Interferometric studies in diffusion. II. Influence of concentration and orientation on diffusion in cellulose acetate

Abstract

The techniques developed in part I are used to study the diffusion of the five penetrants, chloroform, acetone, methylene chloride, water and methyl alcohol into stretched cellulose acetate sheet. The diffusion coefficient—concentration relationship for diffusion at right angles to the direction of stretch is obtained at 25° C for the first three penetrants and also at 40° C for chloroform. In each case a marked increase in diffusion coefficient occurs at a volume concentration of penetrant of about 60 to 70 %. The results for chloroform indicate a mean activation energy of about 6 kcal. Photographs of interferometer fringe systems in the neighbourhood of a corner of the cellulose acetate sheet are shown for each penetrant. Each photograph reveals anisotropic behaviour. This behaviour is due partly to the diffusion coefficient being less in the direction of stretch, than perpendicular to it, and partly to the overall range of concentration being less as a result of the restricted swelling of the polymer in the former direction. The degree of anisotropy is studied quantitatively for chloroform and acetone. New evidence is provided on the nature of the sharp boundaries seen under the microscope when a penetrant enters a polymer. In particular the middle boundary, which approximately indicates the concentration at which the polymer relaxes and becomes isotropic, is found to occur at a volume concentration of 60 to 70 % for each penetrant examined quantitatively. On inspecting a model of a cellulose acetate molecule it seems likely that at this volume concentration the polymer chains are sufficiently well separated to allow one repeating unit of the polymer chain to rotate about the axis of the chain without hindrance from neighbouring chains. An analogy with second-order transition phenomena is drawn.