High-pressure sound velocity of PMMA studied by using brillouin spectroscopy

The acoustic properties of poly(methyl methacrylate) (PMMA) were investigated as functions of pressure by using a Fabry-Perot interferometer and a diamond anvil cell. The longitudinal sound velocity was measured for the first time from ambient pressure up to 11 GPa, over which range it changed from 2630 to 7240 m/s upon compression. The rate of change in the sound velocity with respect to pressure exhibited large values in the low-pressure range below about 0.5 GPa and decreased markedly upon further compression at pressures above ∼0.5 GPa. This significant change was tentatively attributed to a substantial decrease in free volume upon applying the pressure. The pressure dependence of the sound velocity did not display any substantial hysteresis between the compression and the decompression processes, but the half width of the Brillouin doublet and the related acoustic attenuation showed some hysteresis, the origin of which remains unclear.