Photoprobe study of siloxane polymers. 3. Local free volume of polymethylsilsesquioxane probed by photoisomerization of azobenzene

We have investigated the local free volume and its thermal fluctuation in an amorphous polymethylsilsesquioxane (PMSQ) consisting only of T-3 (CH3SiO3/2) and T-2 (CH3Si(OH)O-2/2) units using photoisomerization of azobenzene molecularly dispersed in the PMSQ films as a photoprobe over a wide temperature range (4-300 K). Photoisomerization profiles are discussed an the basis of three parameters, i.e., first-order plots, final cis fractions, and three-component analysis, for uncured and silanol-condensation-cured PMSQ's. The final cis fraction in the uncured PMSQ decreases markedly below 250 K, which has never been observed in linear polysiloxanes, MQ resins, or carbon-based polymers. The effect of low-molecular-weight components in the PMSQ was evaluated using fractionated PMSQ's by molecular weight. The effect of hydrogen bonding by a silanol group was also studied by temperature dependence of infrared spectra. These results suggest that the low-molecular-weight components with silanol groups fill local vacant space resulting in the anomalous decrease in the final cia fraction for the uncured PMSQ below 250 K.