Abstract
We have investigated the local free volume and its thermal fluctuation in an amorphous polymethylsilsesquioxane (PMSQ) consisting only of T3 (CH3SiO3/2) and T2 (CH3Si(OH)O2/2) 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 on 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 vacan...
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