Conventional rules of survival tend to favor the strongest, but Univ. of
Pittsburgh-based researchers recently found that in the emerging world of
self-healing materials, it is the somewhat frail that survive.
The team presents in Langmuir a new model
laying out the inner workings of self-healing materials made of nanoscale gel
particles that can regenerate after taking damage and are being pursued as a
coating or composite material. Moreover, the researchers discovered that an
ideal amount of weak bonds actually make for an overall stronger material that
can withstand more stress.
Although self-healing nanogel materials have already been realized in the
lab, the exact mechanical nature and ideal structure had remained unknown,
explained Anna Balazs, corresponding author and
Distinguished Professor of Chemical Engineering in Pitt’s Swanson School of
Engineering. The team’s findings not only reveal how self-healing nanogel
materials work, but also provide a blueprint for creating more resilient
designs, she said. Balazs worked with lead author and Pitt postdoctoral
researcher Isaac Salib; Chet Gnegy, a Pitt chemical and petroleum engineering
sophomore; German Kolmakov, a postdoctoral researcher in Balazs’ lab;
and Krzysztof Matyjaszewski, a chemistry professor at Carnegie Mellon
Univ. with an adjunct appointment in Pitt’s Department of Chemical and Petroleum
The team worked from a computational model Gnegy, Kolmakov, and Salib
created based on a self-healing material Matyjaszewski developed known as nanogel, a
composition 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, highly reactive bonds
that can break and easily reform, which act as shock absorbers.
The computer model allowed the researchers to test the performance of
various bond arrangements. The polymers were first laid out 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, however, that though these
bonds could recover from short-lived stress, they could not withstand drawn out
tension such as stretching or pulling. Instead, the team found that when
particles were joined by several parallel bonds, the nanogel could absorb more
stress and still self-repair.
The team then sought the most effective concentration of parallel labile
bonds, Balazs said. According to the computational model, even a small number
of labile bonds greatly increased resilience. For instance, a sample in which
only 30% of the bonds were labile—with parallel labile bonds placed in groups
of four—could withstand pressure up to 200% greater than what could fracture a
sample comprised only of stable bonds. A film shows that as this sample is stretched, the
labile bonds (red) rearrange themselves to hold the material together.
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 researchers report.
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 said. “In short, a little bit of weakness
gives a material better mechanical properties. Nature knows this trick.”