Morphology and ionic conductivity of polymerized ionic liquids

Polymerized ionic liquids (PILs) are polymers synthesized with ionic liquid (IL) moieties wherein the IL moieties are covalently incorporated in the polymer backbone or polymer side chains. The structural, mechanical and thermal properties of PILs can be modified and probed by systematically varying select functional groups in the ionic liquid moiety. The ionic conductivity has two processes: (i) ion diffusion and (ii) ion dissociation. In order to fully understand these processes, this research sought to correlate the morphology and the ionic conductivity by identifying and understanding how the chemical structure of the PILs dictates the morphologies that affect ion mobility. The relationship between morphology and ionic conductivity is studied in twenty-one PIL homopolymers using their material analysis, X-ray scattering, electrochemical impedance spectroscopy and dielectric relaxation spectroscopy. The X-ray scattering is studied as a function of temperature and relative humidity using a designed and constructed in situ environmental chamber. The chemical structure and morphology are studied as a function of cation type, alkyl chain length, anion type, and percent water uptake. The morphologies of these PILs have up to three characteristic length scales: (i) backbone-to-backbone, (ii) anion-to-anion and (iii) pendant-to-pendant. Our thorough X-ray scattering study provides morphological information to aide in the understanding of the ionic conductivity. The ionic conductivity results show that the glass transition temperature (Tg) is a dominant, but not exclusive, parameter in determining ion transport. We found that while increasing the pendant length (n) or anion size can decrease Tg to increase conductivity, this also increases the intermolecular separation (db), which acts to decrease ionic conductivity. Further interpretation of the ionic conductivity using the Vogel-Fulcher-Tammann (VFT) equation enabled the correlation between polymer morphology and ionic conductivity, which highlights the importance of anion hoping between adjacent polymer backbones and ion mass. Ion hopping involves two types of motion, namely the intramolecular and the intermolecular ion hopping modes. Our result shows that the Tg− independent ionic conductivity decreases with an increase in the intermolecular backbone-to-backbone, intramolecular anion-to-anion distances and anion mass. This work provided a mean to distinguish these three contributions