Penetrant diffusion in polyethylene spherulites assessed by a novel off-lattice Monte-Carlo technique

Semi-crystalline polymers have a complex hierarchical structure. The purpose of this study was to mimic the real structure of polyethylene spherulites by computer simulation using an off-lattice method in order to predict their diffusion properties. The principles used to build the spherulites were based on established findings obtained by electron microscopy. Spherulites in the crystallinity range of 0–55 vol% were built. Diffusion of small-molecule penetrants assuming no interfacial trapping at the amorphous-crystal boundary was studied using a Monte-Carlo technique. The main findings were: (i) diffusion was isotropic; (ii) diffusion was independent of the aspect ratio of the crystal building bricks, clearly in disagreement with the Fricke model; (iii) the geometrical impedance factor showed a dependence on the average free path length of the penetrant molecules in the amorphous phase; and (iv) data for the geometrical impedance factor obtained by simulation compared favourably with experimental data obtained for several penetrants showing limited interfacial trapping.