Effect of Ionomer–Solvent Interactions in PFSA Dispersions: Dispersion Viscosity

Perfluorosulfonic acid ionomer (PFSA) dispersions are essential to the coating processes used to fabricate membranes, catalyst layers, and thin films for hydrogen fuel cell and water electrolyzer applications. The PFSA dispersion viscosity can significantly affect coating parameters including wetting, leveling, and compatibility with coating equipment. The effect of PFSA concentration, chemical structure, and solvent composition on dispersion viscosity is examined as a function of five different PFSAs and three different binary alcohol-water solvent systems, using n-propanol, isopropanol, or ethanol as the alcohol. The zero-shear viscosity, η₀, is observed to increase with decreasing side chain length, increasing side chain content, and increasing alcohol concentration in the binary alcohol-water solvent. A direct comparison is made between the PFSA colloidal morphology discussed in a previous publication by the present authors and η₀. At a fixed, nondilute PFSA concentration, two regimes of weak and strong dependence of η₀ on alcohol concentration in the solvent are identified. In the regime where η₀ weakly depends on alcohol concentration, an increase in η₀ is associated with an increase in the aggregate surface area normalized by side chain content. The orders of magnitude increases in η₀ with increasing alcohol concentration in the regime of strong dependence of η₀ are attributed to both aggregate morphology and interaggregate ionic associations. By independently considering the PFSA side chain and backbone solubility parameters, two regimes corresponding to relatively favorable solvent-side chain and relatively favorable solvent-backbone interactions are defined as a function of alcohol-water solvent composition. An analysis of PFSA-solvent interaction parameters shows that interaggregate ionic associations occur when solvent-side chain interactions are unfavorable relative to solvent-backbone interactionsfor example, at high alcohol concentrations in the solvent. The alcohol concentration corresponding to the crossover between the weak and strong regimes of η₀ is found to agree within ±5 wt % alcohol with the crossover between the regimes of favorable solvent-side chain and favorable solvent-backbone interactions.