Pressure effect on the coupling between rotational and translational motions of supercooled viscous fluids

Abstract

Spin‐lattice relaxation times,viscosities, and densities have been measured as a function of temperature and pressure for several supercooled viscous fluids in order to investigate the adequacy of the modified Debye equation to describe the microscopic behavior of these systems. By supercooling the range of experimentally accessible viscosities has been extended to 10 000 cP. It has been shown that for anisotropically reorienting molecules, the rotational‐translational coupling parameter increases with increasing density, in contrast to the case of symmetric molecules which possess an axis of symmetry for relatively unhindered reorientations, for which the coupling parameter decreases with increasing density. By examining a sufficiently broad viscosity range, it has been possible to show that the rotational‐translational coupling parameter is not a pressure independent quantity, as previously reported. Under isochoric conditions, the coupling parameter was found to be insensitive to kinetic energy changes. It was concluded that volume rather than kinetic energy plays the decisive role in determining the nature and the degree of the coupling.