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UV Heals New Polymer Repeatedly

Friday, January 21, 2011

U.S. and Japanese scientists have developed a polymer that can heal itself over and over again when irradiated with UV light.

This is the first material in which capped covalent bonds reattach repeatedly, even allowing fully separated pieces to be fused back together, the team reports in Angewandte Chemie, the peer-reviewed journal of the German Chemical Society.

In "Repeatable Photoinduced Self-Healing of Covalently Cross-Linked Polymers through Reshuffling of Trithiocarbonate Units", the research team explains how the new polymeric material repairs itself repeatedly.

The technology could have significant implications for protective coatings and other functional materials subject to damage and weathering.

Restructuring Under UV Light

Previous materials have been able to repair themselves once. Typically, these contain tiny capsules that tear open to release a chemical agent when the material is damaged, explains journal publisher Wiley. Other materials “can repair themselves repeatedly but lack the covalent bonds that increase materials strength and stability.”

The new polymer, however, is cross-linked through trithiocarbonate units, Wiley explains. These carbon atoms are bonded to three sulfur atoms, two of which use their second bonding position to attach to another carbon atom.

“These groups have a special property: They can restructure under UV light,” according to Wiley. “The light breaks one carbon–sulfur bond in the trithiocarbonate groups. This produces two radicals—molecules with a free, unpaired electron.

“The radicals are very reactive and attack other trithiocarbonate groups to form new carbon–sulfur bonds while breaking others to form more free radicals. The chain reaction stops when two radicals react with each other.”

Healing Cut Fragments

The material was developed by Krzysztof Matyjaszewski and colleagues at Carnegie Mellon University in Pittsburgh, PA, and Kyushu University in Japan.

“The researchers were able to heal cut polymer fragments with irradiation—either immersed in liquid or in bulk,” the journal reported. “They only had to firmly press the cut edges together and irradiate them” to cause the edges to grow back together.

Even shredded polymer samples could simply be pressed together and irradiated to be fused into a continuous piece, the publisher said.

“The resulting object was in the shape of the cylindrical tube in which the procedure was carried out. This self-healing process can be carried out repeatedly on the same sample. The material is thus also interesting as a new recyclable product.”