Tethered Cation Size Affects the Imbibition of Polymerized Ionic Liquids and the Ionic Conductivity in Nanopores

There is a growing interest in new polymerized ionic liquids (PILs) with enhanced ion transport properties especially at low temperatures, in the vicinity of the liquid-to-glass temperature, T _g. We employed two structurally similar ionic liquids (ILs), namely, 1-butyl-3-methylimidazolium bis-(trifluoromethylsulfonyl)-imide ([BMIM]⁺[TFSI]^-) and 1-(4-vinylbenzyl)-3-butylimidazolium bis-(trifluoromethane)-sulfonimide ([VBBI]⁺[TFSI]^-) and synthesized the corresponding PILs, poly-[BVIM]⁺[TFSI]^-, and poly-[VBBI]⁺[TFSI]^-. The main difference was the positioning of the cationand hence the cation/anion coordinationwith respect to the backbone. This small structural variation had implications in anion transport in the bulk. Ion coordination in proximity to the backbone restricted backbone mobility, increased T _g, and reduced ionic conductivity. A strategy toward increasing ion conductivity at lower temperatures was by nanometer confinement. We employed as a confining medium self-ordered anodic aluminum oxide (AAO) nanopore templates and investigated the kinetics of imbibition and the ion dynamics following imbibition by ex situ polarizing optical microscopy and by in situ nanodielectric spectroscopy. These methods provided access to the effective viscosity and to the ionic conductivity of a PIL during and following imbibition in nanopores. PILs penetrated nanopores with a lower speed than expected from their bulk viscosity. At lower temperatures, in the vicinity of T _g, confinement effects took over and decoupled the ion dynamics from the arrested backbone dynamics. Under these conditions, the temperature dependence of ion conductivity deviated from the Vogel-Fulcher-Tammann law and followed an Arrhenius temperature dependence with an activation energy that was reduced from 142 kJ/mol in the bulk to ∼108 kJ/mol under confinement. The results offer new insights into how molecular structure and confinement affect ion transport in PILs.