Isothermal enthalpy relaxation of amorphous polystyrene studied using temperature-modulated fast scanning calorimetry

Abstract

The kinetics of enthalpy relaxation of atactic polystyrene after temperature jump-down and jump-up in the glass state is studied using temperature-modulated fast scanning calorimetry (T-M FSC). During the isothermal relaxation, the dynamic heat capacity obtained by T-M FSC underwent a systematic change suggesting a shift of relaxation time. The relaxation time in the nonequilibrium glass state is dependent on the sample's actual temperature T and the nonequilibrium structure that has been indexed by a fictive temperature Tf. The changes in the magnitude and phase angle of the dynamic heat capacity on relaxation prove the influence of the nonequilibrium structure. The influence is indexed by an effective temperature Teff, which is derived from the obtained dynamic heat capacity compared with its temperature dispersion in the liquid state. Teff also corresponds to the mean temperature of the actual and fictive temperatures and can be expressed by Teff≅xT+(1−x)Tf with the nonlinearity parameter x of the Tool–Narayanaswamy–Moynihan model that partitions the activation factor of relaxation into the contributions of T and Tf. Evaluation of Tf and Teff will provide valuable insights into the relaxation time in glass state, which plays an essential role in the glass transition behaviors of amorphous materials.