Density-mediated transport and the glass transition: high pressure viscosity measurements in the diamond anvil cell

Abstract

Three methods have been recently demonstrated for obtaining accurate high pressure viscosity data which combine to give a viscosity range of ten decades. Dynamic light scattering is useful in the range 10 −1–10 2 cP, the rolling-ball diamond-cell viscometer covers the range 10 −1–10 7 cP and the centrifugal-force diamond-cell viscometer operates in the 10 5–10 9 cP range. The overall accuracy of these devices is 10% or better. Within this accuracy, it is found that the simple free-volume model describes the high-pressure data over wide compression ranges. Several features of these fits are noteworthy. First, extrapolation of the fitted function to 10 15 cP yields a calculated pressure, P ∗ , which correlates well with the glass transition pressure obtained experimentally by other techniques. Second, the effective molecule diameter obtained from the free-volume fits is considerably different from that obtained from thermal data, but the diameter is comparable to the effective hard-sphere diameter obtained from a modified Carnahan-Starling—van der Waals equation of state model. Third, it is found that a scaling behavior, analogous to that of fragility for thermal data, is obtained when the viscosity data are plotted versus the volume normalized to that calculated at P ∗ .