Differential scanning calorimetry (DSC) heating curves obtained at 10°C/min for polyetherimide cooled at various rates are fit using the Tool-Narayanaswamy-Moynihan model of structural recovery coupled with Scherer's equation which describes the dependence of the characteristic relaxation time on structure and temperature. The parameters for Scherer's equation are obtained from the temperature dependence of shift factors needed to form reduced curves for isothermal enthalpy recovery experiments and isothermal creep data. Hence the equation for the relaxation time correctly describes the limiting behavior in both glass and equilibrium regimes over large ranges of temperature. Still, the model parameter β which describes the nonexponentiality is found to decrease as the cooling rate decreases. Thermal gradients in the DSC sample have been postulated to be responsible but incorporation of the effect of a thermal gradient in the model calculation did not impact the dependence of β on thermal history.
|Number of pages||11|
|Journal||ACS Symposium Series|
|State||Published - 1998|