Making Hydrogels Stronger through Hydrophilicity-Hydrophobicity Transformation, Thermoresponsive Morphomechanics and Crack Multifurcation
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The development of mechanically strong, flexible and crack-resistant hydrogels is of great academic and practical significance and demands for the biomimetic exploration of energy dissipation pathways. The rational design of strong hydrogels is also limited by insufficient mechanism study, resulting from the lack of powerful technique to “see” hydrogels at morphological level. Herein, we constructed a thermoresponsive mechanically strong hydrogel from poly(N-isopropylacrylamide) (PNIPAM) and poly(N,N-dimethylacrylamide). Its hydrophilicity-hydrophobicity transformation and composition-dependent microphase separation are directly visualized by using luminogens with aggregation-induced emission as fluorescent indicators. Based on the morphological observation and mechanical measurements, the concept of morphomechanics with a comprehensive mechanism clarification is proposed. In this regard, thermoresponsive strengthened mechanical properties are attributed to the entanglement of PNIPAM chains and the formation of multiple noncovalent interactions, mainly hydrogen bonds. The enhanced fracture energy by crack multifurcation is related to the disruption of weak interfaces between two separated phases.