The glass transition beyond the time trap: Opto-acoustic dispersion

High-resolution Brillouin spectroscopy has been used as a crucial experimen tal technique to investigate the nature of the thermal glass transition in an epoxy. In order to study the coupling between the alpha-relaxation proce ss and the thermal glass transition around the quasi-static glass transitio n temperature T-g we introduce the opto-acoustic dispersion function as a v ery sensitive measure of the alpha-relaxations and show that close to but a bove T-g the related relaxation spectrum still contains components within t he microwave frequency region which finally is truncated by the thermal gla ss transition.