CYCLIC POLYSILOXANES .4. GLASS-TRANSITION TEMPERATURES OF POLY(PHENYLMETHYLSILOXANES)

The glass transition temperature (T(g)) for cyclic and linear poly(phenylmethylsiloxanes) (PPMS) were determined using differential scanning calorimetry. The samples had number-average molar masses in the range 300 less-than-or-equal-to M(n) less-than-or-equal-to 27 100 g mol-1, with heterogeneity indices 1.03 less-than-or-equal-to M(w)/M(n) less-than-or-equal-to 1.16; they were all found to be amorphous materials, apart from the linear oligomer (CH3)3SiO[(C6H5)(CH3)SiO]Si(CH3)3. The T(g) values for the linear PPMS fractions showed a linear dependence upon reciprocal number-average molar mass M(n)-1, in agreement with theoretical predictions for linear polymers. The T(g) values for the cyclic PPMS fractions, by contrast, were found to show a non-linear dependence upon M(n)-1. The asymptotic values of the glass transition temperatures T(g)(infinity) for both cyclic PPMS and linear PPMS were found to be in close agreement with the literature value for high-molar-mass linear PPMS. High-resolution H-1 nuclear magnetic resonance spectroscopy was used to demonstrate that the cyclic and linear PPMS fractions used in this investigation-and also in a variety of other studies in this series-are stereochemically atactic polymers.