Connecting Features of Ionomer Scattering Profiles to the Real-Space Structural Features of Ionomer Domains

Membranes made of hydrated ionomers are frequently used in ion-exchange applications, such as hydrogen fuel cells and water electrolyzers. Perfluorinated sulfonic acid (PFSA) ionomer membranes are suited for these applications as they possess chemical stability and mechanical stability at elevated pressures and temperatures and have ion transport capabilities. Hydrated PFSA ionomers form a nanophase-separated structure that has an ion-conducting hydrophilic phase and a nonconducting, semicrystalline hydrophobic phase. This complex nanophase structure of hydrated ionomers is a topic of great interest, and there have been a number of studies attempting to elucidate the morphology of hydrated ionomers by using microscopy, simulations, and scattering measurements. In this work, we seek to understand the connection between the ionomer design and the resulting nanophase structure under various hydration conditions using coarse-grained molecular simulations. We study the effects of varying the side chain spacing and side chain lengths on the measured scattering profiles and real-space hydrophilic domain properties. To enable correct real-space interpretation of the scattering measurement structure, we relate features observed in the scattering profiles, such as the ionomer peak location and intensity, to the hydrophilic domain size distribution, tortuosity, and onset of percolation. We also show how new ionomer designs with tailored variations in side chain spacing and side chain length affect domain size distributions and connectivity of the hydrophilic domains.