APPLICATION OF THE VAN-DER-WAALS-EQUATION OF STATE TO POLYMERS .2. PREDICTION

For polymers, the energy and co-volume parameters of cubic equations of state can be reliably estimated using only two low-pressure volumetric data points. This procedure is applied to the van der Waals equation of state, and bubble-point pressure calculations are performed for a number of polymer solutions. The van der Waals one-fluid mixing rules are used. The Berthelot combining rule is used for estimating the cross energy parameter, while the usual arithmetic mean combining rule is used for evaluating the cross co-volume parameter. Deviations from Berthelot's combining rule are taken into account via a simple method requiring only the molecular weight of the solvent. It is shown that when the proposed method is applied, the van der Waals equation of state can predict the equilibrium pressures of polymer solutions. The accuracy of the predictions is very good, comparable to those of other more complex equations of state, but it is in general inferior to those of the free-volume activity coefficient models. The simplicity, accuracy and general applicability of the proposed method makes it an attractive alternative to previously proposed complex models for prediction of equilibrium pressures of nearly athermal and non-polar polymer solutions.