Correlating ionic conductivity and nanoscale morphology of backbone type polymerized ionic liquids

Polymerized ionic liquids (PILs) are single ion conducting ionomers in which one ionic species (typically the anion) is free for transport, while the other is either located directly in the polymer chain (backbone PILs) or as a pendant group off the main chain (pendant PILs). In this study, backbone PILs with 3-methyl-pentyl spacers between the imidazolium cation and varying counter anions [trifluoromethane sulfonyl imide (TFSI^-), nonafluorobuthane sulfonate (NfO^-), and 1,1,2,2,3,3-hexafluoropropane-1,3-disulfonimide (CPFSI^-)] were synthesized. T_g-independent ionic conductivities were then characterized using broadband dielectric spectroscopy and correlated to morphology by wide-angle X-ray scattering. It was found that changing the size of the counter anion can yield an up to 1.5 order of magnitude difference in T_g-independent ionic conductivity. Moreover, after comparing these results to pendant PILs of equivalent structure, it was found that backbone PILs result in a lower ionic conductivity, suggesting that pendant PILs yield a more correlated path for charge transport.