The effect of varying linker length on ion-transport properties in polymeric ionic liquids

We report results of atomistic molecular dynamics simulations on polymerized 1-butyl-3-(n-alkyl)imidazolium ionic liquids with $PF_6^-$ counterions. Consistent with experimental observations, we observe that the mobility of the $PF_6^-$ ions increases with increasing n-alkyl linker length. Analysis of our results suggests that the motion of $PF_6^-$ ions is driven by intermolecular ion hopping between chains, which in turn is influenced by ion-pair coordination numbers and intermolecular ionic separation distances. With increasing linker length, we observe 1) the anions coordinating less closely with cations and 2) intermolecular hopping distances decreasing.