Nanomaterial bulks up under stress

RICE (US) — A synthetic material gets stronger from repeated strain much like the body strengthens bone and muscle after repeated workouts.

The trick to stiffening polymer-based nanocomposites with carbon nanotube fillers lies in the complex, dynamic interface between nanostructures and polymers in carefully engineered nanocomposite materials.

Details are reported in the journal ACS Nano.

Researchers discovered the interesting property while testing the high-cycle fatigue properties of a composite made by infiltrating vertically aligned, multi-walled nanotubes with polydimethylsiloxane, an inert rubber polymer. Instead of damaging the material, repeatedly loading it seemed to make it stiffer.

Using dynamic mechanical analysis (DMA) to test the material, the researchers found that after 3.5 million compressions (five per second) over about a week’s time, the stiffness of the composite had increased by 12 percent and showed the potential for even further improvement.

“It took a bit of tweaking to get the instrument to do this,” says Brent Carey, a graduate student at Rice University working in the lab of Pulickel Ajayan, professor of mechanical engineering and materials science and of chemistry at Rice University.

“DMA generally assumes that your material isn’t changing in any permanent way. In the early tests, the software kept telling me, ‘I’ve damaged the sample!’ as the stiffness increased. I also had to trick it with an unsolvable program loop to achieve the high number of cycles.”

Materials scientists know that metals can strain-harden during repeated deformation, a result of the creation and jamming of defects—known as dislocations—in their crystalline lattice. Polymers, which are made of long, repeating chains of atoms, don’t behave the same way.

Researchers are not sure precisely why their synthetic material behaves as it does.

“We were able to rule out further cross-linking in the polymer as an explanation,” Carey says. “The data shows that there’s very little chemical interaction, if any, between the polymer and the nanotubes, and it seems that this fluid interface is evolving during stressing.”

“The use of nanomaterials as a filler increases this interfacial area tremendously for the same amount of filler material added,” Ajayan says. “Hence, the resulting interfacial effects are amplified as compared with conventional composites.

“For engineered materials, people would love to have a composite like this. This work shows how nanomaterials in composites can be creatively used.”

Simply compressing the material didn’t change its properties; only dynamic stress—deforming it again and again—made it stiffer.

Carey drew an analogy between the material and bones. “As long as you’re regularly stressing a bone in the body, it will remain strong. For example, the bones in the racket arm of a tennis player are denser. Essentially, this is an adaptive effect our body uses to withstand the loads applied to it.

Graduate student Brent Carey compressed a nanocomposite for a week at a rate of 5 times per second. (Credit: Jeff Fitlow)

“Our material is similar in the sense that a static load on our composite doesn’t cause a change. You have to dynamically stress it in order to improve it.”

Cartilage may be even a better comparison—and possibly even a future candidate for nanocomposite replacement.

“We can envision this response being attractive for developing artificial cartilage that can respond to the forces being applied to it but remains pliable in areas that are not being stressed,” Carey says.

“This is a beautiful result,” says Ajayan. “It tells us that it’s feasible to engineer interfaces that make the material do unconventional things.”

Researchers at the University of Bridgeport and the Federal University of Minas Gerais, Brazil contributed to the study, that was funded in part by the NASA Graduate Student Researchers Program.

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