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Materials

Rubber Heals Itself

Small molecules yield self-mending property

by Rachel A. Petkewich
February 25, 2008 | APPEARED IN VOLUME 86, ISSUE 8

Credit: Jean-François Tremblay & Ludwik Leibler, CNRS/ESPCI

SEVER, UNITE, STRETCH, and repeat. That's what you can do with a new class of materials called self-healing rubber (Nature 2008, 451, 977).

Chemistry professor Ludwik Leibler and colleagues at the Paris-based Industrial Physics & Chemistry Higher Educational Institution have created small-molecule-based supramolecular network materials with rubberlike properties that can repair themselves at room temperature. Rubber elasticity is traditionally a property of macromolecules, as small-molecule assemblies are generally too crystalline to bend. And although self-healing materials have been made before, they usually require heat to repair cracks and breaks.

The new small molecules—which are made from fatty acids and a combination of one, two, or three different amide groups—hydrogen-bond to one another to form the supramolecular networks. Small molecules that assemble in a network and undergo reversible cross-linking can behave like rubber, Leibler says. "The self-mending property is possible thanks to the presence of a long-lived and large excess of nonlinked, open hydrogen bonds on the fractured surfaces," he explains. The pieces must be physically placed together to mend.

 Self-healing supramolecular networks assemble from fatty acid-based small molecules

Building Blocks
Self-healing supramolecular networks assemble from fatty acid-based small molecules containing combinations of three amides-amidoethyl imidazolidone (top left), di(amidoethyl) urea (top right), and diamidotetraethyl triurea.

Leibler says the new materials are not adhesives, because only cut or broken surfaces attach. The surfaces can self-heal if they have been apart less than one week at room temperature, or thermal equilibrium will prevent hydrogen bonding.

From a chemistry point of view, using fatty acid derivatives to build multifunctional materials and new functional groups able to associate via multiple hydrogen bonds clearly opens a new avenue for product development, Leibler says.

Polymer chemist Krzysztof Matyjaszewski of Carnegie Mellon University commends the researchers' use of a simple and renewable resource—vegetable oil—as a source of fatty acids. He says previous self-healing material systems have generally relied on complex chemistry and processing.

Arkema, a French chemical maker, is planning to develop commercial products based on the new supramolecular chemical technology.

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