Gelation mechanism of tetra-armed poly(ethylene glycol) in aprotic ionic liquid containing nonvolatile proton source, protic ionic liquid.

We report the gelation mechanism of tetra-armed prepolymer chains in typical aprotic ionic liquid (aIL), i.e., A-B type cross-end coupling reaction of tetra-armed poly(ethylene glycol)s with amine and activated ester terminals (TetraPEG-NH2 and TetraPEG-NHS, respectively) in 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([C2mIm][TFSA]). In the ion gel system, we focused on the pH (or H(+) concentration) dependence of the gelation reaction. We thus applied the protic ionic liquid (pIL), 1-ethylimidazolium TFSA ([C2ImH][TFSA]), as a nonvolatile H(+) source, and added it into the solvent aIL. It was found that the gelation time of TetraPEG ion gel can be successfully controlled from 1 min to 3 h depending on the concentration of pIL (cpIL = 0-3 mM). This suggests that the acid-base properties of TetraPEG-NH2 showing acid-base equilibrium (-NH2 + H(+) ⇆ -NH3(+)) in the solutions play a key role in the gelation process. The acid dissociation constants, pKa's of TetraPEG-NH3(+) and C2ImH(+) (cation of pIL) in aIL were directly determined by potentiometric titration to be 16.4 and 13.7, respectively. This indicates that most of the H(+) ions bind to TetraPEG-NH2 and then C2ImH(+) exists as neutral C2Im. The reaction efficiency of amide bond (cross-linked point) systematically decreased with increasing cpIL, which was reflected to the mechanical strength of the ion gels. From these results, we discuss the gelation mechanism of TetraPEG in aIL to point out the relationship between polymer network structure and [H(+)] in the solutions.