Glass-Transition Temperatures and Conformational Flexibility

Polysiloxanes are among the most flexible polymers known and this is reflected by the very low glass-transition temperatures of this polymer family. Data for some representative examples of polysiloxanes along with some related polymers are given in Table 6.2. 

The most well-studied polysiloxane viz., [Mc2SiO]n has a Tg of -123 °C [1]. This is one of the lowest values for any polymer. As mentioned earlier in Chapter 2 the glass-transition temperature of a polymer corresponds to a description of its amorphous state. The Tg of a polymer can be taken as a measure of the torsional freedom of polymer chain segments. Above its Tg a polymer has reorientational freedom of motion of its chain segments, while below its Tg this is frozen. Usually elastomeric materials have low glass-transition temperatures. For example, natural rubber has a glass-transition temperature of -72 °C. Similarly, polyisobutylene has a ass-transition temperature of -70 °C. Thus, poly(dimethylsiloxane) has consid- 

Irrespective of the subtle differenee in the all-trans or trans-syn structures that PDMS possesses at a local level, its overall structure is believed to have a regular coiled helical conformation where the backbone Si-0 bonds are effectively shielded from outside influence by the umbrella-type protection afforded by the lipophilic sheath of methyl groups [35]. Since the methyl groups are nonpolar, intermolecular interactions are minimiun and this also accounts for the low melting temperature seen for poly(dimethylsiloxane) (-40 °C). 

The unusual conformational behavior of polysiloxanes and its implication on the polymer properties can be summarized as follows. 

2) The Si-0 and Si-C bond lengths are considerably longer than those (C-0 or C-C) found in organic polymers. 

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