Free‐volume estimates in heterogeneous polymer systems. I. Diffusion in crystalline ethylene–propylene copolymers

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Abstract
Apparent thermodynamic diffusion coefficients were obtained from carbon tetrachloride, benzene, and n‐hexane sorption‐desorption kinetics in crystalline and amorphous ethylene‐propylene copolymers (with propylene content from 1 to 70 wt. %, and crystallinity from 0% to 77%), in high‐density and low‐density polyethylene, and in polypropylene. The dependence of the diffusion coefficient vs. concentration curves on crystallinity and propylene content in the copolymers is reported. The diffusion coefficient at zero penetrant concentration increases with decrease in crystallinity. The apparent diffusion activation energies in the temperature interval investigated (25 to 75°C) are independent of temperature and are constant for crystalline copolymers. A modified Fujita‐like free‐volume theory for diffusion in crystalline polymer systems is introduced, and the theoretical estimates of diffusion coefficients show satisfactory agreement with experiment.