Sorption and diffusion in polymers

Abstract

Sorption and diffusion of gases and vapours in polymers are frequently the subjects of a single investigation. The rate at which a substance is absorbed by or permeates through a polymer is a function of the solubility and diffusion coefficients of the substance in the polymer. Gases in polymers closely obey the ideal laws of Henry and Fick. The mechanism of their diffusion in homogeneous polymers is well understood and information about the motions and distribution of the polymer chains has been obtained from studies of diffusion as a function of temperature. Organic vapours diffuse in polymers by a place-exchange mechanism. At high temperatures and concentrations of diffusate Fick's law is obeyed but with a diffusion coefficient which increases with concentration. The concentration dependence of the diffusion coefficient is a function of free volume in a way related to a number of other properties of polymers+organic diluents. Sorption cannot be described satisfactorily by Henry's law but obeys the Flory—Huggins theory quite well. At lower temperatures and concentrations Fick's law is not obeyed and the apparent diffusion coefficient varies with time as a result of the slow relaxation processes which accompany the swelling of the polymer by diffusate. Because the characteristics of these relaxation processes are a function of the local concentration of diffusate, complex phenomena are observed in systems undergoing transient diffusion. The range of phenomena which may be observed are illustrated by discussing the rates of sorption of vapours by polymers and their interpretation. The examples chosen are restricted to amorphous polymers.