MODULE III ENGINEERING MATERIALS High polymers

MODULE III
ENGINEERING MATERIALS
High polymers
Polymerisation is the process of uniting or linking together of a large number of simple modules of same or
different types with or without the elimination of small modules like H2O, Hcl, NH3 etc. resulting a compound with
high molecular mass.
For example: Polyethylene is a polymer formed by linking together of a large number of ethylene molecule.
n CH2=CH2
[ CH2-CH2 ] n
Ethylene
Polyethylene
A polymer is a large molecule which is formed by repeated linking of small molecules called monomers.
Example: Ethylene is the monomer of polyethylene.
The number of repeating units in a chain of polymer is called degree of polymerisation. The number of
monomer units present in a polymer may range from hundreds to thousands. Polymers with a high degree of
polymersation are called high polymers. High polymers are macromolecules with very high molecular mass usually
ranging from 50000 to 20000. Only this polymers are useful for practical application.
FUNCTIONALITY
The number of bonding sites in a monomer is referred to as its functionality. In olefin, the double bond
present acts as a site for 2 free valencies. When the double bond is broken, two single bonds become available for
combination.
n CH2=CH2→[-CH2-CH2-]
Thus ethylene is bifunctional. Functionality determines structure and property of a polymer. Linear,
branched and cross linked polymers are formed by bifunctional and polyfunctional monomers respectively.
TACTICITY
The monomeric units in a polymer molecule can be arranged in an orderly or disorderly fashion with
respect to the main chain. The different configurations which arise are called tacticity.
a) Isotactic polymer
Here the monomeric units are arranged in such a fashion that all the side groups lie on the same
side of the chain that is Cis arrangement.
H
H
CH3
CH3
CH3
H
H
H
H
H
b) Atactic Polymer
Here the different monomeric units are arranged in a random fashion.
CH3
H
H
CH3
CH3
H
H
H
c)
H
H
H
H
Syndiotactic polymer
Here the monomeric units are arranged in such a way that the side groups lie in an alternating fashion.
CH3
H
CH3
CH3
H
H
H
H
H
H
Classification of Polymers
I. Classification based on origin
a.
Natural polymers
Polymers isolated from natural materials are called natural polymers. Example: Natural rubber, starch,
cellulose, proteins etc.
b.
Synthetic polymers
These are man made polymers. Example: Polyethylene, polystyrene, pvc etc
II. Classification based on structure
The structure of a polymer will be either branched chain or three dimensional networks depending on the
functionality of a polymer.
a.
Linear Polymers
Bifunctional monomers give this type of structure in which the monomeric units are linked together to form
long straight chain.
Example: ─M─M─M─M─M─
b.
Branched chain polymers
These are polymers in which the monomeric units are linked to form long chain with side chain also. This
type may also result when a trifunctional monomer is mixed with a bifunctional monomer.
Example:
M─M─M
│
─M─M─M─M─M─M─M─M
│
M
│
M─M─M
│
M
c.
│
│
│
M
M─M
M
│
│
M─M
M
Cross linked polymers or 3- dimensional network polymers
When the monomers are linked together to form a 3-dimensional network structure, the polymer formed is
called cross-linked polymer. This type of structure is given by the linkage of poly functional monomers.
Example:
M─M─M─M
│
│
M
M
│
│
M
─M─M─M─M─M─ M─M─M─ M─M─
|
M
|
M
|
M
|
|
|
M
|
M
|
M
|
M
|
M
M
|
|
M─M─ M
M
|
M
M
M
|
M
|
M
|
M
TYPES OF POLYMERISATION
Depending upon the nature of reactions taking place during the formation of a polymer, there are 3 types of
polymerization
1.
Addition Polymerization
In addition polymerization, the polymer is formed from the monomer without the elimination of any
material and the product is an exact multiple of the original monomeric module. The monomer usually contains one
or more double bonds which may break to give free valencies. The reaction is activated by the application of heat,
light, pressure or catalyst for breaking down the double bonds of monomers. An addition polymerrisation leads to
the formation of a polymer with linear structure. A few examples for addition polymerization are:-
a.
Formation of polyethylene from ethylene
n CH2=CH2→[─CH2─CH2─] n
b.
Formation of PVC from vinyl chloride
n CH2=CH─Cl→[─CH2─CH─] n
│
Cl
2.
Condensation polymerization
It is reaction occurring between simple monomers containing polar groups like OH, NH2, COOH to form a
polymer with the eluimination of smaller molecules like H2O, Hcl, NH3 etc.
An example is the condensation between hexamethylene diamine and adipic acid to form polymer nylon66.
H─HN─[CH2]6─NH─H+HOOC─(CH2)4─COOH→
─[HN─ (CH2)6─NH─OC─ (CH2)4─CO] ─ n
Nylon-66
The condensation polymerization is an inter molecular combination and it takes place through the different
functional groups in the monomers having affinity for each other. When monomers contain 3 functional groups, the
polymer formed has a cross linked structure.
3.
Co-Polymerization
It is the joint polymerization of two or more monomers which are of different types. High molecular weight
compounds obtained by copolymerization are called copolymers. These polymers possess some special properties.
So that they can be used for various purposes. Examples are:
1.
Formations of Buna-S from 1, 3- butadiene and styrene
n CH2=CH ─CH=CH2+ m H2C=CH →
O
─ CH2─CH=CH─CH2─CH2─CH─
n
O
m
PLASTICS
Depending up on the intermolecular forces the polymers have been classified into elastomers, fibres and
plastics. Pure polymers were mixed with additives and are called plastics.
Plastics are a class of high molecular weight organic compounds which can be moulded in to any desired
shape by the application of temp, pressure and catalyst.
Plastics have attained great importance due to certain unique properties like light in wt, corrosion
resistance, good thermal and electrical insulation, easy moulding, good strength and toughness etc.
Plastics have assured very important position as engineering materials. Some of the important uses of
plastics are:
1.
For making electrical goods.
2.
Automobile parts.
3.
Aircraft part.
4.
Furniture
5.
House hold articles.
6.
Hoses or pipes
7.
Bearings
8.
Hospital equipments
9.
Surgical instruments
10. Artificial limbs and human parts.
Classification of plastics
Depending on the action towards heat, plastics are classified in to two types- thermo plastics and
thermosetting plastics.
Thermoplastics
These are polymers which can be easily softened repeatedly by heating and hardened on cooling without
change in properties. Repeated heating and cooling do not alter the chemical nature of these materials because the
changes involved are purely of physical nature. There fore thermoplastics can be moulded to any desired shape and
can be processed again and again. These are formed by addition polymerization. Neighbouring poltymeric chains of
thermoplastics are held together by weak vander waals forces or hydrogen bonding and can slide over each other.
Weak intermolecular
Example: Polyethylene, polystyrene, pvc etc.
2.
Thermosetting Plastics
These are polymers which undergo permanent change on heating. They become hard and infusible on
heating. They cannot be remelted and remoulded. These are formed by condensation polymerization. Neighbouring
chains of thermosetting plastics are held together by covalent bonds. When these materials are moulded, additional
cross linkings are formed between the long chains leading to further increase in molecular mass. When cross
linkings are formed, the thermosetting plastics acquire some of their characteristic properties, such as hardness,
toughness, non-swelling and non-softening properties, brightness etc. The strength of these bonds are retained even
on heating and hence they cannot be remelted and remoulded.
Cross linking
by covalent
bond
Example: Bakelite, urea- formaldehyde resin etc.
Moulding Constituents of Plastics or Compounding of Plastics
Plastics may contain a number of constituents such as binders, fillers, dyes and pigments, plasticizers,
lubricants, stabilizers, cataltst etc.
1) Binders or Resins
Resin is the binder which holds the other constituents of the plastics together and it is the major constituent.
The binders used may be natural or synthetic resins with very high molecularmass. They undergo condensation and
polymerization during moulding of plastics. The resin gives the desired properties like plasticity and electrical
insulating properties to the plastic.
2) Plasticizers
These are materials which are added to resins to increase their plasticity and flexibility. These neatralise a
part of the intermolecular force of attraction between macromolecules of resins and allow greater freedom of
movement. The most commonly used plasticizers are vegetable oils, esters, dibutyl phthalate etc.
3) Fillers
Fillers are inert materials add to plastic to increase the bulk and there by to reduce the cost of production
and also to impart certain specific properties to the finished product. Commonly used fillers are mica, graphite,
quqrtz, clay etc.
4) Lubricants
Lubricants such as oils, waxes, stearates, soap etc help in easy moulding and give better glossy finish.
5) Catalyst or Accelerators
These are used in the case of thermosettings plastics to accelerate the condensation polymerization to form the
linked products.
Example: H2O2, benzoyl peroxide, metals like Ag, Cu, Pb.
6) Stabilizers
These are substances added to plastic to improve the thermal stability during moulding.
Example: PbO, lead silicate, stearates of Pb, Cl, Ba etc
7) Dyes and Pigments
The main colouring materials are organic dye stuffs and opaque inorganic pigments.
Moulding Techniques
The process of converting the given polymeric material into suitable designs is called moulding. Moulding
can be done in several ways.
I. Compression Moulding
This method is applied to both thermoplastics and thermosettings plastics. It involves forcing a proper
quantity of plastic granules in to the mould. The mould consists of two halves, the upper half and the lower half.
The lower half contains a cavity and the upper half has a projection which fits in to the cavity when the mould is
closed. The gap between the projected upper half and the cavity in the lower half gives the shape of the moulded
article.
Pressure
Guide pins
Outer
part of
mould
Plastic
granules
Lower
part of
mould
Curing is done either by heating as in the case of thermosetting or cooling as in the case of thermoplastics.
Extraction pin
After curing, the moulded article is taken out by opening the mould parts, Now a days fully automatic compression
moulding presses are available.
II. Injection Moulding
This method is mainly applicable to thermoplastic resins. The process involves the injection of the molten
plastic material in to a steel mould cavity under high pressure with the help of a screw arrangement.
hopper
plastic
Upper part
of mould
nozzle
Moulding
plastic
screw
Lower part
of mould
Cylinder
heater
Liquid paste
Extraction pin
The moulding plastic power is fed in to a heated cylinder through the hopper. It is then converted in to
liquid plastic by electrical heating. The liquid plastic is then injected at a controlled rate through the nozzle in to the
lightly locked mould by means of a screw arrangement. The mould is kept cold to allow the hot plastic to cure and
become rigid. The cured rigid article can be taken out by opening half of the mould.
III. Extrusion Moulding
hopper
plastic
Metallic wire
Liquid
plastic
heater
Cylinder
Plastic coated cable
This method is mainly used for the continuous moulding of thermoplastic materials in to articles of uniform
crosssection like tubes, rods, strips, insulated electric cables etc. This is the most efficient and rapid method for
fabricating lengthy articles.
In this method, the thermoplastic ingredients in the form of powder or granules are fed through the hopper
in to a cylinder provided with electrical heater. The ingredients are heated to plastic condition and then pushed by
means of a screw conveyor in to a mould having the required outer shape of the article coming out of the mould get
cooled due to atmospheric exposure and carried away continuously by a long conveyor. The final product can be cut
in to desired length or wound in the form of rolls. Horizontal and vertical extrusion moulds are used for fabrication.
5. Transfer Moulding
This method is applicable only to thermosettings resins and uses the principle of injection moulding. In this
method the moulding powder is placed in a heated chamber maintained at minimum temperature at which the
powder begins to melt and become plasticized. This plastic material is then injected through an orifice in to the hot
mould by a plunger working at a high pressure. Curing takes place under the influence of heat and pressure.
Plunger
cylinder
heater
orifice
Plastic granules
Top part of
mould
Bottom part
of mould
Liquid plate
LAMINATION
Lamination is a process in which thin layers of a material are joined together by placing one over the other
until the desired thickness is obtained by using an adhesive. Thus laminate is a product obtained by joining two or
more layers are parallel to each other, it is called parallel laminates. If the grains are at right angles to each other, it
is called cross laminates. Thus the combination of adhesive and adherents is a laminate. Plywood is an example.
a) Plywood
It is obtained by bonding together an odd number of veneers in such a way that grains of alternate
layers are at right angles to each other. Thus plywood is a cross laminate.
b) Laminated Plastics
These are obtained from sheets of wood, paper, asbestos, glass, nylon etc by applying a suitable
resin. Thermosetting resins like area formaldehyde , phenol formaldehyde etc are commonly used. The roll of
paper or cloth is passed through an alcoholic solution of resin and dried at a temp below the curing temperature
of the resin. The resulting dried sheets are then cut in to suitable size and piled one over the another. These are
then subjected to curing.
c)
Laminated glass
It is obtained joining glass plates or sheets with a layer of plastic in between them. Safety glass is
a kind of laminated glass obtained by fixing a dry sheet of plastic, usually vinyl acetate resin between two glass
sheets and is cured by heating in an oven.
REINFORCED PLASTICS
Strengthening a weak material by mixing it with a more strong material is known as reinforcement.
Reinforcing increases thermal stability, mechanical strength, hardness, tensile strength etc. The commonly used
reinforcing materials are asbestos powder, china clay, silicon carbide, alumina, fibres, marble power etc
Fibre Reinforced Plastics(FRP)
Reinforcing a plastic matrix with a high strength fibre material results in the formation of fibre reinforced
plastics. The important fibres used are glass fibres, carbon fibres, cotton fibres etc.
Glass Reinforced Plastics(GRP)
Reinforcing a plastic matrix with glass fibre results GRP. Glass fibres is the most extensively used fibre
because of its attractive properties like low coefficient of thermal expansion, high dimensional stability, low cost,
good tensile strength,dielectric constant, corrosion resistance. More over glass fibers are durable and are acid proof,
water proof and fire-proof. They neither shrink nor stretch. Glass can be made so soft as it could be drawn in to
threads or fibres in the form of thin filaments. The commonly used resin matrices are polyesters, epoxy resins,
phenolie resins, silicon resins etc.
ELASTOMERS
CH2
CH2An elastomer is a polymer which exhibits elasticity and other rubber like properties. omers possess the
tendency to recover their original shape after they have been greatly deformed. The elastic deformation in an
elastomer arises from the fact that in an unstrained condition an elastomer molecule possess a coiled structure.
Consequently it can be stretched like a spring. Elastomers have weakest intermolecular forces between the polymer
chains.
Natural rubber, synthetic rubbers like Buna-S, Buna-N, Butyl rubber, Thiokol, silicone rubbers are
examples.
1.
Natural Rubber
Natural rubber is the linear polymer of isoprene.
H2C = C - CH = CH2
|
CH3
In natural rubber the isoprene units are joined in a head to tail fashion. Due to cis-configuration about the
double bonds the chains do not fit together well. Hence there are only weak van der walls forces.
H2C
CH2
C = C
H3C
H2C
C =
H
CH2
C
CH2
C
CH3
H
CH3
Gutla Percha
It is transpoly isoprene.
CH3
CH3
CH2
|
CH2
C = C
H
CH2
|
C =
CH2
C
H
Valcanization of Rubber
Charles Good year in 1839 found that when rubber is heated with sulphur its tensile strength, elasticity and
resistance to swelling increases.
Vulcaniztion is the process which converts an elastomer in to a strong, elastic, tough engineering material.
Vulcanization can be done by using sulphur, oxidizing agents, free radical generators etc.
Vulcanization of rubber using sulphur brings stiffering of the rubber by anchoring and restricting the
intermolecular movement of the rubber cahins. This is because of the chemical combination of sulphur at the double
bonds of different rubber springs in a way to bring cross links between the different rubber chains.
Vulcanization is carried out by heating the rubber with an accelerator, a fatty acid and sulphur at about 1000
140 c. During vulcanization sulphur adds at the double bonds of rubber chains and form bridges and cross links.
- H2C -
CH3
CH3
|
|
C = CH - CH2 - CH2 - C = CH - CH2 ……….
+
- H2C -
C = CH - CH2 - CH2 - C = CH - CH2 …………
|
|
CH3
CH3
CH3
|
|
Sulphur - H2C - C = CH - CH2 - CH2 - C = CH |
|
|
Vulcanize
S
S
|
|
- H2C - C = CH - CH2 - CH2 - C = CH
|
|
CH3
CH3
CH2 …………
S
S
S
|
|
- CH2 …………
CH3
Vulcanized rubber
Synthetic Rubbers
Synthetic rubber is any vulcanized man made rubber like polymer which can be stretched to at least twice
its length, but it returns to its original shape and dimensionas as soos as the stretching force is released.
1.
Styrene- Butadiene rubber or SBR or Buna-S
It is obtained by the copolymerization of 75% by weight butadiene and 25% by weight styrene in a mixing
vessel containing an aqueous solution of an emulsifying agent.
|
n
H2C = CH - CH = CH2 +
CH - CH2
copolymerisation
Styrene
CH2 -
CH = CH - CH2 - CH -
CH =
CH2
|
n
SBR
2.
Buna-N or NBR or Nitrile Rubber
Buna-N is a copolymer of 75% butadiene and 75% acrylonitrile.
Copolymerization
n H2C = CH - CH - CH2 + n H2C = CH – CN
butadiene
acrylonetrile
CH2 - CH = CH - CH2 - CH2
-
CH
|
CN
Buna - N
3.
n
Butyl Rubber or GR-I
It is prepared by the copolymerization of isobutylene (97%) with isoprene (2-5%). Isoprene is added to
introduce necessary unsaturation for vulcanization.
H2C = C
|
CH3
- CH3 + H2C = C - CH =
|
CH3
isobutene
CH2
isoprene
- CH2 -
CH3
|
C - CH2 - C = CH
|
|
CH3
CH3
- CH2 -
Butyl rubber
4.
Thiokol or polysulfide Rubber
Thiokols are linear condensation polymers produced when alkyl dihalides react with alkali
polysulfides. Thiokols are obtained by the reaction between sodium tetrasulfide and ethylene dichloride.
n
Cl - CH2 – CH2 - Cl + n Na - S - S - Na
S
S
ethylene dichloride
- NaCl - CH2 – CH2 -
Sodium tetrasulfide
S - S S
n
S
Thiokol
5.
Siulicone Rubbers
Solicones are a group of organosilicon polymers having si─O─si─O bonds and are used as lubricants,
hydraulie fluids and in cosmetics.
These are obtained by milling together a dimethyl silicone polymer, an inorganic filler, and a vulcanizer like benzoyl
peroxide. Curing is followed by heating it in a mould in the absence of air to about 1500c. This eventually leads to
abstraction of hydrogen atoms from methyl groups followed by cross linking of the polymer at these points.
Dimethyl dihydroxy silicon is prepared from dimethyl silicon dichloride.
- Cl -
CH3
|
Si - Cl
|
CH3
2n H2O
HO
-
CH3
|
Si - OH
-2nHCl
CH3
Unstable
It is highly unstable undergo polymerization.
HO -
CH3
|
Si - OH
|
CH3
CH3
CH3
|
|
- Si - O - Si - O |
|
n
CH3
CH3
Silicone rubber
Properties
1.
Silicone rubbers are known for their outstanding stability at elevated temp.
2.
They remain flexible even at as low a temp as -900c to as high a temp of 250 0c.
3.
They are hightly water repellant.
4.
Their tensile strength and elongation are inferior when compared to other organic rubbers.
5.
Silicone rubbers can be made in a variety of forms, ranging from thin sheets to heavy mouldable stocks.
CONDUCTING POLYMERS
An organic polymer with highly delocalized π - electron system having electrical conductance of
the order of a conductor is called a conducting polymer.
Eg :- Polyaniline, polyacetylene, polypyrrole, etc
They are classified into two types :1.
Intrinsically conducting polymers
2.
Extrinsically conducting polymers
1. Intrinsically conducting polymers
These have extensive conjugation in the backbone which is responsible for conductance. These polymers
can be divided into two :a)
Conducting polymers having conjugated π – electrons in the backbone.
eg :- polyacetylene, polyaniline etc.
These type of polymers have backbones of continuous sp2 hybridized carbon centers. One
valence electron on each center resides in a Pz orbital. Overlapping of conjugated π – electrons over the entire
backbone results in the formation of valence bonds as well as conduction bands, which extends over the entire
polymer molecule. But since the valence band and the conduction band are separated by a significant band gap,
conductivity of these polymers is not very high.
b) Doped conducting polymers
Conductivities of polymers having conjugated π – electrons in the backbone can be increased by
creating either +ve or –ve charges on the polymer backbone by oxidation or reduction. This process is called doping.
It can be done in two ways :i)
Oxidative doping (P – doping )
It involves treating the conjugated polymer with a Lewis acid like FeCl3 thereby oxidation takes
place and +ve charges are created on the back bone.
ii) Reductive doping (n – doping)
It involves treating the polymer with a Lewis base like RNH2 thereby reduction takes
place and -ve charges are created on the polymer back bone.
2.
Extrinsically conducting polymers
The conductivity of these polymers is due to the addition of certain external ingredients to the
original polymer.
Properties
1.
Good electrical conductance
2.
Ability to store charge
3.
Ability to exchange ions
4.
Absorption of visible light to give coloured products.
5.
Lighter as compared to metals.
Applications
Conducting polymers are used for various purposes by considering the advantages like flexibility, ease of
fabrication, stability, low cost etc.
1.
Used in organic solar cells
2.
Electrolytic capacitors
3.
Electrostatic loud speakers
4.
Flexible transparent displays
5.
Biosensors
6.
For making ion exchangers
Examples
1.
Polyacetylene
Polyacetylene is a conducting polymer with the repeating unit of
–CH=CH– and thus having a conjugated π – electron back bone.
Polyacetylene can be prepared by the ring opening polymerization (ROMP)
of molecules like cyclo octatetraene
polyacetylene
Cyclooctatetraene
The conjugated double bonds in polyacetylene make it possible to conduct electricity. But its conductivity is low.
However its conductivity can be increased to a greater extend either by P-doping or by N-doping.
During oxidative doping (P – doping) removal of electrons from the polyacetylene π –
backbone results in the formation of positive charges (holes) in between valence band and conduction band. The
presence of holes in the band gap allows easier jumps of electrons from the valence band into the holes. Hence the
conductivity increases.
2. Polyaniline
Polyaniline is considered as an organic metal. It refers to a class of polymers which in the non
conducting form have the following composition.
Conducting polyaniline can be prepared in the presence of an oxidant and protonic acid to the desired molecular
weight. Polyaniline is of three types :i)
Leucoemeraldine (fully reduced form)
ii)
Emeraldine (partially oxidized and reduced form)
iii)
Pernigraniline (fully oxidized form)
Pure polyaniline is a poor semiconductor but when dopped with PHBSA the conductivity is increased. Major
advantageous features of polyaniline are
1.
It has a wide and controllable range of conductivity.
2.
It shows a number of interesting properties like multicolour electrochromism, chemical sensitivity etc.
3.
It can be easily disposed.
Applications
1.
It is used in sensors, smart windows, printed circuit boards, electrochromic display devices, optical
computers etc.
2.
As a secondary electrode in rechargeable batteries.
3.
In the preparation of adhesives.
CARBON NANOTUBES (CNT)
Carbon nanotubes, also known as bucky tubes are allotropes of carbon with a cylindrical nanostructure.
Carbon nanotubes look like the graphene sheet ( a single sheet of graphite ) rolled like a cylinder. The diameter of a
nanotube is of the order of a few nano meters while they can be up to 18 cms in length. Chemical bonds in carbon
nanotubes are similar to that of graphite and thus involve sp2 hybridized carbon atoms in hexagonal arrangement.
Carbon nanotubes have been classified into 2 types :i)
Single walled carbon nanotubes (SWCNT)
it is a one atom thick sheet of graphene rolled up into a cylindrical form with diameter in the range
of 1-5 nanometer. It is stable and has a better defined shape of a cylinder.
ii)
Multi walled carbon nanotubes (MWCNT)
Multi walled carbon nanotubes are formed by the curling of the single sheet concentrically nested
like rings of a tree trunk. The inner diameter of a WCNT varies from 1.5-5 nm and its outer diameter varies from
2.5-30 nm. The inter layer distance in MWCNT is 3.4 A°
Synthesis of carbon nanotubes
Arc synthesis
An electric arc is produced at the gap of two graphite electrodes kept in an inert atmosphere at a
pressure of 50-700 mbar by passing a DC voltage of 12 to 25 V. Both types of carbon nanotubes are produces at the
cathode.
Properties
1.
They have high strength to weight ratio
2.
They have high thermal conductivity.
3.
More durable and light in weight.
4.
High resistance to chemical attack.
5.
They exhibit semimetallic and metallic conductive properties.
6.
Exhibits superconductivity at low temperatures.
Uses
1.
CNT has a huge potential in the electrical and electronic applications such as sensors, semi conducting
devices, energy conversion devices like fuel cells and batteries with improved life time.
2.
In the manufacture of reinforced plastics.
3.
Used in biological applications such as delivery tools, imaging agents, and tissue engineering materials.
4.
Used to make vacuum tube lamps.
5.
Used for storing Li, H2, etc.
6.
Nanotubes can also serve as catalysts in some chemical reactions.