It's always helpful if materials let you know when they're under stress, so that changes can be made before catastrophic failures occur. A new polymer is designed to provide such a warning, as it glows when stretched.
First of all, scientists have previously developed polymers that change their optical properties in response to mechanical stress. Generally, though, the change is triggered by the breaking of molecular bonds within the material, so it can only happen once. Additionally, those bonds can also be broken by stimuli such as heat and light, resulting in false alarms.
With these limitations in mind, researchers from Switzerland's University of Fribourg and Japan's Hokkaido University developed the glowing polymer. A type of polyurethane, it incorporates a rotaxane molecular structure – this means that it's made up of long dumbbell-shaped molecules, onto which are threaded ring-shaped fluorescent molecules called macrocycles.
As long as the polymer is in a relaxed state, the macrocycles stay grouped together, adjacent to "quencher" molecules in the middle of the dumbbell. Those quenchers keep the macrocycles from fluorescing.
Once the polymer (and thus the dumbbell) is stretched, however, the macrocycles are drawn apart and away from the quenchers, allowing them to fluoresce under ultraviolet light. The more that the material is stretched, the brighter they glow, and they go dark again once the stretch is released. The process can be repeated indefinitely.
By altering the type of macrocycles used, the polymer can be made to fluoresce blue, green or orange (pictured above). A white glow can also be produced, by combining them.
The research, which was led by Hokkaido's Yoshimitsu Sagara and Fribourg's Christoph Weder, is described in a paper that was recently published in the journal ACS Central Science. You can see the polymer in all its glowing glory, in the video below.
Sources: American Chemical Society, Hokkaido University