Self-healing polymers are extremely sought after by scientists, as they have many useful—not to mention lucrative—applications. Back in 2009, we reported a polyurethane-based polymeric material that heals itself in roughly an hour when exposed to UV light. That particular polymer, made by Biswajit Ghosh and Marek W. Urban, would be useful as a protective coating for phones, cars, etc. It worked based on the principle of having a reactive chemical component that would split open when physically damaged to create two reactive ends that can then covalently link together under UV light to repair itself.
In a recent issue of Nature, Mark Burnworth and his colleagues report a different type of self-healing material, one that can repair itself in about a minute under UV light. Burnworth’s polymeric material also doesn’t function on the basis of forming chemical bonds between organic compounds for repair. Instead, it relies on localized heating and metal-ligand interactions.
Burnworth and his team used rubbery oligomers, poly(ethylene-co-butylene), as the core of their material. They attached ligands, 2,6-bis(1’-methylbenzimidazolyl)pyridine (Mebip), that can bind to metals at the ends of the oligomers. To form long polymers, the researchers added either zinc (Zn2+) or lanthanum (La3+) ions to the solution of oligomers. The metal ions form metal-ligand complexes with the Mebip, linking the oligomers with one another.
For their self-healing tests, Burnworth and his team shaped the polymers into films that were 350 to 400 µM thick. They purposefully cut the polymer to about 50 to 70 percent of the overall thickness of the film. When the cuts were exposed to two consecutive 30-second rounds of UV light (320 to 390 nm wavelength at an intensity of 950 mW cm-2), the cuts sealed up. The healed material was comparable in toughness to the original polymeric film, and images from atomic force spectroscopy show that the cuts essentially disappeared.
This process works because complexes of Mebip with metals are chromophoric, so they can absorb light of a specific wavelength, such as light in the UV range. Once they absorb light, they get into a higher energetic state and then lose that energy by giving off heat. Thus, when the researchers exposed the cuts to UV light, there was heating at the surface of the polymeric film—enough heating to reach over 220°C in 30 seconds. The heat quickly depolymerizes the area around the cut. Once the UV light is turned off, the liquidized area cools, reforms the ligand-metal complexes, and seals up the cut.
The healing process can be localized, as you only need to hit damaged areas with UV light. The researchers also show that the healing process would still work if the polymer was under a load of about 8 kPa. They suggest that different ligands could be used to cover a range of absorbable wavelengths. Thus, you could selectively tailor the wavelength of light to heal different types of damaged materials.
Burnworth and his colleagues have made a significant step in getting us closer to having self-healing polymers on the market. Their approach is quite different than that of Ghosh and Urban, which we described previously. While Burnworth’s method certainly heals quicker, it also produces a dramatic increase in surface temperature during healing. Such a huge temperature surge could be disadvantageous for certain applications.
Listing image by Dominique Bersier and Gina Fiore for Adolphe Merkle Institute, Case Western Reserve University, US Army Research Laboratory.