The effects of structural recovery and thermal lag in temperature-modulated DSC measurements

It is well known that structural recovery in polymeric glass formers leads to the enthalpy overshoot in nonisothermal experiments. One common model for describing this phenomenon is the Tool-Narayanaswamy-Moynihan (TNM) equation. Here we apply the TNM equation to analyze a typical temperature-modulated DSC (TMDSC) trace and examine the influence of the material nonlinearities on the dynamic heat flow. We show that, in the glass-transition region, the oscillating heat-flow response to the sinusoidal temperature input is a distorted sine wave. Hence, linear analysis of the phase lag between heat flow and temperature is not physically meaningful. The degree of distortion of the sinusoidal heat flow increases as the magnitude of the excess enthalpy annealing peak increases. In addition, we perform a thermal analysis of samples having different geometries and show that an apparent phase lag can result in the measured response due to the presence of thermal gradients in the sample. The significance of the results is discussed.