Simulation Study of Sulfonate Cluster Swelling in Ionomers

TL;DR

This simulation study investigates how humidity influences the structure and water distribution in low-sulfonate Nafion-like ionomers, revealing cluster splitting, swelling behavior, and water-rich shells around clusters.

Contribution

It provides new insights into the structural response of low-sulfonate ionomers to humidity, highlighting cluster behavior and water distribution patterns.

Findings

Clusters split into smaller parts at hydration onset

Water distribution depends strongly on sulfonate concentration

Formation of water-rich shells around clusters

Abstract

We have performed simulations to study how increasing humidity affects the structure of Nafion-like ionomers under conditions of low sulfonate concentration and low humidity. At the onset of membrane hydration, the clusters split into smaller parts. These subsequently swell, but then maintain constant the number of sulfonates per cluster. We find that the distribution of water in low-sulfonate membranes depends strongly on the sulfonate concentration. For a relatively low sulfonate concentration, nearly all the side-chain terminal groups are within cluster formations, and the average water loading per cluster matches the water content of membrane. However, for a relatively higher sulfonate concentration the water-to-sulfonate ratio becomes non-uniform. The clusters become wetter, while the inter-cluster bridges become drier. We note the formation of unusual shells of water-rich material that surround the sulfonate clusters.