Correlation of solvent activities in polymer solutions: a comparison of models

The activity coefficient models that have been proposed for correlation of vapor–liquid equilibrium (VLE) of polymer solutions are often based on an ad hoc coupling of combinatorial (free volume) and residual terms available in the literature. The goal of this work is to present a comparison of the performances of all the possible models that can be obtained based on the best and most used combinatorial (free volume) and residual terms. The combinatorial terms studied here are the entropic free volume, p-free volume and Freed FH and the residual terms the NRF, UNIQUAC and Wu-NRTL along with a segment-based UNIQUAC term, sUNIQUAC, here proposed for the first time. All the models investigated have two adjustable parameters and a two-parameter Flory–Huggins model was used as benchmark for comparative purposes. A database of 70 VLE data systems was used to evaluate the different models. shown that models based on the p-free volume combinatorial term coupled with a segment-based NRTL or UNIQUAC residual part can provide an excellent correlation of VLE data for polymer systems and describe a wide range of molecular weights using a single pair of interaction parameters. The NRF-based models are shown to suffer from some deficiencies that limit their use in concentrated solutions.