Pressure dependence of acoustic behaviors and refractive index of amorphous Kel F-800 copolymer studied by Brillouin spectroscopy

The pressure dependences of the longitudinal sound velocity and the refractive index of amorphous Kel F-800 copolymer were investigated for pressures up to 13 GPa using high-pressure Brillouin spectroscopy combined with a diamond-anvil-cell technique. The sound velocity increased rapidly with increasing pressure at pressures below ∼2 GPa, whereas it showed a mild change at higher pressure values. This was attributed to substantial collapse of effective free volume in this amorphous material. The refractive index could be measured by comparing the Brillouin spectra measured from backward and forward, symmetric scattering geometries. The pressure–density relationship could be obtained by using Lorentz–Lorenz equation based on an assumption of constant polarizability, which was consistent with that reported by previous study.