Abstract
Master curves of the relaxation time, , or viscosity, , versus , where is temperature, the specific volume, and 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 or at the crossover is independent of both and ; 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 , whereas by interpolating within 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 or ) 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.
5 More- Received 15 June 2004
DOI:https://doi.org/10.1103/PhysRevB.71.014210
©2005 American Physical Society