Molecular Dynamics Study of Oxygen Scattering Behavior on Perfluorosulfonic Acid Ionomer Thin Films

The scattering dynamics of oxygen molecules on perfluorosulfonic acid (PFSA) ionomer thin films was investigated using molecular dynamic simulations. We constructed PFSA ionomer thin films with different water contents on a carbon surface at 300 K and then shot oxygen molecules into the films with different incident angles and incident energies corresponding to a temperature range from 150 to 600 K. The effective softness of the film for gas-surface collisions increased on increasing the hydration level of the film. The surface regions exposing PFSA polymers and water molecules had different softness because of polymer entanglement and the mobility of water molecules, respectively. PFSA polymers make the surface more rigid than water molecules and lead to poor accommodation of oxygen molecules during the collisions. In contrast, impinging molecules are more likely to be trapped on the water molecules and trapped molecules are better accommodated than molecules reflected directly from the surface. The angular distributions of scattered molecules that were reflected directly or trapped on the surface both showed good agreement with Knudsen's cosine distribution. The diffusive reflection from the surface was caused by the roughness of the ionomer film rather than by the thermal accommodation with the surface.