Pressure dependence of structural relaxation time in terms of the Adam-Gibbs model

A new equation describing the behavior of the structural relaxation time, τ(T,P), as a function of both pressure and temperature, is discussed. This equation has been derived from the Adam-Gibbs theory by writing the configurational entropy, S_c, in terms of the excess thermal heat capacity and of the molar thermal expansion. Consequently, the parameters introduced in the expression are directly related to specific physical properties of the material, such as the thermal expansion coefficient α and the isothermal bulk modulus K₀. At a fixed pressure, for low pressures, the found equation reduces to a Vogel-Fulcher-Tammann equation of τ versus temperature with the fragility parameter independent from pressure. The equation for τ(T,P) was successfully tested directly by fitting the dielectric relaxation time data for two isothermal and one isobaric measurements on diglycidyl ether of bisphenol-A, carried out in previous experiments. The parameters estimated by the best fit were in reasonable agreement with the values determined from the known physical properties of the material. Finally, the expression for the change versus pressure of the temperatures at which the same value of τ_max is obtained (e.g., the change versus pressure of the glass transition temperature) agrees with several expressions previously proposed in the literature to provide a phenomenological description of the observed phenomena.