THE EFFECTS OF STRUCTURAL RECOVERY AND THERMAL LAG IN TEMPERATURE-MODULATED DSC MEASUREMENTS

It is well known that structural recovery in polymeric glass formers l eads to the enthalpy overshoot in nonisothermal experiments. One commo n model for describing this phenomenon is the Tool-Narayanaswamy-Moyni han (TNM) equation, Here we apply the TNM equation to analyze a typica l temperature-modulated DSC (TMDSC) trace and examine the influence of the material nonlinearities on the dynamic heat flow, We show that, i n the glass-transition region, the oscillating heat-flow response to t he sinusoidal temperature input is a distorted sine wave. Hence, linea r analysis of the phase lag between heat flow and temperature is not p hysically 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 havin g different geometries and show that an apparent phase lag can result in the measured response due to the presence of thermal gradients in t he sample. The significance of the results is discussed. (C) 1997 Publ ished by Elsevier Science B.V.