Analysis of structural and water diffusional properties of ionomer thin film by coarse-grained molecular dynamics simulation

Nanoscale structural and diffusional properties of Nafion ionomer thin films were studied via coarse-grained molecular dynamics method which allowed the larger spatial scale of the simulation system compared with ordinary molecular dynamics simulations. The ionomer films presented the hydrophobic configurations at the film/vapor interface and the multilayer structure with alternating hydrophilic and hydrophobic domain at the film/substrate interface. The strong orientation of Nafion side chain pointed to the water-rich layer due to the strong interactions between water and Nafion sulfonate groups. The interface structures of Nafion ionomer thin films resulted in the locally low water transport properties at interface region, especially at hydrophilic substrate. The fundamental difference in orientation and packing of Nafion aggregates on various substrates in this study provided insight into the molecular and structural origins of the water diffusion properties in the Nafion ionomer thin film.