Phase-equilibrium calculations for solutions of poly(ethylene-co-vinyl acetate) copolymers in supercritical ethylene using a cubic equation of state

The cubic equation of state proposed by Sako, Wu and Prausnitz (SWP-EOS) is used to calculate the phase equilibrium for quasibinary solutions of poly(ethylene-co-vinyl acetate) copolymers (EVA) in supercritical ethylene. The copolymer is considered to be polydisperse with respect to molecular weight and, additionally, with respect to chemical composition. Equations for the cloud-point curve and for the shadow curve are derived with the aid of continuous thermodynamics incorporating both kinds of polydispersity by the use of a bivariate distribution function. The calculations are performed for EVA+ethylene systems with different averages of molecular weight and chemical composition of the copolymer at five temperatures (393–473 K). The binary interaction parameter is found to be a function of the temperature and of the chemical composition of the copolymer but to be independent of the molecular weight. Predictions of cloud-point curves and shadow curves of EVA+ethylene systems that were not included in parameter fit are very successful.