Mechanochromic elastomers that exhibit forceinduced cross-linking reactions in the bulk state are introduced. The synthesis of segmented polyurethanes (SPUs) that contain difluorenylsuccinonitrile (DFSN) moieties in the main chain and methacryloyl groups in the side chains was carried out. DFSN was selected as the mechanophore because it dissociates under mechanical stimuli to form pink cyanofluorene (CF) radicals, which can also initiate the radical polymerization of methacrylate monomers. The obtained elastomers generated CF radicals and changed color by compression or extension; they also became insoluble due to the mechanically induced cross-linking reactions. Additionally, an SPU containing diphenylmethane units also exhibited highly sensitive mechanofluorescence. To the best of our knowledge, this is the first report to demonstrate damage detection ability and changes in the mechanical properties of bulk elastomers induced by simple compression or extension.

A rigid and brittle cross-linking structure was introduced into the flexible poly­(n-hexyl methacrylate) (PHMA) network by the mechanochromic cross-linker difluorenylsuccinonitrile-containing methacrylate (DFMA), whose central C–C bond acted as a dynamic covalent bond and could generate pink radicals when fractured. PHMA with DFMA showed a remarkable hysteresis loop and mechanical enhancement. After deformation, the reassociation of dynamic covalent bonds and the reorganization of the network structure slowed down the recovery of the polymer to its initial state. The correlations between tension stimulation, energy dissipation, and mechanoresponsive color change were discussed. Under stress, the polymer changed from light gray to pink. The broad distribution of red channel intensity under large deformation detected on the surface confirmed that the rupture of dynamic covalent bonds occurred evenly throughout the polymer and suppressed stress concentration. The color showed a strong dependence on stress, which started to appear at around 1.5 to 2.0 MPa. The incorporation of DFMA promised mechanical enhancement and noncontact stress detection ability of the PHMA soft material.

Stimuli-recovery polymer networks with enhanced mechanical performance were designed and synthesized through UV-curing photo-polymerization. Thanks to a mechanochromic cross-linker difluorenylsuccinonitrile-containing dimethacrylate (DFMA), poly(stearyl methacrylate-co-N,N-dimethyl acrylamide) (P(SMA−DMAA)) networks visualized the stress, showing improvements in toughness and higher energy dissipation. The molar ratios determined the transition temperatures, crystal structures, and mechanical performance of the polymer networks. A more efficient and scientific analysis based on achromatic gray-scale colorspace was first proposed to evaluate the mechano-responsive color quantitatively. Uniform evaluation criteria were expected to be established based on the method. The stress distribution and dissociation of dynamic covalent bonds were recorded precisely and expressed clearly on gray-scale color maps, providing a clear warning for when materials were threatening to break. Mechanochromic P(SMA−DMAA) networks showed a prolonged recovery at room temperature. While under heat stimulation, they presented excellent recovery ability with 90% strength and 95% energy. Additionally, the mechano-responsive color changes repeated, showing a similar changing trend to that in the first cycle. The mechanochromic stimuli-recovery P(SMA−DMAA) networks had enhanced mechanical performance and a reliable visual fracture warning function.

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