A segmental interaction model for liquid–liquid equilibrium calculations for polymer solutions

This paper gives a detailed account of a newly developed segment-based thermodynamic model containing the combinatorial, free-volume, and energetic contributions to the excess Gibbs energy for correlation/prediction of liquid–liquid equilibrium (LLE) of polymer solutions. The model is derived from the entropic free-volume (entropic-FV) model, following the idea to associate the nonideality of polymer–solvent mixture with polymer segment–solvent interaction parameters. The energetic contribution is based on interactions between individual segments (repeating units) of polymer or copolymer and solvent molecule, as in the application of the mean field theory. Segment activity coefficients are calculated through the UNIQUAC model. In the present state of development, 29 binary segmental interaction parameters have been estimated. The capability of the model is demonstrated with successful representation of LLE correlation/prediction for more than 60 polymer–solvent systems. The model is capable to correlate and to predict the most common types of phase diagrams of LLE of polymer solutions (i.e. phase diagrams of the UCST, LCST, combined UCST and LCST, and “hourglass” types) with satisfactory accuracy and provides a thermodynamic framework to describe the LLE phase behavior of polymer solutions, using weakly temperature dependent FV-UNIQUAC parameters.