Atomistic simulation and molecular dynamics of model systems for perfluorinated ionomer membranes

An atomistic model for perfluorinated ionomer membranes (PIMs), in particul ar Nafion materials, is presented and used in conjunction with NVT molecula r dynamics simulations to investigate the dynamic and configurational prope rties of these polymers. It is found that the electrostatic term in the for ce field is responsible for the formation of an apparently phase separated morphology which is selectively conductive, favouring the passage of cation s. Specifically, the mobility of H3O+ ions is found to be similar to 3.2 ti mes greater than that of OH- ions, under the application of an external ele ctric field. This phenomenon is shown to be consistent with a jump diffusio n model of ion transport in PIMs. There is also evidence for the existence of water in two distinct environments in the simulations: both tightly boun d to ion exchange groups, and more loosely associated with the fluorocarbon matrix.