Photo-probe study of siloxane polymers. I. Local free volume of an MQ-type silicone resin containing crosslinked nanoparticles probed by photoisomerization of azobenzene

We have investigated the local free volume of an MQ-type silicone resin consisting only of M (R 3 SiO 1/2 ) and Q (SiO 4/2 ) units by using photoisomerization of azobenzene molecularly dispersed in the matrices. Photoisomerization profiles in the uncured and cured MQ resin over a wide temperature range (4–300 K) are discussed based on three parameters: first-order plots, final cis fraction and quantum yields for trans-to-cis photoisomerization of azobenzene. For uncured MQ resins or prepolymers, for which the structure was proposed to be an assembly of crosslinked nanoparticles, a large fluctuation of local free volume was observed between room temperature and 20 K. At 4 K, a drastic decrease in the final cis fraction was observed in both the uncured prepolymer mixture and the cured MQ resin, implying the complete suppression of the free-volume fluctuation at 4 K. The formation of heterogeneous sites after curing is shown by the deviation from first-order kinetics for the cured MQ resins. Data for the cured MQ resin are analyzed quantitatively with the three-component approximation and the validity of a free-volume fluctuation model is verified. Comparison of the results for linear polysiloxanes with those for poly(methyl methacrylate) shows that siloxane chains are very flexible at low temperatures down to 20 K.