic H of benzophenone next to –OCN groups), 6.98-7.07 (2 aromatic H of 1,3-meta
substituted central ring, H ortho to diary ether linkage, br), 6.76-6.78 (1 aromatic H of 1,3-meta
substituted central ring, H ortho to diary ether linkage, br). 13C NMR (CDCl3): δ [ppm] 193.3,
161.3,160.3, 156.9, 136.5, 132.7, 132.2, 131.7, 131.2, 118.8, 116.0, 115.3 111.8, 107.6 (–OCN).
CEM 5: IR (film): 3064 (C=CH), 2265 (–CN) 1642 (diaryl ketone C=O), 1582, 1476 (aromatic C=C),
1213, 1156 (C-O), 1012, 963, 926 (C-OCN), 855 cm-1 (aromatic). 1H NMR (CDCl3): δ [ppm] = 7.647.85 (14 aromatic H, 12H flanking C=O groups of benzophenone and 2 H of central ring meta to diary
ether linkage, br), 7.30-7.37 (4 aromatic H of benzophenone next to –OCN groups), 6.98-7.07 (8
aromatic H of benzophenone next to the diaryl ether linkage, br) 6.82-6.87 (4 aromatic H of 1,3-meta
disubstituted rings, H ortho to diary ether linkage, br), 6.72-6.76 (2 aromatic H of two 1,3-meta
disubstituted rings, H ortho to diary ether linkage, br). 13C NMR (CDCl3): δ [ppm] 193.8, 163.5,
161.8, 159.3, 158.3, 157.1, 135.6, 134.7, 134.7, 133.7, 132.8, 131, 119.4, 116.9, 116.6, 112.9, 109.5 (–
OCN)
CEM 10: IR (film): 3064 (C=CH), 2271, 2235 (–CN) 1649 (diaryl ketone C=O), 1594, 1491 (aromatic
C=C), 1216, 1159 (C-O), 1006, 951, 927 (C-OCN), 842 cm-1 (aromatic). NMR (CDCl3): δ [ppm]
= 7.74-7.87 (8 aromatic H flanking C=O groups of benzophenone, br), 7.36-7.45 (4 aromatic H of
benzophenone next to terminal –OCN groups), 7.05-7.10 (4 aromatic H of benzophenone next to the
diaryl ether linkage, br) 6.75-6.76 (2 aromatic H ortho to the pendant –OCN groups on the 1,3-meta
disubstituted rings, br), 6.64-6.65 (1 aromatic H of the 1,3-meta disubstituted rings, H para to the
pedant –OCN groups, br). 13C NMR (CDCl3): δ [ppm] = 193.3, 161.3, 156.9, 155.0, 136.5, 132.6,
132.1, 131.7, 131.2, 117.8, 115.9, 115.3, 111.8, 107.9 (–OCN)
CEM 13: 3070 (C=CH), 2274, 2235 (–CN) 1588, 1451, 1488 (aromatic C=C), 1232, 1168, 1116 (C-O),
1003 (C-OCN), 829 cm-1 (aromatic). NMR (CD2Cl2): δ [ppm] = 7.18-7.25 (6 aromatic H of outer
rings next to the diaryl ether linkage, br), 6.99-6.06 (6 aromatic H of outer rings next to the –OCN
groups, br), 6.28 (3 aromatic H of the core ring). 13C NMR (CD2Cl2): δ [ppm] = 160.4, 155.5, 150,
122, 188.1, 109.6 (–OCN), 105.01.
CEM 18: 3070 (C=CH), 2268, 2238 (–CN), 1646 (diaryl ketone C=O), 1585, 1485 (aromatic C=C),
1213, 1155 (C-O), 1000, 924 (C-OCN), 835 cm-1 (aromatic). 1H NMR (CD2Cl2): δ [ppm] = 7.68-7.73
(12 aromatic H flanking C=O groups of benzophenone, br), 7.23-7.28 (6 aromatic H meta to the
hydroxy groups of the 3 outer most rings, br) 7.02-7.11 (12 aromatic H of benzophenone next to the
diaryl ether linkage, br) 6.93-6.98 (6 aromatic H ortho to the –OCN groups on the outer ring, br), 6.55
(3 aromatic H of core ring, s). 13C NMR (CD2Cl2): δ [ppm] = 194.0, 161.0, 160.3, 158.9, 154.6, 149.5,
133.6, 133.2, 132.6, 121.9, 118.5, 118, 117.4, 109.2 (–OCN), 106.7.
CEM 19: 3073 (C=CH), 2975-2936 (aliphatic CH) 2265, 2235 (–CN), 1497 (aromatic C=C), 1250,
1171 (C-O), 1024 (C-OCN), 823 cm-1 (aromatic). 1H NMR (d6-Acetone 2): δ [ppm] = 7.17-7.24 (2
aromatic H, m), 6.91-6.98 (2 aromatic H, m) 3.7 (3 H of –OCH3, s). 13C NMR (CD2Cl2): δ [ppm] =
159.4, 148, 117.6, 116.5, 110.3 (–OCN), 56.4.
CEM 20: 3071 (C=CH), 2999-2830 (aliphatic CH) 2269, 2227 (–CN), 1753 (C=O) 1491 (aromatic
C=C), 1201, 1159 (C-O), 1012, 902 (C-OCN), 845 cm-1 (aromatic). 1H NMR (d6-Acetone): δ [ppm]
= 7.29-7.36 (2 aromatic H, m), 7.17-7.24 (2 aromatic H, m) 2.14 (3 H of –OOCCH3, s). 13C NMR
(CD2Cl2): δ [ppm] = 170, 151.5, 150.6, 125.4, 117.7, 109.8 (–OCN), 21.2
Curing of CEMs to PC thermosets:
90
The CEMs were neat cured by the following temperature program in Teflon molds in a tube furnace
under argon to yield the corresponding PCs. After degassing at 80°C for 1h, the curing was induced
according to the program 180°C/2hr → 260°C/8hr → cooling to room temperature. The PCs obtained
were subjected to thermal analysis.
4.8. Reference
1.
Marcos-Fernandez, A.; Posadas, P.; Rodriguez, A.; Gonzalez, L., Synthesis and
Characterization of New Dicyanate Monomers. A Way to Obtain Fully Aromatic Crosslinked
Poly(Ether Ketone)S. Journal of Polymer Science Part A-Polymer Chemistry 1999, 37, (16), 31553168.
2.
Laskoski, M.; Keller, T. M. In Oligomeric Cyanate Ester Resins: A New Class of
Thermosetting Polymers, 227th ACS National Meeting, Anaheim, CA, Mar 28-Apr 01, 2004; Amer
Chemical Soc: Anaheim, CA, 2004; pp 26-POLY.
3.
Kelkar, A. A.; Patil, N. M.; Chaudhari, R. V., Copper-Catalyzed Amination of Aryl Halides:
Single-Step Synthesis of Triarylamines. Tetrahedron Letters 2002, 43, (40), 7143-7146.
4.
Klapars, A.; Antilla, J. C.; Huang, X. H.; Buchwald, S. L., A General and Efficient Copper
Catalyst for the Amidation of Aryl Halides and the N-Arylation of Nitrogen Heterocycles. Journal of
the American Chemical Society 2001, 123, (31), 7727-7729.
5.
Wolter, M.; Nordmann, G.; Job, G. E.; Buchwald, S. L., Copper-Catalyzed Coupling of Aryl
Iodides with Aliphatic Alcohols. Organic Letters 2002, 4, (6), 973-976.
6.
Tisserand, S.; Baati, R.; Nicolas, M.; Mioskowski, C., Expedient Total Syntheses of Rhein
and Diacerhein Via Fries Rearrangement. Journal of Organic Chemistry 2004, 69, (25), 8982-8983.
7.
Chen, Y. M.; Chen, C. F.; Liu, W. H.; Xi, F., Synthesis of Large Building Blocks for
Dendritic Polyethers. Polymer Bulletin 1996, 37, (5), 557-563.
8.
Gilman, J. W.; Lomakin, S.; Kashiwagi, T.; VanderHart, D. L.; Nagy, V., Characterization of
Flame Retarded Polymer Combustion Chars by Solid-State C-13 and Si-29 Nmr and Epr. Abstracts of
Papers of the American Chemical Society 1997, 213, 418-POLY.
9.
Bauer, J.; Hoper, L.; Bauer, M., Cyclotrimerization Reactivities of Mono- and Difunctional
Cyanates. Macromolecular Chemistry and Physics 1998, 199, (11), 2417-2423.
10.
11.
Grenierloustalot, M. F.; Lartigau, C.; Grenier, P., A Study of the Mechanisms and Kinetics of
the Molten State Reaction of Noncatalyzed Cyanate and Epoxy-Cyanate Systems. European Polymer
Journal 1995, 31, (11), 1139-1153.
91
Chapter 5
5.
Polycyanurate Thermoset Networks with High
Thermal, Mechanical, and Hydrolytic Stability Based
on Liquid Multifunctional Cyanate Ester Monomers
with Bisphenol A and AF Units
The synthesis and characterization of two new cyanate ester thermosets is presented,
which are liquid at room temperature and can be conveniently moulded into shapes with
dimensions down to the nanometer scale. After curing these thermosets yield polycyanurates
with excellent mechanical properties and thermal stability up to 400°C in air.
92
The results presented in this chapter are in continuation to the part of the project
where the development easy to process CERs in intended and it is in the same vein
with the work presented in chapter 4. As it was demonstrated in chapter 4, the
versatility of the synthetic method developed by Grigat et al. (Scheme 1.5) made it
possible to incorporate different aromatic structural entities into CEMs, offering a
control over the chemical, physical, and thermal properties of CEMs and PCs by
careful selection of the precursor phenols. The development of ambient temperature
processable CEMs, which could produce PC with good thermal and mechanical
properties, is an active area of current CE resin research. 2,2’-Bis(4-cyanatophenyl)1,1,1,3,3,3-hexafluoropropane
(BAFCY,
Tm
=
87°C)
and
2,2’-bis(4-
cyanatophenyl)isopropylidene (BACY, Tm = 79°C) are among the most studied and
first commercialized CEMs.1, 2 The PCs derived from these bisphenol A derivatives
have attracted great technological interest as structural materials due to their high Tg
(270°C/BAFCY-PC and 289°C/BACY-PC), high mechanical strength (Young’s
modulus 3.11 GPa/BAFCY-PC and 3.17 GPa/BACY-PC), good thermooxidative
stability (up to 400°C) and good moisture resistance. On the other hand, the
corresponding precursor CEMs in their non-crosslinked state suffer from poor
processability due to their crystalline nature at room temperature.1-3 M. Laskoski et
al.4,
5
have produced ambient temperature processable BAFCY- and BACY-based
CEMs by placing oligomeric aromatic ether (OAE) spacers between the terminal
cyanate groups. As the crosslink density is reduced due to an increased chain length
between the crosslinks, their efforts have produced CEMs with enhanced
processability at the expense of a decrease in Tg in the corresponding PCs
(175°C/BAFCY-OAE-PC and 140°C/BACY-OAE-PC). The strategy of A. J.
Guenthner et al.6 to replace in BACY the quaternary carbon center with quaternary
silicon produced a CEM which is still a crystalline solid, but with a lower melting
point of 59.9°C (a 20K lower Tm than BACY for improved monomer processability).
At the same time the Tg in the cross-linked PC is only lowered by 10K and the cured
material essentially maintains its mechanical properties, like a tensile modulus of 2.8
GPa. These reports present important strategies for the development of CE resin
systems with improved properties by specifically tailoring the structure of CEMs at
the molecular level. The key physical properties that serve as a basis for identifying
“improved” CE resin systems are comprehensively outlined by A. J. Guenthner et al.
and include: a) ease of uncured CEM processing, b) which produce PCs with high
93
glass transition temperatures (generally in the range of 200-300°C), c) good
mechanical properties, d) good thermooxidative stability, and e) good resistance to
moisture.6 In particular a high Tg is critical for technical applications of CE resins, as
a low Tg in PCs commonly results in poor mechanical properties at elevated
temperatures.2
Based on these criteria and exploiting the knowledge gained through the work
presented in chapter 4, it was intended developing BAFCY and BACY based CEMs,
which are processable at ambient temperature while possessing a high Tg, good
thermo-oxidative, mechanical, and hydrolytic stability in the cured PC state. To
achieve this goal the strategy, introduced and developed in chapter 4, based on OAE
derivatives of bisphenol AF and A with pendant and terminal cyanate groups was
exploited. The flexible and mixed oligomeric nature of the OAE spacers will provide
CEMs with a low processing temperature whereas multiple reactive cyanate groups
will improve the thermo-mechanical stability of the cured PCs by an increased
crosslinking density. The thermal, mechanical, dielectric and hydrolytic properties of
developed CE resins are discussed and compared with the existing structurally related
PCs.
5.1. Synthesis and characterization of bisphenol AF and bisphenol A
based room temperature processable CEMs
The synthesis of CEMs 3a and 3b is depicted in Scheme 5.1. In the first step 1,3dibromo-5-methoxybenzene was reacted with bisphenol AF or bisphenol A in a
modified Ullmann condensation reaction catalyzed by a soluble Cu(I) complex,7-9
generated in situ from copper (I) iodide and 1,10-phenanthroline. K2CO3 was used as
base. The reaction was carried out at 135-145°C in a mixture of DMF and toluene,
with latter being used to remove water formed during the reaction by azeotropic
distillation. The formed OAE mixture 1a or 1b with terminal hydroxyl and pendant
methoxy groups contained chain lengths of n ≈ 0-5, as determined by FD mass
spectrometry. The pendant methoxy groups were then cleaved by refluxing a
dichloromethane solution of 1a or 1b in the presence of BBr3 to give the OAE
mixture 2a or 2b with terminal as well as pendant hydroxyl groups in quantitative
yield. The following reaction with cyanogen bromide in the presence of triethylamine
in dry acetone at a temperature between –20 to –30°C afforded the CEM 3a or 3b as
oil in more than 90% yield.
94
Scheme 5.1: Synthesis of the CEMs 3 with pendant as well as terminal cyanate groups and their
thermal curing to the polycyanurate thermoset 4.
All the oligomeric intermediates and products were purified by simple
precipitation and washing with high yields, which facilitates scale up to large-scale
synthesis for potential commercialization. The CEMs 3a and 3b are viscous oils with
room temperature viscosities of 288 Pa.s and 3 Pa.s respectively. The higher
molecular weight of 3a, higher steric hindrance and rigidity induced by the CF3
substituents at the quaternary carbon center might account for a higher viscosity of
CEM 3a compared to 3b, with CH3 groups instead. A similar effect is also observable
95
in the parent CEMs, BAFCY and BACY. The melting temperature for BAFCY
(87°C) is 8K higher than the melting temperature of BACY (79°C) and the only
difference between these two CEMs is the CF3 (BAFCY) versus CH3 (BACY)
substituents at the quaternary carbon center. Consequently, the nature of substituents
at the quaternary carbon center is an important factor to be considered when designing
a CEM based on bisphenol A derivatives.
The formation of CEMs 3a and 3b from 2a and 2b can be well observed by IR
spectroscopy (Figure 5.1 and Figure 5.2) with the disappearance of the hydroxyl
stretching band (at 3338 cm-1 for CEM 3a and 3335 cm-1 for CEM 3b) and the
appearance of the -OCN bands (located at 2277-2241 cm-1 for CEM 3a and 22622235 cm-1 for CEM 3b). These -OCN bands vanish while curing the CEMs 3 to PCs 4
with the concurrent appearance of a characteristic stretching band for the triazine ring
(at 1360 cm-1). These experiments demonstrated that IR spectroscopy is a very
convenient tool for the rapid chemical characterization and determination of
functional group conversion in these oligomer mixtures without the need for
separation and tedious purification of the individual oligomeric components. As the
curing reaction is sensitive to residual phenolic groups, it is important to note that
within the resolution of the characterization methods (NMR and IR) no free phenolic
groups were detectable in the CEMs 3a and 3b.
Figure 5.1: FT-IR spectra of 1a to 4a. With the conversion of 2a to CEM 3a the –OH stretching band
at 3338 cm-1 disappears and the –OCN bands appear at 2277-2241 cm-1. During the thermal curing of
96
CEM 3a to PC 4a the characteristic stretching band for the triazine ring appears at around 1357 cm-1
while –OCN bands vanish.
Figure 5.2: FT-IR spectra of 1b to 4b. With the conversion of 2b to 3b the –OH stretching band at 3338
cm-1 disappears and the –OCN bands appear at 2262-2235 cm-1. During the thermal curing of 3b to 4b
the characteristic stretching band for the triazine ring appears at around 1357 cm-1 while the –OCN
bands vanish.
5.1.1.
FTIR monitoring of Cure kinetics of bisphenol AF and
bisphenol A based CEMs
Figure 5.3 shows corresponding time dependent FT-IR absorption spectra for the OCN and triazine ring vibrations of CEM 3a and 3b during the thermal curing process
according to the temperature program described in the experimental section. The
arrow on each plot indicates the curing time direction. Based on the normalization of
the spectra to the persistent CF3 and CH3 groups, the conversion was quantified by the
peak area of the respective functional group (-OCN or triazine ring) for the given
curing time. The prominent feature of the –OCN group signal in Figure 5.3a is a rapid
reduction of peak intensity within the first hour of curing for CEM 3a. The residual
peak intensity vanishes at a much lower rate over the remaining curing period of 7h.
An essentially analogous behaviour is found for CEM 3b as visible in Figure 5.4c.
Interestingly, the signal for the concurrent triazine ring formation shows a different
time dependence, with a lag time of about 2h before a strong signal appears (Figure
5.3) in PC 4a. In the following curing time the signal only slightly increases. In
contrast, compound 3b immediately shows a triazine signal without a lag time, see
Figure 5.3d.
97
Figure 5.3: FT-IR spectra recorded at every hour during thermal curing at 260°C for 8h: (a)
Disappearance of –OCN bands between 2325-2190 cm-1 and (b) appearance of stretching band for
triazine ring at 1360 cm-1 for CEM 3a. (c) Disappearance of -OCN bands between 2360-2113 cm-1 and
(d) appearance of stretching band for triazine ring at 1360 cm-1 for CEM 3b.
In order to get a more detailed picture of the curing kinetics, the α(t) factor
(equation 5.1) for the –OCN group conversion and triazine ring formation were
plotted against curing time in Figure 5.4. The reported data for each thermal curing
step is an average of three separate film samples. The cyanate group conversion
followed a fast kinetic rate during the early stages of curing, and Figure 5.4a shows
almost 80% conversion of the cyanate groups for both CEMs 3a and 3b within the
first hour of curing. This high conversion at the initial curing stage resulted in a rapid
vitrification of the CEM 3a and 3b film samples. The conversion of the remaining
cyanate groups was slow and a diffusion controlled curing process dominated the later
stages due to vitrification. The complete conversion of the cyanate groups took 5h for
the slightly more reactive CEM 3a and 7h for CEM 3b (see discussion of DSC
analysis and Figure 5.5 further below for reactivity comparison of CEM 3a and 3b).
On contrary, the triazine ring formation followed slower kinetics from the very
beginning of the curing process (Figure 5.4b). This is explained by the rather complex
98
reaction sequence of the cyanate moieties via various intermediates leading to the
triazine structure, instead of a direct concerted ring formation in a single step
involving three cyanate groups, as discussed further below. In the case of CEM 3a an
induction period of 2h was observed before a significant triazine ring formation
occurred, while no induction period was found for CEM 3b. Interestingly, despite the
2h induction period for CEM 3a, the α(t) factor of 60% for the triazine ring formation
was approximately the same for both CEMs 3a and 3b after 3h of curing. An
analogous trend for α(t) was followed by both CEMs 3a and 3b during the remaining
curing period, but the α(t) factor for the triazine ring formation in CEM 3a was
always slightly smaller than in CEM 3b at any particular curing time. The completion
of triazine ring formation took 8h in total for both CEMs. The different kinetics are
probably due to the restricted chain mobility of CEM 3a in the vitrified state, induced
by a higher steric hindrance of the CF3 substituent at the quaternary carbon center
compared to CEM 3b with CH3 groups. The comparison of Figure 5.4 a and b
reveales that the α(t) factors for triazine ring formation and for cyanate group
conversion at any particular curing time are not proportional. The cyanate group
conversion is always higher than the triazine ring formation. For instance, after 3h of
curing the α(t) factor for cyanate group conversion in both CEMs 3a and 3b
corresponds to 90% conversion, whereas the α(t) factor for triazine ring formation at
the same curing time corresponds to ~60% triazine ring formation. Considering a
direct trimerization reaction, a linear relationship between the cyanate groups
decrease and the triazine ring formation would be expected at any point in time,
which is not observed here. An essentially similar behaviour was reported by M. F.
Grenier-Loustalot et al., where NMR, HPLC, and IR spectroscopy was used to study
the mechanism and kinetics of non-catalyzed CEMs curing.10, 11 They proved that the
conversion of cyanate groups into the triazine ring is not a direct trimerization step but
passes through 4-membered ring- and carbamate intermediates. This explains the
substantially higher conversion factor α(t) for the cyanate group conversion compared
to the triazine ring formation at a particular curing time. In order to ensure complete
conversion, the samples of CEM 3a and 3b were further kept at 290°C for 1h, but no
changes were observed for the IR signals.
99
Figure 5.4: (a) Kinetic profile from IR measurements for CEM 3a and CEM 3b for the percent fraction
of residual -OCN groups and (b) the triazine ring formation during 8 hours at 260°C and 1 hour at
290°C (average of three measurements).
5.1.2.
Thermal properties of bisphenol AF and bisphenol A based
CERs
Since the synthesized CEMs 3a and 3b are viscous oils (room temperature
viscosities=288Pa.s/3a and 3Pa.s/3b) under ambient conditions, they consequently do
not show endothermic melting transitions in their DSC thermograms above room
temperature (Figure 5.5 a and b).
Figure 5.5: DSC thermograms of CEMs 3a and 3b in a nitrogen atmosphere. The exothermic
transitions for the cyclotrimerization of the cyanate groups peak at 227°C for CEM 3a (a) and 265°C
for CEM 3b (b).
Only one exothermic transition was observed, which relates to the curing reaction
leading to the PCs 4a and 4b by formal cyclotrimerization of the cyanate groups. In
the case of CEM 3a the exothermic transition starts at around 170°C and peaks at
227°C while for 3b it begins with a small shoulder around 160°C, but rapidly
increases at around 230°C and peaks at 265°C. It is worthwhile to note that these
curing peak temperatures are about 40 K lower than BAFCY- and BACY-based OAE
CEMs with only terminal cyanate groups, which show curing temperatures as
100
exothermic transitions peaking at 270°C and 310°C respectively. This indicates a
comparatively higher reactivity of CEMs 3, which allows for lower processing
temperatures in technical applications.4,
5
The higher reactivity of CEMs 3 is
attributed to the electron withdrawing effect of the two meta-phenoxy groups on the
pendant cyanate moieties, which is in accord with previous findings of higher
reactivity in aromatic CEMs having electron withdrawing groups at the meta-position
to the cyanate groups.12 The terminal cyanate groups, on the other hand, are attached
to the bisphenol A subunits with the electron-donating quaternary carbon center in
para-position, reducing their reactivity. When comparing the bisphenol AF with the A
derivatives, CEM 3a with CF3 groups is curing about 40 K below its structural
analogue 3b with CH3 groups. Apparently, the high electronegative fluorine atoms
have a long-range electronic effect on the cyanate groups mediated through the
conjugated system of the aromatic rings. Such an electronic effect of the F
substituents is supported by the distinct features in the 1H NMR spectra (Figure 5.6),
where the aromatic protons in the resorcin substructure of 3a (assigned as protons "a"
in the NMR spectra of Figure 5.6) split into two lines at about δ = 6.6 ppm compared
to the single line for 3b (protons "b" in Figure 5.7). Also, the protons of the bisphenol
AF subunit in 3a (protons "c" and "d" in Figure 5.6) show a distinct splitting
compared to those in 3b (protons "d" in Figure 5.7).
Figure 5.6: 1H NMR (CDCl3) spectrum of CEM 3a
101
Figure 5.7: 1H NMR (CDCl3) spectrum of CEM 3b
The concept of higher reactivity in CEMs with molecular arrangements, that render
the cyanate group carbon more electrophilic, is also supported by the initiation
mechanism of cyanate group cyclotrimerization by traces of water or residual
phenol.10, 11 The higher electrophilicity of the cyanate carbon in CEMs 3 favors the
nucleophilic attack by water or residual phenol resulting in the formation of
carbamate intermediates that autocatalyse the reaction, ultimately leading to lower
curing temperatures. The commonly high curing temperatures in other technically
applied CEMs usually demand a catalyst for curing within reasonable times and
temperatures. A major disadvantage of the catalyst, which will remain in the
thermoset PC, is its activation of hydrolysis reactions and hence accelerated ageing.13
With the lower curing temperatures found for the present CEMs 3 it was possible to
produce PCs under comparable conditions, but without using a curing catalyst. The
DSC of the fully cured PCs 4a and 4b did not reveal any features and no Tg could be
deduced from such thermal analysis (for Tg determination see rheological
measurements further below).
The catalyst-free cured PC 4a was investigated for its thermal stability by TGA, as
shown in Figure 5.8a. No weight loss was observed up to 400°C irrespective of the
gaseous environment (air or nitrogen) being used during the TGA measurements,
which is very similar to BAFCY and BAFCY-OAE without pendant cyanate groups.
The main degradation occured between 400 to 600°C. A char yield of 52% was
obtained after heating to 900°C under N2, analogously to the PC of BAFCY, but
slightly higher than the BAFCY-OAE-PC without pendant cyanate ester groups (47%
char yield).3, 4 In comparison, PC 4b is thermally less stable and started to decompose
above 350°C in both N2 and air atmosphere (Figure 5.8b). A char yield of 44% was
102
observed when heating to 900°C under N2, which is higher than the BACY-OAE-PC
without pendant cyanate groups (32% char yield) and the BACY-PC (41% char
yield).3, 5 The higher char yield indicates a better flame resistance with less volatile
polymer pyrolysis fragments and is thus desirable, as in the case of PCs 4a.14
Figure 5.8: TGA thermograms of PC thermosets 4a and 4b under N2 and air, reflecting thermooxidative stability of 4a up to 400°C (a) and of 4b up to 350°C (b).
5.1.3.
Rheological measurements of PCs derived from bisphenol AF
and bisphenol A based CEMs
Rheological measurements by torsional deformation were performed on the neat
cured PCs 4a and 4b under a dry nitrogen atmosphere over a temperature range of 50
to 330°C. At 50°C the PCs 4a and 4b show storage moduli (G´) of 1.252 and 1.362
GPa, respectively. During heating to 330°C, a sharp reduction in these moduli is
observed (Figure 5.9), which is correlated to the alpha-relaxation at a measurement
frequency of 10 rad s-1 and characteristic of a Tg. As the exact value of a measured Tg
depends critically on the experimental conditions (like technique, heating or cooling
rates) Tgs are hereafter referred (in accordance with the literature) to as "rheologically
determined Tgs".4,
5
These rheologically determined Tgs may be shifted to higher
temperatures compared to Tgs obtained by DSC measurements, but only small shifts
between the DSC and rheological values were found for other CEMs and all values
cited below for comparison are obtained in the same way.4,
5
The rheologically
determined Tg values of the PCs correspond to the midpoint of the sharpest decrease
in the storage moduli curves and peak maximum in the tan δ plots (G´/G´´). They
were found at 286°C for 4a and 287°C for 4b, which is 111K higher for 4a and 147K
higher for 4b than the reported Tgs of BAFCY-OAE-PC and BACY-OAE-PC without
pendant cyanate groups.4,
5
In comparison to the Tg of PCs derived from parent
BAFCY (Tg =270°C) the Tg of 4a is 16K higher, while the Tg of 4b is only 2K lower
103
than the PC derived from parent BACY (289°C).3 The high Tgs determined by
rheology validate the concept of pendant cyanate groups to increase the Tg by
enhancing the crosslinking density in the cured thermoset.
Figure 5.9: Storage modulus (G′), loss modulus (G′′) and damping factor (tan δ) of polycyanurate
thermosets 4a and 4b as a function of temperature.
5.1.4.
Young’s moduli and coeffecients of thermal expansion for
PCs derived from bisphenol AF and bisphenol A based CEMs
Figure 5.10 shows the nominal stress as a function of drawing ratio for PC
thermosets. The Young's moduli (E), as a measure of stiffness, were calculated from
the linear slope at small strains (red solid lines). They were found to be 3.47GPa for
PCs 4a and 3.46GPa for 4b with a slightly higher stiffness than the PCs derived from
parent BAFCY and BACY (3.11GPa and 3.17GPa).3 The PC 4a possesses a higher
density (1.4493g cm-3) than 4b (1.2045g cm-3) and shows a slightly lower coefficient
of thermal expansion (113 × 10-6 K-1) in comparison to 4b (121 × 10-6 K-1). Most of
the PCs derived from commercial CEMs show coefficients of thermal expansion in
the range of 60 × 10-6 K-1 to 70 × 10-6 K-1.15 The higher coefficients of thermal
expansion for PCs 4a and 4b are due to the higher free volume inherent with the
diaryl ether spacers incorporated in CEMs 3a and 3b. Due to the limited number of
tensile tests performed and the defects incorporated in the test specimens, which
effects the break point, the elongation at break of PCs 4 can only be roughly stated as
about 1 to 2%. The small elongation at break reflects brittleness, which is a common
feature of cyanate ester resins and can be counteracted by using additives.16
When comparing the two types of PC 4a and 4b, it is interesting to note, that they
have very similar mechanical properties (G', E, rheologically determined Tg) despite
their chemical differences, which is partially to be explained by the very high
crosslink density averaging out these chemical differences. The elastic modulus E in
the glassy state, which depends on the cohesive energy density and the intensity of
104
sub-glass transitions, is almost the same for both 4a and 4b PC networks.17 At the
very high crosslink densities of the systems presented here (one crosslink per
repeating unit), it may indirectly also depend on this crosslink density, as its increase
leads to a higher number of covalent bonds per volume element and thus may
contribute to the cohesive energy density being a sum over all covalent (strongest)
and non-covalent (weaker) interactions in the system.
Figure 5.10: Stress as a function of drawing ratio for PC thermosets 4a (a) and 4b (b), with the
Young’s moduli calculated from the linear slops of these plots at small strains (red lines).
5.1.5.
Dielectric measurements PCs derived from bisphenol AF and
bisphenol A based CEMs
The relative permitivity, or relative dielectric constants (ε') at room temperature of
4a and 4b were measured at 1 MHz frequency as 3.41 and 3.75, respectively after
being stored at ambient conditions (Figure 5.11). During heating of the samples a
contribution of absorbed moisture to ε' became apparent by a non-linear temperature
dependence and ε' decreased to 3.33 for 4a and 3.65 for 4b at 150°C. In the absence
of moisture ε' showed a weak linear temperature response from 3.30 at -100°C to 3.33
at 150°C for 4a and from 3.63 at -100°C to 3.65 at 150°C for 4b.18 The PC 4a
exhibits a comparatively lower ε' due to the fluorinated isopropylidene linkage in the
BAFCY subunit. In summary the ε' of 4a and 4b are not as low as those of other
members of the CE resin family, which generally lie between 2.5-3.1, and also fall at
the upper margin of the required ε' range (2.5-3.6) for microelectronic applications.
Still, these values are lower than the ε' of common epoxy resins cured with active
hydrogen converters (generally in the range of 3.9-4.2),1 which make them
competitive substituents in epoxy-based electronics. The higher ε' of PCs 4a and 4b
105
are thought to be due to the aryl ether linkages, which increase the polarizability of
the thermosets.19
Figure 5.11: Dielectric constants of PC thermosets 4a and 4b as a function of temperature measured at
1MHz between –100 to 150°C. The hump in the first heating scan shows the contribution of absorbed
moisture to the dielectric constants which vanishes upon drying (straight lines).
5.1.6.
Template assisted fabrication of polycyanurate nanorods and
their hydrolytic stability
PCs are considered as resin matrix for multi-layer electric circuit boards in
microelectronics industry due to many favorable properties, but a poor long term
hydrolytic stability, which ultimately causes blistering of the circuit board assemblies,
often limits such a PC application.13 In order to assess the hydrolytic stability of the
PCs derived from developed CEMs 3, polycyanurate nanorods (PCN) were produced
by curing CEMs 3 in the channels of nanoporous alumina templates by following the
reported method for the templated synthesis of nanostructures (Scheme 5.2).20-22 The
geometric dimensions of these PCNs with the length of around 100 µm and diameters
of 65 and 380 nm correspond to the shapes of the template pores (Figure 5.12). The
PCNs shown in the SEM images demonstrate the high processability of the liquid
CEMs 3, which have completely penetrated such small dimensions of the nanoporous
template and fully replicated the template structure. These nanostructures with their
large surface-to-volume ratio are ideal test objects to assess the hydrolytic stability of
the PC material, since a very large fraction of material is directly exposed to the
surrounding water, diffusion paths within the bulk are comparably short (essentially
limited by the rod radius), and any substantial hydrolysis would have a clearly
observable influence on the PCN shape. The PCNs composed of PC 4 presented good
hydrolytic and dimensional stability when subjected to accelerated (100h boiling
106
water) or long-term (three months at room temperature in water) hydrolytic conditions
and no blistering or shape changes were observed (Figure 5.12 b and d).
Scheme 5.2: Schematic representation of the nanomolding process to produce polycyanurate nanorods
(PCN) in nanoporous alumina templates.
Figure 5.12: SEM images of template molded PCNs of 4a before (a) and after (b) storage in water for
three months. PCNs of 4b before (c) and after (d) 100h boiling water treatment.
The final architecture of the PCNs with either compact rod- or hollow tube
morphology (Figure 5.13) can be conveniently controlled by the temperature during
the template pore filling process. When the more viscous CEM 3a was subjected to a
higher initial pore wetting temperature (i.e., 120°C), complete pore filling resulted in
a rod-like structure, while at lower pore wetting temperature (i.e., 80°C) tubular
structures were favoured by only surface wetting of the template. Since CEM 3b has a
lower viscosity, it formed already under the low temperature condition (80°C) a
compact rod morphology.
107
Figure 5.13: SEM images of PC 4a derived from CEM 3a processed during template filling at (a) 80
°C and (b) 120°C.
5.2. Conclusions
The convenient synthesis route presented here provides bisphenol A- and AFbased CEMs in high yields, with the simple purification steps bearing large potential
for scale-up towards commercialization.
The specific features due to their chemical structure are:
1. High functional group density of reactive cyanate ester units along the main chain.
2. The OAE units impart high chain flexibility in the uncured state, which together with
3. the formation of oligomeric mixtures in the synthesis hinders the crystallization and may only lead to
vitrification below room temperature, which substantially facilitates processing of the liquid thermoset
under ambient conditions.
4. The large number of cyanate groups in the oligomers
a) allow rapid vitrification in the curing process, desirable for fast setting and mechanical stability
in the early curing stage (no flow of the thermoset), and
b) lead to high crosslink density in the polycyanurate network with very high Tg desirable for hightemperature applications.
Replacing the CH3 substituents in the bisphenol A subunit (CEM 3b) by CF3
groups of bisphenol AF (CEM 3a) has substantial consequences on the properties of
the uncured CEM as well as the final PC material:
1. CEM 3a has an about 100-times higher viscosity at room temperature,
2. and consequently a slightly slower rate of triazine ring formation (from IR kinetics),
3. while the cyanate groups react already at lower temperature (from DSC measurments).
4. The cured PC 4a shows a substantially higher thermal stability in air (up to 400°C in the TGA),
5. a higher char yield (as indicator for good flame resistance), and
6. a lower dielectric constant.
These effects result most probably from:
1. the conformational restriction in the bisphenol AF subunit due to the larger space requirement of the
CF3 groups compared to the CH3 moieties, and
2. by an electronic effect of the highly electronegative F atoms
108
Interestingly, besides these distinct differences due to the F substituents, both PC
4a and 4b have very similar mechanical properties, like elastic modulus G', Young's
modulus E, and Tg. Apparently, the differences in chemical structure are
overcompensated by the very high crosslink density (one crosslink per repeating unit)
with short and comparably rigid chain segments interconnecting the network points.
Due to the combination of many positive aspects in this material class, the novel
CEMs presented here show great potential as high-performance thermosets for a large
range of technical application, like metal-to-polymer adhesives or as matrix in fiber
reinforced composites.
5.3.
Experimental
5.3.1.
Synthesis of oligomeric aromatic ether with pendant methoxy and terminal
hydroxy groups 1a and 1b:
Bisphenol AF (10.09g, 30 mmole), 1,3-dibromo-5-methoxybenzene (3.98g, 15 mmole), 1,10phenanthroline (0.240g, 1.33 mmole), toluene (7mL) and DMF (55mL) were added to a 250 mL threenecked round bottom flask fitted with a thermometer, a Dean–Stark trap with condenser, and an argon
inlet. The resulting mixture was degassed thoroughly with argon for ten minutes followed by the
addition of copper (I) iodide (0.227g, 1.19 mmole). After filling the Dean-Stark trap with toluene, the
mixture was heated for 30 min to 1 h at 135-145oC in order to completely dissolve all the starting
materials. The mixture was cooled to 100°C and potassium carbonate (3.1g, 22.43 mmole) was added
in one portion. The resulting mixture was again heated at 135-145°C for 3 h and the water formed in
the reaction was removed by azeotropic distillation. After this time the reaction mixture was cooled
again to 100°C and another portion of potassium carbonate (3.1g, 22.43 mmole) was added. The
reaction mixture was again heated to 135-145°C for 12-14 h until no further water deposited in the
Dean-Stark trap. The remaining toluene was then removed by distillation and the reaction mixture was
cooled to ambient temperature. Water was added (200mL) to the reaction mixture, which was made
acidic by the addition of 2 M HCl (200 mL) and extracted with ether (3 × 100mL). The combined ether
extracts were washed with water until neutral and dried over anhydrous MgSO4. The solvent was
evaporated after passing through a short silica plug to yield a brown semisolid, which was vacuum
dried at 80°C over night to yield a pure oligomeric mixture with pendant methoxy and terminal
hydroxyl groups (16.31g, 70%).
IR (film): 3378 (O-H), 3018 (C=CH), 1597, 1506, 1448 (aromatic C=C), 1241 (C-F), 1207, 1168, 1143
(C-O), 1000, 966 (C-OH), 829 cm-1 (aromatic). FDMS: m/z = 336 (n=0), 777.5 (n=1), 1218.1 (n=2),
1658.8 (n=3), 2099.3 (n=4), 2540.3 (n=5), 2982.2 (n=6). 1H NMR (250 MHz, CDCl3): δ [ppm] = 7.167.29 (8 aromatic H flanking CF3 groups of bisphenol A6F, br), 6.90-6.94 (4 aromatic H next to –O– of
diarylether groups, br), 6.71-6.75 (4 aromatic H next to –OH groups, br), 6.30-6.33 (3 aromatic H of
the ring with pendant –OCH3 groups, br), 5.11 (2 H of –OH, s), 3.69 (3 H of pendant –OCH3, s).
1b was synthesized in the same manner only bisphenol A was used instead of bisphenol AF (76%). IR
(film): 3362 (OH), 3021 (C=CH), 2966 (CH3), 1588, 1503, 1463 (aromatic C=C), 1363 (CH3), 1210,
1171, 1143, 1119 (C-O), 1003, 951 (C-OH), 829 cm-1 (aromatic). FDMS: m/z = 228 (n=0), 561 (n=1),
1227.8 (n=2), 1557.9 (n=3), 1891.7 (n=4), 2224.8 (n=5). 1H NMR (250 MHz, CDCl3): δ [ppm] = 6.997.12 (8 aromatic H flanking CH3 groups of bisphenol A, br), 6.81-6.86 (4 aromatic H next to –O– of
diarylether groups, br), 6.63-6.67 (4 aromatic H next to –OH groups, br), 6.19 (3 aromatic H of ring
with pendant –OCH3 groups, s), 4.65-4.70 (2 H of –OH, s), 3.65-3.71 (3 H of pendant –OCH3, s), 1.541.58 (6 H of CH3 groups of bisphenol A).
5.3.2.
Deprotection of pendant methoxy groups to yield 2a and 2b:
Borontribromide (12.96g, 51.75 mmole) was added to a solution of 1a (8.93g, 11.5 mmole) in dry
dichloromethane (100mL). The reaction mixture was stirred under reflux for 12 h. After 12 h the
reaction mixture was cooled down to room temperature and hydrolyzed with 5% aq. HCl solution
109
(Caution: care should be taken while adding HCl solution. A very slow dropwise addition with ice
cooling is recommended) and extracted with ether (3 × 100mL). The organic phase was washed with
dilute sodium bicarbonate (1 × 100mL) and water until neutral and dried over MgSO4. The solvent was
removed after passing through a short silica plug to yield a brown solid, which was vacuum dried at
80°C for overnight to yield the pure oligomeric mixture of 2a with pendant and terminal hydroxyl
groups. (8.77g, quantitive yield).
IR (film): 3338 (O-H), 3051 (C=CH), 1597, 1509, 1457 (aromatic C=C), 1241 (C-F), 1204, 1171, 1134
(C-O), 1006, 970 (C-OH), 829 cm-1 (aromatic). FDMS: m/z = 336 (n=0), 761.9 (n=1), 1188.2 (n=2),
1613.2 (n=3), 2040.8 (n=4), 2468.4 (n=5). 1H NMR (250 MHz, CDCl3): δ [ppm] = 7.19-7.31 (8
aromatic H flanking CF3 groups of bisphenol AF, br), 6.92-6.95 (4 aromatic H next to –O– of
diarylether groups, br), 6.73-6.76 (4 aromatic H next to terminal OH groups, br), 6.24-6.30 (3 aromatic
H of the ring with pendant –OH groups, br), 4.82-4.90 (3 H of –OH, s).
2b was also synthesized in the same manner as above in quantitive yield. IR (film): 3335 (OH), 3036
(C=CH), 2966 (CH3), 1596, 1503, 1460 (aromatic C=C), 1363 (CH3), 1216, 1171, 1137, 1119 (C-O),
1003, 906 (C-OH), 829 cm-1 (aromatic). FDMS: m/z = 546.9 (n=1), 866.3 (n=2), 1184.4 (n=3). 1H
NMR (250 MHz, CDCl3): δ [ppm] = 6.99-7.11 (8 aromatic H flanking CH3 groups of bisphenol A,
br), 6.81-6.85 (4 aromatic H next to –O– of diarylether groups, br), 6.62-6.67 (4 aromatic H next to –
O– of diarylether groups, br), 6.06-6.16 (3 aromatic H of the ring with pendant –OH groups, br), 4.75.06 (3H of –OH, s), 1.54-1.56 (6 H of CH3 groups of bisphenol A, s).
5.3.3.
Synthesis of oligomeric aromatic ether with pendant and terminal cyanate
groups 3a:
2a (8.77g, 11.5 mmole) and cyanogen bromide (4.26g, 40.22 mmole) were dissolved in dry acetone
(50mL) and were transferred under argon to an oven dried 100mL three neck round bottom flask
equipped with a thermometer, magnetic stirrer and argon inlet. The solution was stirred and cooled to –
20 to –30oC. Dry triethylamine (4.42g, 43.68 mmole) dissolved in acetone (5mL) was added dropwise
over a period of 1h while maintaining the temperature of the reaction mixture below -20oC. After the
addition was complete the reaction mixture was further stirred for 1h below –20oC and 1h at room
temperature while Et3N+Br– salt precipitated. The solvent was removed in vacuo. The resulting residue
was stirred with 250mL of a hexane/dichloromethane mixture (1:1). The mixture was then filtered
through a short silica plug to remove the Et3N+Br– salt. The solvent was removed in vacuo to yield the
oligomeric aromatic ether with pendant and terminal cyanate ester groups 3a (8.67g, 90%) as yellow
oil. IR (film): 3067 (C=CH), 2277, 2241 (CN), 1594, 1506, 1457 (aromatic C=C), 1241 (C-F), 1207,
1171, 1134 (C-O), 1012, 966, 927 (C-OCN), 829 cm-1 (aromatic). 1H NMR (250 MHz, CDCl3): δ
[ppm] = 7.28-7.49 (12 aromatic H, br), 6.98-7.01 (4 aromatic H next to terminal OCN groups, br),
6.62-6.70 (3 aromatic H of the ring with pendant –OCN groups, br).
3b was also synthesized in the same manner as above in 92% yield as light yellow oil. IR (film): 3036
(C=CH), 2969 (CH3), 2262, 2235 (CN), 1591, 1500, 1463 (aromatic C=C), 1363 (CH3), 1213, 1195,
1168, 1143 (C-O), 1012, 936, 912 (C-OCN), 829 cm-1 (aromatic). 1H NMR (250 MHz, CDCl3): δ
[ppm] = 7.11-7.25 (12 aromatic H, br), 6.86-6.89 (4 aromatic H next to OCN, br), 6.47 (3 aromatic H
of the ring with pendant –OCN groups, br), 1.60 (6 H of CH3 groups, s).
5.3.4.
Curing of CEMs to PC thermosets:
The CEMs 3a and 3b were neat cured by the following temperature program in a Teflon mold with a
cavity dimension of 70 mm × 20 mm × 20 mm in a tube furnace under argon to yield the corresponding
PCs. After degassing at 80oC for 1h, the curing was induced according to the program 180°C/2hr →
260°C/8hr → 290°C/1hr → cooling to room temperature. The PCs obtained were cut and sanded
accordingly for TGA, rheometric, dielectric, Young’s modulus (E) and coefficient of thermal
expansion measurements.
5.3.5.
Monitoring the kinetics of thermal curing of CEMs 3:
Thin films of 3a and 3b were prepared via neat spin coating (3000rpm, 60sec) of liquid CEMs
without solvent on silicon substrates. The film samples were placed in a tube furnace purged with N2
gas and subsequently cured at 260°C for 8 hours and at 290°C for 1h. An FT-IR spectrum was taken
every hour in order to monitor the polymerisation kinetics. The reduction in the IR absorbance of the
cyanate groups between 2325-2190 cm-1 for 3a and between 2360-2113 cm-1 for 3b and the appearance
110
of the triazine ring signal at around 1360 cm-1 were monitored to determine the extent of curing. Since
the number of CH3 and CF3 groups remains constant before and after the thermal curing, each spectrum
was normalized by division with the factor obtained from dividing the area of the CH3 and CF3 group
absorption at a particular curing time by the area of the CH3 and CF3 group absorption in the monomers
3a and 3b prior to thermal treatment (t=0). The percent fraction of residual cyanate groups α(t) at a
given time (t) was calculated from the normalized area of cyanate absorbance before (AOCN)t0 and after
(AOCN)t the thermal treatment according to the following equation

α (t ) = 1 −

( AOCN ) t 0 − ( AOCN ) t
( A OCN ) t 0

 × 100 Equation 5.1


In a similar way the triazine ring formation was quantified from the IR spectra.
5.3.6.
Template assisted fabrication of polycyanurate nanorods (PCNs) and
determination of their hydrolytic stability:
Self-ordered nanoporous alumina templates with a pore diameter of 65 and 380 nm and a pore depth
of 100 µm were fabricated by anodization of aluminum according to the procedure reported
elsewhere.23-25 CEMs 3a and 3b were applied on top of nanoporous alumina templates via neat spin
coating (3000rpm, 2min). In order to ensure complete pore wetting and degassing, the nanoporous
alumina templates with CEMs were additionally kept under vacuum at 80°C for tube morphology or
120°C for rod morphology for 12 h, before subjecting to the curing program as mentioned above for
the neat material. Individually dispersed polycyanurate nanorods were obtained by selective dissolution
of the alumina template using NaOH (6M) solution at room temperature for 1h. The suspended PCNs
were centrifuged (20,000 rpm, 15min) and the supernatant liquid was removed. The isolated PCNs
were redispersed in deionized water and again centrifuged. This procedure was repeated until the
supernatant liquid became neutral. Finally, the PCNs were collected and dried. For hydrolytic stability
assessment a small portion of the PCNs was kept in boiling water for 100 h for accelerated hydrolysis
measurements and stored in water for three months for long-term hydrolysis measurements. After
drying at 120°C for 5h the PCNs were investigated by SEM measurements.
5.4. References
1.
International 1998, 47, (4), 465-473.
2.
Vol. 155, pp 1-99.
3.
Science 1995, 20, (1), 61-118.
4.
Laskoski, M.; Dominguez, D. D.; Keller, T. M., Development of an Oligomeric Cyanate Ester
Resin with Enhanced Processability. Journal of Materials Chemistry 2005, 15, (16), 1611-1613.
5.
Laskoski, M.; Dominguez, D. D.; Keller, T. M., Synthesis and Properties of a Liquid
Oligomeric Cyanate Ester Resin. Polymer 2006, 47, (11), 3727-3733.
6.
Guenthner, A. J.; Yandek, G. R.; Wright, M. E.; Petteys, B. J.; Quintana, R.; Connor, D.;
Gilardi, R. D.; Marchant, D., A New Silicon-Containing Bis(Cyanate) Ester Resin with Improved
Thermal Oxidation and Moisture Resistance. Macromolecules 2006, 39, (18), 6046-6053.
7.
Kelkar, A. A.; Patil, N. M.; Chaudhari, R. V., Copper-Catalyzed Amination of Aryl Halides:
Single-Step Synthesis of Triarylamines. Tetrahedron Letters 2002, 43, (40), 7143-7146.
8.
Klapars, A.; Antilla, J. C.; Huang, X. H.; Buchwald, S. L., A General and Efficient Copper
Catalyst for the Amidation of Aryl Halides and the N-Arylation of Nitrogen Heterocycles. Journal of
the American Chemical Society 2001, 123, (31), 7727-7729.
9.
Wolter, M.; Nordmann, G.; Job, G. E.; Buchwald, S. L., Copper-Catalyzed Coupling of Aryl
Iodides with Aliphatic Alcohols. Organic Letters 2002, 4, (6), 973-976.
10.
11.
Grenierloustalot, M. F.; Lartigau, C.; Grenier, P., A Study of the Mechanisms and Kinetics of
the Molten State Reaction of Noncatalyzed Cyanate and Epoxy-Cyanate Systems. European Polymer
Journal 1995, 31, (11), 1139-1153.
111
12.
Bauer, J.; Hoper, L.; Bauer, M., Cyclotrimerization Reactivities of Mono- and Difunctional
Cyanates. Macromolecular Chemistry and Physics 1998, 199, (11), 2417-2423.
13.
Kasehagen, L. J.; Haury, I.; Macosko, C. W.; Shimp, D. A., Hydrolysis and Blistering of
Cyanate Ester Networks. Journal of Applied Polymer Science 1997, 64, (1), 107-113.
14.
Gilman, J. W.; Lomakin, S.; Kashiwagi, T.; VanderHart, D. L.; Nagy, V., Characterization of
Flame Retarded Polymer Combustion Chars by Solid-State C-13 and Si-29 Nmr and Epr. Abstracts of
Papers of the American Chemical Society 1997, 213, 418-POLY.
15.
Stuart, M. L., International Encyclopedia of Composites. In VCH: NewYork, 1990; Vol. 1, pp
552-553.
16.
Wooster, T. J.; Abrol, S.; MacFarlane, D. R., Polymeric Toughening of Particle Filled
Cyanate Ester Composites. Macromolecular Materials and Engineering 2005, 290, (10), 961-969.
17.
Arias, M. L.; Frontini, P. M.; Williams, R. J. J., Analysis of the Damage Zone around the
Crack Tip for Two Rubber-Modified Epoxy Matrices Exhibiting Different Toughenability. Polymer
2003, 44, (5), 1537-1546.
18.
Carter, K. R.; DiPietro, R. A.; Sanchez, M. I.; Swanson, S. A., Nanoporous Polyimides
Derived from Highly Fluorinated Polyimide/Poly(Propylene Oxide) Copolymers. Chemistry of
Materials 2001, 13, (1), 213-221.
19.
Simpson, J. O.; St Clair, A. K., Fundamental Insight on Developing Low Dielectric Constant
Polyimides. Thin Solid Films 1997, 308, 480-485.
20.
Martin, C. R., Template Synthesis of Polymeric and Metal Microtubules. Advanced Materials
1991, 3, (9), 457-459.
21.
Martin, C. R., Nanomaterials - a Membrane-Based Synthetic Approach. Science 1994, 266,
(5193), 1961-1966.
22.
Steinhart, M.; Wehrspohn, R. B.; Gosele, U.; Wendorff, J. H., Nanotubes by Template
Wetting: A Modular Assembly System. Angewandte Chemie-International Edition 2004, 43, (11),
1334-1344.
23.
Masuda, H.; Fukuda, K., Ordered Metal Nanohole Arrays Made by a 2-Step Replication of
Honeycomb Structures of Anodic Alumina. Science 1995, 268, (5216), 1466-1468.
24.
Masuda, H.; Hasegwa, F.; Ono, S., Self-Ordering of Cell Arrangement of Anodic Porous
Alumina Formed in Sulfuric Acid Solution. Journal of the Electrochemical Society 1997, 144, (5),
L127-L130.
25.
Nielsch, K.; Choi, J.; Schwirn, K.; Wehrspohn, R. B.; Gosele, U., Self-Ordering Regimes of
Porous Alumina: The 10% Porosity Rule. Nano Letters 2002, 2, (7), 677-680.
112
Chapter 6
6. Polyether
ether
ketone
(PEEK)
Surface
Functionalization via Surface Initiated Atom Transfer
Radical Polymerization
The interfacial properties of PEEK were tailored by tethering polymeric chains i.e. polymer
brushes to the PEEK surface via SI-ATRP. A simple two-step method is presented for the
covalent immobilization of an ATRP initiator at the surface of PEEK followed by SI-ATRP of
3-(methacryloyloxy)propane-1-sulfonate (MPS), mono-methoxy terminated oligo(ethylene
glycol)methacrylate (MeOEGMA), and N-isopropylacrylamide (NIPAAm). The change in the
surface properties of PEEK as a result of polymer brush growth was further demonstrated by;
1) staining by electrostatic interaction of the negatively charged polyMPS brush with the
positively charged Rhodamine 6G dye. 2) PEEK grafted with polyMeOEGMA was subjected
to bacterial growth for the evaluation of its bio-repellency. 3) Thermally responsive
wettability of the PEEK surface grafted with a polyNIPAAm brush was demonstrated by the
measuring the water CA at temperatures below and above the lower critical solution
temperature (LCST).
113
Like many other polymeric materials, PEEK exhibits a hydrophobic, chemically
inert surface nature, which present problems in adhesion, coating, painting, coloring,
biocompatibility etc. To date, mainly two different strategies have been explored for
specifically tuning the surface properties of PEEK, which include exposure to highenergy species (plasmas, ozone, UV light, electrons, and γ-rays) 1-6 and wet chemical
methods.7-10 High-energy species have been applied mainly to improve adhesion
whereas wet chemical methods have been applied to effect rational control over
surface chemical properties through selective organic surface chemistry.
In chapter 3, an improvement of adhesive joint strength, PEEK-CER-PEEK, by
exploiting a control over PEEK surface chemical nature through controlled surface
organic chemistry was demonstrated. In the same framework, the interfacial
properties of PEEK were further tailored by tethering polymeric chains i.e. polymer
brushes to the PEEK surface. Among the various methods of polymer brush
fabrication ATRP based “grafting-from” approach has emerged as the most versatile
(See chapter 1 for detail).11 Despite of several wet chemical surface modifications
reported, controlled living radical graft polymerization technique like ATRP have not
yet been explored for the surface functionalization of PEEK. Here, a simple two-step
method is presented for the covalent immobilization of an ATRP initiator at the
surface of PEEK (Scheme 6.1) followed by SI-ATRP of 3-(methacryloyloxy)propane1-sulfonate (MPS),12 mono-methoxy terminated oligo(ethylene glycol)methacrylate
(MeOEGMA),13 and N-isopropylacrylamide (NIPAAm).14 The polymer brush
modified PEEK surfaces were characterized by atomic force microscopy (AFM),
scanning electron microscopy (SEM), Fourier transform infrared spectroscopy
(FTIR), water contact angle (CA) measurements and X-ray photoelectron
spectroscopy (XPS). The change in the surface properties of PEEK as a result of
polymer brush growth was further demonstrated by; 1) staining by electrostatic
interaction of the negatively charged polyMPS brush with the positively charged
Rhodamine 6G dye. 2) PEEK grafted with polyMeOEGMA was subjected to bacterial
growth for the evaluation of its bio-repellency. 3) Thermally responsive wettability of
the PEEK surface grafted with a polyNIPAAm brush was demonstrated by the
temperature (LCST).
114
6.1. Immobilization of ATRP initiator and subsequent polymer brush
growth by SI-ATRP
The covalent immobilization of ATRP initiator at the PEEK membrane surface
was achieved in a two-step process (Scheme 6.1). In the first step keto groups at the
PEEK surface were subjected to sodium borohydride assisted reduction in DMSO,
which resulted in hydroxy groups at the surface (PEEK-OH). The resulting hydroxy
groups at the PEEK-OH surface were reacted, in the second step, with 2bromoisobutyryl bromide (BIBr) in the presence of equivalent amount of
triethylamine, which acts as scavenger for HBr formed during the acid bromide/-OH
esterification reaction. The reaction was carried out in dichloromethane and yielded in
PEEK membranes with surface immobilized 2-bromoisobutyryl groups (PEEK-Br)
suitable for the SI-ATRP. Subsequently, polymer brushes were grown at the surface
of PEEK-Br membranes by SI-ATRP of MPS, MeOEGMA, and NIPAAm under
aqueous ATRP conditions (Scheme 6.1).
Scheme 6.1: Schematic representation of keto group reduction at PEEK surface (A), anchoring of
ATRP initiator (B), and SI-ATRP of MPS, MeOEGMA, and NIPAAm (C).
6.2. Characterization of PEEK surface modified by SI-ATRP
6.2.1.
Surface topography by AFM and SEM
After SI-ATRP, a change in surface morphology was evident in both AFM and
SEM images (Figure 6.1). For AFM analysis, the phase images are given as insets to
height images. There was no difference in the surface topography of pristine PEEK
and PEEK-Br therefore AFM and SEM images of pristine PEEK are omitted. PEEKBr showed a fibrous topography in both AFM and SEM images (Figure 6.1A). The
115
higher extent of SI-ATRP of MPS and MeOEGMA resulted in a higher surface
coverage and the PEEK-polyMPS and PEEK-polyMeOEGMA surfaces lost the
fibrous topography of PEEK-Br (Figure 6.1B and C) whereas the surface retained the
traces of the fibrous topography after SI-ATRP of NIPAAm (Figure 6.1D).
Figure 6.1: AFM (left column) and SEM (right column) images of PEEK-Br (A), PEEK-PolyMPS (B),
PEEK-PolyMeOEGMA (C), PEEK-PolyNIPAAm (D). With the growth of polymer brush, the surface
(A) tend to loss fibrous topography (B, C & D).
6.2.2.
Surface chemical characterization by FTIR, XPS and Contact
angle goniometry
After characterizing changes in surface topography, the surface chemical
characterization was carried out. The ATR-IR spectrum of PEEK, PEEK-OH and
PEEK-Br were identical, therefore, only PEEK-Br ATR-IR spectrum is shown.
However, the growth of polyMPS and polyMeOEGMA brushes resulted in the
appearance of carbonyl bands at 1724 cm-1 and 1727 cm-1 in the ATR-IR spectra of
PEEK-polyMPS and PEEK-polyMeOEGMA, which correspond to the ester linkage
in these brushes (Figure 6.2A). On contrary, no additional carbonyl group was
116
observed in the ATR-IR spectrum of PEEK-polyNIPAAm where the aromatic ketone
group of PEEK membrane masked the amide carbonyl of the polyNIPAAm brush as
both appear in the same range in ATR-IR spectrum (Figure 6.2A). In addition, the
growth of polyMPS brush also resulted in the appearance of symmetrical sulfonate
stretching around 1047 cm-1 in ATR-IR spectrum of PEEK-polyMPS (Figure 6.2B).
With the incorporation of -CH2-O- linkages on the surface, a change in the aliphatic CH region was also observed for polyMeOEGMA brush (Figure 6.2A).
Figure 6.2: ATR-IR monitoring of surface chemical modification of PEEK by SI-ATRP. (A) The growth
of polyMPS and polyMeOEGMA brushes resulted in the appearance of carbonyl bands at 1724 cm-1
and 1727 cm-1 in the ATR-IR spectra. No additional carbonyl group was observed in the ATR-IR
spectrum of PEEK-polyNIPAAm, which contains amide bond instead. (B) PolyMPS brush growth also
resulted in the appearance of symmetrical sulfonate stretching around 1047 cm-1 in ATR-IR spectrum.
Successful initiator immobilization and subsequent polymer grafting were further
ascertained by XPS. While reducing the surface keto groups into hydroxy groups,
there is no change in the atomic concentration of elements and no new element is
incorporated, hence no change was observed in XPS. After reacting PEEK-OH with
BIBr, the successful anchoring of ATPR initiator was indicated by the appearance of a
Br3d core level absorption between 70-75 eV and for Br3p between 182-198 eV
(Figure 6.3B and inset). Except for the observation of bromine in the high resolution
XPS, the surface chemical composition (see Table 6.1) was similar to that of pristine
PEEK. The experimentally determined atomic concentration of Br was also less than
theoretical value. This is consistent with the fact that the surface chemical
transformations during keto group reduction to hydroxy groups and subsequent
117
initiator anchoring at the reduced PEEK surface is confined to the outermost surface
layers. Moreover, the depth of surface modification is just at the margin of XPS
sampling depth ~4 nm at 45° take off angle relative to the surface normal.
After the growth of polyMPS brush, the XPS survey-scan of the PEEK-PolyMPS
surface revealed the presence of sulfur (S2s: 233.6 eV and S2p: 170.9 eV) and
potassium (K2s: 379.7 eV and K2p: 295.4eV) incorporated onto the surface through
the sulfonate groups (Figure 6.3C). The grafting of PolyMPS was further evident from
the surface chemical composition that matched that of the bulk PolyMPS (See Table
6.1). Furthermore, the C1s high-resolution scan of PEEK-PolySPM surface could be
curve fitted into three peak components attributable to aliphatic (C-C/C-H), ester (OC=O) and the carbon linked to the sulfonate group (C-S) corresponding to binding
energies at 285.3, 286.8, and 289.1 eV, respectively (Figure 6.3D).
Similarly, the XPS analysis of the PEEK-PolyMeOEGMA surface showed a higher
oxygen content than the pristine PEEK surface (Figure 6.3E and the Table 6.1). The
grafting was confirmed by a reasonable agreement of the theoretical and
experimentally
determined
surface
chemical
composition
corresponding
to
polyMeOEGMA film (Table 6.1). The C1s high-resolution scan of PEEKPolyMeOEGMA could be curve fitted into three peak components attributable to
aliphatic (C-C/C-H), ether (C-O), and ester (O-C=O) carbons corresponding to
binding energies at 284.9, 286.5, and 288.8 eV, respectively (Figure 6.3F).
The survey scan of PEEK-polyNIPAAm surface showed a clear signal for
incorporated nitrogen at 400.8 eV (Figure 6.3G). The experimentally determined
surface chemical composition of PEEK-polyNIPAAM corresponded to that the
theoretical chemical composition of a pure polyNIPAAm film (Table 6.1). The C1s
high-resolution scan of PEEK-PolyNIPAAm could be curve fitted into two peak
components attributeable to aliphatic (C-C/C-H) and amide (NH-C=O) carbons
corresponding to binding energies at 285.0 and 287.3 eV (Figure 6.3H).
The growth of polymer brushes also resulted in a change in surface wettability.
Static water contact angles of the PEEK-brush surfaces determined at room
temperature are given in Table 6.1. PEEK-polyMPS surface exhibited a high
hydrophilicity and water contact angle was approximately 0°. The static water contact
angle of PEEK-polyMeOEGMA (57°) was consistent with the literature values where
polyethylene oxide was immobilized onto the fluorinated ethylene propylene
copolymer surface.15 Similarly, the water contact angle of PEEK-polyNIPAAm (65°)
118
surfaces was comparable to the literature value where polyNIPAAm brush was grown
on the surface of silicon wafer via SI-ATRP.16
119
Figure 6.3: XPS analysis of the modified PEEK surfaces. (Left)XPS survey scans, (right) XPS high
resolution scans. No change was directly evident from survey scan of PEEK-Br (A). Anchoring of
ATRP initiator was confirmed by the appearance of a Br3d core level absorption between 70-75 eV
and for Br3p between 182-198 eV (B). After polymer brush growth XPS survey scans (C, E, & G)
revealed the surface chemical composition attributable to respective polymer brush and corresponding
C1s high resolution scans (D, F, & H) could be curve fitted for carbon components of the respective
brushes.
Table 6.1: Water contact angles and XPS surface atomic concentrations
Sample
Static CA
XPS Surface atomic concentration
(deg)
PEEK
PEEK-Br
PEEK-polyMPS
PEEK-polyMeOEGMA
PEEK-polyNIPAAm
93±3
65 ±2
~0
57 ±2
65 ±2
(%)
C
O
Br
N
S (2s,2p)
K (2s,2p)
86
14
--
--
--
--
86.36
13.64
86
13
1
--
--
--
82.14
14.28
3.5
48
30
--
--
6, 6
5, 5
50.00
35.71
7.14
7.14
68
32
70.00
30.00
76
13
75.00
12.5
--
--
--
--
--
11
--
--
12.5
Theoretic atomic concentrations are shown in italics beneath the experimental values
6.3. Demonstration of properties imparted to PEEK surface by polymer
brushes
After characterizing the surface chemical composition, changes in surface
properties of PEEK as imparted by the tethered polymer brushes were demonstrated
(Figure 6.4).
The growth of polyMPS, which bears sulfonate groups resulted in a negatively
charged PEEK surface. The negatively charged PEEK-polyMPS surface immediately
stained red when immersed in a 0.1mM aqueous solution of positively charged
Rhodamine 6G (Figure 6.4A II), while the pristine PEEK retained its original colour
120
after an immersion time of 2h (Figure 6.4A I). Such surface property would be
interesting for manipulation of electrostatic interactions at PEEK surface.17
The surfaces coated with poly(ethylene glycol) (PEG) or oligo-(ethylene glycol) are
known to prevent bioadhesion.18, 19 The growth of PEG like polyMeOEGMA brushes
rendered the PEEK surface antifouling (Figure 6.4B), which was demonstrated by
exposing PEEK-polyMeOEGMA to a culture medium of E. coli – BL12 (DE3)
bacteria. While a significant number of bacteria attached to the surface of pristine
PEEK (Figure 6.4B I), no bacteria adhered to the surface of PEEK-polyMeOEGMA
(Figure 6.4B II). This result is potentially important in the field of biomedical devices
where PEEK can be applied a structural material.
PolyNIPAAm grafted on solid substrates results in temperature dependent surface
wettability.20 Thermally responsive wettability of PEEK-polyNIPAAm surface is
demonstrated by measuring water CAs at temperatures below and above the LCST
(32°C).
16,21
A change in water drop profile can be seen when temperature was
elevated from 0°C to 25°C and then to 40°C with water CAs of 42°, 64° and 87°. The
molecular mechanism of the thermally responsive wettability of a PNIPAAm thin
film involves a reversible formation of intermolecular hydrogen bonding between
polyNIPAAm chains and water molecules (left) and intramolecular hydrogen bonding
between C=O and N-H groups in PNIPAAm chains (right) below and above the
LCST. The mechanism is also schematically presented in Figure 6.4C.22
121
Figure 6.4: Demonstration of change in surface properties of PEEK after growing polymer brush via
SI-ATRP. (A) Electrostatic interaction of prinstine PEEK (I) and PEEK-PolyMPS (II) with the aqueous
solution of Rhodamine 6G. (B) SEM images of pristine PEEK (I) and PEEK-polyMeOEGMA (II)
surface previously exposed to E. coli bacteria culture. The scale bars in both the images are 10µm. (C)
Water drop profiles of PEEK-polyNIPAAm surface at 0, 25, and 40°C corresponding to water CAs of
42°, 64.° and 87° (above). The molecular mechanism of the thermally responsive wettability of a
PNIPAAm thin film (below).
6.4. Conclusions
The surface of polyether ether ketone (PEEK) was successfully functionalized with
polymer brushes grown by surface initiated atom transfer radical polymerization (SIATRP). The 2-bromoisobutyryl groups, as SI-ATRP initiator, could be covalently
anchored at the PEEK surface by a convenient two-step process. The subsequent
growth of the polymer brushes by SI-ATRP enabled a precise control over surface
properties and rendered the PEEK surface suitable for electrostatic interactions,
imparted antifouling character, and led to a thermally responsive wettability. The
technological relevance of PEEK with a control over accurate tuning of surface
properties via a powerful tool like ATRP, where a plethora of monomers can be
122
employed to fabricate a wide variety of brushes, envisions a new direction for PEEK
with functional surfaces.
6.5.
Experimental
6.5.1.
Reduction of PEEK surface carbonyl groups to the hydroxy groups (PEEKOH)
A 50 mL Schlenck tube was equipped with a reflux condenser, argon inlet and outlet. 30 mL of
freshly distilled DMSO and 60 mg (0.0016 mole) of sodium borohydride were added to the Schlenck
tube. The reaction mixture was heated at 120°C under stirring until dissolution occurred. A piece of
PEEK membrane was immersed in the gently stirred reaction mixture and heated at 120°C for 3h under
argon. After removing from the reaction mixture the PEEK membrane was successively immersed in
stirred methanol for 15 minutes, in distilled water for 10 minutes, in 0.5 N HCl for 10 minutes, in water
for 10 minutes and in ethanol for 10 minutes. The samples were dried at 60 °C under vacuum for 2h
and stored under N2. These samples are referred to as PEEK-OH.
6.5.2.
Immobilization of ATRP Initiator on the PEEK-OH Membrane
A solution of 2-bromoisobutyryl bromide (BIBr) (0.185 mL, 3 mmol) and triethylamine (0.205
mL, 3 mmol) in dry dichloromethane (30 mL) was injected over the PEEK-OH membrane under N2 at
room temperature and left for 2.5h. The membrane was washed with dichloromethane followed by
absolute ethanol, and dried under a stream of N2. The PEEK membrane with the initiator group
anchored on the surface is referred to as PEEK-Br
6.5.3.
SI-ATRP on the surface of PEEK-Br membrane
PEEK-PolyMPS brush
17.29 g of the sulfonate monomer MPS was dissolved by stirring in 20 mL of methanol and 10 mL
of water at room temperature. To this solution 0.651 g of BiPy and 0.0114 g of Cu(II)Cl2 were added.
The mixture was stirred and degassed by N2 bubbling for an hour before 0.1648 g of Cu(I)Cl was
added. The mixture was degassed with N2 bubbling for another 15 minutes. A PEEK-Br membrane was
sealed in a Schlenk tube and degassed by four high vacuum pump/N2 refill cycles. The reaction mixture
was transferred by a syringe into this Schlenk tube, adding enough to cover the membrane completely,
and the mixture was left overnight under N2. The samples were then removed and thoroughly rinsed
with deionized water.
PolyMeOEGMA brush
Monomethoxy oilgo(ethylene glycol) methacrylate (MeOEGMA) (11g, 37 mmole) was dissolved
in water (11 mL) at 30°C. To this solution Bipy (160 mg, 1.0 mmol) and Cu(II)Br2 (9 mg, 42 µmol)
were added. The mixture was stirred and degassed by N2 bubbling for 1h before Cu(I)Cl (41 mg, 420
µmol) was added.
The polymerization and purification conditions were the same as for the polyMPS brush, yielding
PEEK-polyMeOEGMA membrane.
PolyNIPAAm brush
NIPAAm (2.5 g, 22.1 mmole) was dissolved by stirring in a solvent mixture of 5 mL of methanol
and 5 mL of water at room temperature. To this solution PMDETA (0.138 g, 0.8 mmole) was added.
The mixture was stirred and degassed by N2 bubbling for an hour before Cu(I)Br (0.032 g, 0.22
mmole) was added.
PEEK-polyNIPAAm membrane.
6.5.4.
Exploiting the surface charge: Electrostatic interaction of PEEK-PolySPM
and Rhodamine 6G
Strips of pristine PEEK and PEEK-PolySPM membranes were immersed in 0.1mM aqueous
solution of Rhodamine 6G for 2h. Both the membranes were washed with plenty of water before taking
a photograph.
123
6.5.5.
Bio-repellency evaluation: Growth E. coli bacteria on the surface of pristine
PEEK and PEEK-MeOEGMA
Bacterial strain from E. coli – BL12 (DE3) was defrosted, transferred into 250 µL of SOC medium
and incubated for 1h at 37 ºC. LB-Agar (Luria/Miller) medium (40g per Liter) was prepared, sterilized
at 121ºC for 5 min and poured into Petri dishes. During cooling of the medium, sterile strips of pristine
PEEK and PEEK-polyMeOEGMA were gently immersed in the medium, 20 µL of bacteria were
spread at the surface and incubated overnight at 37 ºC. Strips of PEEK and PEEK-polyMeOEGMA
membranes were washed with PBS buffer (Phosphate saline buffer, pH 7.5) before subjecting to SEM
imaging.
6.5.6.
Thermally responsive switching between hydrophilicity and hydrophobicity
of PEEK-PolyNIPAAm: Measurement of static water contact angles above and
below the LCST:
A PEEK-PolyNIPAAm membrane, mounted on a glass slide, was placed in the thermostat
chamber. Static water contact angle measurements were carried out at 0°C, 25°C and 40°C. The sample
was equilibrated at each temperature for 20 minutes. Five sample positions were measured at each
temperature.
6.6. Reference
1.
Inagaki, N.; Tasaka, S.; Horiuchi, T.; Suyama, R., Surface Modification of Poly(Aryl Ether
Ether Ketone) Film by Remote Oxygen Plasma. Journal of Applied Polymer Science 1998, 68, (2),
271-279.
2.
3.
Kim, S.; Lee, K. J.; Seo, Y., Polyetheretherketone (Peek) Surface Functionalization by LowEnergy Ion-Beam Irradiation under a Reactive O-2 Environment and Its Effect on the Peek/Copper
Adhesives. Langmuir 2004, 20, (1), 157-163.
4.
Mathieson, I.; Bradley, R. H., Surface Oxidation of Polyether Ketone Films Using Ultraviolet
Ozone. Journal of Materials Chemistry 1994, 4, (7), 1157-1157.
5.
Pawson, D. J.; Ameen, A. P.; Short, R. D.; Denison, P.; Jones, F. R., An Investigation of the
Surface-Chemistry of Poly(Ether Etherketone) .1. The Effect of Oxygen Plasma Treatment on SurfaceStructure. Surface and Interface Analysis 1992, 18, (1), 13-22.
6.
Baalmann, A.; Vissing, K. D.; Born, E.; Gross, A. In Surface-Treatment of
Polyetheretherketone (Peek) Composites by Plasma Activation, 1994; Gordon Breach Sci Publ Ltd:
1994; pp 57-66.
7.
Franchina, N. L.; McCarthy, T. J., Surface Modifications of Poly(Ether Ether Ketone).
Macromolecules 1991, 24, (11), 3045-3049.
8.
MarchandBrynaert, J.; Pantano, G.; Noiset, O., Surface Fluorination of Peek Film by Selective
Wet-Chemistry. Polymer 1997, 38, (6), 1387-1394.
9.
Noiset, O.; Henneuse, C.; Schneider, Y. J.; MarchandBrynaert, J., Surface Reduction of
Poly(Aryl Ether Ether Ketone) Film: Uv Spectrophotometric, H-3 Radiochemical, and X-Ray
Photoelectron Spectroscopic Assays of the Hydroxyl Functions. Macromolecules 1997, 30, (3), 540548.
10.
Noiset, O.; Schneider, Y. J.; MarchandBrynaert, J., Surface Modification of Poly(Aryl Ether
Ether Ketone) (Peek) Film by Covalent Coupling of Amines and Amino Acids through a Spacer Arm.
Journal of Polymer Science Part A-Polymer Chemistry 1997, 35, (17), 3779-3790.
11.
Edmondson, S.; Osborne, V. L.; Huck, W. T. S., Polymer Brushes Via Surface-Initiated
Polymerizations. Chemical Society Reviews 2004, 33, (1), 14-22.
12.
Ramstedt, M.; Cheng, N.; Azzaroni, O.; Mossialos, D.; Mathieu, H. J.; Huck, W. T. S.,
Synthesis and Characterization of Poly(3-Sulfopropylmethacrylate) Brushes for Potential Antibacterial
Applications. Langmuir 2007, 23, (6), 3314-3321.
13.
14.
Cui, Y.; Tao, C.; Zheng, S. P.; He, Q.; Ai, S. F.; Li, J. B., Synthesis of Thermosensitive
Pnipam-Co-Mbaa Nanotubes by Atom Transfer Radical Polymerization within a Porous Membrane.
124
15.
Gong, X.; Dai, L.; Griesser, H. J.; Mau, A. W. H., Surface Immobilization of Poly(Ethylene
Oxide): Structure and Properties. Journal of Polymer Science Part B-Polymer Physics 2000, 38, (17),
2323-2332.
16.
Sun, T. L.; Wang, G. J.; Feng, L.; Liu, B. Q.; Ma, Y. M.; Jiang, L.; Zhu, D. B., Reversible
Switching between Superhydrophilicity and Superhydrophobicity. Angewandte Chemie-International
Edition 2004, 43, (3), 357-360.
17.
Decher, G., Fuzzy Nanoassemblies: Toward Layered Polymeric Multicomposites. Science
1997, 277, (5330), 1232-1237.
18.
Langer, R.; Tirrell, D. A., Designing Materials for Biology and Medicine. Nature 2004, 428,
(6982), 487-492.
19.
Prime, K. L.; Whitesides, G. M., Self-Assembled Organic Monolayers - Model Systems for
Studying Adsorption of Proteins at Surfaces. Science 1991, 252, (5009), 1164-1167.
20.
Takei, Y. G.; Aoki, T.; Sanui, K.; Ogata, N.; Sakurai, Y.; Okano, T., Dynamic Contact-Angle
Measurement of Temperature-Responsive Surface-Properties for Poly(N-Isopropylacrylamide) Grafted
Surfaces. Macromolecules 1994, 27, (21), 6163-6166.
21.
Gangemi, F.; Longhi, G.; Abbate, S.; Lebon, F.; Cordone, R.; Ghilardi, G. P.; Fornili, S. L.,
Molecular Dynamics Simulation of Aqueous Solutions of 26-Unit Segments of P(Nipaam) and of
P(Nipaam) "Doped" With Amino Acid Based Comonomers. Journal of Physical Chemistry B 2008,
112, (38), 11896-11906.
22.
Lin, S. Y.; Chen, K. S.; Liang, R. C., Thermal Micro Atr/Ft-Ir Spectroscopic System for
Quantitative Study of the Molecular Structure of Poly(N-Isopropylacrylamide) in Water. Polymer
1999, 40, (10), 2619-2624.
125
Chapter 7
7. Plasma Polymerised Polyallylamine as ad-layer for
Anchoring Surface initiated polymerization initiator
on polymeric (PEEK, PI, PET) Surfaces: A General
Route to Polymer Surface Functionalization via SIATRP
Pulse plasma polymerized PAA adlayer deposited on polymeric substrates has been explored
as a general route to amino functionalized polymeric surfaces. Amino groups at the surfaces
are the most suitable moieties facilitating the anchoring of a radical polymerization initiator
through a relatively stable amide linkage. For proof of concept, plasma polymerized PAA was
deposited on the surface of three different polymeric substrates namely PEEK, PET, and PI.
This resulted in amino functionalized polymeric surfaces where a suitable ATRP initiator was
anchored. Subsequent SI-ATRP of MeOEGMA monomer resulted in polyMeOEGMA brush
grafted PEEK, PET, and PI surfaces.
126
Surface initiated polymerization (SIP), as introduced in chapter 1 and demonstrated
for SI-ATRP in chapter 6, is a versatile tool for accurately controlling the material
surface properties. While conducting SIP on the surface of polymeric materials, the
most important step is to find a suitable wet chemical surface pretreatment that may
surface immobilize an appropriate SIP initiator. This surface pretreatment is often a
wet chemical organic reaction targeting specific functional group at the polymer
surface and hence is substrate specific.1 Versatility and a high relevance of SIP in
controlling surface properties urges the development of a rather convenient and
general mean of adapting a surface suitable for a SIP. In this chapter a “pulse plasma
polymerization” based substrate independent strategy for anchoring SIP initiator on
polymeric surfaces is presented.
It is well known that conventional continuous wave plasma polymerization
produces a variety of functional groups at the surface due to severe fragmentation of
the precursor molecule within the electrical discharge.2 However, this inherent
limitation is overcome by pulsing the electric discharge to yield high levels of
functional retention.3-6 In mechanistic terms, this marked enhancement in selectivity
can be ascribed to low levels of precursor fragmentation occurring during the short
duty cycle on-period combined with conventional polymerization reaction pathways
predominating within the much longer pulsed plasma off-period. The Pulse plasma
polymerization of a variety of monomers has been reported to functionalize solid
surfaces with the thin films possessing specific chemical functionalities like
perfluoroalkyl-,7 epoxide-,8 anhydride-,9 carboxylic-,10 and cyano-11 groups. Pulse
plasma deposited polyallylamine (PAA) thin film is another example, which has been
demonstrated to retain high amino group functionality. It has been subjected to
stability evaluations and deposited on different substrates for a large variety of
applications including improved cell adhesion12,
14
biomolecules such as polysaccharides
13
and DNA
and for the immobilization of
15, 16
. In the context of present
framework, the pulse plasma polymerized PAA adlayer deposited on polymeric
substrates was exploited as a general route to amino functionalized polymeric
surfaces. Amino groups at the surfaces are the most suitable moieties facilitating the
anchoring of a radical polymerization initiator through a relatively stable amide
linkage. For proof of concept, plasma polymerized PAA was deposited on the surface
of three different polymeric substrates namely PEEK, PET, and PI. This resulted in
amino functionalized polymeric surfaces where a suitable ATRP initiator was
127
anchored.
Subsequent
SI-ATRP
of
MeOEGMA
monomer
resulted
in
polyMeOEGMA brush grafted PEEK, PET, and PI surfaces.
7.1. Substrate independent anchoring of ATRP initiator and subsequent
SI-ATRP
A 5 to 10 nm thick layer of PAA was deposited on the cleaned polymer surface.
The versatility of the approach was demonstrated by employing three polymer
substrates i.e. PEEK, PET, and PI. After plasma deposition of PAA on the surface of
all the three polymeric substrate, they were immersed in a solution of 2bromoisobutyrylbromide in dichloromethane containing triethylamine. This resulted
in surface immobilized with 2-bromoisobutyryl groups that are suitable for SI-ATRP.
Polymer brushes of polyMeOEGMA were grown on the initiator coated surfaces
employing aqueous ATRP conditions (See the experimental section at the end of this
chapter). The whole scheme is outlined in Figure 7.1.
Figure 7.1: Schematic illustration of employing plasma polymerized polyallylamine as adlayer for
anchoring ATRP initiator on polymeric surfaces and subsequent SI-ATRP of MeOEGMA (above).
Polymer substrate subjected to the proposed modification are also presented (below).
7.1.1.
XPS analysis
The XPS analysis of the pristine PEEK, PET, and PI surface were concomitant to
their chemical structures (Figure 7.2).17-19
128
Figure 7.2: C1s high resolution scans of pristine PEEK (a), PET (b), and PI (c). The C1s peak could be
curve fitted into components in accord with the polymer chemical structures.
The peak in C1s high resolution scans of pristine polymer surfaces could be curve
fitted to the respective carbon components constituting the polymer chemical
structures (Figure 7.2). In case of pristine PEEK C1s high resolution scans could be
curve fitted into two components attributable to C-C and C-O carbon linkages
corresponding to the binding energies at 284.9eV and 286.6 eV (Figure 7.2a). The
C1s high resolution scans of pristine PET could be curve fitted into three components
originating from C-C aromatic (284.9 eV), C-C aliphatic (286.5 eV) and C-O
carbonyl (288.9 eV) carbons (Figure 7.2b). Similarly, the C1s high resolution scans of
pristine PI could be curve fitted into three components corresponding to the binding
energies of 284.3 eV, 285.4 eV, and 288.3 eV resulting from C-C, C-O/C-N, and C-O
(carbonyl) carbons (Figure 7.2c). The accurate agreement of carbon components
appeared in XPS C1s high resolution scan with that of chemical structure of
respective polymers affirmed the pure and contamination free surfaces. This was
129
further supported by a close agreement between theoretical and experimental atomic
concentrations revealed by XPS (Table 7.1).
Table 7.1: XPS surface atomic concentrations
Sample
XPS Surface atomic concentration
(%)
C
O
N
Br
PEEK
86 (86.4)
14 (13.6)
--
--
PET
73 (71.4)
27 (28.6)
--
--
PI
75 (75.8)
18 (17.2)
7 (6.89)
--
PEEK-PAA
73
8
19
--
PET -PAA
72
9
19
--
PI-PAA
72
9
19
--
PEEK-PAA-Br
68
11
15
3, 3
PET -PAA-Br
63
14
15
4, 4
PI-PAA-Br
69
13
12
4, 4
PEEK-PAA-polyMeOEGMA
70
30
--
--
PET-PAA-polyMeOEGMA
69
31
--
--
PI-PAA-polyMeOEGMA
68
32
--
--
Poly-MeOEGMA theoretical
70
30
--
--
Theoretical atomic concentrations (%) are given in parenthesis.
An important issue, while depositing plasma polymerized thin film, is the stability
of the film under different solvents. The fact that highly reactive species (such as free
radicals) are generated during plasma polymerization,20-24 it is believed that plasma
deposited PAA is covalently bound to the organic polymer surfaces and is stable in a
variety of solvents. This hypothesis was evaluated by the XPS analysis. After
anchoring the ATRP initiator and before proceeding to SI-ATRP, the polymer
substrates (hereafter designated as polymer-PAA-Br) were rinsed with excess of THF,
CH2Cl2, DMF and ethanol. The resulting XPS survey scans are depicted in Figure 7.3.
130
Figure 7.3: XPS Survey scans of pristine PEEK (a), PET (c), and PI (e). XPS survey scans of polymer
surfaces after plasma deposition of PAA and subsequent ATRP initiator anchoring: PEEK-PAA-Br (b),
PET-PAA-Br (d), and PI-PAA-Br (f). Bromine incorporated at the surface after initiator anchoring was
evident from Br3p and Br3d signals.
While plasma depositing PAA and subsequent anchoring of initiator, the nitrogen
incorporated on the surface of PEEK and PET can be clearly observed in the XPS
survey scans of PEEK-PAA-Br and PET-PAA-Br surfaces (Figure 7.3b & d). PEEK
and PET do not contain any nitrogen in their chemical structures and XPS survey
scans of their pristine surfaces did not show any sign of nitrogen (Figure 7.3 a & c).
PI contains nitrogen as one of its constituent elements therefore the nitrogen region
for this polymer was not considered for comparison (Figure 7.3a). However, the
successful anchoring of initiator at the surface of all the three polymeric surfaces
(PEEK, PET, and PI) modified with plasma deposited PAA was substantiated by the
131
appearance of a Br3d core level absorption between 70-75 eV and for Br3p between
182-198 eV (Figure 7.3b, d, & f). These absorption signals not only confirmed the
presence of incorporated bromine but also corroborated a reasonable chemical
stability of plasma deposited PAA bearing ATRP initiator towards different solvents.
The XPS analysis also revealed a substantial change in the surface atomic
concentrations of the modified polymeric surfaces as compared to the pristine
polymer substrates i.e PEEK, PET, and PI (Table 7.1). Moreover, after plasma
deposition of PAA and ATRP initiator anchoring, PEEK, PET, and PI exhibited
comparable
surface
atomic
concentrations.
This
observation
evidenced
a
homogeneous nature of these modifications.
After validating the robustness of the polymeric surface-PAA adlayer-anchored
initiator platform, the surfaces were grafted with polyMeOEGMA brushes via SIATRP. The XPS analysis of the polyMeOEGMA brush grafted PEEK, PET, and PI
surfaces revealed a complete transformation of surface chemical nature to that of bulk
polyMeOEGMA. The XPS survey scans of all the three polymeric surfaces after the
growth of polyMeOEGMA brush were identical and revealed carbon and oxygen as
the only surface constituents as predictable for pure polyMeOEGMA brush. High
graft density as inherent with grafting from approach resulted in complete
disappearance of nitrogen signals around 400eV. In addition, the surface atomic
concentrations of PEEK, PET, and PI after polyMeOEGM brush growth were
concomitant with that of bulk polyMeOEGMA (Table 7.1). Furthermore, the C1s high
resolution scan of PEEK, PET, and PI surfaces after polyMeOEGMA brush growth
could be curve fitted into three peak components attributable to aliphatic (C-C/C-H),
ether (C-O), and ester (O-C=O) carbons corresponding to binding energies at 284.9,
286.5, and 288.9 eV, respectively.
132
Figure 7.4: XPS Survey scans of PEEK (a), PET(c), and PI (e) surfaces after polyMeOEGMA brush
growth via SI-ATRP. The corresponding C1s high resolution scans could be curve fitted into the
carbon components characteristic for pure polyMeOEGMA brush (b, d, and f)
7.2. Conclusions
Pulse plasma polymerized polyallylamine could be successfully employed as an
adlayer for anchoring ATRP initiator at polymeric surfaces. The polymeric surfacePAA adlayer-anchored initiator platform was found to be stable in different solvents
(THF, CH2Cl2, DMF and ethanol) and suitable for SI-ATRP. The SI-ATRP could be
demonstrated by growing polyMeOEGMA brushes and was validated by XPS surface
analysis. Versatility of the approach was established by including three
technologically relevant polymeric substrates namely PEEK, PET, and PI.
133
7.3.
Experimental
7.3.1.
Deposition of plasma polymerised PAAm on polymer surface
The allylamine monomer (AA) (3-amino-1-propene) was degassed by three freeze-thaw cycles
before use. Plasma polymerization of AA was carried out using a custom made plasma reactor. A 13.56
MHz r.f. generator was inductively coupled to the gas via an LC matching circuit and a copper coil
wound externally around the pyrex glass cylindrical reaction chamber, with a specific distance from
each other, spanning 8-16 cm from the gas inlet. The position of the substrate in the reactor was chosen
by optimizing depositions on glass substrates. In the case of pulsed plasma polymerization
experiments, a signal generator was attached to the r.f. power source, and a cathode ray oscilloscope
was used to monitor the pulse duration, interval and amplitude. AA was plasma polymerized by using
duty cycle [ton/(ton+toff)] of 5/55ms.
7.3.2.
Immobilization of ATRP Initiator on the PAAm Functionalized Membranes
A solution of 2-bromoisobutyryl bromide (BIBr) (0.185 mL, 3 mmol) and triethylamine (0.205 mL, 3
mmol) in dry dichloromethane (30 mL) was injected over polymer substrates modified with plasma
deposition of PAA under N2 at room temperature and left to react for 2.5h. The substrates were then
washed with dichloromethane followed by soaking in DMF, THF, and absolute ethanol (2h each). The
substrates were finally dried under a stream of N2.
7.3.3.
PolyMeOEGMA brush25
Monomethoxy oilgo(ethylene glycol) methacrylates (MeOEGMA) (11g, 37 mmole) was dissolved in
water (11 mL) at 30°C. To this solution Bipy (160 mg, 1.0 mmol) and Cu(II)Br2 (9 mg, 42 µmol) were
added. The mixture was stirred and degassed by N2(g) bubbling for 1h before Cu(I)Cl (41 mg, 420
µmol) was added. The mixture was degassed with N2(g) bubbling for another 15 minutes. PolymerPAA-Br was sealed in a Schlenk tube and degassed by four high vacuum pump/N2(g) refill cycles. The
reaction mixture was syringed into this Schlenk tube, adding enough to cover the membrane
completely, and the mixture was left overnight under N2(g) at 30°C. The sample was removed and
thoroughly rinsed with deionized water.
7.4. Reference
1.
Xu, F. J.; Zhao, J. P.; Kang, E. T.; Neoh, K. G.; Li, J., Functionalization of Nylon Membranes
Via Surface-Initiated Atom-Transfer Radical Polymerization. Langmuir 2007, 23, (16), 8585-8592.
2.
Lopez, G. P.; Ratner, B. D.; Rapoza, R. J.; Horbett, T. A., Plasma Deposition of Ultrathin
Films of Poly(2-Hydroxyethyl Methacrylate) - Surface-Analysis and Protein Adsorption
Measurements. Macromolecules 1993, 26, (13), 3247-3253.
3.
Han, L. M.; Timmons, R. B.; Bogdal, D.; Pielichowski, J., Ring Retention Via Pulsed Plasma
Polymerization of Heterocyclic Aromatic Compounds. Chemistry of Materials 1998, 10, (5), 14221429.
4.
Hynes, A.; Badyal, J. P. S., Selective Incorporation of Perfluorinated Phenyl Rings During
Pulsed Plasma Polymerization of Perfluoroallylbenzene. Chemistry of Materials 1998, 10, (8), 21772182.
5.
Hynes, A. M.; Shenton, M. J.; Badyal, J. P. S., Pulsed Plasma Polymerization of
Perfluorocyclohexane. Macromolecules 1996, 29, (12), 4220-4225.
6.
Lau, K. K. S.; Gleason, K. K., High-Resolution F-19 Mas Nmr Spectroscopy of Fluorocarbon
Films from Pulsed Pecvd of Hexafluoropropylene Oxide. Journal of Physical Chemistry B 1998, 102,
(31), 5977-5984.
7.
Coulson, S. R.; Woodward, I. S.; Badyal, J. P. S.; Brewer, S. A.; Willis, C., Ultralow Surface
Energy Plasma Polymer Films. Chemistry of Materials 2000, 12, (7), 2031-2038.
8.
Tarducci, C.; Kinmond, E. J.; Badyal, J. P. S.; Brewer, S. A.; Willis, C., EpoxideFunctionalized Solid Surfaces. Chemistry of Materials 2000, 12, (7), 1884-1889.
9.
Ryan, M. E.; Hynes, A. M.; Badyal, J. P. S., Pulsed Plasma Polymerization of Maleic
Anhydride. Chemistry of Materials 1996, 8, (1), 37-42.
10.
Hutton, S. J.; Crowther, J. M.; Badyal, J. P. S., Complexation of Fluorosurfactants to
Functionalized Solid Surfaces: Smart Behavior. Chemistry of Materials 2000, 12, (8), 2282-2286.
11.
Tarducci, C.; Schofield, W. C. E.; Badyal, J. P. S.; Brewer, S. A.; Willis, C., CyanoFunctionalized Solid Surfaces. Chemistry of Materials 2001, 13, (5), 1800-1803.
134
12.
Griesser, H. J.; Chatelier, R. C.; Gengenbach, T. R.; Johnson, G.; Steele, J. G., Growth of
Human-Cells on Plasma Polymers - Putative Role of Amine and Amide Groups. Journal of
Biomaterials Science-Polymer Edition 1994, 5, (6), 531-554.
13.
Harsch, A.; Calderon, J.; Timmons, R. B.; Gross, G. W., Pulsed Plasma Deposition of
Allylamine on Polysiloxane: A Stable Surface for Neuronal Cell Adhesion. Journal of Neuroscience
Methods 2000, 98, (2), 135-144.
14.
Dai, L. M.; StJohn, H. A. W.; Bi, J. J.; Zientek, P.; Chatelier, R. C.; Griesser, H. J.,
Biomedical Coatings by the Covalent Immobilization of Polysaccharides onto Gas-Plasma-Activated
Polymer Surfaces. Surface and Interface Analysis 2000, 29, (1), 46-55.
15.
Chen, Q.; Forch, R.; Knoll, W., Characterization of Pulsed Plasma Polymerization Allylamine
as an Adhesion Layer for DNA Adsorption/Hybridization. Chemistry of Materials 2004, 16, (4), 614620.
16.
Zhang, Z. H.; Chen, Q.; Knoll, W.; Foerch, R.; Holcomb, R.; Roitman, D., Plasma Polymer
Film Structure and DNA Probe Immobilization. Macromolecules 2003, 36, (20), 7689-7694.
17.
Louette, P.; Bodino, F.; Pireaux, J. J., Poly(Ethylene Terephthalate) (Pet) Xps Reference Core
Level and Energy Loss Spectra. Surface Science Spectra|Surface Science Spectra 2005, 12, (1), 1-5.
18.
Louette, P.; Bodino, F.; Pireaux, J. J., Polyimide Xps Reference Core Level and Energy Loss
Spectra. Surface Science Spectra|Surface Science Spectra 2005, 12, (1), 121-6.
19.
Louette, P.; Bodino, F.; Pireaux, J. J., Poly(Ether Ether Ketone) (Peek) Xps Reference Core
Level and Energy Loss Spectra. Surface Science Spectra|Surface Science Spectra 2005, 12, (1), 14953.
20.
Kurosawa, S.; Choi, B. G.; Park, J. W.; Aizawa, H.; Shim, K. B.; Yamamoto, K., Synthesis
and Characterization of Plasma-Polymerized Hexamethyldisiloxane Films. Thin Solid Films 2006, 506,
176-179.
21.
Wertheimer, M. R.; Thomas, H. R.; Perri, M. J.; KlembergSapieha, J. E.; Martinu, L., Plasmas
and Polymers: From Laboratory to Large Scale Commercialization. Pure and Applied Chemistry 1996,
68, (5), 1047-1053.
22.
Yasuda, H., Plasma Polymerization. Academic Press: New York, 1985.
23.
Yasuda, H.; Bumgarner, M. O.; Hillman, J. J., Polymerization of Organic Compounds in an
Electrodeless Glow-Discharge .4. Hydrocarbons in a Closed System. Journal of Applied Polymer
Science 1975, 19, (2), 531-543.
24.
Yasuda, H.; Yu, Q. S., Creation of Polymerizable Species in Plasma Polymerization. Plasma
Chemistry and Plasma Processing 2004, 24, (2), 325-351.
25.
135
Chapter 8
8. A facile route for
the
preparation
of
azide-
terminated polymers. “Clicking” macromolecular
building
blocks
on
planar
surfaces
and
nanochannels
The use of an azo initiator bearing azide groups is presented for the facile synthesis of
azide-terminated polymers. Central to this work is to demonstrate that cRP provides a onestep process to prepare azide end-functional polymers that can be used for surface
functionalization of different substrates. To illustrate the versatility of this approach
unprecedented azide end group-functionalised sodium polystyrene sulfonate (PSSNa) and
poly(2-methacryloyloxyethyl-trimethylammonium chloride) (PMETAC) were prepared. The
azide–alkyne click reaction of the telechelic polymers functionalized with azide end groups
was demonstrated by grafting them onto alkyne-terminated silicon surfaces and polyethylene
terephthalate nanochannels.
136
Chapters 6 and 7 demonstrated the regulation of surface chemical composition and
functionality via SI-ATRP as “grafting-from” approach of polymer brush fabrication.
Grafting-from technique with inherent high graft density is superior to the alternative
“grafting-to” approach. The low grafting density in case of “grafting-to” approach is
due to steric hindrance introduced by the grafted polymer chains that precludes the
further tethering of other chains.1 Notwithstanding this limitation, the “grafting-to”
method is widely used for surface modification due to it is simple, practical and can
be conducted under mild conditions. It is also worthwhile noting that some
applications, as is the case of bioseparations or functionalized biointerfaces, the
polymer chains do not require a very high density because they are often derivatized
with large-sized moieties or conjugated to bulky building blocks, like proteins.2, 3
Azide/alkyne “click” reaction is one of the very few organic reactions, which has
been adapted rather smoothly from organic chemistry to macromolecular-based
research.4 There are many advantages that make “click” chemistry particularly
attractive in fields like macromolecular chemistry or materials science.5-7 These
particular features are: a) the reaction is solvent-insensitive and proceeds with high
yields with no byproducts, b) the chemical strategy exhibits functional group
orthogonality, which indicates good tolerance to different chemical groups, and c) the
reaction also works in heterogeneous conditions with high yields.6 The latter one is a
key feature indicating that “click” chemistry is also a powerful toolbox to deal with
colloidal and surface science problems. During last years considerable progress has
been made in the field of “click” chemistry related to surface science aspects.8-12 The
high versatility of “click” chemistry to tailor surface properties and design materials
with molecular accuracy relies on our ability to generate the adequate building blocks
to “click” together, i.e.: azide- and alkyne-functionalized molecules, in a predictable
manner. In this regard, the further evolution and application of “click” chemistry in
scientific disciplines different from organic chemistry will greatly depend on finding
new routes to create azide- and alkyne- terminated building blocks in a low-cost,
straightforward and simple manner. Macromolecular science provided “click”
chemistry with a more ambitious outlook through the incorporation of more
sophisticated and powerful methods to achieve tailored molecular design of the
“clickable” building blocks. The click chemistry approach has been successfully
combined with many controlled radical polymerization reactions including ATRP,13-15
NMP,16,
17
RAFT18-20 or ROMP21,
22
. These methods of controlled radical
137
polymerizations have demonstrated to be a powerful tool to generate well-defined
macromolecular architectures; however, in some cases they also present
incompatibilities among the participating species in the reaction mixture. In other
words, the interplay between catalysts, monomers and solvent can play a decisive role
to successfully achieve the end-functionalized macromolecule suitable for the “click”
reaction.
On the other hand, conventional radical polymerization (cRP) initiated by azo
initiators is still the most robust synthetic route to prepare polymers, which is
applicable to almost any vinyl monomer. Within this framework, it is considered that
merging the fidelity of alkyn/azide “click” reactions with the robustness of cRP will
give a new dimension to the molecular design of materials by opening a gate to nearly
unlimited “clickable” polymeric architectures obtained by a traditional synthetic
procedure.
Hereby, the use of an azo initiator bearing azide groups for the facile synthesis of
azide-terminated polymers is presented. Central to this work is to demonstrate that
cRP provides a one-step process to prepare azide end-functional polymers that can be
used for surface functionalization of different substrates. To illustrate the versatility of
this approach unprecedented azide end group-functionalised sodium polystyrene
sulfonate (PSSNa) and poly(2-methacryloyloxyethyl-trimethylammonium chloride)
(PMETAC) were prepared. The azide–alkyne click reaction of the telechelic polymers
functionalized with azide end groups was demonstrated by grafting them onto alkyneterminated silicon surfaces and polyethylene terephthalate nanochannels.
8.1. Synthesis of the azide-terminated azo initiator.
As previously discussed, cRP provides a robust alternative to grow a wide variety
of polymers bearing different chemical groups. This simple route relies almost
exclusively on the generation of reactive free radicals originated from the thermally
induced dissociation of an adequate initiator. In our case, the free radical initiator was
an azo compound bearing terminal azide groups synthesized via a three-step route
(Scheme 8.1). First, 2,2’azobis(4-cyanovaleric acid) (1) was treated with a slight
excess of PCl5 in CH2Cl2 at 0oC to obtain an acyl halide-terminated azoinitiator (2).
Then, the azide-terminated linker (4) was obtained by reacting 3-bromo-propylamine
hydrobromide with sodium azide.
138
Scheme 8.1: Synthesis of the azide-terminated azo initiator 5.
Finally, the synthesis of the initiator was accomplished by the reaction of acyl
chloride 2 with an excess of 4. A CH2Cl2 solution of 4 was added drop wise to a
cooled (<0°C) solution of 2 in CH2Cl2. After the complete addition the reaction was
allowed to proceed at room temperature under stirring during 12h. The reaction
mixture was then extracted with 1 % HCl followed by rotary evaporation to remove
the solvent, CH2Cl2. The solid residue obtained was re-suspended in ethanol and
precipitated by adding hexane to the solution. This procedure enabled the preparation
of the azide-terminated azo initiator (5) with reasonable yield (92%) and purity as
indicated by the 1H-NMR and 13C-NMR spectra (Figure 8.1).
a,g
5.0
4.0
3.0
118.5631
(b)
4.0
δ (ppm)
2.0
1.0
36.2791
33.5065
30.3295
28.6447
23.6482
5.0
1.9
2.4
6.0
72.3042
7.0
170.3288
Integral
0.99
8.0
f,b
48.7175
e
d
d6-DMSO
c
(a)
1
3 57
2
6
0.0
8
4
9
200
180
160
140
120
100
δ (ppm)
80
60
40
20
0
Figure 8.1: (a) 1H-NMR (b) 13C-NMR spectra in d6-DMSO of the azide-terminated azo initiator 5.
139
FT-IR characterization of the purified solid also corroborated the presence of the
chemical entities corresponding to the azide-terminated initiator (Figure 8.2).
(D)
(E)
Extinction /a.u.
(F)
(G)
(A) (B)
3500
(C)
3000
2500
2000
1500
-1
Wavenumber /cm
Figure 8.2: FT-IR spectrum of the azide-terminated azo initiator 5. The different labels correspond to:
(A & B) 3257 & 3079 cm-1 (N-H) stretching vibration, (C) 2987-2865 cm-1(C-H) stretching vibration,
(D) 2097 cm-1 (-N3) antisymmetric stretching vibration, (E) 1631 cm-1 (C=O) stretching vibration –
amide I, (F) 1564 cm-1 (N-H) bending vibration, amide II, (G) 1447 cm-1 (C-H) bending vibration.
8.2. Synthesis of azide-terminated polyelectrolytes
The characterization of the azo initiator was followed by the synthesis of the azideterminated polymers. In present case, to illustrate the potentialities of the approach
sodium
polystyrene
sulfonate
(PSSNa)
and
poly(2-methacryloyloxyethyl-
trimethylammonium chloride) (PMETAC) functionalized with azide terminal groups
were synthesized (Scheme 8.2).
Scheme 8.2: Scheme describing the polymerization of (a) azide-terminated sodium polystyrene
sulfonate (PSSNa-N3) and (b) azide-terminated poly(2-methacryloyloxyethyl-trimethylammonium
chloride) (PMETAC-N3).
140
The synthesis was carried out in a Schlenk tube in the absence of oxygen. The
monomer and the initiator were dissolved in DMSO and the solution was stirred and
degassed by N2 (g) bubbling during one hour. Then, the Schlenk tube was closed
under a positive pressure of N2 and the polymerization was carried out at 60oC. After
a preset polymerization time the solution was quenched by rapid cooling in ice bath
and exposing to ambient conditions. Finally, the purification of the azide-terminated
polyelectrolytes was accomplished by precipitation in THF or acetone for PSSNa-N3
or PMETAC-N3, respectively. Once purified, the polymers were characterized by gel
permeation chromatography to elucidate their molecular weight (Figure 8.3).
10
2
10
3
104
105
106
Molecular weight (g/mol)
Figure 8.3: Gel permeation chromatography trace describing the molecular weight distribution of
azide-terminated sodium polystyrene sulfonate synthesized by conventional radical polymerization
using the azide-terminated azo initiator 5.
Table 8.1 summarizes the results obtained from both polyelectrolytes.
Polymerization times between 20 min and 2h enabled the facile synthesis of azideterminated polyelectrolytes with number average molecular weight (Mn) ranging from
a few kDa to ~ 20 kDa. Moreover, it can be seen that polydispersities are within the
expected values for cRP initiated from typical free radical initiators. This indicates
that the chemical modifications introduced in the 2,2’azobis(4-cyanovaleric acid)
leading to the synthesis of the azide-terminated initiator does not have detrimental
effects on its free radical initiation characteristics.
141
Table 8.1: Experimental conditions and molecular weight characterization of the
different azide-terminated polyelectrolytes.
Entry
1
2
3
4
5
Polymer
[Initiator]:[M]
PSSNa-N3
PSSNa-N3
PSSNa-N3
PMETAC-N3
PMETAC-N3
Conditions
o
1:2
1:2
1:1
1:5
1:1
120 min, 70 C
o
20 min, 60 C
o
20 min, 60 C
o
20 min, 60 C
20 min, 60oC
Mn
Mw
PDI
21559
14819
3491
5986
3557
50906
34479
7510
8496
6921
2.36
2.33
2.15
1.42
2.00
The azide-terminated polyelectrolytes were also characterized by FTIR
spectroscopy (Figure 8.4). The spectra depicted the typical IR frequencies
encountered in PMETAC and PSSNa polymers plus the band associated to the
antisymmetric stretching of the azide group. Hence, this typical fingerprint
corresponding to the –N3 corroborates the end functionalization of the polyelectrolyte
chains.
(B)
(D)
Extinction /a.u.
(C) (E)
(A)
(a)
(J)
(F)
(H)
(I)
(G)
(b)
2400
2100
1800
1500
1200
900
Wavenumber /cm-1
Figure 8.4: FTIR spectra corresponding to: (a) azide-terminated sodium polystyrene sulfonate
(PSSNa-N3) and (b) azide-terminated poly(2-methacryloyloxyethyl-trimethylammonium chloride)
(PMETAC-N3). The different labels in infrared spectra are: (A) 1640 cm-1 (C-C) stretching vibration of
aromatic skeleton, (B) 1175 cm-1 SO3- group symmetric vibration, (C) 1127 cm-1 in-plane skeleton
vibration of phenyl ring, (D) 1036 cm-1 SO3- group antisymmetric vibration, (E) 1008 cm-1 in-plane
bending vibration of phenyl ring, (F) 1719 cm-1 (C=O) stretching vibration, (G) 1655 cm-1 asymmetric
bending vibration of the quaternary amine cation (QA+), (H) 1477 cm-1 (C-H) in-plane bending
vibration of QA+, (I) 1232 cm-1 (C-N) stretching vibration, (J) 1146 cm-1 (O=C-O-) stretching
vibration. The gray frame indicates the region corresponding to the antisymmetric stretching vibration
of the -N3 groups (~ 2100 cm-1).
142
8.3. “Clicking” polyelectrolyte chains on planar Si surfaces
Solid substrates modified with polyelectrolyte chains have encountered a wide
variety of applications in different technological fields like lubrication or colloidal
stabilization.23-28 In this regard, anchoring polymer chains to surfaces using a
“grafting-to” approach has proven to be an effective strategy to create highly
functional interfaces. As previously described, the “click” reaction involves two
reactants, the azide- and the alkyne-modified building blocks. The polyelectrolytes
were functionalized with terminal azide groups, so the substrates were modified with
alkyne functionalities. This was readily accomplished by derivatization of the Si
substrates with alkyne-terminated self-assembled monolayers (SAMs) (Figure 8.5).
The procedure for assembling the alkyne functionalities was based on the
condensation of ethynyldimethylchlorosilane with silanol groups from the surface of
O2-plasma-treated silicon wafers. The use of monochlorosilane enables attaining
SAMs with better homogeneity and improves the stability of the self-assembling
solution, which in our case was in toluene. As is well-known trichlorosilanes are
prone to hydrolyze and crosslink in the presence of traces of water leading to the
formation of aggregates in solution that are deposited on the substrates.29
Figure 8.5: Scheme describing the functionalization of the silicon surface with alkyne groups followed
by the “clicking” of the azide terminated polyelectrolyte chains.
After an 18h assembly period, the substrates were rinsed with toluene and the
wettability was examined. A water contact angles of ~ 90o corroborated the
effectiveness of the surface functionalization with the alkyne-functionalized SAM
(Figure 8.6). The macromolecular building blocks were then “clicked”on the alkyne
functionalized Si surface. The alkyne-terminated surfaces were immersed in a water
143
solution containing the azide-terminated polymer, CuSO4.5H2O, and sodium
ascorbate and left overnight at room temperature (Figure 8.5). The modified silicon
wafers were rinsed thoroughly with water and dried under stream of N2.
(a)
90o ± 3o
(b)
42o ± 1o
(c)
49o ± 1o
Figure 8.6: Atomic force imaging and contact angle measurements corresponding to: (a) alkynefunctionalized Si surface, (b) PSSNa-N3-modified Si surface and (c) PMETAC-N3-modified Si surface.
Once the polyelectrolytes were “clicked” on the surfaces the wetting characteristics
were studied (Figure 8.6). Both surfaces, PSSNa- and PMETAC-modified silicon
surfaces, showed a marked increase in hydrophilicty when compared to the alkyneterminated platforms. The contact angles corresponding to the PSSNa- and PMETACmodified substrates were 42o and 49o, respectively (Figure 8.6). This change in
wettability indicated that the grafting of the polyelectrolyte chains had a noticeable
impact on the macroscopic properties of the substrate. To further corroborate the
successful
surface functionalization
of
the substrates X-ray photoelectron
spectroscopy (XPS) analysis of the “clicked” samples was performed. XPS data
indicated the presence of the elements constituting the macromolecular building
blocks (Figure 8.7). In addition, detailed XPS analysis of the C1s signal revealed the
presence of different chemical environments for carbon, which is in close agreement
with that expected for PSSNa and PMETAC macromolecules.
144
Figure 8.7: XPS analysis corresponding to: (a) PSSNa-modified silicon, survey scan, (b) PSSNamodified silicon, C1s high resolution scan, (c) PMETAC-modified silicon, survey scan, (d) PMETACmodified silicon, C1s high resolution scan.
In a similar fashion, AFM imaging revealed significant topographic changes after
grafting the polyelectrolyte. As a consequence nodular aggregates were found which
were evenly distributed on the surface (Figure 8.6). On average, grafting
polyelectrolyte chains of ~15 kDa led to ~ 2 nm thick films (dry thickness).
Considering the grafting density as given by eq. 1:
σ = (hρNA)/Mn
(1)
where σ is the grafting density, h is the films thickness, ρ is the density of the
polymer film, NA is the Avogadro number and Mn is the number average molecular
weight of the polymer chains. The “clicking” of the azide-terminated polyelectrolyte
chains on the alkyne-modified substrates resulted in an estimated grafting density of
0.06-0.08 chains/nm2. This grafting density estimation is in agreement with typical
values obtained in “grafting-to” approaches in which the polymer chains are dissolved
in a “good” solvent and the grafting proceeds in the absence of segmental
adsorption.30
145
8.4. Functionalization of single conical polymer nanochannels via a
“click” chemistry approach.
There is a growing interest in developing nanodevices based on synthetic
nanopores as they are considered promising candidates for a wide variety of
applications, including separation techniques and chemical sensing. Tailoring the
chemical characteristics of nanopore surfaces is of great interest as it means that the
surface composition is no longer fixed by the choice of the substrate material. Within
this framework, Siwy et al.31 demonstrated that a polymeric membrane containing a
single conical nanopore can act as an ion rectifier, and this rectifying behaviour was
dependent on the nature of the fixed charges of the nanochannel wall.
Figure 8.8: Scheme describing the surface chemical modification of the PET conical nanochannel
bearing surface –COOH groups (a) with alkyne groups (b) followed by the “clicking” of the
polyelectrolyte chains (c).
This experimental evidence clearly indicates that developing strategies to
manipulate the surface charges of nanochannels is of paramount importance in order
to control and regulate the transport of ions and molecules through nanopores.42 To
monitor the “clicking” of the polyelectrolytes on the nanochannels all the surface
modification steps were carried out on the surface of a single nanopore containing
PET membrane which was mounted on the electrochemical cell (Figure 8.8). This
enabled to track the changes occurring at the nanochannel surface due to consecutive
chemical modifications. Firstly the nanopore containing PET membrane was
functionalized with alkyne groups. This was accomplished by first activating the
carboxyl groups on the etched surface with EDC and PFP during one hour at room
146
temperature. Afterwards, the alkyne groups were introduced on the surface by
coupling between the activate esters and propargylamine (Figure 8.8). This step
required an overnight reaction time to achieve a successful coupling of the alkyne
moieties on the pore PET surface. This chemical modification was evidenced as a
change from a rectifying to a non-rectifying behaviour (Figure 8.9, red and blue
traces). This was due to the fact that the -COO- groups imparting negative surface
charges were reacted with propargylamine, which neutralized the pore surface.
Subsequently, the alkyne-modified membrane was subjected to “click” chemistry by
immersing the membrane in a solution containing azide-terminated polyelectrolytes
and the corresponding catalyst system i.e CuSO4.5H2O and sodium ascorbate (Figure
8.8). After washing the “clicked” membranes several times with water the
corresponding I-V curves were measured. As expected, clicking the polyelectrolyte
chains on the nanochannel surface promoted drastic changes on their rectifying
characteristics. It is clearly observed that incorporating the charged building blocks
into the PET nanopres enables a straightforward manipulation of their permselective
properties. Nanochannels modified with PSSNa-N3 described a rectifying behaviour
similar to that displayed by PET nanochannels with surface -COO- groups but
achieving lower rectified currents at similar voltages (Figure 8.9a).
147
8
(a)
Current /nA
6
4
2
0
-2
-3
-2
-1
0
1
2
3
1
2
3
Voltage /V
6
(b)
Current /nA
4
2
0
-2
-3
-2
-1
0
Voltage /V
Figure 8.9: Current-potential curves corresponding to nanochannels modified with: (a) PSSNa-N3 and
(b) PMETAC-N3. The different colors indicate: (●) PET nanochannel with surface –COOH groups, (●)
alkyne-modified nanochannel and (●) polyelectrolyte-modified nanochannel.
This observation can be attributed to the fact that grafting polyelectrolyte chains
not solely changes the surface charge of the nanochannel but it also alters the effective
cross-section of the nanochannels. It is well known that these two contributions has an
impact on the magnitude and characteristics of the rectified currents.32-38 In a similar
fashion, PMETAC-N3-modified nanochannels displayed a well-defined rectifying
behaviour in which anions were permselectively transported across the positively
charged channel (Figure 8.9b). Applying a positive transmembrane potential (2 V)
revealed no ionic transport through the pore, while at -2 V the ionic transport suffered
significant changes and currents of nearly -1.46 nA were detected. This experimental
evidence strongly supports the idea that “clicking” polyelectrolytes on the
nanochannel can be an avenue to the facile creation of permselective macromolecular
gates.
148
8.5. Conclusions
In summary, in this work a new route for the facile one-step preparation of azideterminated polymers has been described. The strategy is based on the use of
conventional radical polymerization in combination with a novel azide-terminated azo
initiator which permitted the straightforward synthesis of different polymers endfunctionalized with azide groups. The facile synthesis of the functionalized
macromolecular building blocks enabled to merge the versatility of “click” chemistry
with the robustness and ease of cRP to pave the way to a wide variety of “clickable”
architectures. This was demonstrated by the “clicking” of azide-terminated
polyelectrolytes on alkyne-terminated surface via a “grafting-to” approach. In
particular, these “clickable” macromolecular building blocks were exploited to tailor
the chemical characteristics of planar silicon surfaces; and to tune the surface charge
of conical PET nanochannels in order to control their permselectivity. It is envisioned
that these results will not only appeal to organic and polymer chemists but also to
material scientists willing to explore the use of “click” chemistry in different colloidal
and surface science applications. As such, it is considered that this approach will have
strong implications on the molecular design of interfaces using macromolecular
architectures.
8.6.
Experimental Section
8.6.1.
Synthesis of acyl chloride-terminated azo initiator (2)
5 g (0.01784 mole) of 4,4′-azobis(4-cyanopentanoic acid) were suspended and stirred with 150 mL
dried dichloromethane in a 3-necked round bottom flask equipped with a stopper and nitrogen inlet and
outlet. The white suspension was cooled to 0°C in an ice bath. 9.29 g of PCl5 (0.0446 mole, 2.5 equiv.)
were added progressively over 30 min. At the end of the addition, the solution was completely clear,
following solubilization of 4,4′-azobis(4-cyanopentanoic acid) in dichloromethane. The reaction
mixture was allowed to warm up to room temperature and the dichloromethane was evaporated under
reduced pressure on a rotary evaporator at room temperature until the volume of the remaining solution
was approximately 30 mL. 70 mL n-hexane (dried overnight over molecular sieves) was added to the
reaction mixture and the flask was cooled down to 0°C to allow the complete crystallization of 2.39 The
solid was then filtered, washed with 10 mL cold n-hexane and dried overnight under vacuum (yield:
90%).
8.6.2.
Synthesis of 3-Amino-1-azide propane (4)
In a 1L Schlenk flask, connected to a dropping funnel, 1-bromo-3-aminopropane hydrobromide 3
(0.15 mol, 32g) was dissolved in water (100 ml) followed by the addition of NaN3 (0.5 mol, 3.2g) in
150ml of water. After the complete addition the dropping funnel was replaced by a reflux condenser
and the reaction mixture was heated to reflux for 16 h followed by the removal of 2/3 of the water on
rotary evaporator. The resulting mixture was cooled in an ice bath and 500mL of diethyl ether was
added to it. 40 g of KOH pellets were added to the above solution in small portions while keeping the
temperature below 10°C. The organic layer was separated and the aqueous phase was extracted with
diethyl ether (2 × 300mL). The combined organic layers were dried over K2CO3 and concentrated to
give 12.45 g of clear yellow oil (yield: 85%).
149
IR (film): 3305 cm-1 (N-H stretching vibration), 2933, 2866 cm-1 (C-H stretching vibration), 2091 cm-1
(-N3 antisymmetric stretching vibration). 1H NMR (250 MHz, CDCl3) δ [ppm]: 3.31 (t, J = 6.8 Hz, 2H,
-CH2N3), 2.74 (t, J = 6.8 Hz, 2H, -CH2NH2), 1.66 (5, J = 6.68 2H, -CH2-), 1.13 (s, 2H, -NH2). 13C
NMR (250 MHz, CDCl3) δ [ppm]: 28.2 (-CH2-) 39.2 (-CH2-NH2), 49.1 (-CH2-N3).40
8.6.3.
Synthesis of azide functionalized azo initiator (5)
Acyl chloride 2 (5.49 g, 0.017 mole) was dissolved in dry CH2Cl2 (45 mL) and transferred to a dry
250 mL Schlenk flask under N2(g). The solution was cooled in an ice/salt bath. To this solution a five
times excess of amine 1 (8.66 g, 0.086 mole) dissolved in dry CH2Cl2 (30 mL) was added dropwise.
After the complete addition the reaction mixture was allowed to warm up to room temperature and
stirred overnight. The reaction mixture was then extracted with 1% HCl (3 × 70 mL) followed by
washing with brine until the aqueous extract is neutral. The solvent was removed at room temperature
on rotary evaporator and solid residue obtained was suspended in small volume of ethanol. Hexane was
added to this suspension to get clear precipitates, which were allowed to settle, and the supernatant
liquid was decanted. This procedure was repeated several times till a white solid was obtained (92%).
The thus obtained azide functionalized azo initiator was of reasonable purity as reflected by 1H and
13
C-NMR spectra.
IR (neat): 3256 (NH) 3056-2869 (CH), 2097 (-N3) 1628, 1561 (amide linkage). 1H NMR (d6-DMSO):
δ [ppm] = 8.05 (1H, NH, br-s), 3.33-3.39 (2H HN-CH2, t, J = 6.8), 3.10-3.12 (2H, HNCO-CH2, q, J =
6.42Hz) 1.91-2.40 (4H, 2H from N3-CH2-CH2-CH2-NHCO and 2H from CH2 attached to the
quarternary carbon, m) 1.66-1.70 (5H, 3H from -CH3 and 2H form N3-CH2, br). 13C NMR (d6-DMSO):
δ [ppm] 170.3, 118.5, 72.3, 48.7, 36.2, 33.5, 30.3, 28.6, 23.6.
8.6.4.
Synthesis of azide-terminated polyelectrolytes
As an example the synthetic procedure for the preparation of azide-terminated sodium polystyrene
sulfonate will be described, see Table 8.1 (entry 3). In a Schlenk tube, 0.12g (0.58 mmole) of sodium
4-vinylbenzenesulfonate monomer and 0.26g (0.58 mmole) initiator 5 were dissolved in DMSO and
solution was stirred and degassed by N2(g) bubbling for an hour. The Schlenk tube was closed under a
positive pressure of N2(g) and polymerization was carried out at 60°C for 20 min. Polymerization was
quenched by rapid cooling in ice bath and exposing to air. The polymerization solution was then
poured into a non-solvent for the polymer i.e THF. The polymers were purified by repeated
precipitations from their water solution into THF. The purified polymer was characterized by FTIR
spectroscopy and GPC. In case of PMETAC-N3 acetone was used as a non-solvent to precipitate out
the polymer from polymerization solution and for subsequent re-precipitations from water for
purification purpose.
8.6.5.
Click chemistry on the silicon surface
8.6.5.1.
Synthesis of ethynyldimethylchlorosilane (EDMS)
In an oven-dried 250 mL three neck round bottom flask, equipped with nitrogen inlet and outlet,
dropping funnel, magnetic stirrer and a rubber septum, 5 g of 1,3-dichloro-1,1,3,3tetramethyldisiloxane (8) (25 mmol) and dry THF (25 mL) were added with the help of syringe. To this
solution 0.5M THF solution of ethynylmagnesium chloride (125 mL, 62.5 mmol) was added dropwise
at room temperature under nitrogen. Then the mixture was heated to 45°C and stirred for a period of
3h. The solution was gently concentrated to about 50 mL under reduced pressure and filtered through a
short silica plug to remove the Mg residue using pentane:diethylether (1:1, v/v) as eluent. The filtrate
was concentrated in vacuo to give 1,1,3,3-tetramethyl-1,3-diethynyldisiloxane (9) as a colorless liquid
in 90% yield (4g), which was stored below 4°C. IR (film): 3295 cm-1 (≡C-H stretching vibration),
2957, 2919, 2861 cm-1 (C-H stretching vibration), 2036 cm-1 (-C≡C- stretching vibration). 1H-NMR
(250 MHz, CDCl3) δ [ppm] 0.24 (s, 12H, -CH3) and 2.36 (s, 2H, -C≡CH). 13C NMR (250 MHz,
CDCl3) δ [ppm]: 1.89 (-CH3), 88.96 (-C≡CH), 92.3 (-C≡CH). A mixture of 3.01g (16.5 mmole) 9, 1.7g
(11.4 mmole) of methyltrichlorosilane, 0.131 µL of HMPA and 11.7 µL of water was stirred at 60°C
for 4h. The reaction mixture was then subjected to distillation to give ethynyldimethylchlorosilane (10)
as a colourless liquid (boiling point: 38-42°C) in 51% yield. IR (film): 3277 cm-1 (≡C-H stretching
vibration), 2962, 2903 cm-1 (C-H stretching vibration), 2035 cm-1 (-C≡C- stretching vibration). 1HNMR (250 MHz, CDCl3) δ [ppm] 0.54 (s, 6H, -CH3) and 2.56 (s, 1H, -C≡CH). 13C NMR (250 MHz,
CDCl3) δ [ppm]: 3.4 (-CH3), 85.83 (-C≡CH), 95.19 (-C≡CH).41, 42
150
8.6.5.2.
Functionalization of silicon wafer with 10 and subsequent click chemistry
Plasma activated silicon wafer was placed in a crystallization dish and covered with a filtered (0.22
µm pore filter) solution of 10 (10 µL) in dry toluene (30 mL) followed by the addition of dry
triethylamine (50 µL). The dish was covered and left at r.t for 18h. The alkyne functionalised silicon
wafer was sonicated for 2 min in toluene, acetone and then ethanol. Finally, the wafer was dried under
a stream of N2. The click chemistry reaction on the surface of alkyn functionalised silicon wafer is
exemplified for a PSSNa-N3 with a molecular weight of 7114 g/mole at peak maximum (Mp) in GPC
elugram. 100 mg (0.014 mmole) of the polymer and 0.87 mg (0.0035 mmole, 0.25 molar equivalent) of
CuSO4.5H2O were dissolved in 10 mL water. To this solution 1.4 mg (0.007 mmole, 0.5 molar
equivalent) of sodium ascorbate was added. The click reaction solution was stirred for 10 min at room
temperature and an alkyne functionalised silicon wafer was immersed in the solution for overnight. The
silicon wafer was washed with water and dried under stream of N2.
8.6.6.
Functionalization of the conical nanochannel inner surface with alkyne
groups and subsequent click chemistry
The fabrication of conical nanopores in PET was accomplised by asymmetric etching of single ion
tracked membranes.43 Please see Chapter 2 for details.
All the surface functionalization reactions were carried out in the same cell used for etching process.
The surface carboxyl groups were activated by derivatizing into pentafluorophenyl esters. An ethanolic
solution containing 0.1M EDC.HCl and 0.2M pentafluorophenol was placed on both sides of the tracketched PET membrane with a nanochannel. The activation was carried out for 1h at room temperature.
After washing with ethanol several times, the solution was replaced with the 0.1M ethanolic solution of
propargylamine on both sides of the membrane and allowed to react overnight. Then, the chemically
modified membranes were washed several times with ethanol followed by distilled water. Finally, the
propargylated membrane was subjected to aqueous click chemistry reaction following the same
procedure as described for click reaction on alkyne functionalized silicon wafer. The click reaction
solution was transferred to both sides of the alkyne functionalized PET membrane (with a conical
nanochannel) while the membrane was still fixed in the cell. After immersing the PET membrane in the
click reaction solution for overnight, the membrane was washed with distilled water several times.
The membrane containing the single conically shaped nanopore was mounted in the two halves of
the conductivity cell, and each half of the cell was filled with electrolyte solution (unbuffered 0.1M
KCl, pH ~ 6.2-6.3). A Ag/AgCl electrode was placed into each side, and the Keithley 6487
picoammeter/voltage source (Keithley Instruments, Cleveland, OH) was used to apply the desired
transmembrane potential in order to obtain a current-voltage (I-V) curve associated with ion transport
through the single nanopore.
8.6.7.
Gel permeation chromatography (GPC)
Molecular weight distributions were measured using GPC and the experimental conditions
depended on the studied polyelectrolyte. For azide-terminated sodium polystyrene sulfonate a series of
columns, TSK Gel G6000 PWXL- TSK Gel G5000 PWXL and TSK Gel G3000, supplied by
TosoHaas (Stuttgart) was used. The GPC was equipped with a Waters 590 pump and UV (S-3702,
SOMA) and RI (ERC 7519, Erma Inc.) detectors. The eluent was a solution constituted by 80% (0.1 M
NaNO3) and 20% (CH3CN) at a flow rate of 1mL/min, and the column temperature was 23oC. Sodium
polystyrene sulfonate was used a calibration standard.
The
experimental
conditions
for
the
azide-terminated
poly(2-methacryloyloxyethyltrimethylammonium chloride) were similar to those described above, except for the eluent (97% (0.8 M
NaNO3): 3% (CH3CN)), the column temperature (60oC) and the calibration standard (polyethylene
oxide, PEO).
8.7. References
1.
Granville, A. M. B., W.J. , Polymer Brushes: Synthesis, Characterization and Applications.
VCH-Wiley: Weinheim, 2004.
2.
Denizli, F.; Arica, Y.; Denizli, A., Removal of Cadmium(Ii) Ions from Human Plasma by
Thionein Modified Phema Based Membranes. Reactive & Functional Polymers 2000, 44, (3), 207-217.
3.
Slater, M.; Snauko, M.; Svec, F.; Frechet, J. M. J., "Click Chemistry" In the Preparation of
Porous Polymer-Based Particulate Stationary Phases for Mu-Hplc Separation of Peptides and Proteins.
Analytical Chemistry 2006, 78, (14), 4969-4975.
4.
Hawker, C. J.; Wooley, K. L., The Convergence of Synthetic Organic and Polymer
Chemistries. Science 2005, 309, (5738), 1200-1205.
151
5.
Binder, W. H.; Sachsenhofer, R., 'Click' Chemistry in Polymer and Materials Science.
6.
Fournier, D.; Hoogenboom, R.; Schubert, U. S., Clicking Polymers: A Straightforward
Approach to Novel Macromolecular Architectures. Chemical Society Reviews 2007, 36, (8), 13691380.
7.
Nandivada, H.; Jiang, X. W.; Lahann, J., Click Chemistry: Versatility and Control in the
Hands of Materials Scientists. Advanced Materials 2007, 19, 2197-2208.
8.
Ciampi, S.; Bocking, T.; Kilian, K. A.; Harper, J. B.; Gooding, J. J., Click Chemistry in
Mesoporous Materials: Functionalization of Porous Silicon Rugate Filters. Langmuir 2008, 24, (11),
5888-5892.
9.
Ciampi, S.; Bocking, T.; Kilian, K. A.; James, M.; Harper, J. B.; Gooding, J. J.,
Functionalization of Acetylene-Terminated Monolayers on Si(100) Surfaces: A Click Chemistry
Approach. Langmuir 2007, 23, (18), 9320-9329.
10.
Haensch, C.; Ott, C.; Hoeppener, S.; Schubert, U. S., Combination of Different Chemical
Surface Reactions for the Fabrication of Chemically Versatile Building Blocks onto Silicon Surfaces.
Langmuir 2008, 24, (18), 10222-10227.
11.
Ostaci, R. V.; Damiron, D.; Capponi, S.; Vignaud, G.; Leger, L.; Grohens, Y.; Drockenmuller,
E., Polymer Brushes Grafted To "Passivated" Silicon Substrates Using Click Chemistry. Langmuir
2008, 24, (6), 2732-2738.
12.
Prakash, S.; Long, T. M.; Selby, J. C.; Moore, J. S.; Shannon, M. A., "Click" Modification of
Silica Surfaces and Glass Microfluidic Channels. Analytical Chemistry 2007, 79, (4), 1661-1667.
13.
Mantovani, G.; Ladmiral, V.; Tao, L.; Haddleton, D. M., One-Pot Tandem Living Radical
Polymerisation-Huisgens
Cycloaddition
Process
(''Click'')
Catalysed
by
N-Alkyl-2Pyridylmethanimine/Cu(I)Br Complexes. Chemical Communications 2005, (16), 2089-2091.
14.
Sumerlin, B. S.; Tsarevsky, N. V.; Louche, G.; Lee, R. Y.; Matyjaszewski, K., Highly
Efficient "Click" Functionalization of Poly(3-Azidopropyl Methacrylate) Prepared by ATRP.
Macromolecules 2005, 38, (18), 7540-7545.
15.
Tsarevsky, N. V.; Bernaerts, K. V.; Dufour, B.; Du Prez, F. E.; Matyjaszewski, K., WellDefined (Co)Polymers with 5-Vinyltetrazole Units Via Combination of Atom Transfer Radical
(Co)Polymerization of Acrylonitrile And "Click Chemistry"-Type Postpolymerization Modification.
Macromolecules 2004, 37, (25), 9308-9313.
16.
Malkoch, M.; Thibault, R. J.; Drockenmuller, E.; Messerschmidt, M.; Voit, B.; Russell, T. P.;
Hawker, C. J., Orthogonal Approaches to the Simultaneous and Cascade Functionalization of
Macromolecules Using Click Chemistry. Journal of the American Chemical Society 2005, 127, (42),
14942-14949.
17.
O'Reilly, R. K.; Joralemon, M. J.; Hawker, C. J.; Wooley, K. L., Fluorogenic 1,3-Dipolar
Cycloaddition within the Hydrophobic Core of a Shell Cross-Linked Nanoparticle. Chemistry-a
European Journal 2006, 12, (26), 6776-6786.
18.
O'Reilly, R. K.; Joralemon, M. J.; Hawker, C. J.; Wooley, K. L., Facile Syntheses of SurfaceFunctionalized Micelles and Shell Cross-Linked Nanoparticles. Journal of Polymer Science Part APolymer Chemistry 2006, 44, (17), 5203-5217.
19.
Quemener, D.; Davis, T. P.; Barner-Kowollik, C.; Stenzel, M. H., RAFT and Click Chemistry:
A Versatile Approach to Well-Defined Block Copolymers. Chemical Communications 2006, (48),
5051-5053.
20.
Ranjan, R.; Brittain, W. J., Combination of Living Radical Polymerization and Click
Chemistry for Surface Modification. Macromolecules 2007, 40, (17), 6217-6223.
21.
Binder, W. H.; Kluger, C., Combining Ring-Opening Metathesis Polymerization (Romp) with
Sharpless-Type "Click" Reactions: An Easy Method for the Preparation of Side Chain Functionalized
Poly(Oxynorbornenes). Macromolecules 2004, 37, (25), 9321-9330.
22.
Binder, W. H.; Kluger, C.; Josipovic, M.; Straif, C. J.; Friedbacher, G., Directing
Supramolecular Nanoparticle Binding onto Polymer Films: Film Formation and Influence of Receptor
Density on Binding Densities. Macromolecules 2006, 39, (23), 8092-8101.
23.
Huang, H. Q.; Penn, L. S., Dense Tethered Layers by The "Grafting-To" Approach.
Macromolecules 2005, 38, (11), 4837-4843.
24.
Huang, J. Y.; Koepsel, R. R.; Murata, H.; Wu, W.; Lee, S. B.; Kowalewski, T.; Russell, A. J.;
Matyjaszewski, K., Nonleaching Antibacterial Glass Surfaces Via "Grafting Onto": The Effect of the
Number of Quaternary Ammonium Groups on Biocidal Activity. Langmuir 2008, 24, (13), 6785-6795.
25.
Iyer, K. S.; Zdyrko, B.; Malz, H.; Pionteck, J.; Luzinov, I., Polystyrene Layers Grafted to
Macromolecular Anchoring Layer. Macromolecules 2003, 36, (17), 6519-6526.
152
26.
Minko, S.; Patil, S.; Datsyuk, V.; Simon, F.; Eichhorn, K. J.; Motornov, M.; Usov, D.;
Tokarev, I.; Stamm, M., Synthesis of Adaptive Polymer Brushes Via "Grafting To" Approach from
Melt. Langmuir 2002, 18, (1), 289-296.
27.
Reichelt, S.; Gohs, U.; Simon, F.; Fleischmann, S.; Eichhorn, K. J.; Voit, B., Immobilization
of a Hyperbranched Polyester Via Grafting-to and Electron Beam Irradiation. Langmuir 2008, 24, (17),
9392-9400.
28.
Urban, A. M. U., M.W. , Stimuli-Responsive Polymeric Films and Coatings. American
Chemical Society: Washington, 2005.
29.
Bunker, B. C.; Carpick, R. W.; Assink, R. A.; Thomas, M. L.; Hankins, M. G.; Voigt, J. A.;
Sipola, D.; de Boer, M. P.; Gulley, G. L., The Impact of Solution Agglomeration on the Deposition of
Self-Assembled Monolayers. Langmuir 2000, 16, (20), 7742-7751.
30.
Penn, L. S. H., H.; Quirk, R.P.; Cheong, T.H., Polymer Brushes: Synthesis, Characterization
and Applications. VCH-Wiley: Weinheim, 2004.
31.
10850-10851.
32.
33.
2008, 3, (2), 73-74.
34.
Daiguji, H.; Yang, P. D.; Majumdar, A., Ion Transport in Nanofluidic Channels. Nano Letters
2004, 4, (1), 137-142.
35.
Karnik, R.; Fan, R.; Yue, M.; Li, D. Y.; Yang, P. D.; Majumdar, A., Electrostatic Control of
Ions and Molecules in Nanofluidic Transistors. Nano Letters 2005, 5, 943-948.
36.
37.
van der Heyden, F. H. J.; Bonthuis, D. J.; Stein, D.; Meyer, C.; Dekker, C., Power Generation
by Pressure-Driven Transport of Ions in Nanofluidic Channels. Nano Letters 2007, 7, (4), 1022-1025.
38.
Vlassiouk, I.; Siwy, Z. S., Nanofluidic Diode. Nano Letters 2007, 7, (3), 552-556.
39.
Koenig, A.; Ziener, U.; Schaz, A.; Landfester, K., Polyurethane-Block-Polystyrene Prepared
by Polymerization in Miniemulsion. Macromolecular Chemistry and Physics 2007, 208, (2), 155-163.
40.
Hatzakis, N. S.; Engelkamp, H.; Velonia, K.; Hofkens, J.; Christianen, P. C. M.; Svendsen, A.;
Patkar, S. A.; Vind, J.; Maan, J. C.; Rowan, A. E.; Nolte, R. J. M., Synthesis and Single Enzyme
Activity of a Clicked Lipase-Bsa Hetero-Dimer. Chemical Communications 2006, (19), 2012-2014.
41.
Sukeda, M.; Ichikawa, S.; Matsuda, A.; Shuto, S., The First Radical Method for the
Introduction of an Ethynyl Group Using a Silicon Tether and Its Application to the Synthesis of 2 'Deoxy-2 '-C-Ethynylnucleosides. Journal of Organic Chemistry 2003, 68, (9), 3465-3475.
42.
Wong, W. Y.; Wong, C. K.; Lu, G. L., Sigma-Acetylide Complexes of Ruthenium and
Osmium Containing Alkynylsilane Ligands. Journal of Organometallic Chemistry 2003, 671, (1-2),
27-34.
43.
153
Chapter 9
9. Facile
Large-Scale
Fabrication
of
Proton
Conducting Channels
PEL Brush-Coated
Channel
SiO2
PEL
brush
1 µm
Proton
Conduction
1 µm
PEL
brush
A new approach for the facile large-scale fabrication of robust silicon membranes with artificial
proton conducting channels is presented. Ordered two dimensional macroporous silicon was rendered
proton conducting by growing a thick uniform polyelectrolyte brush using surface initiated atom
transfer radical polymerization throughout the porous matrix. The fabricated silicon-poly(3(methacryloyloxy)propane-1-sulfonate) hybrid membranes were evaluated for their proton
conductivity, ion exchange capacity and water uptake. With proton conductivities in the range of 10-2
S/cm, these proof-of-concept experiments highlight a promising alternative for producing tailorable
proton conducting membranes. This approach constitutes a benchmark for the preparation and study
of model systems and, in addition, for the large-scale fabrication of membranes suitable for a wide
range of technological applications.
154
In the context of present thesis, so far it has been demonstrated that the
manipulation of the chemical composition of surfaces plays a pivotal role in different
technological areas, which include adhesion, wettability, antifouling, etc. Most of the
work was carried out on the flat surfaces. From here onwards the results from second
part of the project will be presented, which deals with the fabrication of functional
materials by tuning the surface properties of the porous substrates. In the present
framework, the porous scaffolds of interest have been introduced in chapter 1. In this
chapter
fabrication
of
proton
exchange
membranes
(PEMs)
by
surface
functionalization of ordered macroporous silicon is presented. PEMs, often referred to
as proton conducting membranes, are key constituting elements in different industrial
applications, and particularly in energy conversion technologies, like fuel cells.1, 2 To
date, the PEMs are typically constituted of perfluorinated polyelectrolytes, like Nafion
(Figure 9.1).3, 4
Figure 9.1: Microstructure of Nafion.
This material, considered as “the golden standard”,5, 6 is characterized by forming
nanoscopic hydrophilic channels suitable for the conduction of protons across the
membrane, even if 60% of the hydrophilic domains at the surface of an operating
Nafion membrane remain inactive.7 The economical and environmental issues related
to this material represent serious drawbacks for the large-scale fabrication of costeffective PEMs.8 In the light of current status,9 it is clear that finding new
technologies enabling the facile and low-cost production of efficient proton
conducting platforms is a high priority goal, and it will constitute a tough scientific
and technological challenge during the coming years. At present, the scientific
community is actively exploring different strategies9 including the use of new
chemistries to achieve perfluorinated polyelectrolytes,10 development of polymeric
155
membranes that could operate at higher temperatures without the need for
humidification e.g. a phosphoric acid doped polybenzimidazole membrane,11 or novel
physicochemical approaches to tailor the proton transport like the use of porous
substrates infiltrated with polyelectrolytes,12 and layer-by-layer polyelectrolyte
films.13-16
On the other hand, the fabrication of self-standing macroporous ceramic
membranes containing highly ordered and monodisperse channels is also attracting
increasing interest.17 In spite of their interesting architectures,18 not much effort has
been devoted to exploiting these channels in order to manipulate the proton
conduction through the membrane. Herein, an investigation is presented on the use of
ordered macroporous silicon modified with polyelectrolyte brushes for channelling
the transport of protons that offers the potential of setting a new trend on the rational
design of proton conducting membranes. It is shown that this strategy, combining
elements from materials science and macromolecular chemistry, enables the robust
and facile large-scale fabrication of artificial proton conducting channels.
9.1. Grafting of polyelectrolyte brush on the surface of ordered
macroporous membranes via SI-ATRP
The
macroporous
silicon
used
in
this
study
was
fabricated
by
a
photoelectrochemical etching process.19 Subsequently, the remaining bulk silicon
from the backside was removed in KOH and thus an ordered porous self-standing
membrane17 was obtained (Figure 9.2). The macroporous area conformed by perfectly
parallel channels was ~ 3 cm2 with a density of ~107 pores/cm2.
Figure 9.2: Scanning electron micrograph of the macroporous silicon membrane used as a platform
for tethering the polyelectrolyte brush by surface-initiated atom transfer radical polymerization. Scale
bar: 25 µm. The inset shows in detail the pore geometry. Scale bar: 800 nm
Next, the proton source was incorporated into the pores, i.e.: the polyelectrolyte.
This was simply accomplished by SI-ATRP.20-22 This polymerization process led to
156
the formation of a dense polyelectrolyte layer (brush) covalently tethered at one end
to the pore sidewall (Figure 9.3). Even if it is well-known that polymer brushes can be
grown by a number of polymerization techniques,23
ATRP24 resulted in a very
attractive alternative due to its simplicity to synthesize different polyelectrolytes
(PELs) in aqueous environments.25, 26 Firstly, the macroporous substrate was modified
with initiator-terminated self-assembled monolayers (SAMs). To achieve this goal 2bromo-2-methyl-N-(3-triethoxysilyl-propyl)-propionamide
monolayers
were
assembled from a toluene solutions. Secondly, the PEL brush was grown (Figure 9.3)
by immersing the initiator-modified macroporous substrates into the corresponding
polymerization solution under conditions described elsewhere.27
Figure 9.3: Simplified cartoon illustrating the construction of the polyelectrolyte brush-coated
channels: The macroporous silicon membrane modified with initiator-terminated self-assembled
monolayers (a) is immersed in the ATRP polymerization solution where the surface-initiated
polyelectrolyte (polyMPS) growth is carried out (b).
The potassium 3-(methacryloyloxy)propane-1-sulfonate (MPS) was chosen as
monomer due to the fact that it is a simple and inexpensive sulfonated monomer.
9.1.1.
Characterization
After polymerization, the chemical composition of the PEL brushes was monitored
by XPS and FTIR spectroscopy. Both spectroscopic techniques corroborated that the
chemical nature of the brush corresponded to polyMPS coordinated with K+ ions
(Figure 9.4). Then the brushes were extensively rinsed with dilute HCl for exchangin
the K+ ions that originally coordinated the monomers with protons.
157
2.5
Intensity /kCounts
(a)
2.0
K 2p
C 1s
O 1s
K 2s
1.5
1.0
S 2s S 2p
0.5
K 3p
K 3s
0.0
600
500
400
300
200
100
0
0.02
~ 736 cm-1
0.04
1437-1487 cm-1
0.06
1043 cm-1
(b)
0.08
1724 cm-1
Extinction /a.u.
0.10
1190 cm-1
Binding Energy /eV
0.00
2500
2000
1500
1000
Wavenumbers /cm
500
-1
Figure 9.4: (a) XPS and (b) FTIR spectra corresponding to the substrate modification by surfaceinitiated ATRP of 3-(methacryloyloxy)propane-1-sulfonate (MPS). In panel (a) the signals correspond
to: carbon, C1s (284.5eV); oxygen, O1s (531eV); sulfur (from the sulfonate group), S2s (227.25eV),
S2p (163.25eV) and potassium (monomer counterion), K2s (373.25eV), K2p (293.75eV) K3s (33.5eV)
K3p (17.5eV). In panel (b) the spectrum shows the stretching of carbonyl groups at 1724 cm-1, CH2
bending vibrations around 1437-1487 and 736 cm-1 and asymmetric and symmetric sulfonate
stretching around 1190 cm-1 and 1043 cm-1, respectively.
The surface-initiated polymerization led to a homogeneous modification of the
silicon membrane resulting in a polymer layer evenly distributed on the porous
substrate. Figure 9.5 clearly shows that the pore (mouth) diameter is significantly
reduced after the polymer growth.
158
(a)
(b)
(c)
Figure 9.5: Scanning electron micrographs of the PEL brush-modified macroporous silicon membrane
imaged at different magnifications. Scale bars are: (a) 12 µm, (b) 2µm, (c) 600 nm.
Cross-sectional analysis by scanning electron microscopy revealed that, in fact, the
inner walls of the channels are uniformly and completely covered by ~ 200 nm thick
brushes (Figure 9.6), thus indicating that the SI-ATRP proceeded smoothly even in
the confined environment of the macroporous silicon.
On the other hand,
polyelectrolyte brushes swell in humid environments leading to a significant increase
in thickness.28 This leads to fully occupied pores where the proton transport across the
membrane is exclusively mediated by the hydronium ions through the polyelectrolyte
brush.
159
(a)
(b)
Channel
SiO2
PEL
Brush
PEL
Brush
1 µm
SiO2
1 µm
Figure 9.6: Scanning electron micrographs corresponding to: (a) cross-sectional imaging of a PEL
brush-modified channel, and (b) longitudinal cross-sectional imaging of the channel.
Once confirmed the modification of the channel wall with the brushes (Figure 9.6),
the effective loading (4.3%), the ion-exchange capacity (0.14 meq/g) and the water
uptake (7%) of the self-standing membrane were estimated. (Please see experimental
details at the end of this chapter for respective procedures).
9.1.2.
Proton conductivity
The proton conductivity of the self standing porous silicon-PEL brush hybrid
membrane was measured using impedance spectroscopy. The impedance plot
described in Figure 9.7 shows that in humidity saturated atmospheres the membrane
displays conductivity values in the range of 10-2 S/cm.
log [Conductivity (mS/cm)]
2
1
0
-1
-2
-1
0
1
2
3
4
5
6
7
log [Frequency (Hz)]
Figure 9.7: Bode plot describing the conducting characteristics of the polyelectrolyte brush-coated
membrane at 100% RH and 25°C. Evaluating the plateau of the Bode plot a specific conductivity of
1.6×10-2 S/cm was calculated.
The variations in proton conductivity of the polyelectrolyte brush-modified
membrane upon changes in the relative humidity (RH) at room temperature were also
studied. A gradual increase in proton conductivity can be clearly observed upon
increasing the RH (Figure 9.8). The proton conductivity at room temperature reaches
160
the value of 2×10-2 S/cm at 95% RH. Importantly, the membrane quickly equilibrates
to humidity changes and at each RH the proton conductivity was fairly constant over
days (Figure 9.8), thus evidencing the good proton conducting characteristics of the
PEL brush-modified macroporous silicon membrane. This approach provides a robust
and highly reproducible strategy to firmly anchoring the PEL layer into the channels.
This is a major improvement with respect to other methods based on polyelectrolyte
impregnation into porous substrates12 where humid environments lead to the
continuous leaching of the polymer (proton source) and, consequently, to a significant
decrease in the proton conductivity. In the present case, the chemical nature of the
silane linkage29 enables the use of the PEL brushes in different environments30
without affecting the stability of the anchoring layer. The simplicity of the procedure
described here is also advantageous when compared to bulk copolymer-based PEMs,
which require tedious synthetic routes to achieve proton conducting nanochannels.
10
10
Relative Humidity
80
-1
-2
10
-3
10
-4
10
-5
10
-6
10
-7
60
40
20
0
0
2
4
6
8
t /d
10
12
14
Conductivity (S/cm)
100
16
Figure 9.8: Variation of the proton conductivity with increase in RH. The different plateaus describe
the stability displayed by the membrane at different preset RHs over the time in days. The membrane
equilibrates quickly to the humidity changes and at each particular level of RH the proton conductivity
was fairly constant over days.
To further investigate the characteristics of the platform, the porous siliconpolyMPS hybrid membrane was subjected to temperature dependent proton
conductivity measurements under highly humidified enviroments. At 90% RH,
varying the temperature from 25°C to 80°C leads to an increase in proton conductivity
from 7 to 15 mS/cm (Figure 9.9). These values further evidence the good proton
conductivity performance of this approach when the platform is operating in the
temperture range of 25°C to 80°C.
161
Conductivity (mS/cm)
16
14
12
10
8
6
20
30
40
50
60
70
80
90
o
T/C
Figure 9.9: Conductivity versus temperature plot measured at RH 90% using a climate chamber.
9.2. Development of self-humidifying proton conducting channels
generated by scaffolded polyelectrolyte brushes “doped” with
hygroscopic monomer units
After conducting the above described proof of concept experiments, the properties
of silicon-PEL brush hybrid membranes were further tuned. As it is evident from
above discussed results that like Nafion the above proposed system shows reasonable
conductivity only at high relative humidity levels. From a technological view this
problem is managed through an external humidification subsystem to ensure constant
humidity conditions and water retention in the PEM. Even though, the humidifier is
considered as a burden due to it requires space and heat supply, thus affecting the
system efficiency and cost. Therefore, developing proton conducting membranes
whose proton conductivity does not depend on humidification level would have a
tremendous impact on the fabrication and efficiency of PEM-based devices.31-33 In the
framework of present system, unprecedented results regarding the facile molecular
design leading to high proton conductivity over a wide range of relative humidity
conditions are presented. The approach is again centered on the SI-ATRP of
polyelectrolyte brushes using photoelectrochemically-etched silicon membranes
(Figure 9.2) as scaffolds to build-up structurally and chemically well-defined
hydrophilic channels. The experimental results show that the polyelectrolyte brushbased pore-filling approach, using sulfonated monomers (MPS) as proton carriers and
polyethylene glycol-based monomers as humidifying agents (Figure 9.10), provides a
unique and extremely powerful tool to fabricate proton conducting membranes with
conductivity values above 10-2 S/cm regardless the humidification level.
162
Figure 9.10: Simplified cartoon illustrating the surface initiated polymerization used to create the
proton conducting channels. The macroporous silicon scaffold was modified with initiator-terminated
self-assembled monolayers (a) and then immersed in the ATRP solution where the surface-initiated
copolymerization of SPM and MeOEGMA was carried out (b). In the figure the chemical structure of
the copolymer brush (n = 0.87, m = 0.13, x = 5) is also described.
As it is well-known, polyethylenglycol (PEG)-related materials have excellent
hydroscopic properties, i.e. they attract and retain moisture from the atmosphere, and
are commonly used as moisturizers and additives in the cosmetics industry.34, 35 In the
presence of PEGylated macromolecular architectures water molecules are able to
develop hydrogen bonding with the ethylene oxide units of the polymer chains
leading to the formation of large clusters and dynamic hydrogen bonded networks.
These properties turn PEG-like species as ideal building blocks to manipulate the
affinity to water of different materials, as is the case of PEGylated membranes
commonly used for the dehumidification of air.36 In a similar fashion, Banerjee and
co-workers exploited the hygroscopic properties of PEG to capture condensed water
droplets and to prevent their evaporation from microwells.37 This would imply that
the presence of PEGylated domains in a polymer film would enhance its water
accumulation capability due to the formation of clusters and networks. Interestingly,
recent results by Rodrigues et al. demonstrated that a blend film containing PEG and
poly(3-hydroxybutyrate) (PHB) in a 0.05:0.95 ratio (PEG:PHB) displayed a 200 %
increase in water permeability when compared to pure PHB films. These results show
that, even in very small amounts, the PEGylated constituents play a key role in
governing the affinity to water of the polymer film.38
In the present case, the role of the PEG-like co-monomer would be acting as a
“humidifying agent” and keeping the brush hydrated while the polyelectrolyte acts as
the proton source. In order to verify this hypothesis, the macroporous membranes
163
were modified with polymer brushes by simply copolymerizing MPS and
monomethoxy oligo(ethylene glycol) methacrylate (MeOEGMA, average Mn ~300) in
a 10:1 monomer ratio (Figure 9.10).
9.2.1.
Characterization and proton conductivity measurements
After polymerization, the PEL brushes were characterized with XPS and FTIR
spectroscopy. The important signals in the spectra from both the spectroscopic
techniques could be assigned to the structural elements of polymer brushes (Figure
9.11).
Figure 9.11: (a) XPS copolymer brush. The signals correspond to: carbon, C1s (283.2eV); oxygen,
O1s (530.4eV); sulfur (from the sulfonate group), S2s (229.6eV), S2p (166.4eV) and potassium
(counterion), K2s (376.8eV), K2p (291.2eV) K3s (33.5eV) K3p (17.5eV). (b) FTIR spectra
corresponding to the substrate modification by surface-initiated copolymerization of 3sulfopropylmethacrylate and monomethoxy oilgo(ethylene glycol) methacrylate . The spectrum shows
the stretching of carbonyl groups at 1727 cm-1, and asymmetric sulfonate stretching around 1157 cm-1.
Beside the SI-ATRP, copolymerisation of MPS and MeOEGMA was also carried
out in the solution under the similar conditions. Ethyl 2-bromopropanoate was used as
initiator. The MPS:MeOEGMA actual ratio in copolymer was estimated by
comparing the integration corresponding to 2 protons (at δ=2.95 ppm) which are next
to sulfonate group in MPS monomer with the protons of -CH2-O- (at δ=3.67 ppm)
linkage in MeOEGMA in 1H-NMR spectra. The actual MPS:MeOEGMA ratio in
copolymer was found to be 0.87:0.13 for 10:1 monomer feed ratio (Figure 9.12).
164
Figure 9.12: 1H-NMR analysis of polyMPS-co-MeOEGMA (below). 1H-NMR of polyMPS (above) is
also shown for comparison.
Like in case of polyMPS, the surface initiated copolymerisation produced a thick
copolymer brush that uniformly covered the whole surface of macroporous silicon
membrane. Scanning electron micrographs depicting top surface and cross section of
the PEB-macroporous silicon hybrid membranes are presented in Figure 9.13.
Figure 9.13: Scanning electron micrographs of the polyMPS-co-MeOEGMA PEL brush-modified
macroporous silicon membrane (a) top view and (b,c) longitudinal cross-sections . Scale bars where
not mentioned are 200 nm.
165
The K+ counter ions were exchanged with the protons by simply soaking the
hybrid membranes in dilute HCl solution. The ion exchange capacity was 0.12 meq/g,
which is slightly lower (~14 %) than its homopolymer MPS-based analogue (0.14
meq/g). As expected, this fact indicates that the replacement of MPS by MeOEGMA
monomers in the brush quantitatively affects the ion exchange capacity of the
membrane. Characterization with impedance spectroscopy (at 100 % RH) revealed
that the proton conductivity of the copolymer brush-modified membrane was very
good and the values were comparable to those observed for Nafion, ~10-2 S/cm
(Figure 9.14).
Conductivity (S/cm)
100
10-1
10-2
10-3
10
-4
10-5
-1
100
10
10
1
102
10
3
10
4
105
10
6
107
Frequency (Hz)
Figure 9.14: Bode plot describing the conducting characteristics of the polyMPS-co-MeOEGMA
brush-modified silicon membrane at 100% RH at 25 °C. Evaluating the plateau of the Bode plot a
specific conductivity of 1.4 × 10-2 S/cm was calculated.
However the remarkable out come of these experiments became more evident
when the proton conductivities of homo- and co-polymer systems were compared at
different humidity levels (Figure 9.15). It was observed that the high conductivity
values (~10-2 S/cm) measured at high humidity conditions (RH 90%) remained almost
constant over the large range of relative humidity, 30–90%. To be more precise, the
polyMPS-co-MeOEGMA brush-modified membrane displayed a proton conductivity
value of 4 × 10-2 S/cm at RH 30 %. This represents an unprecedented result regarding
the molecular design of new proton conducting platforms with optimized properties.
For the sake of clarity and to better describe the relevance of these results, the
conductivity values obtained at different RHs for a MPS brush-modified hybrid
membrane and a Nafion 117 film (data for Nafion is taken from ref39) are also
depicted in Figure 9.15. Direct comparison reveals that, at low RHs, the incorporation
of oligoethylene glycol to a minor extent into the brush architecture promotes drastic
changes in the proton conductivity of MPS brush- modified membranes. In fact,
replacement of ~13 % of the MPS monomer units by MeOEGMA was reflected as a
166
five orders-of-magnitude increase in proton conductivity. This evidences the critical
role of the molecular design of the proton conducting channels and the versatility of
SI-ATRP to easily achieve this goal. In a similar fashion, comparison with Nafion
also revealed striking differences. This perfluorinated polyelectrolyte has been
historically considered as the golden standard in proton conducting membranes. As
observed in Figure 9.15a, proton conductivity in Nafion sensitively decreases when
RH is varied from ~90 to ~30 % reaching values of ~10-4 at low humidity conditions.
It is worthwhile mentioning that this conductivity is 2 orders of magnitude lower than
that measured in the polyMPS-co-MeOEGMA brush-modified membrane under
similar humidity conditions. This fact highlights the remarkable and outstanding
performance of the polyMPS-co-MeOEGMA brush platform as compared to the
performance of polymeric materials currently used by the industry in PEM-based
devices. Their proton conductivities are appreciable only at high RHs. Furthermore, in
contrast to Nafion, the conductivity values of the polyMPS-co-MeOEGMA brushbased membrane are not affected by significant changes in RH which makes the
platform performance independent of the humidity conditions. This particular feature
is extremely relevant in order to achieve highly stable and reliable PEMs. To further
explore the reliability of the polyMPS-co-MeOEGMA brush platform the proton
conductivity values at different RHs over a period of nearly one week were monitored
(Figure 9.15b). Figure 9.15b clearly shows that the proton conductivity is nearly
constant, with values of ~10-2, during an eight-day monitoring period in which RH
was varied from 30 to 90%. These results further corroborate the robustness and
reliability of the hybrid membrane modified with polyMPS-co-MeOEGMA brushes.
167
log[Conductivity(S/cm)]
-1
-2
-3
-4
-5
-6
(a)
-7
20
30
40
50
60
70
80
90
100
Relative Humidity (%)
10
-0
100
-1
80
10-2
10-3
60
10-4
10
10
-5
40
-6
10
(b)
-7
Relative Humidity (%)
Conductivity (S/cm)
10
20
0
2
4
6
8
t (d)
Figure 9.15: (a) Conductivity versus relative humidity plots corresponding to: (●) polyMPS-coMeOEGMA brush-modified silicon membrane, (○) polyMPS brush-modified silicon membrane (●)
Nafion® 117 membrane (data taken from Ref.39 (b) Variation of the proton conductivity of polyMPS-coMeOEGMA brush-based membrane upon changes in RH. The proton conductivity remains fairly
constant over days regardless of the RH level.
The monomers used and presented so far in the present study contain ester
linkages, which might not be the most suitable ones for different industrial
applications. Even though, the presented results obtained at different temperatures
after soaking the PEL brush in highly acidic solutions (0.1 N HCl) indicate that
polyMPS brushes are robust enough to attain stable and reproducible proton
conductivity values (Figure 9.9). However, hydrolytically more stable linkages were
incorporated in the presented system. In this context a SI-ATRP of sodium 2acrylamino-2-methylpropane sulfonate (AMPS) was developed. AMPS is another
commercially available sulfonate group containing acryl monomer. In polyAMPS
brushes the sulfonate groups are attached to the polymer back bone through amide
linkage, which is known to be hydrolytically more stable than the ester linkages. The
results obtained are presented here.
168
9.3.
SI-ATRP
of
2-acrylamino-2-methylpropane
sulfonate
in
macroporous silicon
The SI-ATRP was carried out under conditions analogues to those used in case of
PolyMPS (please see the experimental section at the end of this chaprter for the
procedure). Briefly, the polymerization solution consisted of the Cu(I)/Bipy catalyst
(promoting the activation), Cu(II)/Bipy deactivator (promoting the capping), and the
monomer (2-acrylamino-2-methylpropane sulfonate, sodium salt) in an aqueous
methanolic solvent. The measurement of ellipsometric thicknesses at different
polymerization times reflected (Figure 9.16) a linear polymer brush growth.
Figure 9.16: Plot of ellipsometric polymer brush thickness versus polymerization time for SI-ATRP of
polyAMPS
The polyAMPS brush was characterized by XPS. The constituting elements of
polyAMPS brush were observable in XPS survey scan (Figure 9.17a).
Figure 9.17: (a) XPS and (b) FTIR spectra corresponding to the substrate modification by surfaceinitiated polymerization of 2-acrylamino-2-methylpropane sulfonate (AMPS). In panel (a) the signals
correspond to: carbon, C1s (284eV); N1s (399.2 eV) oxygen, O1s (531eV); sulfur (from the sulfonate
group), and S2s (231.25eV), S2p (167.2eV). In panel (b) the spectrum shows the amide I stretching of
carbonyl groups at 1661 cm-1, amide II bending vibration of –NH- at 1539 cm-1, and asymmetric and
symmetric sulfonate stretching around 1197 cm-1 and 1037 cm-1, respectively.
169
The polyAMPS brush was further characterized by FTIR spectroscopy. In FTIR
spectrum the characteristics bands of amide (1661cm-1/amide I, 1539 cm-1/amide II)
and sulfonate (1197cm-1, 1030cm-1) groups were clearly observable (Figure 9.17a).
The SI-ATRP of AMPS on the surface of initiator coated macroporous silicon
membrane resulted in a homogeneous modification of the silicon membrane with
polyAMPS brush. A significant reduction in the pore (mouth) diameter after polymer
growth is clearly evident from Figure 9.18.
Figure 9.18: Scanning electron micrographs of (a) unmodified macroporous silicon membrane as
reference and (b) the polyAMPS PEL brush-modified macroporous silicon membrane.
The cross sections were also imaged, which revealed a ~200nm thick polyAMPS
brush after 12h of polymerization time (Figure 9.19).
Figure 9.19:Scanning electron micrographs corresponding to the cross-sectional imaging of a
polyAMPS brush-modified channel.
The characteristics like effective loeading, ion exchange capacity, water uptake,
and proton conductivity are being carried out and system is still under investigation.
170
9.4. Conclusions
The results presented in this chapter describe the use of highly ordered
macroporous silicon modified with polyelectrolyte brushes as a promising alternative
for producing tailorable, robust and self-standing proton conducting membranes. This
very simple and straightforward strategy allows for the generation of artificial proton
conducting channels, as demonstrated by proton conductivity values of ~ 10-2 S/cm.
Considering the versatile chemical nature of ATRP and the wide diversity of
macroporous silicon architectures that can be fabricated, it is believed that this
approach constitutes a benchmark for the preparation and study of model proton
conducting systems and, furthermore, for the large-scale fabrication of membranes
suitable for different industrial applications. It is worth noting that the goal of these
proof-of-concept experiments is to provide a framework for future development in the
PEM research field, where perfluorinated polymers, with their economical and
environmental impacts, are still the materials of choice.
9.5.
9.5.1.
Experimental
Synthesis of initiator (1) for SI-ATRP
2 g of (3-aminopropyl)triethoxysilane and 1.13 g of triethylamine in 40 mL of dry dichloromethane
were mixed, stirred and gradually cooled to 0°C. A 50% by volume solution of 2.56 g of 2bromopropionyl bromide in dry dichloromethane was dropped in the reaction mixture over a period of
30 minutes and the reaction mixture was allowed to stir at 0°C for 6h under N2(g). The reaction mixture
was then filtered to remove the Et3N+Br– salt precipitated during the reaction and residue was washed
with small amount of dichloromethane. The filtrate was washed with brine (3 × 25 mL). The organic
phase was dried over MgSO4 and the solvent was removed in vacuo to yield 2-bromo-2-methyl-N-(3triethoxysilyl-propyl)-propionamide 1 as a colorless oil-like liquid (yield = 66%). 1H NMR (250 MHz,
CDCl3): δ 6.8 (1H, s), 3.76 (6H, q, J = 6.9), 3.21 (2H, q, J = 6.6Hz), 1.88 (6H, s), 1.57 (2H, m), 1.16
(9H, t, J = 6.9), 0.58 (2H, t, J =8 Hz).
9.5.2.
Anchoring 1 onto the surface of macroporous silica and subsequent PEB
growth by SI-ATRP
The plasma-activated macroporous silica membranes were placed in a Schlenk tube containing 5
µL of 1 in 10 mL of dry toluene at 120°C for 6h under N2(g). The membranes were then subjected to
ultrasonication for 5 minutes in each of toluene, ethanol and THF. After drying with a stream of N2(g),
the membranes were stored under N2(g) till further use.
PolyMPS brush by SI-ATRP
Thick sulfonate PEB was grown on macroporous silica functionalised with 1. The polymerization
was carried out using aqueous ATRP as reported in literature16: 17.29 g of the sulfonate monomer (3sulfopropylmethacrylate) was dissolved by stirring in 20 mL of methanol and 10 mL of water at room
temperature. To this solution 0.651 g of BiPy and 0.0114 g of Cu(II)Cl2 were added. The mixture was
stirred and degassed by N2(g) bubbling for an hour before 0.1648 g of Cu(I)Cl was added. The mixture
was degassed with N2(g) bubbling for another 15 minutes. Initiator-coated macroporous silica samples
were sealed in a Schlenk tube and degassed by four high vacuum pump/N2(g) refill cycles. The reaction
171
mixture was syringed into this Schlenk tube, adding enough to cover the sample completely, and the
mixture was left overnight under N2(g). The samples were removed and thoroughly rinsed with
deionized water. After the polymerization, the macroporous silica with PEB was extensively rinsed
with water and kept overnight in 0.01N aq. HCl for exchanging the K+ ions that originally coordinated
the monomer with H+.
PolyMPS-co-polyMeOEGMA brush by SI-ATRP
Thick polySPM-co-MeOEGMA was grown on macroporous silica functionalised with 1. The
polymerization was carried out using aqueous ATRP conditions: 5.3 (20 mmole) g of the sulfonate
monomer (3-sulfopropylmethacrylate) and MeOEGMA (290 µL, 0.3 g, 1 mmole for 5 mole % or 580
µL, 0.6 g, 2 mmole for 10 mole %) were dissolved by stirring in 8.7 mL of 2:1 methanol/water mixture
at room temperature. To this solution 191.5 mg g of BiPy and 3.5 mg of Cu(II)Cl2 were added. The
mixture was stirred and degassed by N2(g) bubbling for an hour before 48.5 mg of Cu(I)Cl was added.
polyMPS-co-MeOEGMA brush modified macroporous silicon membrane.
PolyAMPS brush by SI-ATRP
3 g of the sodium 2-acrylamino-2-methylpropane sulfonate was dissolved by stirring in 5.2 mL of
2:1 methanol/water solvent mixture at room temperature. To this solution 0.12 g of BiPy and 2.1 mg of
Cu(II)Cl2 were added. The mixture was stirred and degassed by N2(g) bubbling for an hour before 31
mg of Cu(I)Cl was added.
polyAMPS brush modified macroporous silicon membrane.
9.5.3.
Effective loading by weighing the macroporous silica membrane after each
step of functionalization
A pre-weighed piece of the initiator-modified porous silicon membrane was weighed again after
growing the PEB by SI-ATRP followed by extensive washing with water, exchange of K+ or Na+
counter ions with H+, drying with a stream of N2 and overnight storage under vacuum at room
temperature. The effective loading was determined from the weight gain during the functionalization of
the membrane with the polyelectrolyte brush.
9.5.4.
Ion exchange capacity
A pre-weighed piece of porous silicon-PEB hybrid membrane in -SO3H form was immersed in 15
mL of 2 mM solution of NaCl. After 24h the concentration of Na+ in the supernatant was reduced as
determined by atomic absorption spectroscopy (AAS). The ion exchange capacities of the membranes
were calculated from eq. 1.
 M − M2 
 × V
IEC =  1
 W

(1)
M1 and M2 are the molarities of the NaCl solution before and after immersing the porous silica-PEB
hybrid membrane, W is the weight of the membrane and V is the volume of the NaCl solution in which
the porous silica-PEB hybrid membrane were immersed.
9.5.5.
Water uptake study
Water uptake (WU) was measured gravimetrically (eq. 1). The porous silicon-PEB hybrid
membrane was weighed after exposing to 100% relative humidity (RH) for 24h (Wh). Subsequently the
membrane was allowed to dry first under ambient atmosphere (31% r.H.) followed by evacuating at
50°C for 2h and weighed (Wdry).
 Wh − Wdry 
 ×100
WU = 

W
h


(2)
9.5.6.
Proton Conductivity measurements
Please see chapter 2 for details.
9.6. References
1.
Jacobson, M. Z.; Colella, W. G.; Golden, D. M., Cleaning the Air and Improving Health with
Hydrogen Fuel-Cell Vehicles. Science 2005, 308, (5730), 1901-1905.
172
2.
Steele, B. C. H.; Heinzel, A., Materials for Fuel-Cell Technologies. Nature 2001, 414, (6861),
345-352.
3.
Kreuer, K. D., On the Development of Proton Conducting Polymer Membranes for Hydrogen
and Methanol Fuel Cells. Journal of Membrane Science 2001, 185, (1), 29-39.
4.
Kreuer, K. D.; Paddison, S. J.; Spohr, E.; Schuster, M., Transport in Proton Conductors for
Fuel-Cell Applications: Simulations, Elementary Reactions, and Phenomenology. Chemical Reviews
2004, 104, (10), 4637-4678.
5.
Diat, O.; Gebel, G., Proton Channels. Nature Materials 2008, 7, (1), 13-14.
6.
Schmidt-Rohr, K.; Chen, Q., Parallel Cylindrical Water Nanochannels in Nafion Fuel-Cell
Membranes. Nature Materials 2008, 7, (1), 75-83.
7.
Bussian, D. A.; O'Dea, J. R.; Metiu, H.; Buratto, S. K., Nanoscale Current Imaging of the
Conducting Channels in Proton Exchange Membrane Fuel Cells. Nano Letters 2007, 7, (2), 227-232.
8.
Kannan, R.; Kakade, B. A.; Pillai, V. K., Polymer Electrolyte Fuel Cells Using Nafion-Based
Composite Membranes with Functionalized Carbon Nanotubes. Angewandte Chemie-International
Edition 2008, 47, (14), 2653-2656.
9.
Hickner, M. A.; Ghassemi, H.; Kim, Y. S.; Einsla, B. R.; McGrath, J. E., Alternative Polymer
Systems for Proton Exchange Membranes (Pems). Chemical Reviews 2004, 104, (10), 4587-4611.
10.
Tsang, E. M. W.; Zhang, Z.; Shi, Z.; Soboleva, T.; Holdcroft, S., Considerations of
Macromolecular Structure in the Design of Proton Conducting Polymer Membranes: Graft Versus
Diblock Polyelectrolytes. Journal of the American Chemical Society 2007, 129, (49), 15106-+.
11.
Wainright, J. S.; Wang, J. T.; Weng, D.; Savinell, R. F.; Litt, M., Acid-Doped
Polybenzimidazoles - a New Polymer Electrolyte. Journal of the Electrochemical Society 1995, 142,
(7), L121-L123.
12.
Yamaguchi, T.; Zhou, H.; Nakazawa, S.; Hara, N., An Extremely Low Methanol Crossover
and Highly Durable Aromatic Pore-Filling Electrolyte Membrane for Direct Methanol Fuel Cells.
Advanced Materials 2007, 19, (4), 592-+.
13.
Argun, A. A.; Ashcraft, J. N.; Hammond, P. T., Highly Conductive, Methanol Resistant
Polyelectrolyte Multilayers. Advanced Materials 2008, 20, (8), 1539-+.
14.
DeLongchamp, D. M.; Hammond, P. T., Fast Ion Conduction in Layer-by-Layer Polymer
Films. Chemistry of Materials 2003, 15, (5), 1165-1173.
15.
DeLongchamp, D. M.; Hammond, P. T., Highly Ion Conductive Poly(Ethylene Oxide)-Based
Solid Polymer Electrolytes from Hydrogen Bonding Layer-by-Layer Assembly. Langmuir 2004, 20,
(13), 5403-5411.
16.
Lutkenhaus, J. L.; Hammond, P. T., Electrochemically Enabled Polyelectrolyte Multilayer
Devices: From Fuel Cells to Sensors. Soft Matter 2007, 3, (7), 804-816.
17.
Birner, A.; Wehrspohn, R. B.; Gosele, U. M.; Busch, K., Silicon-Based Photonic Crystals.
Advanced Materials 2001, 13, (6), 377-388.
18.
Fan, R.; Huh, S.; Yan, R.; Arnold, J.; Yang, P. D., Gated Proton Transport in Aligned
Mesoporous Silica Films. Nature Materials 2008, 7, (4), 303-307.
19.
Lehmann, V.; Foll, H., Formation Mechanism and Properties of Electrochemically Etched
Trenches in N-Type Silicon. Journal of the Electrochemical Society 1990, 137, (2), 653-659.
20.
Azzaroni, O.; Brown, A. A.; Huck, W. T. S., Ucst Wetting Transitions of Polyzwitterionic
Brushes Driven by Self-Association. Angewandte Chemie-International Edition 2006, 45, (11), 17701774.
21.
Azzaroni, O.; Moya, S.; Farhan, T.; Brown, A. A.; Huck, W. T. S., Switching the Properties of
Polyelectrolyte Brushes Via "Hydrophobic Collapse". Macromolecules 2005, 38, (24), 10192-10199.
22.
Farhan, T.; Azzaroni, O.; Huck, W. T. S., Afm Study of Cationically Charged Polymer
Brushes: Switching between Soft and Hard Matter. Soft Matter 2005, 1, (1), 66-68.
23.
Advincula, R., Surface-Initiated Polymerization I. Springer-Verlag: Heidelberg, 2006.
24.
25.
Cai, Y. L.; Armes, S. P., Synthesis of Well-Defined Y-Shaped Zwitterionic Block Copolymers
Via Atom-Transfer Radical Polymerization. Macromolecules 2005, 38, (2), 271-279.
26.
Vo, C. D.; Schmid, A.; Armes, S. P.; Sakai, K.; Biggs, S., Surface ATRP of Hydrophilic
Monomers from Ultrafine Aqueous Silica Sols Using Anionic Polyelectrolytic Macroinitiators.
Langmuir 2007, 23, (2), 408-413.
27.
Ramstedt, M.; Cheng, N.; Azzaroni, O.; Mossialos, D.; Mathieu, H. J.; Huck, W. T. S.,
Synthesis and Characterization of Poly(3-Sulfopropylmethacrylate) Brushes for Potential Antibacterial
Applications. Langmuir 2007, 23, (6), 3314-3321.
173
28.
Biesalski, M.; Ruhe, J., Swelling of a Polyelectrolyte Brush in Humid Air. Langmuir 2000,
16, (4), 1943-1950.
29.
Tredgold, R. H., Order in Thin Organic Films. Cambridge University Press: Cambridge,
1994.
30.
Kluth, G. J.; Sander, M.; Sung, M. M.; Maboudian, R. In Study of the Desorption Mechanism
of Alkylsiloxane Self-Assembled Monolayers through Isotopic Labeling and High Resolution Electron
Energy-Loss Spectroscopy Experiments, 1998; 1998; pp 932-936.
31.
Miyatake, K.; Watanabe, M., Recent Progress in Proton Conducting Membranes for Pefcs.
Electrochemistry 2005, 73, (1), 12-19.
32.
Noda, A.; Susan, A. B.; Kudo, K.; Mitsushima, S.; Hayamizu, K.; Watanabe, M., Bronsted
Acid-Base Ionic Liquids as Proton-Conducting Nonaqueous Electrolytes. Journal of Physical
Chemistry B 2003, 107, (17), 4024-4033.
33.
Schuster, M. E.; Meyer, W. H., Anhydrous Proton-Conducting Polymers. Annual Review of
Materials Research 2003, 33, 233-261.
34.
Hooker, E., Final Report of the Amended Safety Assessment of Peg-5, -10, -16, -25, -30, and 40 Soy Sterol. International Journal of Toxicology 2004, 23, 23-47.
35.
Lanigan, R. S.; Cosmetic Ingredient Review, E., Final Report on the Safety Assessment of
Peg-20 Sorbitan Cocoate; Peg-40 Sorbitan Diisostearate; Peg-2,-5, and-20 Sorbitan Isostearate; Peg-40
and-75 Sorbitan Lanolate; Peg-10,-40,-44,-75, and-80 Sorbitan Laurate; Peg-3, and-6 Sorbitan Oleate;
Peg-80 Sorbitan Palmitate; Peg-40 Sorbitan Perisostearate; Peg-40 Sorbitan Peroleate; Peg-3,-6,-40,
and-60 Sorbitan Stearate; Peg-20,-30,-40, and-60 Sorbitan Tetraoleate; Peg-60 Sorbitan Tetrastearate;
Peg-20 and-160 Sorbitantriisostearate; Peg-18 Sorbitan Trioleate; Peg-40 and-50 Sorbitol Hexaoleate;
Peg-30 Sorbitol Tetraoleate Laurate; and Peg-60 Sorbitol Tetrastearate - Addendum to the Final Report
on the Safety Assessment of Polysorbates. International Journal of Toxicology 2000, 19, 43-89.
36.
Ito, A., Dehumidification of Air by a Hygroscopic Liquid Membrane Supported on Surface of
a Hydrophobic Microporous Membrane. Journal of Membrane Science 2000, 175, (1), 35-42.
37.
Rivas-Cardona, J. A.; Banerjee, D., Microfluidic Device for Delivery of Multiple Inks for Dip
Pen Nanolithography. Journal of Micro-Nanolithography Mems and Moems 2007, 6.
38.
Rodrigues, J.; Parra, D. F.; Lugao, A. B., Crystallization on Films of Phb/Peg Blends. Journal
of Thermal Analysis and Calorimetry 2005, 79, (2), 379-381.
39.
Anantaraman, A. V.; Gardner, C. L., Studies on Ion-Exchange Membranes .1. Effect of
Humidity on the Conductivity of Nafion(R). Journal of Electroanalytical Chemistry 1996, 414, (2),
115-120.
174
Chapter 10
10. Stimuli responsive artificially fabricated solidstate nanopores
(a)
solid-state
nanopore
Initiator surface group
Surface-Initiated Atom Transfer
(b)
thermally
nanoactuated
molecular gate
Poly(N-isopropylacrylamide)
Brushes
-
H+
+ H+
Current /nA
200
pH 2
pH 10
100
0
-100
-200
-2
-1
0
1
2
Transmembrane Potential /V
Mimicking the ever-versatile ever-inspiring nature: Exploiting a combination of elements
from macromolecular and materials research fields 1) thermally actuated macromolecularbased nanogates, and 2) ionic gates displaying pH tunable rectification were fabricated.
Results obtained and the potential of the developed system are discussed in detail.
175
In this chapter the contributions regarding stimuli responsive artificially fabricated
solid-state nanopores are presented. More specifically, this includes artificially
fabricated a) thermally actuated macromolecular-based nanogates, and b) ionic gates
displaying pH tunable rectification. A combination of elements from macromolecular
and materials research fields were exploited in order to mimic the ever-versatile everinspiring nature. The results presented here were achieved by confining a highly
resourceful discipline of surface initiated polymerization (SIP) inside the unique
scaffold of track etched solid-state nanopores.
10.1. Ionic transport through single solid state nanopore controlled with
thermally nanoactuated macromolecular gate
10.1.1.
Nature: The Inspiration I
Biological nanopores acting as membrane channels play a determinant role in all
living systems. They operate as very sensitive devices in charge of regulating key
functions such as electric potential, ionic flow, and molecular transport through the
membranes.1-4 Ion channels of biological membranes are formed by pore-like single
proteins that span cell membranes and open and close in response to stimuli like
changes in the transmembrane potential, binding of a ligand, or mechanical stress.5, 6
These stimuli-sensitive biological building blocks embedded into the membrane
enable the modulation of ion transport through the protein channel.5,
6
They are
responsible for providing the conducting pathways for the ionic species. When they
are in an open conformation, ions pass through the pore, and when they are resting in
a closed, nonconducting conformation, ion transport is precluded.7,
8
This indicates
that nanopore channels change their conformations between different states in order to
enable or prohibit the transport of species through the biological membrane, thus
acting as a gate. In addition, the conformational transition between the states is
usually referred to as gating. Hence, these biological channels work as nanodevices
that control the molecular transport through a physical interface.9 Biological
nanopores, such as α-hemolysin, provide unique features in regard to single molecule
sensitivity;10 however they are labile and difficult to handle in environments different
from the lipid membrane and not fully compatible with nanodevice systems.2 This
explains the growing interest in developing robust chemical devices that mimic the
function of biological ion channels.11-14 The possibility of creating fully artificial
176
(“man-made”) nanogating devices is one of the remaining challenges in
nanotechnology,15-19 in which the generation of bioinspired switchable and easily
controllable molecular gates would provide new tools for the creation of complex
nanomachineries. In this context, solid-state nanopores have demonstrated to be a
robust alternative to biological nanopores.20-29 Besides, they also offer better control
over channel geometry and compatibility with the integration into functional
systems.25 However, controlling the internal architecture and the chemical features of
synthetic nanopores, as precisely as α-hemolysin or other biological ion channels, is a
non-trivial task.
This introduces a new nanotechnological demand focused on the quest for novel
alternative switchable nanopore machineries capable of being “nanoactuated” by
external stimuli in a controllable manner. In this regard, one stimulus of particular
interest in biological and non-biological systems is temperature.30 Biological ionic
channels activated by temperature changes transduce this information into
conformational changes that open/close the channel pore. A typical example is
thermosensation that is carried out by the direct activation of thermally-gated ion
channels in the surface membranes of sensory neurones.31 This complex task is
accomplished by temperature-sensitive ion channels which are members of the
extensive TRP family (transient receptor potential channels).32 Another good example
concerns N-methyl-D-aspartate (NMDA) receptors which are glutamate-gated Ca2+
permeable ion channels constituted by heteromultimers involving NR1 and NR2
subunits activated by glutamate and glycine. Recently, Cais et al. demonstrated that
these subunits undergo significant conformational transitions between 21.9–46.5°C,
which could have strong implications on the neuronal excitability at physiological
temperatures.33 With the inspiration of examples from nature, the scientific
community started to build-up biomimetic thermosensitive pores, as is the case of
Movileanu and co-workers who created temperature-responsive protein pores
containing elastin-like polypeptide (ELP) loops.34 These ELP loops were placed
within the cavity of the lumen of the α-hemolysin pore. Below the transition
temperature the ELP loop is fully expanded and blocks the pore, above its transition
temperature the ELP is dehydrated and the structure collapses, enabling a substantial
flow of ions through the pore.
177
In this context, the application of solid-state nanopores modified with
thermoresponsive brushes as molecular gates nanoactuated by temperature-driven
conformational transitions is described herein. The creation of “fully artificial” smart
nanopores with tunable nanoscale diameters controlled by temperature changes in the
physiological range is demonstrated. These results demonstrate the huge potential of
“soft nanotechnology”35,
36
to emulate and replicate complex biological functions
using soft matter-based man-made systems.
10.1.2.
Grafting of polyNIPAAm brush at the surface of PI membrane
bearing a single conical nanopore via SI-ATRP
Solid-state single conical nanopores used in this study were fabricated in polyimide
(PI) membranes by asymmetric chemical etching of the latent track of an energetic
heavy ion.20 After etching, carboxyl groups were generated on the nanopores surface,
which were then converted into amino groups by covalent linkage of diamine using
conventional EDC/PFP coupling chemistry.37 Next, the single pore-containing
membrane was modified with the thermo-responsive brushes. This was easily
accomplished by using SI-ATRP38,
39
of N-isopropylacrylamide (NIPAAm) under
aqueous condition leading to the formation of a dense polymer layer (polyNIPAAm
brush) covalently tethered at one end to the nanopore sidewall (Figure 10.1). The
solvents used in the aqueous ATRP are fully compatible with the membrane material,
thus avoiding any detrimental effect on the single pore characteristics, i.e.: closure,
due to the swelling of the PI. Firstly, the membrane having a single aminated channel
was modified with the initiator groups. Secondly, the polymer brush was grown
(Figure 10.1) by immersing the initiator-modified membrane into the corresponding
polymerization solution under conditions described elsewhere (see experimental
section for details).40 The choice of a polyNIPAAm based thermoresponsive system
was motivated by the fact that it requires a simple and widely available monomer and
that its temperature-driven conformational changes are well-documented in the
literature.41-43 After polymerization, the brushes were extensively rinsed with water
and methanol for removing the polymerization solution and kept under water prior to
performing the thermoactuated gating experiments.
178
(a)
solid-state
nanopore
Initiator surface group
Surface-Initiated Atom Transfer
(b)
thermally
nanoactuated
molecular gate
Poly(N-isopropylacrylamide)
Brushes
Figure 10.1: Scheme illustrating the surface modification of the nanopore in PI membrane by surface
initiated ATRP resulting in polyNIPAAm brushes. Firstly, the aminated pore wall is modified with the
initiator groups (a). Then, the aqueous SI-ATRP is carried out (b). The figure also describes the
chemical structures of the ATRP initiator and the polyNIPAAm brushes.
10.1.3.
Pore dimensions
The track etching technique allows control over the shape of the nanopores, and in
our experiments the etched single nanopore was conelike. Its large opening (base) was
usually ~ 1.0 to 1.5 micrometer, and the narrow opening a few tens of nanometers.
Diameter measurements of single conical nanopore were conducted with a commonly
used electrochemical measurement.25 An electrochemical method described by Apel
et al.44 was used to calculate the tip diameter, d, of the single conical nanopores. The
method is described in chapter 2 Materials and methods.
10.1.4.
Thermally induced nano-actuation read by current-voltage (I-V)
curve measurements
The PI membrane containing single conical nanopore was mounted between two
halves of the electrochemical cell. An electrode was immersed in each half of the cell,
which were filled with 1M KCl solution. At neutral pH, the surface of PI nanopore
fabricated by track etch method is characterized by negative charge inherent with the
resulting surface carboxylate groups. This results in conical nanopore rectifying at
positive
transmembrane
potential
(Figure
10.2).
After
modification
with
179
ethylenediamine, the pore walls were positively charged (due to the protonated amino
groups) which resulted in the inversion of rectification as shown in Figure 10.2.
400
Current /nA
300
200
100
0
-100
-200
-2
-1
0
1
2
Voltage /V
Figure 10.2: Current-Voltage characteristics of a polyimide (PI) conical nanopore in 1M KCl having
d ~ 48 nm and D ~ 1.45 µm, prior to (●) and after (●)modification with ethylenediamine. The terms d
and D refer to the diameters of the small and large opening of the pore, respectively.
The gating performance of the polyNIPAAm brush-modified nanopore (d ~ 8 nm)
was studied using the same experimental setup by measuring the ionic current across
the channel at different temperatures (Figure 10.3).
150
Current /nA
100
50
0
o
23 C
-50
o
32 C
-100
o
40 C
-150
-2
-1
0
1
2
Voltage /V
Figure 10.3: I-V curves in 1M KCl for a polyimide (PI) conical nanopore after modification with
polyNIPAAm brushes at different temperatures.
Figure 10.3 shows the I-V curves of single PI nanopore modified with
polyNIPAAm brushes obtained at different temperatures. PolyNIPAAm brushes
neutralize the surface charge of the pores resulting in the loss of the rectifying
behaviour and, consequently, the pore exhibits a linear current-voltage (I-V)
characteristic (Figure 10.3). At room temperature (23°C) polyNIPAAm brushes
remain swollen, thus decreasing the effective cross-section of the nanopore. This is
described by the low slope of the I-V curve which is associated with a low
180
conductance of the nanopore, 17 nS. Rising the temperature above the lower critical
solubility temperature promotes drastic changes on the conformational state of the
polyNIPAAm brushes. In this case, the brushes suffer a transition into a collapsed
state,41-43 which also has an impact on the effective diameter of the nanopore (Figure
10.4).
Swollen Brush
T < LCST
Collapsed Brush
T >LCST
Heating up
dT> LCST
dT< LCST
Cooling down
Figure 10.4: Cartoon describing the thermally-driven nanoactuation of the polyNIPAAm brushes in the
nanopore.
The conformational transition into a collapsed and more compact state promotes
the widening of the nanopore which is evidenced by an increase in conductance (76
nS), as derived from the slope of the I-V plots at 40°C. Regarding this latter, it is
worth mentioning that the increased slope of the I-V plots could be also attributed to
changes in the specific conductivity of the electrolyte.45 In order to estimate the
contributions to the conductance coming from the specific conductivity changes due
to temperature-driven actuations45 the nanopore conductance was estimated
considering the corrected values of the specific conductivity (Table 10.1),
GT =
κ Tπ D d T
4L
[1]
The superscript T indicates the parameters estimated at temperature T. In
expression [1] it is assumed that the effective diameter of the large opening (base) and
the thickness of the PI membrane film are not sensitively affected by the
conformational changes of the polyNIPAAm brushes. In this context, a more realistic
estimation of the variation on the effective cross section of the nanopore is given by:
o
C
o
C
G 23
G 40
o
o
o
o
κ 23 C d 23 C 17 nS
= 40 C 40 C =
76 nS
κ
d
[2]
181
Considering that the specific conductivity of 1M KCl at 23 and 40 °C is 0.1073
and 0.1417 S·cm, respectively, the effective diameter of the nanopore changed was
estimated from the electrochemical measurements and was found to be:
d 40
o
o
C
d 23 C
~ 3.4
[3]
These results clearly evidence the nanoactuating behaviour of the polyNIPAAm
brush that above the LCST undergoes a sharp change in its conformational state
leading to a 3-fold increase of the effective cross-section of the nanopore. The
thermoresponsive brush is acting as a thermally-driven macromolecular gate
controlling the ionic flow through the nanopore. The very sensitive nature of the
thermoactuated nanopore is also evidenced when studied at temperatures near the
LCST. Figure 10.3 depicts the I-V plots for the polyNIPAAm-modified pore
measured at 32°C. In that case a very stable I-V curve was recorded with a slope
corresponding to a nanopore conductance of 56 nS, which is in between 17 nS (23°C)
and 76 nS (40°C). This fact further evidences the molecular-level control of the gating
process in the nanoconfined environment driven by the fine tuning of the temperature.
In other words, temperature variations in the physiological range (23-40°C) can lead
to an accurate control of the macromolecular nanogate through temperature-driven
intermediate conformational states (Table 10.1).
Table 10.1 Changes in conductance (G), specific conductivity
(κ), pore opening and pore diamemter (d) upon variations in
temperature
dT
T /°C
G /nS
κ/ Scm-1
d 23 C
d (nm)
23
17
0.1073
1
~8
32
56
0.1252
2.8
~ 22
40
76
0.1417
3.4
~ 27
o
Summarizing this part of the chapter, the combined use of thermoresponsive
polymer brushes together with single solid-state nanopores in order to create fully
artificial stimuli-responsive nanopore, which resemble those commonly encountered
in nature was described. It was demonstrated that the polyNIPAAm-modified
182
nanopore act as thermally-driven molecular gates with closure stages that can be
remotely controlled by simply tuning the working temperature in the 23-40°C range.
Achieving a delicate control of the nanoactuating characteristics of this molecular
device in the physiological temperature range is an interesting feature that could be of
much interest for the implementation of artificial nanosystems propelled and/or
controlled by temperature variations in living systems, like human beings. In this
context, one interesting application could be in the area of biomedical science and
nanomedicine through the implementation of these fully “abiotic” nanogating devices
as “intelligent” pores enabling the temperature-tuned release of drugs.
10.2. Single
conical
nanopores
displaying
pH-tunable
rectifying
characteristics. Manipulating ionic transport with zwitterionic
polymer brushes
10.2.1.
Nature-The Inspiration II
Single conical nanopores are able to rectify the ion transport flowing through them,
which is in close resemblance to voltage-gated biological ion channels.25, 46 One of the
remarkable aspects of the ever-growing field of nanotechnology relies on the creation
of architectures with length scales and functional features comparable to biological
machineries.21, 47 The progress in this field is enabled by the reproducible fabrication48
of
synthetic
nanopores
displaying properties
that
mimic their biological
counterparts.18
In this context, generating “fully synthetic” architectures with functionalities
comparable to biological entities triggered the interest of scientific community related
to diverse research fields, including life sciences, chemistry and applied biophysics.
One of the central features that determine the rectifying characteristics of the conical
nanopores is the nanoscale control over the surface charge properties of the pore
walls.27 As a consequence, finding new avenues to manipulate the surface charges of
conical nanopores is of paramount importance to further expand the potentialities of
these nanosized systems.15, 17, 20, 27, 29, 49, 50 The development of functionalized conical
nanopores with “smart” properties leading to the modulation of the fixed charges
provides an exciting new approach to gain control over the ion transport through the
nanopores. Controlling the permeation through membranes with pH-tunable moieties
has attracted considerable attention during recent years.51 Seminal work of Martin and
183
co-workers described the pH-switchable permselectivity of cylindrical cysteinemodified gold-coated nanotubules demonstrating the huge potential of zwitterionic
moieties to tailor the surface charge properties of confined environments.52 On the
contrary, in spite of its relevance, little is known about the behaviour of rectifying
conical nanopores bearing tailorable surface charges. In addition, the tailoring of the
surface charge in single conical nanopores has been exclusively performed using
monolayer assemblies. Within this framework, it has to be noted that polymer brushes
provide a versatile toolbox to molecularly design interfaces with nanoscale control
being applicable to a plethora of “smart” chemical functionalities.53-55 More
important, considering that the degree of rectification is highly dependent on the
surface charge density,49 polymer brushes provide a simple mean to sensitively
increase the number of fixed charges on the nanopore walls. These particular features
of polymer brushes merged with the transport properties of conical nanopores would
open the door to completely new signal-responsive chemical nanodevices. In this
work, it is demonstrate that the growth of zwitterionic polymer brushes provides a
useful strategy to finely tune the rectifying characteristics of the nanopores and,
consequently, to manipulate the mass or ion transport through them by simply varying
the environmental pH.
184
10.2.2.
Grafting of poly-L-lysine brush at the surface of PI membrane
bearing
a
single
conical
nanopore
via
surface
initiated
conventional radical polymerization and current-voltage (I-V)
curves measurement
Poly(methacryoyl-L-Lysine)
Brushes
pH-dependent
surface charge
pH-tunable
pore wall
characteristics
Figure 10.5: Schematic cartoon describing the polymer brush-modified conical nanopore. The
chemical structure of the polymer brush and the equilibrium associated to the pH-dependent behaviour
of the zwitterion in monomer units are also indicated.
Polyimide (PI) membrane bearing track etched single conical nanopore, with a tip
diameter (d) of ~20 nm and a base diameter (D) of ~1.65 µm, was modified with 4,4´azobis(4-cyanopentanoic
acid)
as
surface-initiated
conventional
radical
polymerization initiator (see the experimental section for details).56, 57 Afterwards, the
brush growth was accomplished by surface-initiated free radical polymerization of the
zwitterionic monomer methacryloyl-L-lysine (Figure 10.5). After a 2h of
polymerization time the membranes were thoroughly rinsed with Milli-Q water and
mounted between two halves of a conductivity cell. The surface-initiated
polymerization resulted in a polymer brush of ~ 7 nm in thickness (d ~ 13 nm).
Figure 10.6 depicts I-V curves of a single conical nanopore in PI modified with
polyzwitterionic brushes using 1M KCl (at different pHs) as electrolyte solution in
both half cells. Considering the zwitterionic nature of the monomer units (Figure
10.5) it can be inferred that at strongly acidic pHs the monomer units covalently
tethered to the pore wall will be bearing positively charged groups corresponding to –
185
NH3+. As is well known, the presence of rectification requires surface charges.27 In
present case it was observed that at pH 2 the I-V curve displayed a well-defined
rectification behavior which would imply the permselective transport of anionic
species through the positively charged nanopore (Figure 10.6). The degree of
rectification (frec),49 defined as the ratio between currents measured at voltages of the
same amplitude but opposite polarities, was 22. At this point, it is worth mentioning
that these results are similar to those previously reported in the literature using
positively charged nanopores except for the higher degree of rectification, as
compared with typical values for conical nanopores modified with monolayer
assemblies, frec ~ 5.
200
pH 2
pH 4
pH 5
pH 6
pH 8
pH 10
-100
-200
-300
300
-300
250
-250
200
-200
150
-150
100
-100
50
-50
0
Current@-2V /nA
0
Current@2V /nA
Current /nA
100
0
0
2
-400
4
6
8
10 12
pH
-2
-1
0
1
2
Transmembrane Potential /V
Figure 10.6: I-V curves corresponding to a single conical nanopore modified with poly(methacryloylL-lysine) brushes measured at different pH values (using 1M KCl as electrolyte). The different pHs are
displayed using colored symbols as indicated in the figure. The inset describes the changes in the
rectified currents upon variation in the environmental pH. The red and blue dots refer to the rectified
currents measured at -2 and 2 V, respectively.
The next goal was to achieve a fine tuning of the surface charges in order to
manipulate the ionic transport through the pore. As described in Figure 10.5, the
equilibrium of the zwitterionic units involves a variety of charged states that are
thermodynamically controlled by the pH value. So, the acidic pH was varied from 2 to
4, where the population of charged “bipolar” monomer units grew at the expense of
the –NH3+ species resulting in a “less positive” surface charge. The I-V curve
indicated that at pH 4 the well-defined rectification behaviour was still observed, but
in this case the rectified current had decreased. This variation in pH led to a
significant decrease in frec from 22 to 9. This fact clearly indicates that the zwitterionic
186
brush enables the tuning of the rectified current under the same permselective
conditions. Then, increasing the pH to 5 was evidenced by the loss of the rectification
behaviour displayed by the pore. At pH 5 the isoelectric point (pI) of the zwitterionic
brush was reached, where the net charges are zero, thus explaining the nearly resistive
behaviour of the I-V response (Figure 10.6) typically observed in non-charged conical
nanopores.25,
46
Increasing the pH above pI promoted the displacement of the
zwitterionic equilibrium towards the formation of negatively charged species (COO-),
thus reversing the permselectivity and rectification characteristics observed at acidic
pH values. Changing the pH from 6 to 10 displayed a sensitive variation in the
rectified currents measured at +2 V from +87 to +187 nA, while the currents sensed at
–2 V remained almost the same. In terms of frec the pH variation promoted a change
from 5 to 17 in the rectification efficiency. This observation further indicates that,
even in reversed permselective conditions, variations in pH can lead to the fine tuning
of the rectification characteristics.
Summarizing: it was shown for the first time the integration of polymer brushes
into conical single nanopore in order to obtain highly functional chemical
nanodevices. The experimental evidences describing the use of zwitterionic brushes,
and exploiting the pH-dependent chemical equilibrium of their monomer units,
demonstrate that the fine tuning of the ionic transport by presetting the environmental
pH is achievable and enables a higher degree of control over the ion transport
properties of the system. It is envisioned that these results will pave the way to new
“smart” nanodevices based on the interplay between the chemical richness of polymer
brushes and the remarkable physical characteristics of conical nanopores.
10.3. Experimental Section
10.3.1.
Functionalization of nanopore bearing PI surface with ethylenediamine
For the activation of the carboxyl groups into pentafluorophenyl esters, an ethanol solution
containing 0.1 M EDC and 0.2 M PFP was placed on both sides of the track-etched polymer
membrane. The reaction was carried out for 60 minutes at room temperature. After washing with
ethanol several times, the solution was replaced with 0.1 M ethylendiamine (EDA) on both sides of the
membrane for overnight. Finally, chemically modified membranes were washed three times with
ethanol followed by distilled water.
10.3.2.
Immobilization of ATRP initiator on the membrane:
A solution of 2-bromoisobutyryl bromide (BIBr) (0.185 mL, 3 mmol) and triethylamine (0.205 mL,
3 mmol) in dry dichloromethane (30 mL) was injected over the membranes with surface -NH2 groups
under N2 at room temperature and left for 2h. The membranes were washed with dichloromethane
followed by absolute ethanol, and dried under a stream of N2.
187
10.3.3.
PolyNIPAAm brush growth
The polymerization was carried out using aqueous ATRP as reported in literature with slight
modifications. Briefly, NIPAAM (2.5 g, 22.1 mmole) was dissolved by stirring in 5 mL of methanol
and 5 mL of water at room temperature. To this solution PMDETA (0.138 g, 0.8 mmole) was added.
The mixture was stirred and degassed by N2(g) bubbling for an hour before Cu(I)Br (0.032 g, 0.22
mmole) was added. The mixture was degassed with N2(g) bubbling for another 15 minutes. The
membrane was sealed in a Schlenk tube and degassed by four high vacuum pump/N2(g) refill cycles.
The reaction mixture was syringed into this Schlenk tube, adding enough to cover the membrane
completely, and the mixture was left overnight under N2(g). The samples were removed and thoroughly
rinsed with deionized methanol and water.
10.3.4.
Anchoring 4,4′-azobis(4-cyanopentanoic acid) on the surface of PI
membrane with single conical nanopore
The surface modification of the PI membrane with azo initiator groups was accomplished following
the procedure reported by Bruening and co-workers with only minor modifications.2 Briefly, 0.5g (1.78
mmole) of 4,4′-Azobis(4-cyanopentanoic acid) and 0.92g (4.5 mmole) of DCC were added to a single
neck Schlenk flask and closed with a rubber septum. The reactants were degassed under vacuum for 15
minutes followed by back filling with N2(g). 40 mL of dry DMF was added to the flask through the
septum with the help of a syringe and the reactants were allowed to dissolve. After complete
dissolution 0.13 mL of dry pyridine was added. PI membrane with a single conical nanopore was
sealed in a Schlenk tube and degassed (4 × high vacuum pump/N2 refill cycles). Then, the reaction
mixture was syringed into the Schlenk flask containing the azo-modified membrane and left overnight
under N2(g) at room temperature. Finally, the membrane was removed from the reaction mixture and
immersed in a beaker containing DMF. The beaker was gently shaken over a period of 2h. The
membrane was then washed twice with DMF followed by washing with water and ethanol. The
initiator functionalized membrane was store under nitrogen below 4 °C until further use.
10.3.5.
Poly-L-Lysine brush growth on initiator functionalized single nanoporecontaining PI membrane
In a 50 mL Schlenk flask, 2.5 g of the monomer (N-methacryloyl-L-lysine) was dissolved in 36 mL
of aqueous HCl solution (pH 2.5). The solution was degassed by N2(g) bubbling for 1h. The initiatorfunctionalized membrane was sealed in a Schlenk flask and degassed (4 × high vacuum pump/N2 refill
cycles). The monomer solution was syringed into the Schlenk flask containing the membrane. Then,
the flask was immersed in an oil bath preheated at 65°C and the polymerization was carried out during
45 min. The PI membrane was then removed from the polymerization solution and left overnight in
aqueous HCl solution with pH of 2.5. The following day the membrane was gently rinsed with
abundant Milli Q water. The poly-L-lysine brush-modified membrane was stored in water at room
temperature until further use. To evaluate the thickness of the brush grown inside the nanopore, the
diameter of the small opening of the conical nanopore was estimated prior to (d ~ 20 nm) and after (d ~
13 nm) the surface-initiated polymerization, thus obtaining a brush thickness of ~ 7 nm.
10.4. References
1.
Banghart, M. R.; Volgraf, M.; Trauner, D., Engineering Light-Gated Ion Channels.
Biochemistry 2006, 45, (51), 15129-15141.
2.
D. Voet, J. D. V., Biochemistry. John Wiley & Sons: New York, 1995.
3.
Perozo, E.; Cortes, D. M.; Sompornpisut, P.; Kloda, A.; Martinac, B., Open Channel Structure
of Mscl and the Gating Mechanism of Mechanosensitive Channels. Nature 2002, 418, (6901), 942-948.
4.
S. Sugawara, H. S., T. Ozawa, Y. Umezawa Frontiers in Biosensorics I: Fundamental
Aspects. Birkhäuser Verlag: Basel, 1997.
5.
G.G. Cross, T. M. F., P.J. Montoya-Pelaez, W.F. van Straaten-Nilenhuis, X. Zhou Interfacial
Design and Chemical Sensing. American Chemical Society: Washington, 1994.
6.
H. Bailey, O. B., S. Cheley, L.-Q. Gu Nanobiotechnology: Concepts, Applications and
Perspectives. VCH-Wiley: Weinheim, 2004.
7.
Goychuk, I.; Hanggi, P., Ion Channel Gating: A First-Passage Time Analysis of the Kramers
Type. Proceedings of the National Academy of Sciences of the United States of America 2002, 99, (6),
3552-3556.
8.
Xia, F.; Guo, W.; Mao, Y. D.; Hou, X.; Xue, J. M.; Xia, H. W.; Wang, L.; Song, Y. L.; Ji, H.;
Qi, O. Y.; Wang, Y. G.; Jiang, L., Gating of Single Synthetic Nanopores by Proton-Driven DNA
Molecular Motors. Journal of the American Chemical Society 2008, 130, (26), 8345-8350.
188
9.
H.T. Tien, A. O., Bioelectrochemistry. VCH-Wiley: Weinheim, 2002.
10.
Bayley, H.; Cremer, P. S., Stochastic Sensors Inspired by Biology. Nature 2001, 413, (6852),
226-230.
11.
Gershow, M.; Golovchenko, J. A., Recapturing and Trapping Single Molecules with a SolidState Nanopore. Nature Nanotechnology 2007, 2, (12), 775-779.
12.
Iqbal, S. M.; Akin, D.; Bashir, R., Solid-State Nanopore Channels with DNA Selectivity.
Nature Nanotechnology 2007, 2, (4), 243-248.
13.
Li, J.; Stein, D.; McMullan, C.; Branton, D.; Aziz, M. J.; Golovchenko, J. A., Ion-Beam
Sculpting at Nanometre Length Scales. Nature 2001, 412, (6843), 166-169.
14.
Li, J. L.; Gershow, M.; Stein, D.; Brandin, E.; Golovchenko, J. A., DNA Molecules and
Configurations in a Solid-State Nanopore Microscope. Nature Materials 2003, 2, (9), 611-615.
15.
Daiguji, H.; Yang, P. D.; Majumdar, A., Ion Transport in Nanofluidic Channels. Nano Letters
2004, 4, (1), 137-142.
16.
Karnik, R.; Castelino, K.; Fan, R.; Yang, P.; Majumdar, A., Effects of Biological Reactions
and Modifications on Conductance of Nanofluidic Channels. Nano Letters 2005, 5, (9), 1638-1642.
17.
Karnik, R.; Fan, R.; Yue, M.; Li, D. Y.; Yang, P. D.; Majumdar, A., Electrostatic Control of
Ions and Molecules in Nanofluidic Transistors. Nano Letters 2005, 5, 943-948.
18.
Sisson, A. L.; Shah, M. R.; Bhosale, S.; Matile, S., Synthetic Ion Channels and Pores (20042005). Chemical Society Reviews 2006, 35, 1269-1286.
19.
van der Heyden, F. H. J.; Bonthuis, D. J.; Stein, D.; Meyer, C.; Dekker, C., Power Generation
by Pressure-Driven Transport of Ions in Nanofluidic Channels. Nano Letters 2007, 7, (4), 1022-1025.
20.
21.
2008, 3, (2), 73-74.
22.
Dekker, C., Solid-State Nanopores. Nature Nanotechnology 2007, 2, (4), 209-215.
23.
Harrell, C. C.; Kohli, P.; Siwy, Z.; Martin, C. R., DNA - Nanotube Artificial Ion Channels.
24.
Hulteen, J. C.; Jirage, K. B.; Martin, C. R., Introducing Chemical Transport Selectivity into
Gold Nanotubule Membranes. Journal of the American Chemical Society 1998, 120, (26), 6603-6604.
25.
26.
Sexton, L. T.; Horne, L. P.; Sherrill, S. A.; Bishop, G. W.; Baker, L. A.; Martin, C. R.,
Resistive-Pulse Studies of Proteins and Protein/Antibody Complexes Using a Conical Nanotube
Sensor. Journal of the American Chemical Society 2007, 129, 13144-13152.
27.
10850-10851.
28.
Siwy, Z.; Trofin, L.; Kohli, P.; Baker, L. A.; Trautmann, C.; Martin, C. R., Protein Biosensors
Based on Biofunctionalized Conical Gold Nanotubes. Journal of the American Chemical Society 2005,
127, (14), 5000-5001.
29.
Vlassiouk, I.; Siwy, Z. S., Nanofluidic Diode. Nano Letters 2007, 7, (3), 552-556.
30.
Urry, D. W., Molecular Machines - How Motion and Other Functions of Living Organisms
Can Result from Reversible Chemical-Changes. Angewandte Chemie-International Edition in English
1993, 32, (6), 819-841.
31.
Huang, J. J.; Zhang, X. M.; McNaughton, P. A., Modulation of Temperature-Sensitive Trp
Channels. Seminars in Cell & Developmental Biology 2006, 17, (6), 638-645.
32.
Latorre, R.; Brauchi, S.; Orta, G.; Zaelzer, C.; Vargas, G., Thermo Trp Channels as Modular
Proteins with Allosteric Gating. Cell Calcium 2007, 42, 427-438.
33.
Cais, O.; Sedlacek, M.; Horak, M.; Dittert, I.; Vyklicky, L., Temperature Dependence of
Nr1/Nr2b Nmda Receptor Channels. Neuroscience 2008, 151, (2), 428-438.
34.
Jung, Y.; Bayley, H.; Movileanu, L., Temperature-Responsive Protein Pores. Journal of the
35.
Comrie, J. E.; Huck, W. T. S., Exploring Actuation and Mechanotransduction Properties of
Polymer Brushes. Macromolecular Rapid Communications 2008, 29, (7), 539-546.
36.
Huck, W. T. S., Responsive Polymers for Nanoscale Actuation. Materials Today 2008, 11, (78), 24-32.
37.
Hermanson, G. T., Bioconjugate Techniques. Academic Press: San Diego, 1996.
38.
K. Matyjaszewski, J. X., Handbook of Radical Polymerization. John Wiley & Sons: New
York, 2002.
189
39.
40.
Cui, Y.; Tao, C.; Zheng, S. P.; He, Q.; Ai, S. F.; Li, J. B., Synthesis of Thermosensitive
Pnipam-Co-Mbaa Nanotubes by Atom Transfer Radical Polymerization within a Porous Membrane.
41.
Jones, D. M.; Smith, J. R.; Huck, W. T. S.; Alexander, C., Variable Adhesion of
Micropatterned Thermoresponsive Polymer Brushes: Afm Investigations of Poly (NIsopropylacrylamide) Brushes Prepared by Surface-Initiated Polymerizations. Advanced Materials
2002, 14, (16), 1130-1134.
42.
Kikuchi, A.; Okano, T., Intelligent Thermoresponsive Polymeric Stationary Phases for
Aqueous Chromatography of Biological Compounds. Progress in Polymer Science 2002, 27, (6), 11651193.
43.
Weissman, J. M.; Sunkara, H. B.; Tse, A. S.; Asher, S. A., Thermally Switchable Periodicities
and Diffraction from Mesoscopically Ordered Materials. Sciensce 1996, 274, (5289), 959-960.
44.
45.
46.
Siwy, Z. S., Ion-Current Rectification in Nanopores and Nanotubes with Broken Symmetry.
Advanced Functional Materials 2006, 16, (6), 735-746.
47.
Martin, C. R.; Siwy, Z. S., Learning Nature's Way: Biosensing with Synthetic Nanopores.
Science 2007, 317, (5836), 331-332.
48.
Spohr, R., Status of Ion Track Technology - Prospects of Single Tracks. Radiation
Measurements 2005, 40, (2-6), 191-202.
49.
Kalman, E. B.; Vlassiouk, I.; Siwy, Z. S., Nanofluidic Bipolar Transistors. Advanced
Materials 2008, 20, (2), 293-+.
50.
Wanunu, M.; Meller, A., Chemically Modified Solid-State Nanopores. Nano Letters 2007, 7,
(6), 1580-1585.
51.
Ramirez, P.; Aguilella-Arzo, M.; Alcaraz, A.; Cervera, J.; Aguilella, V. M., Theoretical
Description of the Ion Transport across Nanopores with Titratable Fixed Charges. Cell Biochemistry
and Biophysics 2006, 44, (2), 287-312.
52.
Lee, S. B.; Martin, C. R., Ph-Switchable, Ion-Permselective Gold Nanotubule Membrane
Based on Chemisorbed Cysteine. Analytical Chemistry 2001, 73, (4), 768-775.
53.
Polymer Brushes: Synthesis, Characterization and Applications. Wiley–VCH: Weinheim,
2004.
54.
Responsive Polymer Materials: Design and Applications. Blackwell Publishing: Ames, 2006.
55.
Azzaroni, O.; Brown, A. A.; Huck, W. T. S., Tunable Wettability by Clicking into
Polyelectrolyte Brushes. Advanced Materials 2007, 19, (1), 151-154.
56.
Huang, W. X.; Skanth, G.; Baker, G. L.; Bruening, M. L., Surface-Initiated Thermal Radical
Polymerization on Gold. Langmuir 2001, 17, (5), 1731-1736.
57.
Prucker, O.; Ruhe, J., Polymer Layers through Self-Assembled Monolayers of Initiators.
Langmuir 1998, 14, (24), 6893-6898.
190
Summary and outlook
11. Summary and outlook
The work presented in this thesis can be divided into two parts. The first part
mainly dealt with the surface modification of planar PEEK substrates in combination
with CERs as thermally stable adhesives. The second part included the surface
modification of polymeric/ceramic surfaces leading to functional materials. In this
regard, plasma treatment/polymerization at surfaces, wet chemical surface
modifications, and polymer brush assisted control over surface properties were
employed as chief methodologies. The main focus of the project was manipulation of
controlled surface property modulation with a facet of technological relevance. A
summary with relevant outlook is presented here.
11.1. Polyether ether ketone-Cyanate ester resins an innate combination:
A proposal
Based on their superior thermal properties compared to the more commonly used
epoxy resins, cyanate ester resins (CERs) were proposed as thermally stable adhesives
for polyether ether ketone (PEEK). In combination with appropriate PEEK surface
pre-treatment, a commercial novolac based PT-30 cyanate ester monomer (CEM) was
presented as an example. Compared to a two-component epoxy, PT-30 CEM
exhibited better adhesive properties at room temperature and at 200°C, where epoxies
generally fail. The PEEK-CER-PEEK joint strength could be further improved by wet
chemical- and plasma-assisted activation of the PEEK surface. During wet chemical
activation the PEEK surface was successfully functionalised with –OH and –OCN
groups. The room temperature joint strengths involving PEEK with surface –OH and
–OCN groups (26 MPa and 28 MPa) were double compared to joints involving
untreated PEEK surfaces (13 MPa). However, within the present framework a highest
room temperature joint strength of 48 MPa was achieved for optimized conditions of
N2/O2 plasma treatment. The joint strength of 8 to 13 MPa at 200°C involving
activated PEEK surfaces also showed a slight improvement over the joints involving
untreated surfaces (6 MPa). It has to be mentioned here that the surface chemical
nature of PEEK as a result of optimized plasma treatment is not yet fully
characterized.
191
The PEEK is a semicrystalline polymer with a high melting temperature (342°C),
which necessitates the use of very high temperatures for processing (340-400°C)
resulting in a high material cost. In the framework of PEEK-CERs project, we have
developed room temperature processable CEMs with PEEK like spacers between the
crosslinks. This was achieved by a careful investigation of “structure-property
relationship” for CERs, which led to develop easy to process PEEK like resin
systems. Central to this strategy was the mixture of oligomeric spacers between the
cyanate group based crosslinking sites. The mixed oligomeric nature of spacers
prevented the crystallization resulting in a liquid CEMs at room temperature. In
addition to the terminal cyanate groups, pendant cyanate groups were introduced to
improve the Tg of the crosslinked thermosets. Thermal properties of these resins were
investigated, however, evaluation of mechanical properties is yet to be realized.
Capitalizing on the mixed oligomer concept with multiple crosslinking sites,
bisphenol A and bisphenol AF based room temperature processable CEMs were also
developed. These CEMs could be crosslinked by cyclotrimerization of the cyanate
groups to form extended polycyanurate networks at lower temperatures (<265 ºC)
than many existing CEMs. The cured polycyanurates have high Tgs (>280 ºC), with
excellent thermal, mechanical, and dielectric properties. Polycyanurate nanorods with
diameters of 65 or 380 nm could be moulded in porous alumina templates from the
CEMs. The high aspect ratio nanorods with a length in the order of 100 µm were
hydrolytically stable upon extended exposure to boiling water.
11.2. Surface initiated polymerization from initiator functionalized flat
polymeric surfaces
The interfacial properties of PEEK were further tailored by growing different
polymer brushes from PEEK surface via SI-ATRP. A two step wet chemical method
for covalent immobilization of 2-bromoisobutyryl groups as ATRP initiator at the
PEEK surface was developed.
This involved, reduction of surface keto groups
followed by anchoring of ATRP initiator via esterification with 2-bromoisobutyryl
bromide. Subsequent SI-ATRP of 3-(methacryloyloxy)propane-1-sulfonate (MPS),
mono-methoxy terminated oligo(ethylene glycol)methacrylate (MeOEGMA), and Nisopropylacrylamide (NIPAAm). The change in the surface properties of PEEK as a
result of polymer brush growth could be demonstrated by; 1) staining by electrostatic
interaction of the negatively charged polyMPS brush with the positively charged
192
Rhodamine 6G dye. 2) PEEK grafted with polyMeOEGMA was subjected to bacterial
growth for the evaluation of its bio-repellency. 3) Thermally responsive wettability of
the PEEK surface grafted with a polyNIPAAm brush was demonstrated by the
temperature (LCST). The technological relevance of PEEK with a control over
accurate tuning of surface properties via a powerful tool like ATRP, where a plethora
of monomers can be employed to fabricate a wide variety of brushes, envisions a new
direction for PEEK with functional surfaces.
Furthermore, a high relevance of SI-ATRP to control the interfacial properties of
polymeric materials and limited work published in this context, motivated the
development of a general route to functionalize the polymeric surfaces with ATRP
initiator for subsequent SI-ATRP. The pulse plasma polymerized polyallylamine
(PAA) adlayer deposited on polymeric substrates was exploited as a general route to
amino functionalized polymeric surfaces. Amino groups at the surfaces are the most
suitable moieties facilitating the anchoring of a radical polymerization initiator
through a relatively stable amide linkage. The polymeric surface-PAA adlayeranchored initiator platform was found to be stable in different solvent (THF, CH2Cl2,
DMF and ethanol) and suitable for SI-ATRP. For proof of concept, PAA was plasma
deposited on the surface of three different polymeric substrates namely PEEK, PET,
and PI. This resulted in amino functionalized polymeric surfaces where a suitable
ATRP initiator could be anchored. The SI-ATRP was demonstrated by growing
polyMeOEGMA brushes and was validated by XPS surface analysis. Versatility of
the approach was established by including three technologically relevant polymeric
substrates namely PEEK, PET, and PI. It is worth mentioning here that the amino
group functionalized substrates fabricated via plasma deposited PAA may be
envisioned as a rather general platform for anchoring initiators for polymerizations
also other than ATRP. It is well established that in case of acidic or basic monomers
there is a problem with compatibility of various ATRP components. This highlights
the robustness of conventional radical polymerization (cRP), which is compatible
with most of the vinyl monomers. For such reasons, one can think of anchoring a cRP
initiator at the surfaces through the amino groups in plasma deposited PAA adlayer.
This may further generalize the scope of the proposed platform. Some preliminary
experiments were carried out in this context, where plasma polymerized PAA was
deposited on a gold substrate. The gold substrate itself was functionalized with 1193
octadecanethiol prior to plasma deposition of PAA. Pre-functionalization of gold with
an organothiol was necessary in order to provide binding sites for a covalent
anchoring of PAA. After anchoring a cRP initiator, we proceeded to SI-cRP of
NIPAAm and methacrylic acid copolymer. The proposed strategy is schematically
illustrated in Figure 11.1.
Figure 11.1: A schematic representation of generalizing the scope of plasma deposited PAA adlayer in
combination of more robust cRP.
A prelimary FTIR spectroscopic analysis revealed the presence of amide linkages
as evidenced by (C=O) stretching vibration for amide I band at 1664 cm-1 and (N-H)
bending vibration for amide II band at 1530 cm-1 originating from polyNIPAAm
(Figure 11.2). An investigation is underway to further authenticate the scope of this
approach.
Figure 11.2: FTIR spectra of plasma deposited PAA (black line) and polyNIPAAm-co-MMA brush
grown by cRP. The C=O amide I band (1664 cm-1) and N-H amide II band (1530 cm-1) are evident.
194
11.3. Click chemistry as a new grafting-to methodology boosted up by
cRP
A new route for the facile one-step preparation of azide-terminated polymers was
developed. The strategy was based on the use of cRP in combination with a novel
azide-terminated azo initiator which permitted the straightforward synthesis of
different polymers end-functionalized with azide groups. The facile synthesis of the
functionalized macromolecular building blocks enabled the merger of “click”
chemistry versatility with the robustness and ease of cRP to pave the way to a wide
variety of “clickable” architectures. This was demonstrated by the “clicking” of azideterminated polyelectrolytes on alkyne-terminated surface via a “grafting-to” approach.
In particular, “clickable” macromolecular building blocks were exploited to tailor the
chemical characteristics of planar silicon surfaces, and to tune the surface charge of
conical PET nanochannels in order to control their permselectivity. It is envisioned
that these results will not only appeal to organic and polymer chemists but also to
material scientists willing to explore the use “click” chemistry in different colloidal
and surface science applications. As such, this approach will have strong implications
on the molecular design of interfaces using macromolecular architectures.
11.4. Functional porous materials achieved by a combination of
elements form macromolecular and materials research fields
A new avenue carved by a joint venture of elements from macromolecular science,
in particular polymer science related to surface initiated polymerization (SIP), with
materials research field dealing in fabrication of solid-state porous membranes was
explored. The results obtained are summarized here.
11.5. Facile Large-Scale Fabrication of Proton Conducting Channels
Highly ordered macroporous silicon scaffolds were modified with polyelectrolyte
brushes grown via SI-ATRP. This very simple and straightforward strategy allows for
the generation of artificial proton conducting channels, as demonstrated by proton
conductivity values of ~ 10-2 S/cm. Considering the versatile chemical nature of
ATRP and the wide diversity of macroporous silicon architectures that can be
fabricated, it is believed that this approach constitutes a benchmark for the preparation
and study of model proton conducting systems and, furthermore, for the large-scale
fabrication of membranes suitable for different industrial applications. It is worth
195
noting that the goal of these proof-of-concept experiments was to provide a
framework for future development in the PEM research field, where perfluorinated
polymers, with their economical and environmental impacts, are still the materials of
choice. In the same vein, application of this concept to porous polymeric membranes
foresees a future transition from hard type silicon to flexible polymeric membranes.
11.6. Mimicing
the
ever-versatile
ever-inspiring
nature:
Stimuli
responsive artificially fabricated solid-state nanopores
For the first time, integration of polymer brushes into track etched conical single
nanopores via SIP in order to obtain functional chemical nanodevices was explored.
In this context, a combination of thermoresponsive polymer brushes together with
single solid-state nanopore was exploited to create fully artificial stimuli-responsive
nanopore. For the proof of concept, it was demonstrated that the polyNIPAAmmodified nanopore acted as thermally-driven molecular gate with closure stages that
can be remotely controlled by simply tuning the working temperature in the 23-40°C
range. This nanopore showed resemblance to those commonly encountered in nature.
The fabrication of thermally-driven molecular gate was achieved via SI-ATRP. The
scope of this strategy was broadened by anchoring a cRP initiator on nanopore surface
and subsequently growing poly-L-lysine based zwitterionic brushes. It was
demonstrated that exploiting the pH-dependent chemical equilibrium of the monomer
units, a fine tuning of the ionic transport by presetting the environmental pH is
achievable and enables a higher degree of control over the ion transport properties of
the system. It is envisioned that these results will pave the way to new “smart”
nanodevices based on the interplay between the chemical richness of polymer brushes
and the remarkable physical characteristics of conical nanopores.
196
Acknowledgement
The work constituting this thesis would have simply been impossible without the scientific
and moral support of a number of highly dedicated individuals. I am deeply obliged to all of
them and with this opportunity of writing the acknowledgement part of my thesis, I would
like to thank all of them.
Let me start from the very beginning: During early April 2005, in his office at second floor
of MPI-P in Mainz, I still can remember my very first meeting with Prof. Wolfgang Knoll
and his words to me “Gentleman, I wish you good luck”. It was rather short but very concise.
Such is the charismatic personality of him that one immediately starts to feel rejuvenated. I
am grateful to him for his support throughout my stay here and wish him a lot of success in
Vienna, Austria.
Now guess who took me to Prof. Knoll’s office……….. right “Dr. Ulrich Jonas” (The
Uli). I remember during my early college studies there was a story in our English Language
book titled in a questioning notion as “Where do those bright ideas come from?” by Lancelot
L. Whyte. One can immediately find the answer after a few minutes of scientific chat with
Uli. He has a brain full of brilliant ideas with an instinct of exploring new directions and on
top of everything a very big heart. Many thanks Uli for all the care and support, scientific or
otherwise.
I am also grateful to Prof. Dr. Holger Frey (Department of Chemistry, University of
Mainz, Germany) for being my supervisor from the University of Mainz.
I am thankful to Dr. Renate Förch (MPI-P) and her group for support during plasmabased experiments.
Many thanks to Anne Mülle and Susan Pinnells for the great support from the secretary
office.
Dr. Andreas Langner, Dr. Frank Müller and Prof. Dr. Ulrich Gösele from Max Planck
Institute for Microstructure Physics, Halle, Germany, are highly acknowledged for an
effective collaboration and providing beautiful ordered macroporous silicon membranes.
Mubarak Ali, Prof. Dr. Wolfgang Ensinger and Prof. Dr. Reinhard Neumann are
people
from
Darmstadt
University
of
Technology
and
Gesellschaft
für
Schwerionenforschung, GSI, Darmstadt, Germany, highly acknowledged for their
cooperation and providing polymeric membranes with single nanopores.
I am thankful to Dr. Omar Azzaroni for introducing me to the fascinating field of
Polymer Brushes, for the countless brain storming sessions, and many corridor meetings.
Dr. Anke Kaltbeitzel (MPI-P Mainz) is highly acknowledged for the proton conductivity
measurements and data analysis.
197
Dr. Kaloian Koynov and Andreas Best are gratefully acknowledged for dielectric and
mechanical property measurements, and data analysis.
I am thankful to Dr. Marta Alvarez-Chamorro (MPI-P, Mainz) for her scientific and
moral support, especially for the AFM imaging.
Dr. Hatice Duran (MPI-P, Mainz) is acknowledged for her cooperation in nano-moulding
experiments.
Hadayat Ullah Khan (MPI-P, Mainz) is acknowledged for plasma polymerization
experiments.
I am thankful to Filipe Natálio from Department of Physiological Chemistry, University
of Mainz, for his timely help in antifouling experiments.
Dr. Bernd Mathiasch (from the Institute of Inorganic and Analytical Chemistry,
University of Mainz) is acknowledged for atomic absorption spectroscopic analysis.
It is hard to express in words the support Daniela Mössner (from Prof. Dr. Jürgen
Rühe’s group at IMTEK, Albert-Ludwigs University, Frieburg) has provided while carrying
out XPS analysis of the samples presented throughout this thesis. Many thanks Daniela. In the
same context Siham Ouardi (from the group of Prof. Dr. Claudia Felser at the Institute of
Inorganic and Analytical Chemistry, university of Mainz) is acknowledged for taking care of
XPS analysis when the instrument at IMTEK was out of order.
Thanks to Petia and Mathieu the co-inhabitants of Lab 1.412.
I am grateful to the highly competent technical staff of MPIP especially Gunnar Glasser
(SEM), Sandra Seywald and Ute Heinz (Polymer analysis), Walter Scholdei (IR
spectroscopy), Martina Knecht (Lab 1.412), and Andreas Hanewald (Mechanical testing).
These people do make your scientific life smoother.
Many thanks to all the group members of AK-Knoll who maintained such a nice working
environment.
Sometimes you are so much indebted to some individuals that they are simply hard to
acknowledge. My wife Lubna and my daughter Shiza: for taking so much care of me, my
parents: for all their prayers. I wonder if I will ever be able to make up for what they have
done for me!
198
List of Publications
From this thesis
1) Polycyanurate Thermoset Networks with High Thermal, Mechanical, and Hydrolytic Stability
Based on Liquid Multifunctional Cyanate Ester Monomers with Bisphenol A and AF Units.
Basit Yameen, Hatice Duran, Andreas Best, Ulrich Jonas, Martin Steinhart, Wolfgang Knoll
Macromolecular chemistry and physics2008, 209, 1673–1685. (Accepted as Cover page)
2) Cyanate Ester Resins as Thermally Stable Adhesives for PEEK.
Basit Yameen, Matthias Tamm, Nicolas Vogel, Arthur Echler, Renate Förch, Ulrich Jonas and
Wolfgang Knoll. A chapter contributed to MTB Book, Wiley, 2009 Edited by Förch, Schönherr,
Jenkins, in print
3) Effect of structural variables on thermal properties of aryl ether ketone based cyanate ester
resins with linear and tri-armed molecular architectures.
Basit Yameen, Ulrich Jonas, Wolfgang Knoll
In preparation
4) Polyether ether ketone (PEEK) Surface Functionalization via Surface Initiated Atom Transfer
Basit Yameen, Marta Alvarez, Omar Azzaroni, Ulrich Jonas, Wolfgang Knoll. In preparation
5) Plasma
Polymerised Polyallylamine
as ad-layer for Anchoring
Surface initiated
polymerization initiator on polymeric (PEEK, PI, PET) Surfaces: A General Route to Polymer
Surface Functionalization via SI-ATRP
Basit Yameen, Hadayat Ullah Khan, Omar Azzaroni, Ulrich Jonas, Wolfgang Knoll. In preparation
6) Surface Initiated Atom Transfer Radical Polymerization of 2-acrylamino-2-methylpropane
sulfonate (AMPS) in macroporous silicon: Facile Large-Scale Fabrication of Hydrolytically
Stable Proton Conducting Channels
Basit Yameen, Marta Alvarez, Omar Azzaroni, Anke Kaltbeitzel, Andreas Langer, Frank Müller,
Ulrich Gösele, Ulrich Jonas, Wolfgang Knoll.
In preparation
7) Facile Large-Scale Fabrication of Proton Conducting Channels.
Basit Yameen, Anke Kaltbeitzel, Andreas Langner, Hatice Duran, Frank Müller, Ulrich Gösele, Omar
Azzaroni, and Wolfgang Knoll
Journal of the Americal Chemical Society, 30 (39), 13140–13144, 2008.
8) Single Conical Nanopores Displaying pH-Tunable Rectifying Characteristics. Manipulating
Molecular Transport with Zwitterionic Polymer Brushes.
Basit Yameen, Mubarak Ali, Reinhard Neumann, Wolfgang Ensinger, Wolfgang Knoll and Omar
Azzaroni
Submitted
9) Ionic Transport through Single Solid-State Nanopores Controlled with Thermally
Nanoactuated Macromolecular Gates.
Basit Yameen, Mubarak Ali, Reinhard Neumann, Wolfgang Ensinger, Wolfgang Knoll and Omar
Azzaroni
Submitted
10) A Facile Route for the Preparation of Azide-Terminated Polymers. “Clicking”
Macromolecular Building Blocks on Planar Surfaces and Nanochannels.
199
Basit Yameen, Mubarak Ali, Marta Álvarez, Reinhard Neumann, Wolfgang Ensinger, Omar Azzaroni
and Wolfgang Knoll.
In preparation
11) Highly Proton-Conducting Self-Humidifying Microchannels Generated by Scaffolded
Copolymer Brushes.
Basit Yameen, Anke Kaltbeitzel, Andreas Langer, Frank Müller, Ulrich Gösele, Wolfgang Knoll and
Omar Azzaroni
In preparation
In collaboration
12) Effect of the Electrostatic Microenvironment on the Observed Redox Potential of
Electroactive Supramolecular Bioconjugates.
Omar Azzaroni, Basit Yameen and Wolfgang Knoll
Physical Chemistry Chemical Physics, 2008, DOI: 10.1039/b806445h
13) Tuning the Unidirectional Electron Transfer at Interfaces with Multilayered Redox-Active
Supramolecular Bionanoassemblies
Omar Azzaroni, Marta Álvarez. A.I. Abou-Kandil, Basit Yameen, W. Knoll
Advanced Functional Materials, 2008, 18, 3487–3496
14) Redox Mediation and Electron Transfer through Supramolecular Arrays of Ferrocenelabeled Streptavidin on Biotinylated Gold Electrodes.
Omar Azzaroni, Marta Álvarez, Mónica Mir, Basit Yameen and Wolfgang Knoll
Journal of Physical Chemistry C, 2008, Vol 112, Issue 40, 15850-15859.
15) Biosensing and Supramolecular Bioconjugation in Single Conical Polymer Nanochannels.
Facile
Incorporation
of
Biorecognition
Elements
into
Nanoconfined
Geometries
Mubarak Ali, Basit Yameen, Reinhard Neumann, Wolfgang Ensinger, Wolfgang Knoll, and Omar
Azzaroni
Journal of the American Chemical Society. Web Release Date: 12-Nov-2008; (Article) DOI:
10.1021/ja8071258
200

Novel Polymer Systems and Surface Modifications of Planar and

Transcription

Similar documents

Polymaster for Hazardous Locations.

ATC-1000 - Peek Traffic

TALIKA Launches NEW Feet Therapy Socks

Kerite LTR3 Round Cables

ChemLine® 784/32 - Advanced Polymer Coatings

HELIXP

The Renewal Plan

Press Release - Reynolds Polymer Technology, Inc.

Presentation of Universidade do Minho, Portugal

8-Year Premium Driveway Resurfacer