Ionic aggregates abd counterion diffusion in model ionomers

Ionomers, polymers containing a small fraction of covalently bound ionic groups, have potential as solid, single ion conducting electrolytes in future batteries. However, the ions tend to form aggregates, making counterion diffusion unacceptably slow. A key materials design question is how molecular properties affect ionic aggregation and counterion dynamics. We perform coarse-grained molecular dynamics simulations of ionomers with either periodically or randomly spaced charged beads. The charged beads are placed either in the polymer backbone (ionenes) or as pendants on the backbone. The well-known "ionomer peak" in the scattering is present in all cases. The peak is significantly more intense for pendant ions with a large periodic spacing of charged beads, which form roughly spherical aggregates. This morphology is in qualitative contrast to the extended aggregates of ionenes that show increased counterion diffusion. Possible implications for ionomer electrolyte design will be discussed.