Modeling of Liquid−Liquid-Phase Separation in Linear Low-Density Polyethylene−Solvent Systems Using the Statistical Associating Fluid Theory Equation of State

Abstract

The statistical associating fluid theory (SAFT) is used to model liquid−liquid-phase equilibria in solutions of linear low-density polyethylene (LLDPE) with hexane, heptane, and octane. The effect of temperature, pressure, polymer concentration, and polymer molecular weight on phase separation is studied. Finally, the effect of polydispersity on cloud point is also considered. SAFT results are compared with experimental data by de Loos et al. (Fluid Phase Equilib. 1996, 117, 40). SAFT can model the phase behavior of the polymer in different solvents at various state conditions with a single adjustable parameter. For a monodisperse polymer in hexane, the critical polymer concentration is linear with respect to the reciprocal of the square root of molecular weight. Calculations with a polydisperse polymer show that the cloud-point curve for a polydisperse polymer differs qualitatively from that of a monodisperse polymer at low polymer concentration. As the polymer concentration is decreased below the critical polymer concentration, the cloud-point pressures decrease for a monodisperse polymer. SAFT predicts that the cloud-point pressures for a polydisperse polymer continue to increase in qualitative agreement with experimental data. Thus, SAFT can be used to predict the phase behavior of LLDPE in different hydrocarbon solvents.