Correlation between glass transition temperature and the width of the glass transition interval

The glass transition is a kinetic phenomenon. Its basic characteristics-the glass transition temperature, T-g, and the width, delta T-g, of the glass transition interval-depend significantly on cooling and heating rates as it is observed experimentally by standard DSC and fast scanning calorimetry. The knowledge of these and related correlations is of outstanding importance both for a theoretical understanding of vitrification and devitrification processes and their control in a variety of technological applications. By these reasons, general kinetic criteria of vitrification and, based on them, theoretical expressions for the glass transition temperature and the width of the glass transition range in dependence on rates of change of temperature have been derived in previous papers. These results are advanced here by establishing a direct correlation between these two quantities, T-g and delta T-g. The ratio delta T-g/T-g is shown, for arbitrary cooling and heating rates, to be a function of an appropriately defined index being a straightforward generalization of the definition introduced by Angell. The theoretical results are tested and confirmed by experiment. The methods employed here can be utilized generally for the description of vitrification caused also by a variation of other external control parameters like pressure.