Sorption and diffusion in ethyl cellulose. Part I. History‐dependence of sorption isotherms and permeation rates

Abstract

The sorption and permeation of benzene, acetone, and methanol in and through ethyl cellulose has been investigated from the point of view of hysteresis and time‐dependence. The three sorbates are imbibed decreasingly in the order given, at any relative pressure, and so swell the polymer to different extents. The medium is one in which anomalous diffusion is to be expected. The study was made in the lower range of concentrations of penetrant. The initial isotherm of acetone at 50°C. lay below subsequent ones at the same temperature. The higher the temperature the smaller this difference tended to be, while the greater the pressure and so the amount imbibed the greater it became. Eventually “settled” reproducible isotherms were found. If the outgassed polymer, following lower temperature isotherms, was “annealed” at a higher temperature and then cooled again, the initial isotherm at 50°C. was more nearly approached. However, subsequent isotherm determinations at the same temperature again established the “settled” isotherm. Permeation rates were determined as a function of pressure under various treatments. Repeated permeation runs caused the permeability of the polymer to decrease to a “settled” value. This behavior may be ascribed to some orientation of polymer, by mass flow, during permeation. After establishing the settled permeability the polymer was outgassed and “annealed” at a higher temperature, and the permeability at the original temperature then lay below the “settled” value. Benzene, which swelled the membrane more than the other sorbates, showed more pronounced effects of the same kind as acetone. On the other hand methanol, which was least sorbed, showed less pronounced time‐ and history‐dependence. Sorption always occurred more slowly than desorption. The various effects have been discussed in terms of a model of the polymer having rather stiff chains with some semipermanent interchain bonds, but other bonds more easily rearranged. The history‐dependent effects can arise at temperatures well above the glass transition usually quoted for ethyl cellulose.