The temperature and pressure dependence of the dynamic properties of branched perfluoropolyethers

Abstract

The relative volume and the limiting low shear viscosity have been accurately determined as functions of temperature and pressure for a series of eight branched perfluoropolyether liquids. The Murnaghan equation of state was fitted to the volumes and three correlations for viscosity were examined: the Doolittle free volume, the Bair and Casalini thermodynamic scaling, and the improved Yasutomi. The ambient-pressure bulk modulus increases gradually with molecular weight and is low compared with conventional lubricants. The Doolittle parameter increases with molecular mass as does the occupied volume fraction. The Ashurst–Hoover interaction parameter decreases with increasing molecular mass. The improved Yasutomi model provides accuracy comparable to models requiring an equation of state. When the kinematic viscosity at 40℃ is greater than about 200 mm2/s, fragility is independent of pressure. When the kinematic viscosity is less than about 200 mm2/s fragility increases with pressure. The pressure counterpart to the Angell plot and to the Vogel, Tammann, and Fulcher equation suggested by Drozd-Rzoska et al. is shown to be workable using stability limit pressures in the tensile domain. Clearly, progress in quantitative elastohydrodynamic lubrication must rely upon accurate measurements of the dynamic properties as a function of temperature and pressure and upon accurate temperature and pressure correlations for these properties. Only then can precise predictions of film thickness and friction be made.