Rice Univ. graduate student Brent Carey positions a piece of nanocomposite material in the dynamic mechanical analysis device. He used the device to compress the material 3.5 million times over about a week, proving that the nanocomposite stiffens under strain. Credit: Jeff Fitlow/Rice Univ. |
Researchers at Rice
Univ. 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 materials science and of chemistry, shows the potential of stiffening
polymer-based nanocomposites with carbon nanotube fillers. The team reported
its discovery in ACS Nano.
The trick, it seems, lies in 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 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 3.5
million compressions (five per second) over about a week’s time, the stiffness
of the composite had increased by 12% 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 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.”
Co-authors of the paper are former Rice postdoctoral researcher Lijie Ci;
Prabir Patra, assistant professor of mechanical engineering at the Univ. of
Bridgeport; and Glaura Goulart Silva, associate professor at the Federal Univ.
of Minas Gerais, Brazil.
