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

The well-known equation of Davies and Jones for the pressure dependence of the glass transition temperature, dTg/dP=Δβ/Δα=TgVΔα/ΔCp, 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.