Polymer Paves the Way for Self-Healing Materials

A ring-like polymer molecule was stretched by an ultrasonic process and then sprung back to be smaller than it was initially (right).

A research team from Duke University and Stanford University has found a polymer molecule that’s so springy it snaps back from stretching much smaller than it was before. Duke graduate student Jeremy Lenhardt and associate professor Stephen Craig have been systematically hunting through a library of polymers in search of a molecule that might be useful for “self-healing” materials. They hope to find a polymer that can trigger a chemical reaction when it is stretched and enable a material to build its own repairs.

Imagine a sheet of plastic wrap that could fix a microscopic puncture before the hole ever got big enough to see. This would require that the polymer molecules immediately around the tear could somehow jump into action and perform new chemistry to build bridges across the hole.

To stretch polymers and see what happens to them, Lenhardt uses an apparatus that pumps up and down on a solution filled with polymers, pressurising it and depressurising it 20,000 times a second which causes tiny bubbles to form fleetingly. The void created by the bubbles exerts a tug on the ends of some of the polymers in the solution and stretches them, if only for a billionth of a second.

“Think of two rafts going down a river with a rope between them,” Craig explains. “As the first raft enters a rapids and accelerates forward, that rope – the polymer – gets pulled taught and stretches.”

Over and over Lenhardt ran the experiment, characterizing different polymer species that became more reactive when stretched, potentially able to do “stress-induced chemistry.”

Then, while looking at polymers that contained tiny ring-shaped molecules called gem-difluorocyclopropanes (gDFC), he was surprised to find that some of these molecules emerged from the stretching noticeably shorter than when they went in.

“I ran up to his office,” Lenhardt says. ” ‘Steve, something funny is going on here. Look at this!’ ” A technique called nuclear magnetic resonance had revealed the shapes of the rings after pulling and shown that they were, in fact, shorter.

More information is available from Duke University.

Tags: Duke University, Jeremy Lenhardt, polymer, polymer molecule, Stanford University, Stephen Craig