Nanomaterials with give survive

U. PITTSBURGH (US) — Self-healing materials have turned the tables on strength and survival by showing that sometimes it helps to be a little bit frail.

Made from nanoscale gel particles that can regenerate after being damaged, the material shows that an ideal amount of weak bonds actually makes an overall stronger material that is able to withstand more stress.

Although self-healing nanogel materials have already been realized in the lab, the exact mechanical nature and ideal structure remains unknown, says Anna Balazs, professor of chemical engineering at University of Pittsburgh.

The findings not only reveal how self-healing nanogel materials work, but also provide a blueprint for creating more resilient designs.

Details of the research are reported in the journal Langmuir.

Researchers worked from a computational model that was created based on nanogel, a self-healing material developed by Krzysztof Matyjaszewski, professor of chemistry at Carnegie Mellon University with an adjunct appointment at the University of Pittsburgh.

The material is a composite of spongy, microscopic polymer particles linked to one another by several tentacle-like bonds. The nanogel particles consist of stable bonds—which provide overall strength—and labile bonds that are highly reactive that can break and easily reform, that act as shock absorbers.

Researchers first tested the performance of various bond arrangements by laying out polymers in an arrangement similar to that in the nanogel, with the tentacles linked end-to-end by a single strong bond.

Simulated stress tests showed, that though these bonds could recover from short-lived stress, they couldn’t withstand drawn out tension such as stretching or pulling. But when particles were joined by several parallel bonds, the nanogel could absorb more stress and was still able to self-repair.

The team then sought the most effective concentration of parallel labile bonds. According to the computational model, even a small number of labile bonds greatly increased resilience.

For instance, a sample in which only 30 percent of the bonds were labile—with parallel labile bonds placed in groups of four—could withstand pressure up to 200 percent greater than what could fracture a sample comprised only of stable bonds.

On the other hand, too many labile linkages were so collectively strong that the self-healing ability was cancelled out and the nanogel became brittle.

The Pitt model is corroborated by nature, which engineered the same principle into the famously tough abalone shell, Balazs said. An amalgamation of microscopic ceramic plates and a small percentage of soft protein, the abalone shell absorbs a blow by stretching and sliding rather than shattering.

“What we found is that if a material can easily break and reform, the overall strength is much better,” she says. “In short, a little bit of weakness gives a material better mechanical properties. Nature knows this trick.”

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