Diffusion in glassy polymers. IV. Diffusion of methyl ethyl ketone in atactic polystyrene above the critical concentration

Integral absorption and desorption of methyl ethyl ketone in atactic polystyrene were studied as functions of film thickness at 25 and 45°C. with concentrations above the critical concentration of the system. In all cases investigated, the reduced absorption and desorption curves for films of different thicknesses did not give a single curve, though the individual absorption and desorption curves appeared to have the shape expected from the Fickian diffusion mechanism. The initial slopes of the reduced curves increased with increasing film thickness for both absorption and desorption. The quantity R 0 , the value of the reciprocal of the half‐time for absorption or desorption extrapolated to zero concentration increment, provides a measure of absorption rate at a given initial concentration. The dependence of R 0 , on film thickness was almost unaltered in the concentration region concerned at both temperatures. The values of the mutual diffusion coefficient D of the system were calculated by assuming absorption and desorption processes for hypothetical film of infinite thickness to be controlled by the purely Fickian mechanism. The values of D evaluated by this method agreed quite well with those deduced by Kishimoto from steady‐state permeation measurements.