Micelle-Cross-Linked Hydrogels with Strain Stiffening Properties Regulated by Intramicellar Cross-Linking

Micelle-cross-linked hydrogels are promising candidates for tough hydrogels with a tailorable chemical composition, nanostructure, mechanical properties, and functionality. In this article, intramicellar cross-linking was demonstrated to be a facile strategy for achieving micelle-cross-linked hydrogels with an adaptive strain stiffening property and decoupled fracture stress and Young's modulus. Core-cross-linked micelles with tunable intramicellar cross-linking density and tailorable chemical composition were synthesized by polymerization-induced self-assembly at a high concentration and were used as the macro-cross-linkers for tough polyacrylamide hydrogels. With the increase in the intramicellar cross-linking, these hydrogels exhibited increasing fracture stress and almost consistent Young's modulus, enabling a decoupled regulation of the fracture stress and modulus. Mooney-Rivlin analyses suggested the enhanced strain stiffening with the intramicellar cross-linking originating from the increasing permanent cross-links, which was confirmed by the relaxation and cyclic tensile tests. The structure-performance correlation was further verified by two additional core-cross-linked micelles with varying chemical compositions. Based on the structure-performance correlation, a photoresponsive micelle-cross-linked hydrogel was designed by using coumarin groups as the photoswitch for the regulation of the intramicellar cross-linking density. Taking advantage of the photoswitched dimerization/cleavage of coumarin, a photomodulated strain stiffening property and the decoupled regulation of the fracture stress and modulus were achieved. This work has provided new insights into the design of tough hydrogels with adaptive mechanical properties.