Nearly Ideal Polymer Network Ion Gel Prepared in pH-Buffering Ionic Liquid

Abstract

We report a high-toughness ion gel with a nearly ideal polymer network prepared in an imidazolium-based aprotic ionic liquid (aIL) with a controlled solution pH. We formed the ion gel from tetra-armed poly(ethylene glycol) (TetraPEG), i.e., an A–B-type cross-end coupling reaction of two different TetraPEG prepolymers. To complete this A–B-type reaction, we needed to optimize the reaction rate such that the two TetraPEGs were mixed homogeneously, which strongly depends on the pH or [H+] in the aIL solution. To control solution pH, we established a “pH-buffering IL” by adding an imidazolium-based protic IL (as a proton source) and its conjugated base to the solvent aIL. We demonstrated that the pH-buffering IL exhibits a successful pH-buffering effect to maintain a constant pH (≈16.2, apparent value) during the gelation reaction. From a kinetic study, we found that the gelation reaction undergoes a simple second-order reaction of the two TetraPEGs in the pH-buffering IL. The gelation rate constant, kgel, in the present ion gel system was 2 orders of magnitude smaller than that in the corresponding hydrogel system, which is ascribed to the difference in the activation entropy, ΔS‡, of the cross-end coupling reactions. The reaction efficiency at the cross-linking point was experimentally estimated to be 92% by spectroscopic measurements. We thus conclude that a nearly ideal polymer network was formed in the pH-buffering IL system. This is reflected in the excellent mechanical property of the ion gel, even at a low polymer content (=6 wt %).