A novel approach to liquid–liquid equilibrium in polymer systems with application to simplified PC-SAFT

A recently developed equation of state (simplified PC-SAFT) has been used to calculate binary liquid–liquid equilibria for polymer–solvent systems. A number of different polymers and solvents were examined as part of the study, including both non-associating and associating solvents. In general simplified PC-SAFT is successful in modelling liquid–liquid equilibrium, successfully predicting the correct behaviour in many systems exhibiting either upper or lower or both upper and lower critical solution temperatures. Where predictions are not accurate, a small value of the binary interaction parameter is required to correlate experimental data. A novel method, which we call the method of alternating tangents, has been developed for finding liquid–liquid equilibrium in binary polymer–solvent systems. The algorithm is robust and traces the full temperature composition curve for both UCST and LCST type systems through the critical solution temperature. The algorithm has been successfully applied for all polymer molecular weights encountered in the experimental literature. The algorithm is applicable to any equation of state for which analytical fugacity coefficients and their derivatives are available, although application was restricted in this study to the simplified PC-SAFT equation of state.