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Color-changing materials react to force

Many uses in store for new polymers that change color when physically stressed, researchers say.

An elastomer made with mechanophore-linked polymers changes color when stretched. Beckman Institute Imaging Technology Group, Darren Stevenson, and Alex Jerez

Scientists at the University of Illinois have developed polymers that change color when the material becomes overstressed.

The materials science invention could be used in things like parachute cords, climbing ropes, or added to smart coatings for bridges.

The polymers contain mechanophores--molecules that create a chemical reaction that makes the synthetic material change color when a certain amount of force is exerted upon it.

One of the polymers offered by the scientists as an example of their work is an amber-colored elastomer that turns progressively more orange as it's pulled and then finally red right before it reaches its point of failure and snaps (see photo). In another example, the group made a hard little bead that turned from translucent to purple when compressed.

The group, whose project is funded by the U.S. Army Research Office MURI program, had previously done work with mechanophore-linked polymers in liquid. This latest invention is with solids .

The University of Illinois research was led by Nancy Sottos, a Willett Professor of materials science and engineering and a professor at the university's Beckman Institute; and Douglas Davis, graduate research assistant and lead author on the project.

Davis noted that the material can go back to its original color once relieved of stress and perform the same function over and over.

"Mechanical stress induces a ring-opening reaction of the spiropyran that changes the color of the material. The reaction is reversible, so we can repeat the opening and closing of the mechanophore," Davis said in a statement.

Keeping that in mind, the group hopes to create mechanophore-linked polymers that could actually self-reinforce each time they're met with increased stress. If created, the material could be used in things like airplanes as a temporary solution to damaged or stressed parts. In a plane, for example, parts made of the polymer could self-reinforce to minimize damage until the plane could safely be landed and fixed.

Details of the group's invention can be found in the May 7 issue of Nature.