Making Hydrogels Stronger through Hydrophilicity-Hydrophobicity Transformation, Thermoresponsive Morphomechanics and Crack Multifurcation
2020
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.
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