The application of modulated differential scanning calorimetry to the glass transition of polymers. I. A single-parameter theoretical model and its predictions

Abstract

A single relaxation time model describing the kinetics of enthalpy relaxation has been applied to modulated differential scanning calorimetry (MDSC) in the glass transition region. The model is able to describe semi-quantitatively all the characteristic features of MDSC: the average or total heat capacity which is very similar to the conventional DSC response at the same average heating rate; a phase angle between heating 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 shows a sigmoidal change from glassy to liquid-like Cp. The model is used to predict the effects of the experimental and material parameters. Of the experimental parameters, the most important are the average heating rate and the period. The former affects significantly only the total heat capacity, and in the same way as in conventional DSC. The latter affects significantly only the storage heat capacity, causing the sigmoidal transition to shift to higher temperatures as the period is reduced. The amplitude of temperature modulation appears to have no significant effect within a reasonable range. These predictions, and those for the effects of some material parameters, namely the initial enthalpic state and the non-linearity parameter, are discussed in the light of published experimental data.