A comprehensive dynamic diffusion model is developed to calculate the diffusion coefficients of low molecular weight penetrants (i.e., α‐olefins) in semi‐crystalline polyolefins from dynamic sorption measurements. The model also takes into account the extent of polymer swelling on the penetrant diffusion flux, resulting in a moving boundary value problem. The free volume theory is employed to calculate the dependence of the diffusion coefficient on the penetrant concentration. The solubilities and diffusivities of ethylene and propylene in semi‐crystalline high density polyethylene films were measured at different temperatures and pressures, using a Rubotherm® magnetic suspension microbalance operated in series with an optical view cell for the measurement of the degree of polymer swelling. It is shown that model predictions are in excellent agreement with the experimental dynamic measurements on the mass uptake of the sorbed species. Moreover, it is shown that the proposed model can predict correctly the diffusion coefficient of α‐olefins in semi‐crystalline polyolefins.
