Scientists at the Adolphe Merkle Institute and the Tokyo Institute of Technology have designed polymers infused with a stress-sensitive molecular unit that respond to external forces by fluorescing proportionally to the force applied, opening the door to the exploration of new force regimes in polymers. The researchers were also able to demonstrate that it is possible to detect both reversible and irreversible polymer deformations.

Besides causing physical motion, mechanical forces can drive chemical changes in controlled and productive ways, allowing for certain material properties. One way to go about this is by introducing a so-called mechanophore into the material, molecular units that are sensitive to stress or strain. Specifically, mechanochromic mechanophores, which alter their optical properties in response to mechanical stimuli, can be useful in quantifying their local mechanical environment.

However, the response mechanism at play in most mechanophores involves severing of chemical bonds. Consequently, they require relatively large mechanical forces to be activated and their response is usually not reversible. To address these issues, researchers led by Prof. Yoshimitsu Sagara from the Tokyo Institute of Technology, an AMI alum, had previously investigated supramolecular mechanophores that show instantly reversible on/off switching of fluorescence without any scission of covalent bonds. The scientists’ next challenge was to determine if both reversible and irreversible mechanoresponses could be elicited from the same molecular motif.

In a study published by the Journal of the American Chemical Society, the researchers, which included AMI’s Prof. Christoph Weder, Dr. Stephen Schrettl and PhD student Hanna Traeger,  explored this question using an unusual molecular architecture called "rotaxane" in which a dumbbell-shaped molecule is threaded through a "ring" such that they are mechanically interlocked. In other words, the "ring" cannot be normally removed. By attaching a quencher-emitter pair to the rotaxane and selecting appropriate sizes of ring and stopper moieties, the team demonstrated a new type of mechanophore response that can be either reversible or irreversible, depending on the magnitude of the applied force "When there is no force applied, the attractive interaction keeps the emitter-containing ring near the quencher fixed on the rotaxane's axle, so that the emission is quenched," explains Sagara. "Upon applying a weak force, the emitter is moved away from the quencher, and its fluorescence is turned on. This effect is reversible, unless the force is sufficiently high to push the ring past the stopper so that irreversible dethreading occurs."

By investigating a carefully designed set of different rotaxanes, the team demonstrated that the combination of appropriately selected ring and stopper moieties with the right size is crucial to obtain interlocked structures that display the dual response. The researchers incorporated the new mechanophores into elastic polyurethane rubbers. These materials which exhibit reversible fluorescence changes over many stretch-and-release cycles to low strains, due to the shuttling function, whereas permanent changes were observed when the rubbers were subjected to repeated high strain deformations due to dethreading of the ring from the axle. "This mechanism allows one, at least conceptually, to monitor the actual deformation of polymer materials, and examine mechanical damage that was inflicted in the past on the basis of an optical signal" says Sagara.

Speculating on the possible implications of their results, Sagara comments, "Extending the current library of mechanophores with our rotaxane-based candidates would be useful for studying the mechanical properties of not only polymers but also cells and tissues, as our mechanophores can respond to much smaller forces compared to those involving chemical bond scission."

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