Anomalous behavior of the longitudinal mode Grüneisen parameter around the glass transition as revealed by Brillouin spectroscopy: Polyvinylacetate

Brillouin spectroscopy was used to investigate the temperature dependence of the longitudinal acoustic mode, the refractive index, the density and the hypersonic mode Gr\"uneisen parameter around the glass transition of polyvinylacetate. In the glass-transition zone the mode Gr\"uneisen parameter shows an apparent weak anomaly and an extrapolated weak discontinuity of about 60%. The temperature position of this apparent anomaly depends on the thermal history of the sample, whereas the strength of it does not. {} 1996 The American Physical Society.