Prediction of Liquid-Liquid Equilibrium for Binary Polymer Solutions with Simple Activity Coefficient Models

Liquid-liquid equilibrium predictions for binary polymer solutions using four simple UNIFAC-based activity coefficient models are presented in this work. The four models are the original UNIFAC, the new UNIFAC recently developed in Lyngby, a modified Flory-Huggins model, and the recently developed entropic-FV model. All four models are purely predictive, since they are based on the group-contribution approach. They employ existing UNIFAC group interaction parameter tables, which have been estimated from vapor-liquid equilibrium data of mixtures with exclusively low molecular weight compounds. The investigated models are capable of predicting UCST qualitatively well, but only the modified Flory-Huggins and the entropic-FV models can predict LCST (near the critical temperature of the solvent) and are, thus, able to describe the combined UCST/LCST behavior often found in polymer solutions. In particular, the entropic-FV model can represent the five types of phase diagrams which are most often encountered in polymer solutions, including closed loops and hourglass type. Furthermore, the predictions with the entropic-FV model are in several cases (especially for nonpolar solutions) semiquantitatively correct and generally far more accurate than those provided by the classical UNIFAC models.