Entropy Basis for the Thermodynamic Scaling of the Dynamics of OTP

TL;DR

This paper demonstrates that the thermodynamic scaling of relaxation times and viscosities in OTP and similar liquids can be understood through entropy, with a focus on separating vibrational contributions from the entropy.

Contribution

It introduces a method to account for the thermodynamic scaling exponents by removing vibrational contributions from entropy, providing a clearer understanding of relaxation dynamics.

Findings

Entropy scales with TV^g_s collapsing data onto a single curve.

The scaling exponent g_s is approximately equal to the thermodynamic Gruneisen parameter.

Removing vibrational contributions from entropy improves the correlation with relaxation times.

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.