Morphology and Ion Dynamics in Neat and Hydrated Sulfonated Polymers

An understanding of morphology and dynamics in single-ion conducting (SIC) polymers is needed to design these polymers for use as electrolytes. In melt SICs, the ions tend to self-assemble into nanoscale ionic aggregates, and the morphology of these aggregates affects the ion dynamics. I will describe atomistic molecular dynamics (MD) simulations of a precise SIC with a polyethylene backbone and sulfonated phenyl groups placed every 5th carbon along the backbone (p5PhSA). The ionic aggregates form percolating clusters, and the scattering profiles calculated from simulations are in good agreement with x-ray scattering. We characterize the shape of the percolated aggregate and provide evidence that the ions move by "shuttling" along the ionic aggregate. When hydrated, the acid form of the polymer has excellent proton conductivity. Increasing water content leads to an increase in the characteristic spacing between hydrophilic domains. This swelling can be obscured in X-ray scattering due to a loss of scattering contrast between the hydrophobic and hydrophilic domains. The MD simulations reveal that the systems are still nano-phase separated, even when X-ray scattering appears to indicate otherwise. Hydronium ion diffusivities calculated from the MD simulations follow the experimentally-measured trends in conductivity and are consistent with changes in the nanoscale morphology with changing water content. I will discuss comparisons with other SICs and implications for the future design of ion-conducting polymers.