Temperature and pressure dependences of acoustic anomalies of PET films studied by using Brillouin spectroscopy

The acoustic properties of biaxially-oriented polyethylene terephthalate (PET) were investigated as a function of either temperature or pressure by using Brillouin spectroscopy. The Brillouin frequency shift of the longitudinal acoustic mode of both biaxially-oriented and amorphous PET materials showed a change in the slope near 80 °C, which was the approximate glass transition temperature. The acoustic damping of amorphous PET exhibited large values near the melting temperature compared to that of semicrystalline PET. This indicated stronger coupling between the acoustic waves and the structural relaxation process in the amorphous state. The pressure dependences of the sound velocities were investigated at pressures up to 8.5 GPa by using a diamond anvil cell. The pressure-density relationship could be obtained based on the Birch-Murnaghan equation of state.