Diffusion in polymer-solvent systems

Abstract

Recent advances in the area of molecular diffusion in polymer-solvent systems will be reviewed. Alfrey's classification scheme for the diffusion of solvent molecules in polymers will be used as a starting point. It will be shown that the various regions on Alfrey's diagram are distinguished by the ratio of two characteristic times, a characteristic relaxation time for the polymer-solvent system and a characteristic diffusion time. Fickian diffusion is realized when this characteristic ratio is a small number, and anomalous behavior occurs when these two characteristic times are the same order of magnitude. Alfrey's “null” region corresponds to those conditions where this dimensionless ratio is much greater than one. The different manifestations of anomalous diffusion with polymer-solvent systems will be discussed, including case II transport as defined by Alfrey, Gurnee, and Lloyd. Finally, diffusion in polymer-solvent systems above the glass transition temperature where classical diffusion theory is applicable will be considered. Emphasis will be placed on the demonstration of the ability of theoretical methods based on free volume concepts to predict the concentration and temperature dependencies of diffusion coefficients. The extension of this free volume theory to describe diffusion in glassy polymers will also be demonstrated. Experimental results for several polymer-solvent systems which exhibit different types of transport behavior will be used to illustrate the correlative and predictive capabilities of these recently developed theories.