Modulating Activation Strain Thresholds in Mechanochromic Polymers through the Prestretching Strategy

Abstract Mechanochromic polymers provide a powerful tool for visualizing stress distribution and signaling mechanical damage through force-induced optical responses. However, their practical applications are often restricted by the relatively high activation strain threshold, which originates from the requirement for polymer chains to reach a fully extended conformation before mechanical forces can be efficiently transmitted to embedded mechanophores. Herein, we report a general prestretching strategy to tune mechanochemical activation by preorienting polymer chains and kinetically locking their extended conformations through a rubbery-to-glassy transition. This strategy can be realized by using rhodamine mechanophore-crosslinked acrylate copolymers with tunable glass transition temperatures, which allows the degree of chain prestretching to be systematically controlled. It is found that prestretching significantly lowers the activation strain threshold and enhances mechanochromic sensitivity. Moreover, mechanophore activation threshold can be further tuned by varying the angle between tensile direction and prestretched chain orientation.The prestretching strategy is further extended to another glassy polymer, copolymer of ethyl acrylate and isobornyl acrylate, showing its broad applicability. This work provides a general strategy toward highly sensitive mechanochemical polymers with reduced activation strains.