Force-sensitive polymers being developed by researchers at the University of Illinois (U. of I.) could one day lead to materials that speak for themselves. For example, parachute cords and smart coatings for bridges would change color when overstressed, offering a warning sign before reaching catastrophic failure. The polymers contain mechanically active molecules known as mechanophores. When pushed or pulled with a certain force, specific chemical reactions are triggered within the molecules.

“This offers a new way to build function directly into synthetic materials,” says Nancy Sottos, materials science professor, at U. of I. “And it opens the door to creating mechanophores that can perform different responsive functions, including self-sensing and self-reinforcing, when stressed.”

In previous work, Sottos and her colleagues showed they could use mechanical force to induce a reaction in mechanophore-linked polymers that were in solution. Now, as reported in the May 7 issue of the journal Nature, researchers showed they could perform a similar feat in a solid polymer. Mechanically induced chemical activation (mechanochemical transduction) enables an extraordinary range of physiological processes, including the senses of touch, hearing, and balance, as well as growth and remodeling of tissue and bone.

In critical material systems, such as polymers used in aircraft components self-sensing and self-reinforcing capabilities could be used to report damage and warn of potential component failure, slow the spread of damage to extend a material’s lifetime, or even repair damage in early stages to avoid catastrophic failure.

In their work, the researchers used molecules called spiropyrans, a promising class of molecular probes that serve as color-generating mechanophores, capable of vivid color changes when they undergo mechanochemical change. Normally colorless, the spiropyran used in the experiments turns red or purple when exposed to certain levels of mechanical stress. To demonstrate the mechanochemical response, the researchers prepared two different mechanophore-linked polymers and subjected them to different levels of mechanical stress.

In one polymer, an elastomer, the material was stretched until it broke in two. A vivid color change in the polymer occurred just before it snapped. The second polymer was formed into rigid beads several hundred microns in diameter. When the beads were squeezed, they changed from colorless to purple. The color change that took place within both polymers could serve as a useful indicator of how much stress a mechanical part or structural component made of the material had undergone. The work was funded by the U.S. Army Research Office MURI program.