Chemistry Reference
In-Depth Information
7.17 High-Performance Polymers
The polymers that are found in common commercial uses, such as fibers, films, or structural resins,
fail to withstand elevated temperatures above 250
C for long periods of time and decompose. There is
a need, however, in various technologies for materials that can tolerate temperatures over 300
C for
reasonably long periods. Many such materials might be too high priced for common commercial use.
They find application, however, in specialized areas that include space, aeronautic, or military
technologies, where a higher price is justified by greatly enhanced performance.
In developing tough, heat-resistant polymers the chemists pursued several goals [
191
]. These were:
1. To improve heat stability of the available polymers by introducing structural modifications.
2. To develop new macromolecules based on chemical structures capable of withstanding high
temperatures.
3. Because many inorganic molecules are more thermally stable than the organic ones, to develop
inorganic and inorganic-organic polymeric materials.
Improvement of thermal stability of existing polymers has to be based on the following
considerations: (1) The primary bond energy between atoms in a polymeric chain is the greatest
source of thermal stability. The strength of these bonds, therefore, imposes an upper limit on the
vibrational energy that a molecule may withstand without bond ruptures. In cases of cyclic repeating
units, as in ladder polymers, a rupture of one bond in a ring may not lead to loss of molecular weight.
In such polymers, two bonds would have to break within the same ring for the chain to rupture, and
the probability of that is low. This means that ladder polymers should exhibit greater heat stability
than single-stranded chains [
183
]. (2) Secondary bond forces or the cohesive energies contribute
additional stability to the molecule. (3) Resonance energy of aromatic and heterocyclic structures
contributes an additional amount of thermal stability and bond strength. (4) Polymers with high
melting or softening temperatures are generally more heat-resistant.
7.17.1 Fluorine Containing Aromatic Polymers
The high strength of the carbon-fluorine bonds and the shielding effect of the highly electronegative
fluorine atoms improve heat stability. One such material was synthesized by heating perfluoroalkyl
amidines above theirmelting points. Ammonia evolves and triazine ring containing polymers form[
174
]:
N
N
N
(CF
2
)
(CF
2
)
x
x
N
N
NN
N
N
N
The thermal stability of these polymers in vacuum is about equivalent to polytetrafluoroethylene [
174
].
