Study of Dynamics of a Glassy Polymer by Brillouin Light Scattering (CAT#: STEM-ST-0138-YJL)

Introduction

BLS can be used to explore the dynamics of glasses in the 10 GHz spectral region, corresponding to an acoustic wavelength of some hundred nanometers. Different relaxation mechanisms are active in glasses in this region. In case of macromolecules, local motions of small molecular units can couple with density fluctuations and give an important contribution to acoustic damping. Also in case of simple molecular glasses, or more traditional SiO2 and GeO2 glasses, appreciable damping mechanisms are given by thermally activated processes or by anharmonicity of interparticle interactions. The 10 GHz region is close to the upper limit where the continuous medium approximation is valid for ordinary glasses. At higher frequencies the wavelength approaches the dimension of topologically disordered regions in the glass, and the nature of collective excitations turns out to be profoundly affected by the local disorder in both position of atoms and intermolecular force constants.