Researchers have created solid plastics that change color under mechanical stress. Further research could lead to plastics that signal when they are about to break or trigger a chemical reaction designed to toughen them where they are under the most stress.
A team led by Jeffrey Moore and Nancy Sottos of the University of Illinois, Urbana Champaign, have been working to equip the polymers with the ability to sense and respond to mechanical stress. Two years ago, some members of the current team showed that they could place small ring-shaped molecules that they call "mechanophores" in the center of polymer chains. In response to mechanical force, these rings break, changing the color of the polymer. But the mechanophores worked only when the polymer was in solution, after the researchers stressed them with high-power ultrasound. For their current study, the team wanted to see if the same strategy would work for solid polymers. One concern, however, was whether extreme stress would cause the polymers to break at random locations. If so, the mechanophore would usually remain unchanged and produce no color change. In tomorrow's issue of Nature, they report that the mechanophores work as hoped in solids.
In the long run, color-changing polymers could find application as coatings bridges to airplane wings, alerting engineers when vital structures are near failure. It should also be possible, using the same strategy, to design a wide variety of other mechanophores that perform different functions, such as triggering a self-healing chemical reaction, and embed them into polymer molecules. When materials such as metals and concrete begin to fail, they typically show signs of stress and fatigue, such as small cracks. But plastics often offer little sign before they give way. The new color-changing polymers could lead to plastics that show when they are under undue stress.