Some Thermodynamic Aspects of the Glass Transition: Free Volume, Entropy, and Enthalpy Theories

Abstract

The well‐known equation of Davies and Jones for the pressure dependence of the glass transition temperature, dT g /dP=Δβ/Δα=T g VΔα/ΔC p , depends on two independently testable hypotheses: that a one‐parameter description of the excess thermodynamic functions is adequate, and that the glass transition occurs when some one thermodynamic function reaches a critical value. It is concluded on the basis of experimental data available that dTg /dP<Δβ/Δα and that dTg /dP=TgVΔα/ΔCp, which suggests that a one‐parameter description will not do, and that either the entropy or the enthalpy rather than the free volume determines the transition. An examination of specific‐heat data leads to the conclusion that the only way to reconcile the above results with the WLF equation is to assume that enthalpy per molecular unit rather than free‐volume fraction is the determining variable.