Rice University lab creates self-strengthening nanocomposite

Researchers at Rice University have created a synthetic material that gets stronger from repeated stress. The trick, it seems, lies in the complex, dynamic interface between nanostructures and polymers in carefully engineered nanocomposite materials.

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.

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. Instead, it made it stiffer.

“For engineered materials, people would love to have a composite like this,” Carey 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.”
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