Highly sensitive mechanochromism and high-contrast multicolor switching in epoxy nanocomposites for stress visualization and damage monitoring

Epoxy resins are widely used in coatings and composites due to their excellent comprehensive properties, but they also present new challenges in stress sensing and damage detection. Mechanochromism, which implies that a material can change color in response to mechanical stimulation, could be a potential tool to solve this problem. However, researches on mechanochromic polymers have primarily focused on elastomers and gels. For rigid epoxy thermosets, it is still difficult to achieve significant mechanochromism, let alone multicolor changes under different mechanical stimuli. In this study, SiO₂ nanoparticles (NPs) are introduced into a rhodamine (Rh)-modified epoxy system to improve its mechanochromic response. When the nanocomposite is mechanically stimulated, concentrated stress around the heterogeneous interfaces induces massive ring-opening reactions of Rh in these areas, resulting in a vivid red color change. In addition, direct observation of the stress-concentration effect near the rigid particles is realized by using the visualization properties of the Rh moiety, and combined with finite element analysis to elucidate the enhancement mechanism of the mechanochromism. Furthermore, how the NPs affect multicolor mechanochromism of epoxy thermosets is also investigated. Nanocomposites with two different mechanochromophores exhibit stress- and time-dependent five-color variations due to the different activation of the Rh and disulfide moieties, whereas samples without NPs only show a triple-color change with lower contrast. This strategy is suitable for use in practical applications owning to its ability to display the stress intensity and stress history of a material through high-contrast multicolor switching. Several proof-of-concept scenarios are presented.