Structure Frustration Enables Thermal History-Dependent Responsive Behavior in Self-Healing Hydrogels

Abstract

Biological soft tissues usually execute their functions via nonequilibrium and dynamic structural transformations. By contrast, functional hydrogels are mainly constructed by implementing static and equilibrium structures in the polymer network. Here, using polyampholyte hydrogel as a model system, we demonstrated that the nonequilibrium structure transformation in self-healing hydrogels enables the gels with many new features, including thermal history dependence, quick and asymmetric thermal response (instant transparent-to-turbid transition but slow turbid-to-transparent transition), tunable cloud point, tunable recovery time, and tiny changes in sample size and mechanical performance. These features make them distinct to conventional thermoresponsive hydrogels based on thermodynamic equilibrium and endow them with a new type of promising thermoresponsive materials. We revealed the structure change and studied the role of the thermal protocol on this thermoresponsive behavior by combining ultraviolet spectrum, small-angle X-ray scattering, rheology, and mechanical measurements. We also presented two conceptual applications of this thermoresponsive hydrogel in thermal imaging and security paper. We believe that this work will inspire future research on creating functional hydrogels via nonequilibrium structure transformations.