Spatiotemporally Resolved Dual Mechanochromic Time Integrators

There is a growing societal demand to accurately monitor and track different strain exposures in materials to improve their quality, reliability, and safety. Currently, there is no mechanochromic material that can simultaneously track multiple strain integration timescales, which is key for effective structural health monitoring, as many materials are subject to impulsive and quasi-static loads. Here, a fluorescent coating that offers a spatiotemporally resolved mechanochromic response, with distinct integration timescales, is reported. The polymer coating consists of photopolymerizable donor and acceptor fluorescent droplets. Owing to differences in the exchange rates of donor and acceptor molecules among binary droplets in polymerized and nonpolymerized regions of the coating, two distinct integration timescales emerge over time. Fluorophore exchange between the binary fluorescent droplets results in a decrease in the average intermolecular distance between the donor and acceptor resulting in a ratiometric fluorescence Förster resonance energy transfer change. The mechanochromic response depends on both the magnitude and duration of applied strain. The mechanochromic response can be preprogrammed by adjusting the concentration of the fluorescent droplets or postprogrammed after fabrication of the coating by changing the UV dose. The coating provides a comprehensive overview of a material's exposure history as it can detect both the degree and duration of strain while exhibiting memorability.