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

Abstract

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/m2, 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.