THERMAL CYCLING OF GLASSES .2. EXPERIMENTAL EVALUATION OF THE STRUCTURE (OR NONLINEARITY) PARAMETER-X

The structure parameter x (also known as the nonlinearity parameter), which in the KAHR model determines the relative contributions of temperature and structure to the retardation times of polymers in the glass transition region, is evaluated experimentally for a narrow molecular weight fraction of atactic polystyrene by the peak-shift method. This method is based on the dependence on heating rate and annealing time of the endothermal peak temperatures Tp, obtained by differential scanning calorimetry during the heating stage of well-defined three-step thermal cycles. The results obtained from the shifts of Tp, with both heating rate and the amount of isothermal recovery during the annealing stage before reheating, are consistent with the theoretical predictions of the KAHR model.