Volume 92 Issue 19 | p. 8 | News of The Week
Issue Date: May 12, 2014 | Web Date: May 8, 2014

Polymers That Heal Themselves

Materials Science: Two-stage process of gelation and polymerization repairs holes as big as 9 mm in polymers
Department: Science & Technology
News Channels: Materials SCENE
Keywords: materials, polymers, self-healing, repair
Solutions containing the ingredients needed to form a gel scaffold and a polymerized solid are mixed together and repair a 7.5-mm hole in a polymeric material.
Credit: Brett Krull/UIUC
In this time sequence of images, a 4.75-mm hole in a polymer is repaired by mixing two solutions (dyed blue and red) that react to form a gel scaffold and a polymerized solid.
Credit: Nathan Bajandas/UIUC
In this time sequence of images, a 4.75-mm hole in a polymer is repaired by mixing two solutions (dyed blue and red) that react to form a gel scaffold and a polymerized solid.
Credit: Nathan Bajandas/UIUC

Biological systems can heal themselves. That’s not usually true for materials made with synthetic polymers. Although researchers have previously reported self-healing polymers, such repairs have been limited to microscopic defects and cracks.

A new system, devised by researchers at the University of Illinois, Urbana-Champaign, enables the self-healing in polymers of holes as big as 9 mm (Science 2014, DOI: 10.1126/science.1251135). Aerospace engineer Scott R. White, chemist Jeffrey S. Moore, materials scientist Nancy R. Sottos, and coworkers developed a two-stage gelation and polymerization process to carry out these repairs. The researchers envision using the process in structural materials and other plastics prone to impact damage.

The process depends upon channels embedded in the polymer. Those channels contain solutions that remain segregated until they are needed for repairs. One channel contains a solution of a gelator, a catalyst, and a promoter. Another channel contains a solution of another gelator and an initiator. The monomers needed for the reaction are divided between the channels. Damage to the polymer breaks open the channels, releasing the solutions and allowing them to mix.

The two gelators form a cross-linked network that turns the solution into a gel. The gelation continues until the hole is filled with a gel scaffold. That scaffold then polymerizes into a solid. These two stages occur on different timescales—seconds to minutes for the gel formation and hours for the polymerization.

“We’re able to tune the system so the gel time is relatively fast but the polymerization time is much slower,” Sottos says. “I don’t think we’ve really hit the optimum yet for the gel formation versus polymerization, but the polymerization time should come much later than the gel or you’ll never get the full scaffold to form.”

The researchers demonstrated damage repair with two different types of polymers—a methacrylate-based polymer and a thiolene-based polymer. In one example, they repaired an open cylindrical hole. In a more realistic example, they repaired materials that they had crushed and punctured with a metal striker.

So far, the method is limited to damage holes smaller than about 9 mm. “Currently, the worst enemy of the concept appears to be gravity,” says Fred Wudl, a materials chemist at the University of California, Santa Barbara. If the hole is too big, gravity pulls unreacted fluid out of the damage zone before it has a chance to gel. “One wonders what this inventive group has up their sleeve to emasculate gravity.”

“This two-stage approach has considerable design versatility,” says Richard P. Wool, a professor of chemical and biomolecular engineering at the University of Delaware. “Many future variants could be visualized, such as self-assembling scaffolds, even fibers in composites and reactive nanostructures.”

Chemical & Engineering News
ISSN 0009-2347
Copyright © American Chemical Society
Charles U. Pittman, Jr.  (May 17, 2014 1:41 PM)
After the initial healing of the damage at temperature one (RT for example), if the healed part is now resujected to a use temperature (for example 100 C higher), how do the relative properties of the original material (before damage) compare with the properties of the self-healed material? How does this depend on the relative properties of the now crosslinked gel and its interfacial bonding to the bulk polymeric material?
Celia Arnaud  (May 22, 2014 12:32 PM)
I asked the researchers your question. They haven't tested the healed materials at temperatures above 100 C. They don't think the bond would be affected, but they don't know for sure. They do say that both the original polymer and the healing system would have to be selected to retain significant stiffness at temperatures above 100 C.
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