Hydrophilicity-Hydrophobicity Transformation, Thermoresponsive Morphomechanics, and Crack Multifurcation Revealed by
Yubing Hu1,2, Lucile Barbier3, Zhao Li1
1Department of Chemistry, the Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration and Reconstruction, Institute for Advanced Study, The Hong Kong University of Science and Technology, Guangdong-Hong Kong-Macro Joint Laboratory of Optoelectronic and Magnetic Functional Materials, Clear Water Bay, Kowloon, Hong Kong, 999077, China.
Abstract:
Biomimetic exploration of stimuli-responsive and crack-resistant hydrogels is of great academic and practical significance, although the rational design of tough hydrogels is limited by insufficient mechanism study due to the lack of imaging techniques to "see" hydrogels at mesoscale level. A series of composite hydrogels with compartmentalized thermal response is designed by incorporating aggregation- and polarity-sensitive fluorescent probes in a poly(N-isopropylacrylamide) (PNIPAM) network grafted with poly(N,N-dimethylacrylamide) side-chains. The fluorescence technique is explored as a powerful tool to directly visualize their hydrophilicity-hydrophobicity transformation and the composition-dependent microphase separation. Based on the morphological observation and mechanical measurements, the concept of morphomechanics with a comprehensive mechanism clarification is proposed. In this regard, the thermoresponsive toughening is attributed to the formation of multiple noncovalent interactions and the conformational changes of PNIPAM chains. The enhanced fracture energy by crack multifurcation is related to the tearing-like disruption of weak interfaces between the separated phases.
