Ion Transport in Pendant and Backbone Polymerized Ionic Liquids

Polymerized ionic liquids (PILs) are single ion conductors, in which one of the ionic species is incorporated in the polymer chain while the other is nominally free to be transported. PILs are attractive as electrolytes in battery applications and other areas where liquid electrolytes are undesirable. In PILs, the ionic species can either be directly incorporated into the polymeric backbone (backbone PILs (B-PILs)) or placed as pendant groups to the chain (pendant PILs (P-PILs)). Here, we examined the ion transport, morphology, and dynamics of imidazolium-based pendant and backbone PILs with TFSI, CPFSI, and NfO counter-anions. We found that P-PILs yielded higher ionic conductivity when scaled to Tg, but B-PILs exhibited higher ionic conductivity on an absolute temperature scale, likely because of differences in the Tgs of the two systems. We also found that ion transport for B-PILs was coupled to the segmental dynamics below Tg, where the decoupling of ionic conductivity from segmental relaxation was observed for P-PILs. This study gleans insight on relating conductivity of equivalent backbone and pendant PIL structures to morphology, leading to a deeper understanding on the fundamental relationship between conductivity and morphology in PILs.