Kinetics of pressure-induced phase separation (PIPS) from polymer solutions by time-resolved light scattering. Polyethylene + n-pentane

Abstract

Kinetics of pressure-induced phase separation (PIPS) in solutions of polyethylene + n-pentane at high pressures have been investigated using a novel experimental system which permits imposing controlled, multiple rapid (and repetitive) pressure quenches of different penetration depths into the region of immiscibility of the system. The evolution of new phase formation is monitored by time-resolved light scattering using a fibre optic scattering cell as a function of the depth of penetration into the two-phase regions. It is shown that the rate of change in the scattered light intensities increases with quench depth, becoming very fast and eventually reaches an apparent limiting value for quenches below a characteristic penetration depth. Determination of these quench pressures below which phase separation is extremely rapid identifies a kinetic phase separation boundary below the binodal pressures which is of practical significance. This is because pressure-induced phase separation by rapid expansion is an important step in processing of materials such as polymers with near- and supercritical fluids. The results are shown for liquid-liquid phase separation for polyethylene solutions of two different molecular weights (108 000 and 16 400) in n-pentane subjected to pressure quenches as deep as 10 MPa.