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 respon se of glasses to temperature-modulated differential scanning calorimet ry in the glass transition region has been made. This yields quasi-con tinuous 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 ann ealing, and the nonlinearity parameter. Increasing the period reduces the mid-point transition temperature for C-p' in a quantitatively simi lar way to the effect of cooling rate on T-g, from which a relationshi p between period and cooling rate has been derived and compared with a fluctuation model for the glass transition. The amount of annealing i s 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 t he enthalpic state of the glass. Finally, the non-linearity parameter has opposite effects on the cooling and heating stages of intrinsic th ermal cycles, with reducing x causing the C-p '' peak to broaden on co oling and to sharpen on heating. For these same intrinsic cycles, the area difference under the C-p'' peaks on heating and cooling may be ei ther positive or negative, depending on the value of x. This last obse rvation 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.