An ionic liquid gel with ultralow concentrations of tetra-arm polymers: Gelation kinetics and mechanical and ion-conducting properties

Abstract

We report an ion gel with ultralow polymer content based on two key materials: 1 wt% tetra-arm poly(ethylene glycol) (TetraPEG) and 99 wt% ionic liquid (herein, 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)amide, [C2mIm][FSA]). We performed kinetics analysis on the TetraPEG network formation in [C2mIm][FSA], i.e., AB-type cross-end coupling reaction of two different TetraPEGs with thiol (SH) and maleimide (MA) terminals (TetraPEG-SH and TetraPEG-MA). In neat [C2mIm][FSA], no gelation occurred at room temperature, whereas the reaction successfully proceeded when 1-ethylimidazole (C2Im) was added in this reaction as a catalyst. The apparent reaction rate constant (k′gel) was determined by varying C2Im concentration, and the network connectivity (p) was estimated to be p ∼ 98% toward the end of the reaction. The results indicated that the TetraPEG prepolymers could be successfully gelled in [C2mIm][FSA] despite extremely low polymer content (1 wt%), which allows us to prepare an ideally homogeneous polymer network gel with a large amount of ionic liquid. The ionic conductivity measurements demonstrated that the 1 wt% TetraPEG ion gel showed ideal liquid-like ion-conducting properties that were comparable to those of neat ionic liquid.