Photo-probe study of siloxane polymers: low-temperature structural relaxation in siloxane polymers probed by persistent spectral hole burning

We have investigated low-temperature structural relaxation in polysiloxane matrices by using persistent spectral hole burning (PSHB). The irreversible increase in hole width and the irreversible decrease in hole area were evaluated in terms of PSHB temperature-cycling experiments starting at 4 or 20 K. MQ-type silicone resins containing cross-linked nanoparticles (an uncured prepolymer mixture and cured MQ resins) and poly(methylphenylsiloxane) (PMPS) were used. The increase in hole width was observed at 15–20 K for all measured polysiloxanes. The increase is attributed to the local relaxation originating from the siloxane chains. The local structural relaxation of siloxane chains in this temperature range is confirmed for the first time. The local structural relaxation observed in the PSHB can be related to the rapid decrease in the final cis fraction between 4 and 20 K observed in the previous azobenzene photo-isomerization study. The curing effects of the MQ resin on the local relaxation are also discussed.