2
Polymers in Special
Uses
2.1 Introduction
There are a number of polymeric materials that distinguish themselves from others by virtue of their
limited use, high prices, or very specific application or properties. The expression specialty polymer is,
however, slightly ambiguous to use for such materials as the definition covers any polymeric material that
does not have high volume use. There are thus some materials that were originally developed as
specialties have now become high volume commodities, while a number of materials developed some
years ago still fall into the specialty category. Some examples of the latter are polytetrafluoroethylene,
polydimethyl siloxane, poly(vinylidene fluoride), and engineering materials such as poly(phenylene
oxide), poly(phenylene sulfide) (PPS), polyether sulfone, polyether ether ketone, and polyetherimide.
In this chapter, attention is focused on a number of polymers that are either themselves characterized
by special properties or are modified for special uses. These include high-temperature and fire-resistant
polymers, electroactive polymers, polymer electrolytes, liquid crystal polymers (LCPs), polymers in
photoresist applications, ionic polymers, and polymers as reagent carriers and catalyst supports.
2.2 High-Temperature and Fire-Resistant Polymers
Compared to traditional materials, especially metals, organic polymers show high sensitivity to
temperature [1–3]. Most importantly, they exhibit very low softening points, which is attributed to
the intrinsic flexibility of their molecular chains. Thus, whereas most metals do not soften appreciably
below their melting points, which may be 10008C or higher, many polymers commonly used as plastics
such as polyethylene, polystyrene, and poly(vinyl chloride), soften sufficiently by about 1008C to be of no
use in any load-bearing applications.
The poor thermal resistance of common polymers has greatly restricted some of their application
potential. In two particular application areas, namely electrical and transport applications, this
restriction has long been particularly evident.
Owing to their unique electric insulation properties, polymers are widely used in electrical products.
However, many electrical components are required to operate at high temperatures, for example, electric
motors and some domestic appliances.
Another characteristic property of polymers, namely their high specific stiffness and strength (which
are due to their low density, especially when used in fiber-reinforced composite materials), has led to the
use of polymers in transport applications, especially in aerospace industries, where weight saving is of
vital importance and materials cost is secondary. However, here again many applications also demand
high temperature resistance.
2-1
,2-2 Industrial Polymers, Specialty Polymers, and Their Applications
Road vehicle manufacturers in their attempt to save weight (and hence reduce fuel consumption) have
been replacing heavy metal components with light plastic ones. The ease of molding plastics into
intricately shaped part has also been used to advantage for fabricating many under-the-bonnet products.
Here again, resistance to elevated temperatures is often also needed, necessitating use of high-
temperature resistant polymers.
Although electrical and transport applications have perhaps provided the biggest demand for
thermally resistant specialty polymers, such polymers are also sought for use in more mundane
consumer goods, especially appliances where exposure to elevated temperature can occur, such as hair
dryers, toasters, and microwave ovens.
2.2.1 Temperature-Resistant Polymers
To measure the thermal stability of polymers, one must define the thermal stress in terms of both time
and temperature. An increase in either of these factors shortens the expected lifetime. In general terms,
for a polymer to be considered thermally stable, it must retain its physical properties at 2508C for
extended periods, or up to 10008C for a very short time (seconds). As compared to this, some of the more
common engineering thermoplastics such as ABS, polyacetal, polycarbonates, and the molding grade
nylons have their upper limit of use temperatures (stable physical properties) at only 808C–1208C.
The principal ways to improve the thermal stability of a polymer are to increase crystallinity, introduce
cross-linking, increase inherent stiffness of the polymer chain, and remove thermoxidative weak links.
Although cross-linking of oligomers is certainly useful and does make a real change in properties (see
Chapter 1 of Plastics Fundamentals, Properties, and Testing), crystallinity development has limited
application for very high temperatures, since higher crystallinity results in lower solubility and more
rigorous processing conditions. Chain stiffening or elimination of weak links is a more fruitful approach.
The weakest bond in a polymer chain determines the overall thermal stability of the polymer molecule.
The aliphatic carbon–carbon bond has a relatively low bond energy (see Table 2.1). Oxidation of alkylene
groups is also observed during prolonged heating in air. Thus the weak links to be avoided are mostly
those present in alkylene, alicyclic, unsaturated, and nonaromatic hydrocarbons. On the other hand, the
functions proven to be desirable are aromatic (benzenoid or heterocyclic) ether, sulfone, and some
carboxylic acid derivatives (amide, imide, etc.). Aromatic rings in the polymer chain also give
intrinsically stiff backbone.
It follows from this reasoning that aromatic polymers will have greater thermal stability. For example,
poly(p-phenylene) synthesized by stereospecific 1,4-cyclopolymerization of cyclohexadiene, followed by
dehydrogenation:
Ziegler/Natta −H2
Catalyst
n n
is infusible and insoluble.
TABLE 2.1 Bond Energies of Common Organic and Inorganic Polymers
Bond Energy
Bond (kcal/mol) (kJ/mol)
Cal–Cal 83 347
Car–Car 98 410
Cal–H 97 405
Car–H 102 426
C–F 116 485
B–N 105 439
Si–O 106 443
, Polymers in Special Uses 2-3
Another example of aromatic polymer is poly (p-xylene), which is usually vacuum deposited as thin
films on substrates. In one application the monomer is di-p-xylylene formed by the pyrolysis of
p-xylylene at 9508C in the presence of steam. When this monomer is heated at about 5508C at a reduced
pressure, a diradical results in the vapor phase, which, when deposited on a surface below 708C,
polymerizes instantaneously and forms a thin, adherent coating.
H2C CH2
On
950°C surface
H3C CH3 H2C CH2 H2C CH2
H2O <70°C
n
H2C CH2
Metals or other substrates can be coated in this way. A major application has been the production of
miniature capacitors that have the polymer as a dielectric.
Commercial polymers based on the principle of synthesis of polyaromatic compounds include the
previously discussed commercial polymers—aromatic polyamides, polyimides, poly(phenylene oxide),
polysulfone, and polybenzimidazole (see Chapter 1).
Another approach to achieve thermal stability is to synthesize ladder polymers, so called because of
their ladderlike structure ( ). For example, pyrolysis of polyacrylonitrile gives a ladder
polymer of high thermal stability:
∆ 250°C
C C C O2
N N N N N N
≡
≡
≡
N N N
The product (black orlon) is so stable that it can be held directly in a flame in the form of woven cloth
and not be changed physically or chemically. Further heating of black orlon to 14008C–18008C and
simultaneous stretching produces graphite HT. If the heating and stretching is conducted at 2400–
25008C, the high modulus graphite HM is obtained. Other carbonizable polymers that produce carbon
fibers on heating include poly(vinyl acetate), poly(vinyl chloride), poly(vinylidene chloride), and
cellulose. Thermosets, such as phenolic resins, generally produce nongraphitizing or glassy carbon.
Ladder polymers are also produced by polycondensation reactions of tetrafunctional monomers. If a
tetrafunctional monomer is reacted with a bifunctional monomer, as in the formation of polyimides, the
derived polymer is referred to as a partial ladder or stepladder polymer.
If two tetrafunctional monomers are used, as in the formation of polyquinoxaline (PQ) from an
aromatic tetramine and an aromatic tetraketone, the resulting polymer is a ladder polymer:
H2N NH2 O O N N
−H2O
+
H2N NH2 O O N N
Other tetraketones have also been used in the preparation of PQs. The PQs have proven to be one of
the better high temperature polymers with respect to both stability and potential application. The PQs
are also one of the three most highly developed systems—the others being benzimidazoles and
oxadiazoles. The interest in the PQs increased considerably after the development in 1967 of the
soluble phenylated polyquinoxaline (PPQ). PQs are stable to 5508C and are used for high-temperature
composites and adhesives.
Polymers in Special
Uses
2.1 Introduction
There are a number of polymeric materials that distinguish themselves from others by virtue of their
limited use, high prices, or very specific application or properties. The expression specialty polymer is,
however, slightly ambiguous to use for such materials as the definition covers any polymeric material that
does not have high volume use. There are thus some materials that were originally developed as
specialties have now become high volume commodities, while a number of materials developed some
years ago still fall into the specialty category. Some examples of the latter are polytetrafluoroethylene,
polydimethyl siloxane, poly(vinylidene fluoride), and engineering materials such as poly(phenylene
oxide), poly(phenylene sulfide) (PPS), polyether sulfone, polyether ether ketone, and polyetherimide.
In this chapter, attention is focused on a number of polymers that are either themselves characterized
by special properties or are modified for special uses. These include high-temperature and fire-resistant
polymers, electroactive polymers, polymer electrolytes, liquid crystal polymers (LCPs), polymers in
photoresist applications, ionic polymers, and polymers as reagent carriers and catalyst supports.
2.2 High-Temperature and Fire-Resistant Polymers
Compared to traditional materials, especially metals, organic polymers show high sensitivity to
temperature [1–3]. Most importantly, they exhibit very low softening points, which is attributed to
the intrinsic flexibility of their molecular chains. Thus, whereas most metals do not soften appreciably
below their melting points, which may be 10008C or higher, many polymers commonly used as plastics
such as polyethylene, polystyrene, and poly(vinyl chloride), soften sufficiently by about 1008C to be of no
use in any load-bearing applications.
The poor thermal resistance of common polymers has greatly restricted some of their application
potential. In two particular application areas, namely electrical and transport applications, this
restriction has long been particularly evident.
Owing to their unique electric insulation properties, polymers are widely used in electrical products.
However, many electrical components are required to operate at high temperatures, for example, electric
motors and some domestic appliances.
Another characteristic property of polymers, namely their high specific stiffness and strength (which
are due to their low density, especially when used in fiber-reinforced composite materials), has led to the
use of polymers in transport applications, especially in aerospace industries, where weight saving is of
vital importance and materials cost is secondary. However, here again many applications also demand
high temperature resistance.
2-1
,2-2 Industrial Polymers, Specialty Polymers, and Their Applications
Road vehicle manufacturers in their attempt to save weight (and hence reduce fuel consumption) have
been replacing heavy metal components with light plastic ones. The ease of molding plastics into
intricately shaped part has also been used to advantage for fabricating many under-the-bonnet products.
Here again, resistance to elevated temperatures is often also needed, necessitating use of high-
temperature resistant polymers.
Although electrical and transport applications have perhaps provided the biggest demand for
thermally resistant specialty polymers, such polymers are also sought for use in more mundane
consumer goods, especially appliances where exposure to elevated temperature can occur, such as hair
dryers, toasters, and microwave ovens.
2.2.1 Temperature-Resistant Polymers
To measure the thermal stability of polymers, one must define the thermal stress in terms of both time
and temperature. An increase in either of these factors shortens the expected lifetime. In general terms,
for a polymer to be considered thermally stable, it must retain its physical properties at 2508C for
extended periods, or up to 10008C for a very short time (seconds). As compared to this, some of the more
common engineering thermoplastics such as ABS, polyacetal, polycarbonates, and the molding grade
nylons have their upper limit of use temperatures (stable physical properties) at only 808C–1208C.
The principal ways to improve the thermal stability of a polymer are to increase crystallinity, introduce
cross-linking, increase inherent stiffness of the polymer chain, and remove thermoxidative weak links.
Although cross-linking of oligomers is certainly useful and does make a real change in properties (see
Chapter 1 of Plastics Fundamentals, Properties, and Testing), crystallinity development has limited
application for very high temperatures, since higher crystallinity results in lower solubility and more
rigorous processing conditions. Chain stiffening or elimination of weak links is a more fruitful approach.
The weakest bond in a polymer chain determines the overall thermal stability of the polymer molecule.
The aliphatic carbon–carbon bond has a relatively low bond energy (see Table 2.1). Oxidation of alkylene
groups is also observed during prolonged heating in air. Thus the weak links to be avoided are mostly
those present in alkylene, alicyclic, unsaturated, and nonaromatic hydrocarbons. On the other hand, the
functions proven to be desirable are aromatic (benzenoid or heterocyclic) ether, sulfone, and some
carboxylic acid derivatives (amide, imide, etc.). Aromatic rings in the polymer chain also give
intrinsically stiff backbone.
It follows from this reasoning that aromatic polymers will have greater thermal stability. For example,
poly(p-phenylene) synthesized by stereospecific 1,4-cyclopolymerization of cyclohexadiene, followed by
dehydrogenation:
Ziegler/Natta −H2
Catalyst
n n
is infusible and insoluble.
TABLE 2.1 Bond Energies of Common Organic and Inorganic Polymers
Bond Energy
Bond (kcal/mol) (kJ/mol)
Cal–Cal 83 347
Car–Car 98 410
Cal–H 97 405
Car–H 102 426
C–F 116 485
B–N 105 439
Si–O 106 443
, Polymers in Special Uses 2-3
Another example of aromatic polymer is poly (p-xylene), which is usually vacuum deposited as thin
films on substrates. In one application the monomer is di-p-xylylene formed by the pyrolysis of
p-xylylene at 9508C in the presence of steam. When this monomer is heated at about 5508C at a reduced
pressure, a diradical results in the vapor phase, which, when deposited on a surface below 708C,
polymerizes instantaneously and forms a thin, adherent coating.
H2C CH2
On
950°C surface
H3C CH3 H2C CH2 H2C CH2
H2O <70°C
n
H2C CH2
Metals or other substrates can be coated in this way. A major application has been the production of
miniature capacitors that have the polymer as a dielectric.
Commercial polymers based on the principle of synthesis of polyaromatic compounds include the
previously discussed commercial polymers—aromatic polyamides, polyimides, poly(phenylene oxide),
polysulfone, and polybenzimidazole (see Chapter 1).
Another approach to achieve thermal stability is to synthesize ladder polymers, so called because of
their ladderlike structure ( ). For example, pyrolysis of polyacrylonitrile gives a ladder
polymer of high thermal stability:
∆ 250°C
C C C O2
N N N N N N
≡
≡
≡
N N N
The product (black orlon) is so stable that it can be held directly in a flame in the form of woven cloth
and not be changed physically or chemically. Further heating of black orlon to 14008C–18008C and
simultaneous stretching produces graphite HT. If the heating and stretching is conducted at 2400–
25008C, the high modulus graphite HM is obtained. Other carbonizable polymers that produce carbon
fibers on heating include poly(vinyl acetate), poly(vinyl chloride), poly(vinylidene chloride), and
cellulose. Thermosets, such as phenolic resins, generally produce nongraphitizing or glassy carbon.
Ladder polymers are also produced by polycondensation reactions of tetrafunctional monomers. If a
tetrafunctional monomer is reacted with a bifunctional monomer, as in the formation of polyimides, the
derived polymer is referred to as a partial ladder or stepladder polymer.
If two tetrafunctional monomers are used, as in the formation of polyquinoxaline (PQ) from an
aromatic tetramine and an aromatic tetraketone, the resulting polymer is a ladder polymer:
H2N NH2 O O N N
−H2O
+
H2N NH2 O O N N
Other tetraketones have also been used in the preparation of PQs. The PQs have proven to be one of
the better high temperature polymers with respect to both stability and potential application. The PQs
are also one of the three most highly developed systems—the others being benzimidazoles and
oxadiazoles. The interest in the PQs increased considerably after the development in 1967 of the
soluble phenylated polyquinoxaline (PPQ). PQs are stable to 5508C and are used for high-temperature
composites and adhesives.
