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

Abstract

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 (T g ) 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 T g 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.

Influence of Tethered Ions on Electric Polarization and Electrorheological Property of Polymerized Ionic Liquids

“…Different from the crystallized structure of conventional ionic solids or polymeric ionic coordination complexes [ 22 , 23 ], most of PILs are amorphous solids. Thereby, WAXS spectra have been frequently employed to analyze morphology of structure [ 24 , 25 ]. We also used WAXS to characterize the micromorphology of PILs as shown in Figure 3 .…”

“…It is seen that n of PF 6 − in P[C 2 VIm][PF 6 ] is the highest among three PILs, but its conductivity is the lowest. This reveals that the real number of free PF 6 − contributing to conductivity is not in accordance with this because of different ion-pair complexation interaction energy ( E c ) [ 24 ]. The value of E c can be calculated by DFT with Gaussian program by considering the different electronegativity of atoms and charge distribution of anion and cation in the real PIL structure as shown in Figure 8 [ 19 , 32 ].…”

Liquid‐to‐Solid Phase Transitions of Imidazolium‐Based Zwitterionic Polymers Induced by Hofmeister Anions

“…Ammonium and imidazolium cations have a local positive charge at the nitrogen atom and a delocalized positive charge within the nitrogen heterocycle, respectively. The molecular structure of ammonium cations better shields electrostatic interactions with anions than that of imidazolium cations [32] . In addition, the nearly flat structure of the imidazolium ring allows most of the atoms to be easily exposed to anions, resulting in a shorter effective distance from the anions compared to ammonium [32] .…”

“…The molecular structure of ammonium cations better shields electrostatic interactions with anions than that of imidazolium cations [32] . In addition, the nearly flat structure of the imidazolium ring allows most of the atoms to be easily exposed to anions, resulting in a shorter effective distance from the anions compared to ammonium [32] . Thus, poly(SBVI) has a stronger inter/intrachain interaction resulting from electrostatic interactions between cations and sulfonate anions than poly(SBMA).…”

Design of Clickable Ionic Liquid Monomers to Enhance Ionic Conductivity for Main-Chain 1,2,3-Triazolium-Based Poly(Ionic Liquid)s

“…Thus, to impart PILs with superior ionic conductivities, various polymer backbones and spacers have been examined that vary in their chemical nature and structure. 22 − 24 A range of cationic/anionic moieties have also been studied, and some trends between the macromolecular design of PILs and the resulting ionic conductivity have been reported. For example, the chemical nature and structure of both the cations and anions greatly affects the ionic conductivities of PILs.…”