Scaling of the supercooled dynamics and its relation to the pressure dependences of the dynamic crossover and the fragility of glass-formers

TL;DR

This study investigates how pressure influences the dynamic crossover and fragility of glass-formers by analyzing relaxation times and viscosities through master curves, revealing that fragility decreases with increasing pressure and proposing an empirical scaling function.

Contribution

The paper extends previous analyses of dynamic crossover to smaller relaxation times and demonstrates that fragility decreases with pressure across various glass-formers using a new empirical scaling approach.

Findings

Dynamic crossover occurs at a characteristic relaxation time independent of T and P.

Fragility decreases as pressure increases for all studied glass-formers.

An empirical function accurately models the master curves across conditions.

Abstract

Master curves of the relaxation time, tau, or viscosity, eta, versus T^-1V^-x, where T is temperature, V the specific volume, and x a material constant, are used to deduce the effect of pressure on the dynamic crossover and the fragility. The crossover is determined from the change in slope of derivative plots of the relaxation times or viscosities. We confirm our previous findings that the value of tau or eta at the crossover is independent of both T and P; that is, the dynamic crossover is associated with a characteristic value of the relaxation time. Previous determinations were limited to liquids having crossovers occurring at large values of tau (> 10 microsec), whereas by interpolating within T^-1V^-x space, we extend the analysis to smaller values of the crossover time. Using the superpositioned data, the dynamic crossover can be observed in isochoric data, where it is found that the relaxation time at the crossover for constant volume is equivalent to the value obtained under (the more usual) condition of constant pressure. Similarly, from the scaling analysis, isobaric relaxation times at high pressure are deduced from experimental measurements at atmospheric pressure. We find for all glass-formers studied, that the fragility (normalized temperature dependence of tau or eta) is a decreasing function of pressure. This conclusion is less subject to uncertainties in the measurements than published determinations of the pressure coefficient of fragility. Finally, we show that an empirical function, having the form of the Cohen-Grest relation but without connection to any free volume model, parameterizes the master curves, and accurately describes the data over all measured conditions.