Local Stiffness Modulates Mechanophore Activation in Glassy Epoxy Thermosets

Mechanochromism is a promising tool for damage monitoring in polymeric materials. However, achieving significant mechanochromism in glassy thermosets remains challenging, owing to the low mechanophore activation. Herein, we investigate the effect of local structures around rhodamine (Rh) on the mechanochromism of Rh-based epoxy thermosets. The different microenvironments surrounding Rh lead to different levels of mechanochromic response under compression. We demonstrated that greater structural rigidity around the mechanophore promotes mechanical responsiveness. Specifically, the AFG90-modified Rh, featuring dense phenyl rings and the highest functionality, exhibits optimal local stiffness, generating a stress-concentration that triggers significant activation. These findings reveal that the local details of how a mechanophore is connected to a surrounding glassy matrix can influence the extent of mechanochemical activation. Tuning of the local network structure might provide an additional strategy for developing desired mechanochromic thermosets without compromising mechanical/thermal properties and guidelines for the design of polymer networks.