A polymer that releases acid in response to a hammer blow can trigger “bruising” that reveals where and when it was struck (J. Am. Chem. Soc. 2020, DOI: 10.1021/jacs.9b12861). This kind of mechanochemistry—in which a mechanical force sets off a chemical reaction—could eventually lead to materials that offer an early warning signal of impending structural failure, for example. “The overall goal here is to create useful stress-responsive materials,” says Stephen L. Craig of Duke University, whose PhD student Yangju Lin led the work.
Polymer mechanochemistry relies on chemical groups called mechanophores, which undergo a chemical change when they are stretched or compressed. Craig’s team developed a new mechanophore called 2-methoxy-substituted gem-di-chlorocyclopropane (MeO-gDCC). The researchers incorporated repeating units of MeO-gDCC into polymer chains and then blasted the resulting molecules with ultrasound. This blast breaks a carbon-carbon bond in the cyclopropane ring to generate an allyl chloride intermediate, which can rearrange to release hydrogen chloride. Given the number of MeO-gDCC units in each polymer molecule, every chain produces an average of 67 protons in this way.
Previous mechanoacids have suffered from limited thermal stability or were only able to release a single proton from each polymer chain, drawbacks that MeO-gDCC overcomes. “This chemistry addresses all the shortcomings of the prior acid-generating mechanophores,” says Jeffrey S. Moore of the University of Illinois at Urbana−Champaign, who developed one of the few other mechanoacid polymers. “It’s a very exciting advance.”
Craig’s team also studied the mechanoacid with single-molecule force spectroscopy, a technique that involves stretching a single polymer chain with an atomic force microscope.
The researchers used this technique to compare their mechanoacid polymer with an equivalent polymer whose mechanophores lacked a methoxy substituent. Under a force of 880 pN, the methoxy-substituted mechanophores popped open about 1 million times as fast, meaning they were much more responsive to stress. The researchers attribute this to the methoxy group stabilizing the mechanophore’s transition state during ring opening. “Just a simple change of design, adding the methoxy, can lead to these new properties,” says Guillaume De Bo of the University of Manchester, who also works on polymer mechanochemistry.
Single-molecule experiments are a long way from real-world applications, though. So the researchers created a 6 mm wide polymer disk by blending their mechanoacid with poly(dimethylsiloxane) and the dye rhodamine B, which turns pink when protonated. When they hit the disk with a hammer, the polymer released acid that slowly spread through the material and reacted with the dye to create a pink bruise. The researchers could estimate when the blow was struck by measuring the extent of the color change, and they were accurate to within a few minutes for periods up to half an hour. It marks the first example of a mechanoacid being used in a bulk material, the team says.
Craig suggests that the ability to time-stamp a mechanical shock in this way could prove useful in pinpointing the particular event that led to the failure of a mechanical part, in areas as diverse as medical devices or remotely operated drones. The protons released by the mechanophore could also be harnessed to trigger self-repair reactions in a damaged polymer, adds De Bo. “It’s simple enough that it could eventually be applied in real systems,” he says.