A theoretical model of temperature-modulated differential scanning calorimetry in the glass transition region

A new analysis of an earlier theoretical model of the enthalpic response of glasses to temperature-modulated differential scanning calorimetry in the glass transition region has been made. This yields quasi-continuous values of the complex specific heat capacity, C-p*, and its in-phase and out-of-phase components, C-p' and C-p ", respectively, as well as those of the phase angle, phi. This analysis has been used to predict the effects of three parameters: the period, the amount of annealing, and the nonlinearity parameter. Increasing the period reduces the mid-point transition temperature for C-p' in a quantitatively similar way to the effect of cooling rate on T-g, from which a relationship between period and cooling rate has been derived and compared with a fluctuation model for the glass transition. The amount of annealing is shown to have no effect on the area under the C-p" peak, confirming that this out-of-phase component does not provide information about the enthalpic state of the glass. Finally, the non-linearity parameter has opposite effects on the cooling and heating stages of intrinsic thermal cycles, with reducing x causing the C-p " peak to broaden on cooling and to sharpen on heating. For these same intrinsic cycles, the area difference under the C-p" peaks on heating and cooling may be either positive or negative, depending on the value of x. This last observation may have implications for the interpretation of C-p" in terms of entropy changes occurring during the modulation periods. (C) 1997 Elsevier Science B.V.