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Stress quantification can be observed during single fiber pull-out test in a polymer matrix composite with stress sensing molecules. 

By covalently attaching spirolactam at the interface of a glass/epoxy composite, we observe real time interfacial mechanophore activationviafluorescence. 

Abstract Understanding the stress distribution within fiber‐reinforced polymers (FRPs) is critical to extending their operational lifespan. The integration of mechanoresponsive molecular force probes, referred to as mechanophores, presents a potential solution by enabling direct monitoring of stress concentrations. In this study, spiropyran (SP) mechanophores (MPs) are embedded within a polydimethylsiloxane (PDMS) matrix to visualize stress localization during loading within a single fiber‐reinforced framework. The SP mechanophore undergoes a transition from a non‐fluorescent state to an active state (merocyanine) through isomerization in response to mechanical forces. Using a single fiber mounted axially within the matrix, the fundamental failure modes observed in conventional fiber‐reinforced composites are replicated. Samples are strained under uniaxial tensile loading along the fiber direction and the localization of stresses is observed via MP activation. Stresses are concentrated in the matrix near the fiber region that gradually decreases away from the fiber surface. Confocal microscopy is used to visualize mechanophore activation and quantitatively assess fluorescence intensity. Finite element modeling is used to develop a calibration to quantify the stresses based on the observed fluorescence intensity. These outcomes underscore the viability of employing these mechanoresponsive molecules as a potential means to visualize real‐time stress distribution, thereby facilitating the design of high‐performance composites. 

Abstract Mechanoresponsive polymeric materials that respond to mechanical deformation are highly valued for their potential in sensors, degradation studies, and optoelectronics. However, direct visualization and detection of these responses remain significant obstacles. In this study, novel mechanoresponsive polybiidenedionediyl (PBIT) derivative topochemical polymers are developed that depolymerize under mechanical forces, exhibiting a distinct and irreversible color change in response to grinding, milling, and compression. This color change is attributed to the alteration of polymer backbone conjugation during elongated Carbon‐Carbon (C─C) single bond cleavage. Quantum chemical pulling simulations on PBIT polymers reveals a force range of 4.3–5.0 nN associated with the selective cleavage of elongated C─C single bonds. This force range is comparable to that observed for typical homolytic mechanophores, supporting the mechanistic interpretation of homolytic bond scission under mechanical stress. C─C bond cleavage kinetic studies of PBIT under compression indicates that strong interchain interactions significantly increase the pressure needed to cleave the elongated C─C bonds. Additionally, PBIT polymer thin films are composited with polydimethylsiloxane to create free‐standing and robust thin films, which can serve as ink‐free and rewritable paper for writing and stress visualization applications. This advancement opens new possibilities for utilizing crystalline and brittle topochemical polymers in practical applications.