†Institute for Solid State Physics, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba, 277-8581, Japan.
‡Graduate School of Science and Engineering, Yamaguchi University, 2-16-1 Tokiwadai, Ube, Yamaguchi 755-8611, Japan.
J Phys Chem B. 2015 Apr 2;119(13):4795-801. doi: 10.1021/acs.jpcb.5b00274. Epub 2015 Mar 24.
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.
我们报告了在典型的非质子离子液体（aIL）中，四臂预聚物链的凝胶化机制，即四臂聚（乙二醇）与胺和活化酯端基（TetraPEG-NH2 和 TetraPEG-NHS，分别）的 A-B 型交叉端偶联反应。在离子凝胶体系中，我们专注于凝胶化反应的 pH（或 H+浓度）依赖性。因此，我们应用质子离子液体（pIL），1-乙基咪唑四氟硼酸盐([C2mIm][TFSA])作为非挥发性 H+源，并将其添加到溶剂 aIL 中。结果发现，TetraPEG 离子凝胶的凝胶化时间可以根据 pIL（cpIL=0-3 mM）的浓度从 1 分钟成功控制到 3 小时。这表明在溶液中显示酸碱平衡（-NH2+H+⇆-NH3(+))的 TetraPEG-NH2 的酸碱性质在凝胶化过程中起着关键作用。通过电位滴定直接测定了 TetraPEG-NH3(+)和 C2ImH+(pIL 的阳离子)在 aIL 中的酸离解常数，pKa 分别为 16.4 和 13.7。这表明大部分 H+离子与 TetraPEG-NH2 结合，然后 C2ImH+以中性 C2Im 存在。酰胺键（交联点）的反应效率随着 cpIL 的增加而系统降低，这反映在离子凝胶的机械强度上。从这些结果中，我们讨论了 TetraPEG 在 aIL 中的凝胶化机制，指出了聚合物网络结构与溶液中[H+]之间的关系。
