Polymers that change their fluorescence color in response to mechanical deformation provide an easy way to directly assess stresses in a material.^1,2 Such mechanochromic responses could be extremely useful in load-bearing parts that directly signal the location and extent of damage and would crucially allow for a timely repair or replacement. The simple blending of aggregachromic dyes has shown to be a convenient, industrially scalable technique for fabricating mechanochromic materials.² However, the approach has largely been limited to semi-crystalline polymer matrices,^3–5 as the dissociation of dye aggregates is thought to require the relatively high shear forces that occur upon disruption of the crystalline domains.
To overcome this challenge, our group has recently focused on the development of mechanophores that feature relatively weak, non-covalent interactions. Expanding beyond the use of simple aggregachromic dyes, we developed a telechelic poly(ethylene-co-butylene) with oligo(phenylene-vinylene) (OPV) dyes at the termini (tOPV) and have shown that blending of this tOPV additive with different thermoplastic elastomers renders the latter mechanochromic.⁶ We now demonstrate the versatility of this tOPV additive as a sensitive, reversible strain sensor in commerical thermoplastic polyurethanes (TPUs) with varying degrees of crystallinity. Using a simple, in situ opto-mechanical testing apparatus, we detect a decisive change in the fluorescence of the TPU-tOPV blend materials at tensile strains as low as 5% and use this visual response to elucidate different processes in the samples.
Moreover, we have developed a derivative of the well-known complementary H-bonding ureido pyrimidinone (UPy) motif that signals dissociation of the non-covalent interaction through a distinct fluorescence signal. The integration of UPy motifs can strengthen a polymer material via non-covalent reinforcement and endow it with self-healing properties.^7,8 Using this motif as the core, we have developed a modular synthetic strategy to introduce excimer-forming pyrene units via short carbon linkers. When two of these UPy-bispyrene (UPB) molecules interact to form a dimer, the pyrene emission shifts to longer wavelengths, thus directly connecting a visual signal to UPy bonding and breakage events without interfering with the material enhancement properties of the latter. When integrated into polymeric materials, this non-covalent binding motif acts both as a supramolecular crosslinker and as a mechanophore. The investigation of the mechanical as well as mechanochromic properties of polymers that feature this motif is currently ongoing.

[1] Calvino, C.; Neumann, L. et al. J. Polym. Sci. Part A Polym. Chem. 2017, 55 (4), 640–652.
[2] Ciardelli, F.; Ruggeri, G.; Pucci, A. Chem. Soc. Rev. 2013, 42 (3), 857–870.
[3] Crenshaw, B. R.; Weder, C. Chem. Mater. 2003, 15 (25), 4717–4724.
[4] Kinami, M.; Crenshaw, B. R.; Weder, C. Chem. Mater. 2006, 18 (4), 946–955.
[5] Crenshaw, B. R.; Weder, C. Macromolecules 2006, 39 (26), 9581–9589.
[6] Calvino, C.; Sagara, Y. et al. Macromol. Rapid Commun. 2019, 40 (1), 1800705.
[7] Balkenende, D. W. R.; Monnier, C. A. et al. Nat. Commun. 2016, 7, 1–9.
[8] Kushner, A. M.; Gabuchian, V. et al. J. Am. Chem. Soc. 2007, 129 (46), 14110–14111.