Dependence of Tg on Chemical Structure

In Section 8.9 some effects of crystallinity and hydrogen bonding on Tg were considered. The effect of molecular weight was discussed in Section 8.7, and the effect of copolymerization was discussed in Section 8.8. This section discusses the effect of chemical structure in homopolymers. 

Boyer (135) suggested a number of general factors that affect Tg (see Table 8.11). In general, factors that increase the energy required for the onset of molecular motion increase Tg, those that decrease the energy requirements lower Tg. 

In monosubstituted vinyl polymers and at least some other classes of polymers, flexible pendant groups reduce the glass transition of the polymer by acting as internal diluents, lowering the frictional interaction between chains. The total effect is to reduce the rotational energy requirements of the backbone. 

The discussion so far in this chapter has assumed atactic polymers, which with a few exceptions are amorphous. Other stereo isomers include isotactic and syndiotactic polymers (see Section 2.3). 

The effect of tacticity on Tg may be significant, as illustrated in Table 8.12 (140,141). Karasz and MacKnight (141) noted that the effect of tacticity on Tg is expected in view of the Gibbs-DiMarzio theory (Section 8.6.3.1). In disub-stituted vinyl polymers, the energy difference between the two predominant rotational isomers is greater for the syndiotactic configuration than for the iso- 

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