Atomistic simulations of structure of solvated sulfonated poly(ether ether ketone) membranes and their comparisons to nafion: I. Nanophase segregation and hydrophilic domains.

The results of extensive all-atom molecular dynamics (MD) simulations of water- and methanol-solvated SPEEK (sulfonated poly(ether ether ketone)) are reported. In this Part I of the two-part article, we present results elucidating the key structural features of hydrophilic domains with varying water, methanol content, and temperature. With increasing hydration, the membrane was observed to swell appreciably and transform from a state containing a large number of water clusters containing just few molecules at low water content to very large water clusters encompassing almost all molecules at the highest water contents. In comparison with the results reported for Nafion, SPEEK was observed to be characterized by more isolated and smaller sized water clusters and less pronounced percolation than Nafion at lower water contents, but the percolation characteristics became comparable to Nafion at higher water content. Water confined in SPEEK showed less internal structure than bulk water or water confined in Nafion. With increasing water content, solvation of sulfonic acid groups was noted. The average sulfur-sulfur separation in SPEEK was found to be higher compared with the results reported for Nafion. The backbone of SPEEK was found to be more rigid and more hydrophobic than that of Nafion. These observations suggest that the nanophase segregation in SPEEK is less pronounced than that in Nafion, which may contribute to the diminished crossover characteristics of SPEEK noted in experiments on direct methanol fuel cells (DMFC) and reported in Part II of this work.