Influence of Cation Type on Ionic Aggregates in Precise Ionomers

We report atomistic molecular dynamics (MD) simulations of model ionomers with precise spacing between charged groups (polyethylene-co-acrylic acid). We explore different counterion types, neutralization levels, and spacer lengths between acid groups and provide a thorough analysis of the resulting ionic aggregate morphologies. Structure factors computed from the simulations are in good agreement with previous experimental Xray scattering data, which provides strong validation of the simulation methods. Aggregate morphologies range from small spherical aggregates to string-like shapes and large percolated networks. The unexpected morphologies of the ionic aggregates suggest the need for a novel interpretation of scattering data for these materials. We quantify cation-anion and oxygen-hydrogen association, the two interactions primarily responsible for aggregate formation, and report detailed information pertaining to local structures around cations. This information is difficult to obtain experimentally and may have important consequences for ion transport.