Ion Transport in Glassy Polymerized Ionic Liquids: Unraveling the Impact of the Molecular Structure

The impact of molecular structure on ion dynamics and morphology in ammonium- and imidazolium-based glassy polymerized ionic liquids (polyILs) is investigated using broadband dielectric spectroscopy (BDS), wide-angle X-ray scattering (WAXS), and classical molecular dynamics (MD) simulations. It is shown that ammonium-based polyILs exhibit higher dc ionic conductivity at their respective glass transition temperatures (Tg) compared to imidazolium systems. In addition, the length of the alkyl spacer has a more drastic impact on ionic conductivity at comparable time scales of segmental dynamics for ammonium than imidazolium polyILs. Agreement between the characteristic ion diffusion lengths estimated from the dielectric data and the ion-to-ion correlation lengths from the WAXS and all-atom MD simulations is observed. A recently proposed approach is employed to determine ionic mobility in a broad frequency range spanning 5 orders of magnitude below the Tg of polyILs studied, providing access to a regime of diffusivities that is inaccessible to many current experimental techniques. The ion mobility is found to be more sensitive to variation of the molecular structure than to the effective number density of the mobile ions. These results showcase the subtle interplay between molecular structure, morphology, and ion dynamics in polymerized ionic liquids.