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 qua si-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 ave rage temperatures, modulated sinusoidally with an amplitude of +/-1.0 K and a period of 100 s. These measurements use, thus only a single ti me scale defined by the modulation frequency. The standard use of TMDS C adds a second time scale to the experiment, the underlying heating a nd cooling rates [q] at 1 K min(-1). In the pseudo-isothermal analysis of standard MDSC, the effects of the underlying heating and cooling r ates are separated by subtraction of averages over full modulation per iods from the modulated temperature and heat flow. The small differenc es in the reversing, apparent heat capacity between MDSC heating and c ooling traces are measured and linked to the kinetic expressions of ir reversible thermodynamics and interpreted using the hole theory. As th e sample moves away from the equilibrium of the liquid state, the kine tics depends increasingly on the thermal history, and the expected dev iations from a first-order kinetic expression are observed. (C) 1997 E lsevier Science B.V.