Characterization of mechanochemically active polymers using combined photoelasticity and fluorescence measurements

Force-sensitive molecules, called mechanophores, exhibit a chemical response to mechanical force and can be incorporated into the polymer chains. Mechanically stressing these polymers in turn can activate the mechanophore, producing an advantageous chemical response. We have previously demonstrated activation of a mechanophore called spiropyran, which undergoes a force-induced, 6-π electrocyclic ring-opening reaction accompanied by a color change, in linear polymers in solution via sonication and in bulk solids via tension and compression. Reliable, fully characterized transfer of macroscopic stress on a bulk solid polymer to the mechanophore remains a topic of active research. The premise for mechanical activation in linear polymers is that aligned polymer chains can better transfer mechanical energy to the mechanophore than a randomly oriented chain. We have combined photoelasticity and fluorescence measurements for the same field of view during uniaxial tension experiments of two bulk linear solid spiropyran-linked polymers, elastomeric poly(methyl acrylate) (PMA) and glassy poly(methyl methacrylate) (PMMA), in order to quantify the influence of polymer chain orientation, determined from optical birefringence, on mechanophore activation evident by color change. These experiments elucidate the critical molecular orientation and macroscopic stress level required to activate the mechanophores, which are critical for the design of systems incorporating mechanochemically active polymers.