Structures and transport properties of hydrated water-soluble dendrimer-grafted polymer membranes for application to polymer electrolyte membrane fuel cells: Classical molecular dynamics approach

We investigate a new molecular architecture in which water-soluble dendrimers are grafted onto a linear polymer for application to polymer electrolyte membrane fuel cells (PEMFC). Using full-atomistic molecular dynamics simulations, we examined the nanophase segregation and transport properties in hydrated membranes with this new architecture. In order to determine how the nature of the linear polymer backbone might affect membrane properties, we considered three different types of linear polymers, poly(epichlorohydrin) (PECH), polystyrene (PS), and poly(tetrafluoroethylene) (PTFE), each in combination with the second-generation sulfonic polyaryl ether dendrimer to form PECH-D2, PS-D2, and PTFE-D2. Our simulations show that the extent of nanophase segregation in the membrane increases in the order of PECH-D2 (similar to 20 angstrom) < PS-D2 (similar to 35 angstrom) < PTFE-D2 (similar to 40 angstrom) at the same water content, which can be compared to similar to 30-50 angstrom for Nafion and similar to 30 angstrom for Dendrion at the same water content. We find that the structure and dynamics of the water molecules and transport of protons are strongly affected by the extent of nanophase segregation and water content of the membrane. As the nanophase-segregation scale increases, the structure in water phase, the water dynamics, and the proton transport approach those in bulk water. On the basis of the predicted proton and water transport rates, we expect that the PTFE-D2 may have a performance comparable with Nafion and Dendrion.