Molecular dynamics simulations of oxygen transport through a fully atomistic polyimide membrane

Oxygen (02) transport through a fully atomistic glassy ODPA-ODA polyimide free-standing membrane has been studied using large-scale molecular dynamics (MD) simulations. Both the polymer matrix and the penetrants were described with a realistic force field, including full excluded-volume and electrostatic interactions, and the total number of atoms amounted to 65 480. The polyimide had the typical features of surface layers, i.e., flattened chain configurations which led to a density corresponding to similar to 97% of the bulk density, surface roughness, and increased mobility at the interface. Oxygen penetration and diffusion through this membrane model was followed over a total of 13 000 ps. Bulk models were also studied for comparison. In the membrane, O-2 exhibits two distinct mobility modes. The first one is associated with the adsorption of the gas at the polymer interface, which is driven by the strong solubility gradient in this region and where the mobility remains fairly close to that in the gas phase. The second one is a much slower and limiting diffusion mode with a boundary situated at the start of the denser part of the matrix. The diffusion here was found to be anomalous at short time intervals in a similar way to that in the bulk. It eventually became Fickian at longer time intervals irrespective of the definition of the membrane width and showed that it is mostly governed by the density of the matrix. Such polymer interfacial structures are thus associated with an increase in both solubility and diffusion.