A NEW SIMPLE EQUATION FOR THE PREDICTION OF SOLVENT ACTIVITIES IN POLYMER-SOLUTIONS

A new equation for an activity coefficient—a so-called free volume activity coefficient— which includes combinatorial and free volume contributions has been derived from a van der Waals free volume type of expression. The new expression is analogous to the Flory–Huggins combinatorial term. The free volumes of solvents and polymers are compared, and it is shown that even for mixtures of solvents the free volume effect cannot always be ignored. The new free volume activity coefficient is superior to the Flory-Huggins combinatorial in predicting solvent activities in polymer solutions without any energy interactions. For polymer solutions with energy interactions a UNIQUAC residual term is added. The energy parameters are obtained from small-molecule homologues. Again the new equation is superior to the Flory-Huggins equation in predicting the solvent activities of the polymer solution homologues. The results obtained with this new equation compare favorably with the Holten-Andersen equation of state and the UNIFAC-FV model by Oishi and Prausnitz. © 1990, American Chemical Society. All rights reserved.