Difficulties associated with the measurement of the diffusion coefficient of solvating liquid or vapor in semicrystalline polymer. I. Permeation methods

The diffusion coefficients D for solvating liquids and vapors in membranes can be measured in a number of ways. The effectiveness of permeation methods for systems involving semicrystalline polymers at near‐ambient temperatures has been assessed, and new experiments have been performed using p‐xylene and high‐density polyethylene. The conclusions are likely to apply to any semicrystalline polymer where the diffusant concentration is not zero. Stresses induced in the downstream side of the membrane by solvent sorption at the upstream surface during the nonsteady period are shown to lead to underestimation of D. The rate of attainment of a steady state is controlled primarily by the rate at which these stresses relax. The extremely slow rate of stress relaxation in polyethylene implies that, under many experimental conditions, a true steady state of permeation is not attained within an experimentally accessible time, a conclusion supported by experimental evidence. Diffusion coefficients calculated in the usual way from apparently steady‐state permeation rates may be grossly underestimated. In liquid permeation experiments, some of the polymer may dissolve in the upstream permeant, and this may lead to overestimation both of the time to reach an apparently steady state and of the steady‐state permeation rate itself.