Phenomenology of the Structural Relaxation Process and the Glass Transition

Abstract

The glass transition is a kinetic phenomenon caused by the inability of the liquid structure to equilibrate on an experimental timescale at sufficiently low temperatures. This results in changes during heating or cooling in the temperature dependence of macroscopic properties such as volume, enthalpy, dielectric constant and loss, etc., over a narrow range in temperature generally referred to as the “glass transition region.” During the past 25 years, fairly straightforward semi-empirical models have been developed for the behavior of liquids and glasses in the glass transition region. These models are able to describe both qualitatively and, if the system is not too far from equilibrium, quantitatively the time and temperature dependence of properties during cooling, heating, and annealing. In addition, analysis of the structural relaxation process using irreversible thermodynamics has shown that different properties, e.g., volume and enthalpy, are expected to exhibit different time dependencies in the glass transition region. The relevance of these models and theories to a meaningful assignment of a “glass transition temperature” is discussed.