Nafion Ionomer Dispersion in Mixtures of 1-Propanol and Water Based on the Martini Coarse-Grained Model

The self-assembly of Nafion ionomer in a mixture of 1-propanol (NPA) and water was investigated using coarse-grained molecular dynamics simulations. Ionomer formation into cylindrical bundle-like aggregates is observed when the ionomer chain size is sufficiently large. The size of ionomer bundles decreases (the diameter decreased from ~2.5 to ~1.9 nm) with increasing NPA content, which indicates that the ionomers tend to be more dispersed at higher NPA content. The results of simulations are in good quantitative agreement with the stable size of an ionomer bundle estimated from our free energy calculations as well as the available experimental data. Furthermore, in contrast to the polarizable NPA model, the standard nonpolarizable NPA model shows the opposite trend that the ionomer size increases with increasing NPA content because of a higher localization of hydronium ions at the bundle surface, revealing that the polarization effect plays a significant role in determining the aggregation behaviors. The present study provides insight into the control of ionomer self-assembly toward obtaining targeted structures for specific purposes.