Mechanical Performance and Visual Fracture Warning Function of Mechanochromic Stimuli-Recovery Polymer Networks

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.