Ion Conduction in Imidazolium Acrylate Ionic Liquids and their Polymers

Abstract

Polymerizable imidazolium acrylates and their polymers with pendant imidazolium cations were synthesized with hexafluorophosphate and bis(trifluoromethanesulfonyl)imide counterions and characterized using calorimetry and dielectric spectroscopy. The ionic polymers containing a diethyleneoxy unit as an N-substituent on the imidazolium cation display higher ionic conductivities than the analogous N-n-butyl polymers. Using a physical model of electrode polarization, we separate the conductivity of single-ion conductors into number density of conducting ions p and their mobility μ. The monomers invariably possess higher conducting ion number density than the polymers, owing to the cation being part of the polymer, but p is insensitive to the N-substituent. In contrast, the diethyleneoxy N-substituent imparts higher mobility than the n-butyl N-substituent, for both monomers and polymers, owing to a lower binding energy between the imidazolium and the counteranions, which is not directly reflected in glass transition temperatures.