91av

EVER wished you could turn back time to reverse the damage on something you’ve broken? A new material that can mend itself after being cracked goes a long way to making that dream possible.

The quest for a material that can “heal” itself has occupied scientists for some time. For instance, engineers at the University of Illinois in Urbana-Champaign have made a polymer that can plug cracks as they form (91av, 17 February 2001, p 23). But the healing mechanism, based on tiny capsules of embedded chemicals and catalysts that help reseal cracks, only works until those chemicals run out.

In a new approach, Fred Wudl, based at the Exotic Materials Institute at the University of California in Los Angeles, has come up with a material that can mend itself as many times as it’s broken.

It works by adapting the Diels-Alder reaction between two organic molecules—a furan and a maleimide—which combine to form long polymer chains. Crucially, this reaction is reversible: heating the plastic breaks it up into the original reactive molecules, so they can react again.

Wudl wondered if he could get the polymer reaction to spread out in three dimensions, with molecules joining up to form a strongly interconnected lattice, rather than just separate chains. He succeeded by reacting clusters of the original organic chemicals: molecules comprising four furan groups and molecules composed of three maleimide groups. The result was a strong transparent plastic.

But would it self-repair? To find out, the researchers stretched slabs of the plastic in a machine until they broke in two. A day or two later, Wudl clamped the broken pieces together and heated the polymer to 120 °C before leaving it to cool.

His idea worked. The crack between the two pieces self-healed on cooling, leaving a faint shadow where the crack had been. But this was no welding job since the polymer didn’t melt. Instead, the plastic partially unset, allowing chemical bonds to reform across the fracture as it cooled.

It works because it’s the bonds between separate organic clusters, and not the molecules themselves, that break when the plastic fractures. “That’s the beauty of it,” Wudl told 91av.

“Being able to reform bonds is totally novel,” says Scott White, who led the team based at Illinois. “The only problem now is that it needs manual intervention to heat it up.”

Another drawback is that the self-healed plastic slabs lose 40 per cent of their strength. That’s because it’s tough to perfectly align the fracture surfaces without leaving gaps that chemical bonds can’t bridge. But a large internal crack that Wudl accidentally made in one piece disappeared completely when he heated it.

Possible applications for the material include computer circuit boards. If these crack, circuitry printed on them can be damaged. Because they routinely heat up and cool down during use, Wudl says his healing plastic would continually mend cracks as they formed.