The Influence of Polymer Molecular Weight on Transport Properties in Polymerized Ionic Liquids*

We report results from atomistic molecular dynamics simulations on polymerized 1-butyl-3-vinylimidazolium-PF₆^- ionic liquids, studying the influence of the polymer molecular weight on the ion mobilities and mechanisms underlying ion transport. We present results for ionic diffusivity, ion-association dynamics, ion hopping, and ion-polymer coordination behavior. With increasing polymer molecular weight, the diffusivity of PF₆^- ions is seen decrease and plateaus above seven repeat units. The diffusivity is seen to correlate well with the ion-association structural relaxation time for pure ionic liquids, but becomes more correlated with the average ion-association lifetime for larger molecular weight polymers. By analyzing the diffusivity of ions based on coordination structure, we unearth a transport mechanism in which the PF₆^- moves by “climbing the ladder” while associated with four polymeric cations from two different polymers.