AN EXTENSION OF CUBIC EQUATIONS OF STATE TO VAPOR-LIQUID-EQUILIBRIA IN POLYMER-SOLVENT MIXTURES

Cubic equations of state (EOS) are extended to describe polymer-solven t vapor-liquid equilibria (VLE). The solvents an described the convent ional way using critical parameters. To describe the pure polymers, on ly the weight-average molecular weight is necessary, though number-ave rage molecular weight, polydispersity and melt density can be incorpor ated if desired. To extend the model to mixtures, a mixing rule that c ombines EOS with excess energy models is used. In this formulation, th e excess Gibbs energy term is considered in two parts: the classical F iery term for the entropic contributions and a residual term that tal; es care of specific interactions between the solvent and the polymer. For athermal mixtures that exhibit no such interactions, the residual term drops out and the model becomes completely predictive. Otherwise, for residual contributions, depending upon the complexity of specific molecular interactions anticipated in the mixture, either a single pa rameter Flory expression or a two-parameter NRTL equation can be used. We conclude that the simple cubic EOS approach presented here is easy to use, yet competes successfully with more sophisticated EOS models developed particularly for polymer solutions. Moreover, it offers more flexibility if one or mon parameters are to be tuned to the process d ata. (C) 1998 Published by Elsevier Science B.V. All rights reserved.