Enthalpy recovery of polyetherimide: experiment and model calculations

Enthalpy recovery of polyetherimide is measured during heating with differential scanning calorimetry (DSC) after cooling at various rates. The resulting annealing peaks are fit using the Moynihan-Tool-Narayanaswamy model of structural recovery. A self-consistent phenomenological equation is used to describe the experimentally observed structure and temperature dependence of the relaxation time in both glass and equilibrium regimes. Temperature gradients in the DSC sample are incorporated into the model calculations. When no thermal gradients are assumed, model parameters are found to vary with thermal history despite the use of the self-consistent equation for the relaxation time. Accounting for the presence of thermal gradients in the DSC sample is found to affect the values of the model parameters needed to fit the data. However, thermal gradients are unable to account for the thermal history dependence of the model parameters or the discrepancy between the observed and calculated shapes of the DSC annealing peaks.