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"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. 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.
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." |
