Correlating fluorescence and residual stiffness in self-reporting microcapsule composites with an intact soft matrix

Self-reporting microcapsule-based systems are highly valuable for providing information about a composite’s health, whether by indicating the location of damage or, in more advanced cases, reflecting the extent of damage through intensity of fluorescence. However, measuring such intensity alone is insufficient for evaluating the deterioration of a composite’s mechanical properties. Using self-reporting stiff capsules containing tetraphenylethylene (TPE) and hexyl acetate embedded in a soft polymeric matrix, we observe that accurately assessing mechanical properties after damage in evaporation-based self-reporting systems requires not only measuring fluorescence brightness but also accounting for the time elapsed since the rupture of the capsules. While the fluorescence–stiffness correlation can be approximated by a linear fit at any given time, the proportionality coefficient gradually evolves, stabilizing only hours after capsule rupture. This study highlights the importance of considering the transient nature of fluorescence–stiffness relationships when leveraging self-reporting composites for advanced damage evaluation.