On the Relationship between Polymer Electrolyte Structure and Hydrated Morphology of Perfluorosulfonic Acid Membranes

Abstract

Molecular dynamics simulations were performed to investigate the relationship between the molecular structure of perfluorosulfonic acid (PFSA) ionomers and the nanoscale morphology of the hydrated membranes. Three structural features are examined including (i) the length of the side chain to which the sulfonic acid group is attached, (ii) the equivalent weight (EW) of the electrolyte ionomer, and (iii) the molecular weight (MW) of the polymer electrolyte. Membrane morphologies are studied from the water content λ = 3 (λ represents number of water molecules per sulfonate group) to saturation (λ = 22). We find that with the longer side chain, there is more clustering of the sulfonate groups and more local water−water clustering, but a more poorly connected aqueous domain. When one decreases the equivalent weight (EW) in either the short side chain (SSC) PFSA or Nafion, there is more clustering of the sulfonate groups and more local water−water clustering and a better connected aqueous domain. Because connectivity enhances and confinement reduces water mobility, a decrease in EW, which enhances connectivity and reduces confinement, results in an increase in diffusivity. An increase in side chain length diminishes connectivity but reduces confinement, which together result in little change in the observed water diffusivity. For the short chains studied, we find these results to be independent of MW.