Theories of Liquid-Glass Transition

Abstract

A number of theories have been proposed to describe atomic (molecular) movements in glassy solids; their common point is that they consider collective processes when the temperature approaches Tg, despite the fact that the heterogeneity of the microscopic structure of glassy materials has been implied by a number of theories. The structure of a glassy solid may be considered as a random distribution of local regions of spatially fluctuating density and high energy (and entropy) in an atomically disordered continuum. These local regions have of ten been described in terms of free volume or alternatively as quasi-point defects (qpd). Above Tg, the concentration, Cd, of qpd is temperature dependent as the system remains in metastable thermodynamic equilibrium, but when the temperature is decreased below Tg, Cd becomes constant. The hierarchical constraints assumption yields a theory of correlated molecular movements relating to a lifetime τmol characterizing those movements to the structural state, i.e. Cd, via a correlation parameter increasing with Cd. Such an approach is compared to those of other authors and can be used in order to describe the rheological behaviour of glasses near Tg.