Reexamination of the Molecular Size Dependence of Diffusivities of Hydrocarbons in Natural Rubber

Diffusion coefficients at zero concentration, D0, for hydrocarbon vapors in natural rubber have been determined, and their dependence on the size and shape (or length) of the penetrants has been reexamined. For normal hydrocarbons, D0 decreased gradually with increasing the number of carbon atoms up to 5 and then leveled off. For branched hydrocarbons, such as iso- and neo-paraffins, D0 was considerably lower than that for the corresponding normal ones. The dependence of D0 on the size and shape of penetrants could be explained by assuming that the principal resistance to diffusion is defined by the cross-sectional area at the most bulky portion of the major axis and also that an additional resistance due to the molecular length contributes for relatively lower hydrocarbons. The latter resistance was practically negligible for hydrocarbons with more than 4 carbon atoms in the major chain. The effective cross-sectional dimension could be estimated successfully in terms of the group dimension of Pauling.