(556f) Atomistic Molecular Dynamics Simulations of Precise Ionomers

Ionomers are being investigated as potential solid electrolytes in battery applications, due to their unique electrical properties. However, the relationships between ionomer chemistry, morphology and ion transport are poorly understood, which has hindered the development of ionomer-based batteries. To this end, we report atomistic molecular dynamics (MD) simulations of a series of polyethylene-based model ionomer melts, for which spacing between functional groups is precisely controlled. We vary the polymer-bound ion chemistry (methylimidazolium, sulfonate and carboxylate), the counterion type, the neutralization level, and the length of the spacer. Where experimental data are available, comparison to the structure factor computed from the simulations is highly favorable. The simulations provide new insights into the shape, size and composition of ionic aggregates. In particular, we observe a wide variety of aggregate morphologies, ranging from small spherical aggregates to string-like shapes and large percolated networks. The unexpected morphologies of these ionic aggregates imply the need for a new interpretation of scattering spectra for these materials. As expected, counterion dynamics depend strongly on the nature of the polymer-bound ionic group. Overall, these simulations elucidate several important connections between physicochemical properties of ionomers, aggregate morphologies, and the nature of counterion transport.