Structural Parameters Affecting the Glass Transition

We have seen that the free-volume, thermodynamic, and kinetic theories serve to rationalize the glass transition phenomenon in a wide variety of polymeric systems. There are, of course, additional effects that cannot be well explained 

The details of the molecular structure of polymers profoundly influence the observed Tg s, as illustrated by Table 5-2, where we may contrast the Tg of polydimethyl siloxane, -123 °C, with that of poly(calcium phosphate), + 525 °C. At least approximately, we may separate the observed effects into intermolecular and intramolecular parts. The latter refer to structural parameters affecting the stiffness of the chain backbone we shall examine these first. 

An increase in side group bulkiness generally serves to raise Tg. Ring-substituted polystyrenes show this effect and, in the case of polyacenaphthalene, the motions of the chain are so severely restricted that the Tg occurs at 264 °C. Chain microstructure is clearly important in determining chain mobility. We have already discussed the case of random copolymers on the basis of the Gibbs-DiMarzio theory and have developed equation (5-19), which shows that the Tg of a random copolymer is intermediate between the Tg s of the corresponding homopolymers. Equation (5-19) is similar in form to the empirical expression of Wood7 

Equation (5-27) is applicable only to random copolymers very different phenomena are observed in the case of block and graft copolymers. Frequently in these situations, two Tg s are observed as a consequence of the occurrence of microphase separation. This subject is beyond the scope of this book, however. 

In addition to copolymer composition, geometrical and steric isomerism play important roles in the determination of chain stiffness and thus of Tg. Polydienes, for example, can exist as cis and trans geometrical isomers. In the case of poly(l, 5-butadiene), the cis isomer has a Tg of-102 °C while the stiffer trans isomer shows a Tg of -48 °C. The effect is not nearly so marked in the 

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Structural Parameters Affecting the Glass Transition Temperature... 

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