A New Physical Model for the Diffusion of Solvents and Solute Probes in Polymer Solutions

Abstract

We propose a new physical model for the interpretation of the diffusion of solvent and other solute molecules in polymer solutions. In this model, the polymer solution is regarded as a network where the diffusing molecules have to overcome periodic energy barriers of equal magnitude, where the distance between the barriers corresponds to the correlation length in polymer solutions as defined in de Gennes' scaling theory. We demonstrate that this model applies to the diffusion of small molecules in polymer matrices, such as ternary aqueous systems of poly(vinyl alcohol) (PVA) and binary organic solutions of poly(methyl methacrylate) (PMMA). The model successfully interprets the diffusion of solute molecules and water in aqueous polymer solutions and that of solvent molecules in organic polymer solutions. In particular, the effects of polymer concentration and temperature on diffusion can be predicted. An energy barrier of 21 kJ/mol is calculated from the variable temperature studies of self-diffusion in the range 23-53 °C carried out on a PVA-water-tert-butyl alcohol ternary system.