Acoustic damping behaviors of Kel-F 800 copolymer studied by high-pressure Brillouin scattering spectroscopy

The acoustic damping behaviors of Kel-F 800 copolymer were investigated in a wide temperature and pressure ranges. The Brillouin frequency shift of the longitudinal acoustic (LA) mode decreased substantially, along with the increase in the linewidth, upon heating in the viscous liquid phase far above the glass transition temperature (Tg). The temperature dependence of the linewidth of the LA mode below Tg was attributed to the remnant, thermally-activated relaxation processes that were still active in the glassy state. Hydrostatic pressure decreased the Brillouin linewidth significantly at the early stage below ~ 2 GPa and did not have any substantial effect above ~ 3 GPa. This crossover phenomenon was attributed to the rapid decrease in the free volume in Kel-F 800, leaving only the intramolecular motions active at high pressures.