Leave your child’s plastic toys out in the backyard over the summer, and the sun’s ultraviolet (UV) rays bleach them and make them brittle. But UV light can be a healer, too, according to a new study. Researchers have created a polymer that mends itself when hit with a bright beam of UV light. The new self-healing plastic could find its way into coatings for automobiles and other everyday devices, making scratches and other minor dings vanish as if they were never there.
Self-healing plastics have been the rage for much of the past decade and generally come in two classes. The first are chains of organic molecules linked together by strong covalent bonds. These can make very tough polymers, good for structural applications, such as outdoor tables and water pipes. But they require adding in microcapsules containing extra polymer building blocks, called monomers, so that when the polymer is damaged, the monomers are released to bind the wound. Unfortunately, once the binding monomers are used up, the polymer can no longer heal itself.
A second class of self-healers is made using more loosely linked supramolecular bonds. In this case, the bonds holding individual polymer chains together aren’t rigid covalent links; rather they’re composed of a more delicate set of connections, known as pi-pi bonds, hydrogen bonds, or metal-receptor interactions. Because such bonds can readily be broken with a burst of heat and restored when cooled, supramolecular self-healing polymers are often better at repeated repairs. But the weakness of the links can also make the resulting polymers softer and less able to carry a load.
In hopes of firming up these rubbery supramolecular polymers, researchers led by Christoph Weder of the University of Fribourg in Switzerland borrowed a trick long known to firm up a class of plastics known as block copolymers, which are made from a mixture of two or more types of polymer molecules tethered together. As these hybrids are mixed, the components spontaneously segregate themselves to form a repeated pattern of alternating blocks or layers, an arrangement that typically makes them tougher.
Weder’s team started by building two-part polymers. The first consisted of typical organic chains. At both ends they attached negatively charged sticky end groups. They then spiked their mixture with positively charged metals, either zinc or lanthanum. The oppositely charged sticky ends and metals snapped together, linking the individual polymers into an extended network. The metals and their sticky ends also preferred hanging out together, so the polymer spontaneously formed layers of metals attached to the sticky ends, alternating with layers of the organic chains.
The new structure, reported online today in Nature, made the supramolecular plastics nearly as tough as some of their covalently bonded counterparts. But the researchers had another trick as well. Weder’s team chose metals that are strong absorbers of UV light. So if the researchers shined a strong beam of UV light on scratched or cracked material, the metals would absorb the light and convert the energy to heat, causing the polymer at the site to melt and reform its normal bonds as it cooled. That made it just as tough as the original material. This self-healing wouldn’t occur if the materials were simply left out in the sun, Weder says, because sunlight doesn't have enough energy to melt the polymer.
Jeffrey Moore, a chemist and self-healing polymer expert at the University of Illinois, Urbana-Champaign, says he is impressed by the work because it’s much easier to direct UV light than heat to heal any damage right where you want it. “This is definitely a future direction for the field to go,” Moore says. Weder says he doesn’t have a deal yet to commercialize his self-healing plastics, but that discussions for licensing the technology are now at an “advanced” stage.