Hydrogen bond reinforced poly(1-vinylimidazole-co-acrylic acid) hydrogels with high toughness, fast self-recovery, and dual pH-responsiveness

As a promising structural material, various tough hydrogels have been developed in recent years by incorporating different kinds of noncovalent bonds as the energy-dissipating segments. Among these systems, it is challenging to use hydrogen bonds due to their instability in aqueous environment. Here we report a tough hydrogel of poly(1-vinylimidazole- co -acrylic acid) (P(VI- co -AAc)) reinforced by the robust hydrogen bonds between imidazole and carboxylic acid groups. The hydrogels with different composition possess excellent mechanical performances, with tensile breaking stress, breaking strain, Young's modulus , and tearing fracture energy being 0.3–1.8 MPa, 920–1400%, 0.1–0.7 MPa, and 0.7–5.6 kJ/m 2 , respectively, which are well maintained over a wide pH range (3 ≤ pH ≤ 10). The high stability of hydrogen bonds should be rooted in the strong proton donor-acceptor pair. However, when pH ≤ 2 or pH ≥ 11, the gels become highly swelled due to the dissociation of imidazole and carboxylic acid groups, respectively, accompanying with dramatic decrease in mechanical properties . The tough hydrogels also show temperature- and deformation rate-dependent mechanical properties , as well as shape memory and self-recovery abilities, due to the dynamic nature of noncovalent bonds. Such hydrogels with remarkable mechanical properties and dual pH-responsiveness should find applications in load-bearing soft actuators, flexible electronics, etc.