Page 268 - Academic Press Encyclopedia of Physical Science and Technology 3rd Polymer

P1: GPQ Final Pages/GNB P2: GTV
Encyclopedia of Physical Science and Technology En012c-604 July 26, 2001 16:2

Polymers, Thermally Stable 777

Bond dissociation energies (kJ/mol)
C S 273 C N 307 C H 416 P O 528 C C 609 B O 777
B H 294 Si H 319 C F 428 P C 580 C N 617
Si C 328 Si N 437 Ti O 672
C Cl 340 Si O 445
C C 349
C O 361
C B 374
B N 386

The choice of the highest bond strength combinations to a discrepancy in the theoretically predicted and experi-
(see below), though important, is certainly not the final mentally observed stability of the polymer. Factors influ-
arbiter of thermal stability since ultimately all polymer encing the choice of aromatic/heteroaromatic ring struc-
systems degrade by lowest energy routes. tures in thermal/thermo-oxidatively stable polymers have
The exploitation of inorganic high-temperature poly- been highlighted above. However, polymer systems incor-
mers has been, for example, severely restricted due to the porating long sequences of directly linked rings are invari-
intervention of preferred low-energy processes leading ably insoluble and infusible and structural modifications
to hydrolytic or oxidative breakdown. With the semior- required to allow processing and fabrication of practically
ganic polysiloxanes, although notably successful as elas- useful materials often result in a reduced stability.
tomers, low-energy paths have resulted in a relatively For aromatic/heteroaromatic ring-containing polymers
more facile cleavage of the Si—O bond with concomitant the following main conclusions regarding structure–
formation of low-molecular-weight cyclics than would property relationships have, therefore, been established:
have been expected from an apparently high (445 kJ/mol)
bond strength. In fully organic systems energetically fa- 1. The highest thermal/thermo-oxidative stabilities are
vored eliminations (e.g., hydrogen fluoride from hydroflu- reserved for ladder (double-strand) polymers although
oro polymers) and unzipping (stepwise) breakdown of synthetic difficulties—incomplete ladder formation—
aliphatic polymer chains are characteristic causes of in- frequently reduce the observed stability to that of “con-
stability. Contrary to the situation described above, how- ventional” aromatic systems.
ever, bond strengths of aromatic and hetero-aromatic 2. For polymers containing phenylene groups, para-
nuclei are significantly strengthened through resonance linked rings produce the highest thermal stability but also
stabilization and polymers that incorporate these systems the highest softening points and lowest solubilities. The
exhibit an enhanced thermo-oxidative stability. Ideally, use of meta-or meta-/para-linked rings provides a com-
fused ring/ladder polymers should be more stable than the promise in processability versus stability.
ring-chainanalogssince,withmultiplebonding,chaindis- 3. Substitution of hydrogen in phenylene groups leads
ruption should not occur on cleavage of a single bond. In to a reduced thermal stability. This is not always reflected
practice, however, the stability of ladder systems seldom in other ring systems (e.g., chlorine substitution in polyxy-
reaches the optimum. lylene or phenyl substitution in polyquinoxalines) where
stability is increased.
4. Interposition of flexible groups midchain leads to a
B. Structure and Thermal Stability
reduction in thermal stability in all cases but that reduction
and Tractability
is minimized by using the following: CO , COO ,
Assumptions regarding potentially thermally stable poly- CONH , S , SO 2 , O ,( CF 2 ) n . The relative
mer systems based on a study of model compounds have stability (ITGA) in air of phenylene polymers containing
been of only limited value since polymer stability depends these linkages is shown in Fig. 1.
critically on the structure, reactivity, and mutual interac-
tion of the macromolecules. Variations in thermal stability
C. Material Developments and Applications
have, for example, been observed in ordered as opposed to
limited order or random copolymers. The extent of cross- High costs associated with the provision of novel raw
linking or chain branching is an important feature as are materials and difficult processing and fabrication re-
those physical characteristics such as molecular weight quirements for thermally stable/heat-resistant polymers
and degree of crystallinity. Frequently the presence of un- have limited their marketability to the relatively narrow,
stable end groups, weaklinks, and trace impurities leads specialized field of aerospace and in particular that of