Block Copolymer Modified Epoxies: The Effect of Resin Chemistry on Morphology

Block Copolymer Modified Epoxies: The Effect of Resin Chemistry on
Morphology
Amelia Labak and Raymond A. Pearson
Lehigh University
Bethlehem, PA, 18015, USA
akl212@lehigh.edu, rp02@lehigh.edu
Introduction
An epoxy matrix is inherently brittle. In order to provide a more versatile material, these epoxies are often
toughened with a rubber. In this rubber toughened system,
an additive, such as a diblock copolymer, is included in the
matrix to provide different toughening mechanisms. These
mechanisms include localized shear yielding, plastic void
growth, and crack bridging. [1]
The type and amount of the additive has a clear impact
on the properties of the system. Some of these particle
variations in a consistent epoxy system have been investigated. [2,3] Diblock copolymers self assemble in a resin
matrix due to the fact that one of the blocks is miscible in
the matrix and the other is not.
In addition to the rubber particle addition, variations
in the epoxy matrix also have an impact on the fracture
behavior of the toughened systems. It is necessary for there
to be sufficient molecular weight between crosslinks in
order to facilitate shear yielding of the epoxy matrix.
For epoxy systems toughened by self-assembling
block copolymers, the size and morphology of the rubber
particles are controlled by the block length and the polarity
of the miscible block [4,7] Similarly, controlling the chemical structure of the chain extender in order to create a
more polar resin system should have a similar effect on
rubber particle size and morphology. Therefore, the purpose of this research is to investigate the effect of the
structure of the epoxy system on the morphology of the
diblock copolymers and, ultimately, the fracture behavior
the toughened epoxy systems.
Experimental
Materials
In this study, an epoxy system of diglyidyl ether of
bisphenol A (DGEBA) was cured using m-phenylene
diamine, mPDA, (Acros Organics), aniline (Alfa Aesar),
and hydroquinone (Alfa Aesar). The DGEBA had the designation of D.E.R. (Dow Epoxy Resin) 331 and was provided by Sigma-Aldrith. The epoxy resin has a molecular
weight of 374 g/mol. The curing agent (mPDA), chain
extender (aniline), and accelerator (hydroquinone) were
added to the epoxy resin in different amounts of mole ratios. Nanostrength D51N (NM27), a poly(butyl acrylate-co-
methacylate) block copolymer, was provided by Arkema
in order to rubber toughen the resin matrix.
Six resin matrices will be investigated to determine
the role of the polarity of the chain extender on the morphology of the diblock copolymer and the mechanical
properties of the epoxy systems. The stoichiometric ratios
of these resin matrices can be seen in Table 1 below. Additionally, each of these systems will be toughened with 10
phr of NM 27. These systems will establish the effect of
varying the crosslink density and the polarity of the resin
matrix on the rubber toughened system. Increasing the
ratio of aniline to mPDA will decrease the crosslink density. Furthermore, as hydroquinone is more polar than aniline, increasing the amount of hydroquinone will increase
the polarity of the system.
Table 1. Stoichiometric Mole Ratios of the Systems
Mole Ratios
System DGEBA mPDA Aniline Hydroquinone
A
3
1
1
0
B
4
1
2
0
C
5
1
3
0
D
4
1
3/2
1/2
E
4
1
5/3
1/3
F
4
1
7/4
1/4
Table 2. Theoretical Crosslink Densities of the Systems
System
Theoretical Molecular Weight Between Crosslinks, Mnc (g/mol)
A
1326
B
1794
C
2262
D
1802
E
1799
F
1798
It can be seen in Table 2 that he systems which include hydroquinone (D, E, and F) should have comparable
crosslink densities to system B which involves a ratio of 2
aniline (chain extender) to 1 mPDA (curing agent). The
slight variation is due to the fact that the hydroquinone has
a slightly higher molecular weight than aniline. In these
systems, hydroquinone replaces 1/4th, 1/6th, and 1/8th of the
aniline for systems D, E, and F, respectively.
Processing
Initially, a master batch of 15 phr NM27 was made by
adding NM27 into DER 331. It was mechanically mixed at
80ºC for 2 hours. Next this mixture was mechanically
mixed at 150ºC under a vacuum for 1 hour. Finally, this
master batch was then diluted to 10 phr following a similar
method (80ºC for 2 hours, then 150ºC under a vacuum for
2 hours).
The master batch mixture was cured using the various
systems discussed above in Table 1. The mPDA and hydroquinone were provided in a solid state and the aniline
was in a liquid state. In DGEBA, mPDA, and aniline systems (systems A, B, and C), the materials were combined
at 80°C under a vacuum and mechanically mixed for 10
minutes. This heat and mechanical mixing dissolves and
disperses the mPDA. For the systems that contain hydroquinone (systems D, E, and F), the aniline and hydroquinone were initially added into the master batch and mechanically mixed under a vacuum for half an hour. After
the solid particles of the hydroquinone were incorporated,
the mPDA was added and mixed under a vacuum at 80°C
for 10 minutes.
Immediately afterwards, the mixture was cast into a 6mm-thick pre-heated Teflon coated aluminum mold and
cured for 12 hours at 50°C followed by a post-cure for 3
hours at 130°C. The samples are then cooled to room temperature while remaining in the oven.
Fracture Toughness Testing
sectional area was exposed to a uniaxial compression with
a crosshead speed of 1 mm/min and a 30 kN load cell. The
yield stress average values were derived from a minimum
of seven samples.
Glass Transition Temperature Testing
TA Instruments DSC, Series Q2000, is used in order to
determine the glass transition temperature, Tg. Samples are
placed in aluminum hermetical sealed pans. The sample
size ranges from 10-20 mg. In order to determine the T g,
the sample is subjected to a heat-cool-heat cycle. The first
heat ramp is to remove any uncured portions or effects of
physical aging. Then, the T g is found from the midpoint of
the step change of the second heat ramp according to
ASTM D-7426.
Dynamic Mechanical Analysis (DMA)
Samples (50mm x 12.75mm x 2.5mm) are tested by
the TA Instruments Advanced Rheometric Expansion System (ARES). The samples are run with a .2% strain rate
until a temperature 15°C above the Tg. Then the strain rate
is increased to .6% until 200°C. The storage and loss modulus, G’ and G”, respectively, and the tan(delta) are recorded. The Tg is determined from this measurement according to the peak of the loss modulus (ASTM E-1640).
The shear modulus, Gc, which is used for crosslink density
calculations, is recorded as the storage modulus when it
reaches a constant value, at 50°C above Tg. The crosslink
density is then calculated using
(1)
The fracture toughness of the modified epoxies is
measured according to the ASTM D5045-99 test method.
The critical stress intensity factor, KIC, is determined using
single-edge-notched, three-point-bend (SEN-3PB) specimens with dimensions of 75.6 mm x 12.7 mm x 6.0mm. In
order to create the pre-cracks in the samples, a two step
process is performed. First, the samples are notched with a
jewelers saw. Next, a razor blade that has been in nitrogen
is hammered into the notch in order to propagate a crack.
In order to ensure that the average crack length is within
the allowable percentage of the sample (45%-55%), the
crack length is averaged from three points across the fracture surface using calipers. The SEN-3PB test is performed
using a screw-driven universal testing machine (Model
5567 Instron) equipped with a 500 N load cell at a crosshead speed of 1 mm/min in compression mode. The KIC
average values are derived from a minimum of five samples to provide accuracy.
Compressive Yield Stress Testing
The compressive yield stress of modified epoxies is
measured according to the ASTM D790 test method. Samples with dimensions of 6 mm x 6 mm x 12 mm are tested
according to this method using a screw-driven universal
testing machine (Instron Model 5567). The square cross-
where Mnc is the molecular weight between crosslinks, q is
the front factor (usually one), and ρ is the density at the
temperature. [4]
Transmission Optical Microscopy (TOM)
TOM samples are made from the fractured surface of
the SEN-3PB by manually grinding the sample until it has
a thickness between 80-120 μm. This thin sample is then
looked at under an optical microscope using two modes.
First, bright field mode is used to illuminate the particle
cavitation. Second, the crossed polars is used to show the
birefringence that arises from the shear plasticity.
Scanning Electrical Microscopy (SEM)
Samples for the SEM are made by mounting the fractures surface from the SEN-3PB samples on an aluminum
stub and sputter coating them for 30 seconds with iridium
(~5 nm) in order to prevent charging. A Hitachi 4300 low
voltage SEM is used with an accelerating voltage of 5kV.
Both the fast fracture and the stress whitened zone of the
fracture surface is analyzed in order to determine the
toughening mechanism of the system.
Bright Field Scanning Transmission Electrical Mi-
croscopy (BF-STEM)
Samples from each system are cryo-microtomed and
OsO4 stained at the University of Massachusetts Medical
School. They are then investigated using a Hitachi S4300SE/N at 30kV. A standard STEM-IN-SEM technique
is used involving a specimen holder in order to produce a
bright field image of one plane of the stained specimen.
These images have a phase-contrast so that the dispersion
and morphology of the diblock copolymers can be observed. [5]
Results and Discussion
Previously, research was conducted to determine the
properties of neat resin systems involving mPDA, aniline,
and hydroquinone. [6] When comparing the stoichiometric
rario seen in system B from Table 1 in neat and rubber
toughened systems, it can be seen that there is a difference
in the properties (Table 3). As expected, the addition of the
rubber toughening diblock copolymer increases the fracture tougheness, yield stress, and glass transition temperature.
Table 3. Comparison for Rubber Toughened and Neat
Systems
Sample
KIC
Yield Stress
Tg
(MPa-m1/2)
(MPa)
(°C)
Neat
0.68 ± 0.03
114.99 ±
97 ± 0.35
0.79
Rubber
1.48 ± 0.08
116.07 ±
113 ± 0.35
Toughened
0.55
Conclusions
By adding a diblock copolymer into a lightly
crosslinked epoxy resin there is an increase in the fracture
toughness of the system. From past research it was seen
that varying the resin systems also has an effect on the
physical and mechanical properties, therefore, it can be
extrapolated that these various resin systems, when rubber
toughened, will also show a range in properties.
Future Work
Moving forward, the effect of the variety in the resin
systems on the diblock copolymer in the rubber toughened
system will be investigated. In the “Materials” section the
systems that will be analyzed are described. In the “Experimental” section the various tests that will be used to quantify these differences are outlined.
Acknowledgements
The research was partially supported for a research
grant from the Semiconductor Research Corporation (SRC
Task No. 2251.001).
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