Self-damping photonic crystals with differentiated reversible crosslinking domains for biomimetic delayed visual perception of underwater impact stress.

Structural color-based impact sensors output light or electrical signals through entropic elasticity storing and releasing of the polymer network, inspiring the design of armors for underwater equipment. Designing self-damping units at the molecular and nanostructural levels will contribute to capturing and analyzing relevant impact and mechanical signals by the naked eye. Herein, inspired by the octopus' sucker, we proposed self-damping photonic crystals (SDPCs) with differentiated reversible crosslinking domains, which can delayed-release entropic elasticity in water and visually perceive stress field evolution via structural color. These domains are generated by weak and strong hydrogen bonds (H-bonds) assigned by differentiated copolymerization, corresponding to weak and strong crosslinking domains, respectively. The compressed network stores entropic elasticity, showing size-effect-induced blueshift structural colors. During entropic elasticity release, the weak/strong crosslinking domains are disrupted successively, resulting in temporary macropore asymmetry and forming transient Laplacian pressure difference (ΔP). The self-damping effect based on the continuous recombination of domains and the equilibrium iteration of ΔP achieves a delayed visual perception of entropy elasticity release. Given this, impact stress sensing and structural color self-erasing techniques have been developed.