Understanding Polymer Design Effects on Hydrated Ionomer Morphology and Hydrophilic Domain Structures

Ionomer membranes are frequently used as proton conducting membranes for fuel cell and water electrolyzer applications [Chemical Reviews 2017, 117 (3), 987-1104] due to their chemical stability, ion conductivity, and mechanical rigidity. The ion transport capabilities and mechanical properties of these membranes are dictated by the morphology of the hydrated ionomer film. The hydrophilic domains of these hydrated ionomers are of particular interest as the structure and connectivity of these domains influence the ion-transport capabilities of the membrane. In this work, we show how desired hydrophilic domain structures can be achieved by tuning the polymer design of the ionomers; and how the morphology of the various ionomer designs studied evolve while varying the extent of hydration. We use coarse-grained molecular dynamics simulations to study ionomers at mesoscopic length scales. These simulations show how the charged sidechain spacing, placement, and length impacts the morphology (i.e., domain sizes and shapes and connectivity of domains) of the polymer backbones, charged sidechains, water molecules, and hydronium beads.