Researchers create self-strengthening nanocomposite

Mar 23, 2011
A small block of nanocomposite material proved its ability to stiffen under strain at a Rice University laboratory. (Credit Ajayan Lab/Rice University)

Researchers at Rice University have created a synthetic material that gets stronger from repeated stress much like the body strengthens bones and muscles after repeated workouts.

Work by the Rice lab of Pulickel Ajayan, professor in mechanical engineering and and of chemistry, shows the potential of stiffening polymer-based with fillers. The team reported its discovery this month in the journal ACS Nano.

The trick, it seems, lies in the complex, dynamic interface between and polymers in carefully engineered nanocomposite materials.

Brent Carey, a graduate student in Ajayan's lab, found the interesting property while testing the high-cycle fatigue properties of a composite he made by infiltrating a forest of vertically aligned, multiwalled with polydimethylsiloxane (PDMS), an inert, rubbery polymer. To his great surprise, repeatedly loading the material didn't seem to damage it at all. In fact, the stress made it stiffer.

Carey, whose research is sponsored by a NASA fellowship, used dynamic mechanical analysis (DMA) to test their material. He found that after an astounding 3.5 million compressions (five per second) over about a week's time, the 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," Carey said. "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.

The team is 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 said. "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 said. "Hence, the resulting interfacial effects are amplified as compared with conventional composites.

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

They also found one other truth about this unique phenomenon: 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 their material and bones. "As long as you're regularly stressing a bone in the body, it will remain strong," he said. "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.

"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 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 said.

Both researchers noted this is the kind of basic research that asks more questions than it answers. While they can easily measure the material's bulk properties, it's an entirely different story to understand how the polymer and nanotubes interact at the nanoscale.

"People have been trying to address the question of how the polymer layer around a nanoparticle behaves," Ajayan said.
"It's a very complicated problem. But fundamentally, it's important if you're an engineer of nanocomposites.

"From that perspective, I think this is a beautiful result. It tells us that it's feasible to engineer interfaces that make the material do unconventional things."

Explore further: Tiny magnetic DNA particles protect olive oil from counterfeiters

More information: Read the abstract at pubs.acs.org/doi/abs/10.1021/nn103104g

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User comments : 9

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Kingsix
not rated yet Mar 23, 2011
Cool
TombSyphon2317
not rated yet Mar 23, 2011
Wow, imagine body armor that gets stronger every time your shot. Talk about cost reduction engines that never wear out......They only get stronger. even orbital drops if you could protect the package inside. You could do some really cool sh!t with stuff.
apex01
not rated yet Mar 23, 2011
DARPA might be investing in this new tech soon, I hope.
PinkElephant
4 / 5 (1) Mar 23, 2011
Any uses of this material would depend on how stiff/strong/stable it is to begin with. For instance, if it's not nearly as strong as steel, then it probably wouldn't be able to replace any structures currently made from steel, aluminum, titanium, carbon epoxy composites, high-tech ceramics, etc.
SteveL
3.5 / 5 (2) Mar 23, 2011
Wow, imagine body armor that gets stronger every time your shot. Talk about cost reduction engines that never wear out......They only get stronger. even orbital drops if you could protect the package inside. You could do some really cool sh!t with stuff.


Getting shot 3.5 million times for a 12% increase in armor stiffness? Sorry, your post just gave me a chuckle.

Still, this is an impressive discovery and begs for further research.
The_P
4 / 5 (1) Mar 23, 2011
I'm curious if it may be some sort of annealing process, where the nanotubes start to align? Would that make any sense?
antialias
3 / 5 (1) Mar 24, 2011
The coparison with bone is a bit iffy. Bone constantly gets remodeled by osteoclasts and osteoblasts. no remodeling istaking plave in this sample. At most we get a reordering of the filler materials.

But statements like these:
I also had to trick it with an unsolvable program loop to achieve the high number of cycles."

don't really give me much faith in the author. If he thinks that it is a 'neat trick' to use an infinite loop then the rest of his research might be equally shoddy.
FastEddy
not rated yet Mar 24, 2011
This may be related to thermal "shock" results (cryogenic metal hardening technics). There are many ways to "stress" materials to produce better, stronger, more resilient materials. Perhaps some combination of thermal and mechanical stress would ....
Beard
not rated yet Mar 26, 2011
This is a complete guess:

What if the material undergoes a kind of natural selection after the millions of iterations? If it's not completely uniform, the weaker configurations would deform more while stronger configurations would survive. After millions of cycles, the stronger variant might replace the weaker throughout the composite? Kind of like a self assembling phenomena.

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