Comparing ion transport in ionic liquids and polymerized ionic liquids

Polymerized ionic liquids (polyILs) combine the unique properties of ionic liquids (ILs) with macromolecular polymers. But anion diffusivities in polyILs can be three orders of magnitude lower than that in ILs. Endeavors to improve ion transport in polyILs urgently need in-depth insights of ion transport in polyILs. As such in the work we compared ion transport in poly (1-butyl-3-vinylimidazolium-tetrafluoroborate) (poly ([BVIM]-[BF₄])) polyIL and 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM]-[BF₄]) IL. The diffusivities of ions in the polyIL and IL were measured and computed. According to the results of the molecular dynamics simulations performed, in the IL the coupling motion between an anion and the ions around determines the ion diffusivities, and the ion association lifetime gives the time scale of ion transport. But in the polyIL, the hopping of an anion among cages composed of cationic branch chains determines the diffusivity, and the associated anion transport time scale is the trap time, which is the time when an anion is caught inside a cage, not the ion association lifetime, as Mogurampelly et al. regarded. The calculation results of average displacements (ADs) of the polyIL chains show that, besides free volume fraction, average amplitudes of the oscillation of chains and chain translation speed lead to various diffusivities at various temperatures.