Modulated differential scanning calorimetry in the glass transition region .4. Pseudo-isothermal analysis of the polystyrene glass transition

The kinetics of the glass transition of Polystyrene is measured by quasi-isothermal, temperature-modulated differential scanning calorimetry (TMDSC) and compared to pseudo-isothermal analyses of standard TMDSC traces. Quasi-isothermal TMDSC is carried out at a series of fixed average temperatures, modulated sinusoidally with an amplitude of +/-1.0 K and a period of 100 s. These measurements use, thus only a single time scale defined by the modulation frequency. The standard use of TMDSC adds a second time scale to the experiment, the underlying heating and cooling rates [q] at 1 K min(-1). In the pseudo-isothermal analysis of standard MDSC, the effects of the underlying heating and cooling rates are separated by subtraction of averages over full modulation periods from the modulated temperature and heat flow. The small differences in the reversing, apparent heat capacity between MDSC heating and cooling traces are measured and linked to the kinetic expressions of irreversible thermodynamics and interpreted using the hole theory. As the sample moves away from the equilibrium of the liquid state, the kinetics depends increasingly on the thermal history, and the expected deviations from a first-order kinetic expression are observed. (C) 1997 Elsevier Science B.V.