(475i) Understanding Molecular Packing within Ionomer Domains Characterized By Small Angle Scattering

Ionomers are a class of charged polymers that typically contain less than 15 mol% ionized or acidic pendant groups bonded to a low dielectric polymer backbone. [1, 2] These ion-containing polymers exhibit nanoscale phase separation properties due to the difference in solvent interaction with the hydrophobic backbone and the covalently attached charged pendant groups. For example, in water, these polymers will phase separate to form nanostructures with hydrophilic and hydrophobic domains. The hydrophilic domains with charged groups facilitate ion-conduction and the hydrophobic domains serve as anchors that can provide mechanical stability. As this nanoscale phase separation enables tuning of the mechanical and ion-conducting properties of the ionomer containing material, there is considerable interest in understanding the morphology of these phase separated domains. Structural characterization of ionomers typically involves small angle and wide-angle scattering experiments, along with microscopy. Small angle x-ray scattering profiles of hydrated ionomers exhibit a characteristic peak known as the “ionomer peak”, which is absent in dry ionomer films. Analytical model fit (e.g., local order model [3]) and data-driven computational methods (e.g., CREASE [4]) are used to interpret the scattering profiles from SAXS experiments to identify sizes and shapes of the domains. To complement these SAXS interpreting models and methods, we are conducting coarse-grained molecular dynamics simulations using MARTINI based polymer models along with explicit polarizable water model to elucidate the molecular packing within these domains as a function of hydration and polymer design.

[1] Zhang, L. H., et al. (2014). "Perspective: Ionomer Research and Applications." Macromolecular Reaction Engineering 8(2): 81-99.

[2] Kusoglu, A. and Weber, A.Z. (2017) “New insights into perfluorinated sulfonic-acid ionomers,” Chemical Reviews, 117(3), pp. 987–1104.

[3] Gebel, G. and Moore, R.B. (2000) “Small-angle scattering study of short pendant chain perfuorosulfonated ionomer membranes,” Macromolecules, 33(13), pp. 4850–4855. Available at: https://doi.org/10.1021/ma9912709.

[4] Heil, C. M., et al. (2022). "Computational Reverse-Engineering Analysis for Scattering Experiments (CREASE) with Machine Learning Enhancement to Determine Structure of Nanoparticle Mixtures and Solutions." ACS Central Science 8(7): 996-1007.