Translational dynamics of ionic liquid imidazolium cations at solid/liquid interface in gel polymer electrolyte

Abstract

Gel polymer electrolyte (GPE) based on ethoxylated bisphenol A dimethacrylate and 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIm][BF4]) ionic liquid (IL) at the mass ratio of 20:80 was prepared by in situ photopolymerization. The obtained material showed an exceptional behavior: its ionic conductivity was approximately twice higher than that of pure IL (as measured by impedance method). To explain the observed effect the translational diffusion of cations was studied indirectly by means of fast field-cycling (FFC) proton (1H) nuclear magnetic resonance (NMR) relaxometry method. The spin–lattice relaxation times of proton in cations were measured for neat IL and the IL involved in the polymer matrix in the temperature range from 248 to 343K. The relaxation data obtained for bulk IL were analyzed in terms of both translational self-diffusion (Torrey’s model) and local molecular reorientations around a long and a short molecular axis of the IL cation (Woessner’s model). The relaxation of IL confined to polymer matrix revealed a low-frequency dispersion (not observed for bulk liquid) which is the fingerprint of the ionic liquid cations/polymer matrix interactions. Therefore, the low-frequency NMR relaxation data in GPE were analyzed assuming the reorientation mediated by translational displacements (RMTD) mechanism. This dynamic process allows to explain a very long correlation time of the order of 10−5s calculated for the cations at the polymer/IL interface and determine their diffusion coefficient. It was found that the latter is unexpectedly higher than that of the self-diffusion constant of cations in pure IL and/or inside pools of IL in the polymer matrix. This finding indicates that in the presence of the GPE matrix new cation-conducting pathways may be created which play an important role in ionic conduction at the microscopic and/or quasi-macroscopic scale.