Abstract
The high-frequency acoustic longitudinal- and shear-wave velocities in amorphous polycarbonate (Lexan) obtained from Brillouin scattering spectra are reported as a function of temperature from room temperature through the glass transition to the melting temperature (250°C). Comparison with low-frequency modulus-derived shear velocities is made: 20% dispersion is seen up to the glass transition and above this temperature completely different behavior is seen. The temperature dependence of the linewidths of the Brillouin components is also reported. A method for estimating the shear-wave Brillouin scattering intensity from tabulated photoelastic constants is given and applied to fused silica, PMMA, polystyrene, and polycarbonate to explain why shear waves have not been detectable in the Brillouin spectra of PMMA and polystyrene.
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