Application of photothermal mathods to the measurements of glass transition and viscoelasticity of polymers

The glass transition temperature Tg can be identified with a sharp increase of the thermal expansion coefficient X. We present photoacoustic and constrained photodeformation measurements of the glass transition in polystyrene in conditions of weak optical absorption. Since the thermoelastic contribution to the signal prevails in those conditions, we show both theoretically and experimentally that the signal amplitudes exhibit a sharp increase near Tg, allowing its clear identification and the observation of a viscoelastic behavior.