EFFECTS OF HEATING RATE ON ENTHALPY RECOVERY IN POLYSTYRENE

Temperature gradients in polystyrene samples were measured at heating rates between 1.25 and 40 K min-1. Enthalpy recovery was measured at these heating rates and compared with phenomenological models that describe the heat capacity overshoot. At low heating rates, where gradients are shown to be negligible, the non-linearity parameter, x, and Kohlrausch-Williams-Watts (KWW) stretched exponential parameter, beta, both change significantly with thermal history. In particular, both x and beta-decrease with increasing heating rate to an extent that cannot be completely accounted for by temperature gradients or thermal resistance effects. It is suggested that the Tool-Narayanaswamy and Kovacs-Aklonis-Hutchinson-Ramos (KAHR) formalisms are deficient for polymers.