Entropy Basis for the Thermodynamic Scaling of the Dynamics of OTP

Abstract

Structural relaxation times and viscosities for non-associated liquids and polymers are a unique function of the product of temperature, T, times specific volume, V, with the latter raised to a constant, g_tau. Similarly, for both neat o-terphenyl (OTP) and a mixture the entropy for different T and pressures, P, collapse to a single curve when expressed versus TV^g_s, with the scaling exponent for the entropy, g_s, essentially equal to the thermodynamic Gruneisen parameter. Since the entropy includes contributions from motions, such as vibrations and secondary relaxations, which do not affect structural relaxation, g_s < g_tau. We show herein that removal of these contributions gives a satisfactory accounting for the magnitude of g_tau. Moreover, the relaxation times of OTP are found to be uniquely defined by the entropy, after subtraction from the latter of a V-independent component.