Design and Synthesis of a Well-Controlled Mechanoluminescent Polymer System Based on Fluorescence Resonance Energy Transfer with Spiropyran as a Force-Activated Acceptor and Nitrobenzoxadiazole as a Fluorescent Donor

Colorimetric mechanophores, which change color through force-induced covalent bond breakage, have been incorporated into polymer chains to create mechanochromic materials. However, the color change of most mechanophores is a turn-on response. In this work, we designed and synthesized a novel mechanoluminescent polymer system, PBA-SP-P(MMA-co-NBD), based on fluorescence resonance energy transfer (FRET) with spiropyran (SP) as a force-activated "on/off mechanophore acceptor and nitrobenzoxadiazole (NBD) as a fluorescent donor. It is a comb polymer having NBD-bearing poly(methyl methacrylate) (PMMA) side chains grafted onto a poly(butyl acrylate) (PBA) backbone through SP groups. The processed polymer yields a morphology having NBD-containing PMMA nanodomains embedded into a PBA matrix with SP at the interface. The distance between NBD and SP functionalities is less than 10 nm with confinement of the nanophase separation, which effectively facilitates their fluorescence resonance energy transfer. Under uniaxial stretch, the materials change color at strains as low as about 18% and undergo a green-to-red fluorescence color switch. The two colors are ratiometric, exhibiting a linear correlation with strain after the onset of mechanoactivation. The color change is greatly enhanced right after failure. The fluorescence emission performance can be well controlled through regulating the PMMA/PBA and SP/NBD ratios in the synthesis. The synthesized polymer system exhibits a good potential as an indicator in monitoring microscale cut damages.