Differential scanning calorimetry of polymer glasses: corrections for thermal lag

Abstract

A theoretical treatment of the kinetics of structural recovery of polymer glasses is applied to ‘intrinsic’ thermal cycles. These cycles involve cooling a sample at constant rate q 1 from an equilibrium state at high temperature through the glass transition region to a lower temperature T 1, and then immediately reheating the sample at constant rate q 2. In differential scanning calorimetry (d.s.c.), a peak in the specific heat capacity is observed during the heating stage, occurring at a temperature T p which depends upon both the cooling and the heating rates. For a fixed ratio of these rates, such intrinsic cycles yield heating isobars of identical shape, but shifted along the temperature scale by an amount which depends upon the heating (or cooling) rate. The invariance of the peak shape, and in particular the peak width, is shown to provide a means of correcting d.s.c. data for thermal lag on heating. Experimental data for a low molecular weight polystyrene, when corrected for thermal lag in this way, are shown to agree with the predictions of the kinetic model for structural recovery. An analytical treatment of heat transfer in the d.s.c. cell is also described, and the theoretical results are compared with the experimental data.