Pressure dependence of acoustic anomalies of polydimethylsiloxane studied by Brillouin spectroscopy

The acoustic properties of polydimethylsiloxane elastomer was investigated as a function of pressure by using a multi-pass Fabry–Perot interferometer and a diamond anvil cell. Pressure dependence of the sound velocity, the Brillouin linewidth, and the refractive index was determined up to ~8.7 GPa. Acoustic properties exhibited a crossover behavior at approximately 1 GPa, which was attributed to the complete collapse of the free volume content in this polymer. The refractive index increased from 1.46 at ambient condition to ~1.63 at 8.67 GPa, which reflected the corresponding increase in density.