Dispersion of Nafion Ionomer Aggregates in 1-Propanol/Water Solutions: Effects of Ionomer Concentration, Alcohol Content, and Salt Addition

Coarse-grained molecular dynamics simulations using explicit solvent models were performed to understand Nafion ionomer morphology in 1-propanol (NPA)/water solutions under various conditions (ionomer concentration, NPA/water fraction, and salt addition). The self-assembly behavior of ionomers into a cylindrical aggregate with a diameter of ∼2-3 nm was observed. At low ionomer concentration (≤5.0 wt %), the ionomer aggregate becomes smaller in size and thinner with increasing NPA fractions. At high ionomer concentration (7.5 wt %), the size of aggregates in a longitudinal direction increases significantly at high NPA fractions, suggesting that excess addition of NPA does not necessarily contribute to the dispersivity of ionomers, particularly at high ionomer concentration. These nonmonotonic behaviors of ionomer aggregation are controlled by the electrostatic repulsion among the sulfonate groups, which is determined by the balance among the dielectric constants of solvents, the distribution of hydronium ions, and the surface density of sulfonate groups. Upon the addition of salts, the size of aggregates increases significantly, and the formation of a larger disk-shaped aggregate and a secondary aggregate of multiple bundles for low and high NPA contents, respectively, was found.