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The free-volume theory for solvent self-diffusion coefficients in polymer-solvent systems is modified to include a more general analysis of the effects of solvent size. The predictions of the theory are compared with self-diffusion data for a diverse set of solvents and four polymers, polystyrene, poly-(vinyl acetate), poly(methyl methacrylate), and poly(p-methylstyrene). The new prediction of size effects is based on the consideration of the asymmetry of solvent molecules.

The free-volume theory for solvent self-diffusion in polymer-solvent systems is used to describe the self-diffusion process in glassy polymer-solvent mixtures. General predictions of the theory and comparisons of theoretical predictions with experimental data are considered.

The free-volume theory for solvent self-diffusion in polymer-solvent systems is applied to rubbery mixtures at temperatures both above and below the polymer glass transition temperature. A new parameter evaluation scheme is introduced, and it is shown that the theory provides good predictions for the solvent self-diffusion coefficient over wide temperature and concentration ranges. IntroductionA free-volume theory of transport has been developed for predicting and correlating solvent self-diffusion coefficients for rubbery polymer-solvent systems.14 There are two principal objectives of this theory: (1) to provide accurate predictions for solvent self-diffusion coefficients over wide temperature and concentration ranges and (2) to provide a method of evaluating the parameters of the theory which requires only a minimal amount of diffusion data. It has been shown recentlyg that the predictions of the free-volume theory for the solvent self-diffusion coefficient are good for temperatures above the glass transition temperature of the pure polymer. However, it has also been noted7*', that difficulties can be encountered for some systems at temperatures below the pure polymer glass transition temperature. These difficulties are caused by uncertainties in the nature of the temperature dependencies of the various expansion coefficients of the theory. The objective of this paper is to provide a reasonable framework for computing free volumes for the polymer in the equilibrium liquid state below its glass transition temperature. This framework should extend the temperature range over which the theory gives reasonable predictions and thus should produce good predictions for the solvent self-diffusion coefficient for rubbery polymer-solvent systems over all temperatures and concentrations of practical interest.The modification of the free-volume theory is considered in the second section of the paper, and the parameter evaluation scheme is summarized in the third section. The predictions of the theory are compared with comprehensive sets of experimental data in the fourth section of the paper.

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