The sorption, diffusion, and permeation of organic vapors in polyethylene

Abstract

AbstractAn investigation has been made of the sorption, diffusion, and permeation of several organic vapors in polyethylenes of different density over wide ranges of vapor activity and concentration. It was found that the equilibrium sorption isotherms, in terms of the solubility coefficient S, could be represented as exponential functions of the sorbed vapor concentration. The limits and physical significance of the relationship are briefly discussed in terms of the two parameters which characterize the sorption for a given solvent, polymer and temperature. The integral diffusion coefficients were estimated by three independent methods encompassing both the transient and the steady‐state of flow. The diffusion coefficients determined by the different methods generally agreed within experimental error, even though the rate of sorption—desorption indicated that some time‐dependent effects were present. These integral diffusion coefficients were found to be exponential functions of vapor activity. By combining the relation describing the dependence of the diffusion coefficient on vapor activity with the expression for the dependence of the solubility on concentration, it is shown that for an ideal vapor or, at least, low concentrations of sorbed vapor, the combined equation reduces to those equations previously reported in the literature describing the concentration dependence of the integral and differential diffusion constants. The observed trends of the permeability constant with vapor activity and concentration are then related to the activity and concentration dependence of the diffusion and solubility coefficients.