Rice University researchers have developed a
self-strengthening nanocomposite that works much like muscles and bones that
strengthen after working out.
Ajayan, study leader and professor of mechanical engineering, materials science
and chemistry at Rice University, and Brent Carey, a graduate student in
Ajayan's lab, have found that a certain synthetic material is capable of strengthening
itself through repeated stress.
material consists of polymer-based
nanocomposites with carbon tube filters. Carey discovered the
strengthening capability when testing the fatigue properties of a composite he
created by allowing the access of polydimethylsiloxane (PDMS), a rubbery
polymer, into several multi-walled and vertical nanotubes.
loading the material repeatedly, it didn't wreck it like Carey thought it
would. Instead, the stress made the material stronger. Carey tested this out
using dynamic mechanical analysis (DMA) and found that 3.5 million compressions
over one week's time increased the
composite's strength, or stiffness, by 12 percent.
took a bit of tweaking to get the instrument to do this," said Carey.
"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."
Ajayan also discovered that compressing the material wasn't enough to change
its properties. Deforming it repeatedly, known as dynamic stress, was the only
way to make the material stiffen.
long as you're regularly stressing a bone in the body, it will remain
strong," said Carey. "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."
both Carey and Ajayan have noted that they're not sure as to why their material
acts the way it does, and that basic research at this level tends to "ask
more questions than it answers," they do know that the dynamic fluid
interface between nanostructures and polymers in the engineered nanocomposites
evolves while the material is stressed, leading them to believe that this
interface is key to what makes it become stiff.
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," said Carey.
also mentioned that nanomaterials used as a filler "increases this
interfacial area tremendously for the same amount of filler material
added," meaning that the interfacial effects become amplified.
hope this invention can eventually be used to develop
artificial cartilage that can strengthen itself through stress.
This study was
published in ACS Nano.