Self-healing polymers

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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 thatheals 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 workedbased 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 repairitself.

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’spolymeric 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.

Burnworthand 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 theends 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 about50 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 cutssealed 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.


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