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Mechanics meets chemistry in new ways to manipulate matter


James E. Kloeppel, Physical Sciences Editor
217-244-1073; kloeppel@illinois.edu

Released 3/21/2007

An overlay of images at successive stages of force-induced chemical change
Click photo to enlarge
Graphic by Ashley Levato
An overlay of images at successive stages of force-induced chemical change. The blue image is the start of the reaction. The yellow image represents the end of the reaction.

CHAMPAIGN, Ill. — The inventors of self-healing plastic have come up with another invention: a new way of doing chemistry.

Researchers at the University of Illinois at Urbana-Champaign have found a novel way to manipulate matter and drive chemical reactions along a desired direction. The new technique utilizes mechanical force to alter the course of chemical reactions and yield products not obtainable through conventional conditions.

Potential applications include materials that more readily repair themselves, or clearly indicate when they have been damaged.

“This is a fundamentally new way of doing chemistry,” said Jeffrey Moore, a William H. and Janet Lycan Professor of Chemistry at Illinois and corresponding author of a paper that describes the technique in the March 22 issue of the journal Nature.

“By harnessing mechanical energy, we can go into molecules and pull on specific bonds to drive desired reactions,” said Moore, who also is a researcher at the Frederick Seitz Materials Laboratory on campus and at the university’s Beckman Institute for Advanced Science and Technology.

The directionally specific nature of mechanical force makes this approach to reaction control fundamentally different from the usual chemical and physical constraints.

researchers
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Photo by L. Brian Stauffer

Nancy Sottos, professor of materials science; Scott White, professor of aerospace engineering, center; and Jeffrey Moore, professor of chemistry, have collaborated again. The inventors of self-healing plastic have come up with another invention: a new way of doing chemistry.

To demonstrate the technique, Moore and colleagues placed a mechanically active molecule – called a mechanophore – at the center of a long polymer chain. The polymer chain was then stretched in opposite directions by a flow field created by the collapse of cavitating bubbles produced by ultrasound, subjecting the mechanophore to a mechanical tug of war.

“We created a situation where a chemical reaction could go down one of two pathways,” Moore said. “By applying force to the mechanophore, we could bias which of those pathways the reaction chose to follow.”

One potential application of the technique is as a trigger to divert mechanical energy stored in stressed polymers into chemical pathways such as self-healing reactions.

Science cover
Click photo to enlarge
Graphic by Benjamin Grosser, Imaging Technology Group, Beckman Institute
For most chemical reactions to proceed the reactants need to surmount an energy barrier. The energy required is usually provided as heat, light, pressure or electrical potential. Now mechanical force can be added to that list - to the surprise of many a chemist. A reaction can literally be given a shove.

In the original self-healing concept, microcapsules of healing agent are ruptured when a crack forms in the material. Capillary action then transports the healing agent to the crack, where it mixes with a chemical catalyst, and polymerization takes place.

With new mechanical triggers, however, mechanical energy would initiate the polymerization directly, thereby skipping many steps. The cross-linking of neighboring chains would prevent further propagation of a crack and avoid additional damage.

“We have demonstrated that it is now possible to use mechanical force to steer chemical reactions along pathways that are unattainable by conventional means,” Moore said. “We look forward to developing additional mechanophores whose chemical reactivity will be activated by external force.”

The other authors of the paper besides Moore are graduate student and lead author Charles Hickenboth, aerospace engineering professor Scott White, materials science and engineering professor Nancy Sottos, and research chemists Scott Wilson and Jerome Baudry. White, Sottos and Moore co-invented self-healing plastic.

The work was supported by the U.S. Air Force Office of Scientific Research and the Petroleum Research Fund.

Editor’s note: To reach Jeffrey Moore, call 217-244-4024; e-mail: jsmoore@illinois.edu.