THE APPLICATION OF MODULATED DIFFERENTIAL SCANNING CALORIMETRY TO THEGLASS-TRANSITION OF POLYMERS .1. A SINGLE-PARAMETER THEORETICAL-MODELAND ITS PREDICTIONS

A single relaxation time model describing the kinetics of enthalpy rel axation has been applied to modulated differential scanning calorimetr y (MDSC) in the glass transition region. The model is able to describe semi-quantitatively all the characteristic features of MDSC: the aver age or total heat capacity which is very similar to the conventional D SC response at the same average heating rate; a phase angle between he ating rate and heat flow modulations which passes through a maximum in the transition region; a ''loss heat capacity'' which shows a similar behaviour to the phase angle; and a ''storage heat capacity'' which s hows a sigmoidal change from glassy to liquid-like C-p. The model is u sed to predict the effects of the experimental and material parameters . Of the experimental parameters, the most important are the average h eating rate and the period. The former affects significantly only the total heat capacity, and in the same way as in conventional DSC. The l atter affects significantly only the storage heat capacity, causing th e sigmoidal transition to shift to higher temperatures as the period i s reduced. The amplitude of temperature modulation appears to have no significant effect within a reasonable range. These predictions, and t hose for the effects of some material parameters, namely the initial e nthalpic state and the non-linearity parameter, are discussed in the l ight of published experimental data.