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

Abstract

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,f. This analysis has been used to predict the effects of three parameters: the period, the amount of annealing, and the non-linearity 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.