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

This study introduced the concept of a dendrimer-grafted polymer using precisely-defined watersoluble dendritic architecture (sulfonic poly aryl ether dendrimer) in a copolymer with a linear polymer backbone for applications such as PEMFC. In order to investigate the effect of backbone polymer on the properties suitable for PEMFC such as nanophase-segregation and transport, we grafted the secondgeneration sulfonic poly aryl ether dendrimers onto three different types of linear backbone polymers, e.g. PECH, PS and PTFE for the preparation of three different types of dendrimer-grafted polymer such as PECH-D2, PS-D2 and PTFE-D2. We found that all these cases the equilibrium systems exhibits nanophase-segregation in which water phase is formed and associated with the hydrophilic dendrimers. Analyzing the pair correlation of sulfur-sulfur in sulfonic acid groups of dendrimers, we found that in the nanophase-segregated structures, the dendrimers contact each other through the membrane, helping the formation of a continuous and percolated water phase. We also found out that the water coordination number for water molecules in the water phase increases with increasing water content for all cases,. In addition, at the same water content, the water coordination number increases in order of PECH-PS < D2-D2 < PTFE-D2, indicating that the similarity of the water structure to the bulk water is also in order of PECH-D2 < PS-D2 < PTFE-D2. The extent of nanophase-segregation in the new copolymer membrane was evaluated quantitatively by analyzing structure factor profiles, S(q) as a function of scattering vector (q) for the membranes with 20 wt % of water content. By determining the characteristic dimension and density contrast in the membrane, we found that the extent of nanophase-segregation increases in order of PECH-D2 (∼20 Å) < PS-D2 (∼35 Å)) < PTFE-D2 (∼40 Å)) which can be compared to 30∼50 Å for Nation and ∼30 Å for Dendrion. We suggest that the structure of the water phase and the structure factor analysis can be understood consistently in terms of the hydrophilicity (or hydrophobicity) of backbone polymer. In comparison to the hydrophilic backbone polymer, PECH, the more hydrophobic PS and the most hydrophobic PTFE have the more extent of nanophase-segregation and thereby achieve the more bulkwater-like structure. Calculating the rotational diffusion coefficient (D_R) and the translational diffusion coefficient (D_T) from the MD trajectory files, we found that the values of these diffusion coefficients increase with increasing water content, and increase in order of PECH-D2 < PS-D2 < PTFE-D2 at the same water content, indicating that the water dynamics in the hydrated membrane is strongly coupled with the structures in water phase. We estimated the proton diffusion according to its mechanism: vehicular and hopping. For both mechanisms, the values of proton diffusion coefficients increase with increasing water content, and increase in order of PECH-D2 < PS-D2 < PTFE-D2, which also shows the dependency of proton transport on the structures in water phase as observed in the water dynamics. Based on the observations from our simulations, we expect that the PTFE-D2 dendrimer-grafted polymer membrane may have comparable performance to Nafion and Dendrion membranes in a PEM fuel cell.