Self-strengthening polymer nanocomposite works best stressed

Rice University lab creates self-strengthening nanocomposite

Researchers at Rice University have created an artificial material that gets stronger from repeated stress very like the body strengthens bones and muscles after repeated workouts.

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

The trick, it sort of feels, lies within the complex, dynamic interface between nanostructures 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 nanotubes with polydimethylsiloxane (PDMS), an inert, rubbery polymer. To his great surprise, repeatedly loading the fabric didn’t appear to damage it in any respect. In truth, the tension made it stiffer.

Carey, whose research is sponsored by a NASA fellowship, used dynamic mechanical analysis (DMA) to check their material. He found that when an astounding 3.5 million compressions (five per second) over a couple of week’s time, the stiffness of the composite had increased by 12 percent and showed the opportunity of even further improvement.

“It took a bit tweaking to get the instrument to try this,” Carey said. “DMA generally assumes that your material isn’t changing in any permanent way. Within the early tests, the software kept telling me, ‘I’ve damaged the sample!’ because the stiffness increased. I also needed to trick it with an unsolvable program loop to reach the high selection of cycles.”

Materials scientists know that metals can strain-harden during repeated deformation, end result of the the creation and jamming of defects — called dislocations — of their crystalline lattice. Polymers, that are fabricated from long, repeating chains of atoms, don’t behave an analogous way.

The team will not be sure precisely why their synthetic material behaves because it does. “We were capable of rule out further cross-linking within the polymer as a proof,” Carey said. “The knowledge shows that there’s little or no chemical interaction, if any, between the polymer and the nanotubes, and it appears that this fluid interface is evolving during stressing.”

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

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

Additionally they found a different truth about this unique phenomenon: Simply compressing the fabric didn’t change its properties; only dynamic stress — deforming it repeatedly — made it stiffer.

Carey drew an analogy between their material and bones. “So long as you’re regularly stressing a bone within the body, it’s going to remain strong,” he said. “As an example, the bones within the racket arm of a tennis player are denser. Essentially, it’s an adaptive effect our body uses to resist the hundreds applied to it.

“Our material is analogous within the sense that a static load on our composite doesn’t cause a metamorphosis. You’ll want to dynamically stress it as a way to improve it.”

Cartilage could be a better comparison — and doubtless even a future candidate for nanocomposite replacement. “We will be able to envision this response being attractive for developing artificial cartilage which will reply to the forces being applied to it but remains pliable in areas that aren’t being stressed,” Carey said.

Both researchers noted here is the type of basic research that asks more questions than it answers. While they could easily measure the fabric’s bulk properties, it’s a wholly different story to comprehend how the polymer and nanotubes interact on the nanoscale.

“People were attempting to address the question of the way the polymer layer around a nanoparticle behaves,” Ajayan said. “It’s an exceedingly complicated problem. But fundamentally, it’s important in case you’re an engineer of nanocomposites.

“From that perspective, i believe this can be a beautiful result. It tells us that it’s feasible to engineer interfaces that make the cloth do unconventional things.”