Brillouin Scattering Near the Glass Transition of Polymethyl Methacrylate

Abstract

Brillouin scattering of laser light has been used to measure the frequency of hypersonic sound waves in the range of 10¹⁰ Hz in PMMA as a function of temperature through the glass‐transition region. A discontinuity in the temperature coefficient of sound velocity is observed at the glass‐transition temperature; this is explained as a consequence of a corresponding discontinuity in the temperature coefficient of the specific volume (thermal expansion coefficient). The ratio of the light scattered by isothermal density fluctuations to that scattered by adiabatic density fluctuations was also measured. This ratio was large and did not change appreciably near the glass‐transition temperature. The value of the Landau‐Placzek ratio is approximately what one would expect from previously observed ultrasonic‐velocity‐dispersion data as a function of temperature well above the glass‐transition temperature. Both the velocity and intensity ratio data indicate that no velocity‐dispersion effects are present for the hypersonic sound waves up to temperatures 35° above the glass‐transition temperature. These results also indicate that the glass transition is not a classical second‐order phase transition.