Just as our bodies grow stronger the more we visit the gym, scientists have now developed a synthetic material that strengthens under repeated stress.
Scientists at Rice University headed up by Professor Pulickel Ajayan have shown the potential of stiffening polymer based nanocomposites with carbon nanotube fillers [Carey et al., ACS Nano (2011) doi: 10.1021/nn103104g].
The theory lies in the dynamic interface created between nanostructures and polymers in specifically engineered nanocomposite materials. The property was actually found whilst testing the high cycle fatigue properties of a composite made by infiltrating a forest of vertically aligned, multiwalled nanotubes with polydimethylsiloxane. To the surprise of the scientists, repeated loading of the material did not seem to cause any significant damage, in fact it seemed to make it stiffer.
The group then went on to test the newfound material using dynamic mechanical analysis. They found that after 3.5 million compressions at a rate of 5 per second over a period of 1 week, the stiffness of the composite had increased by 12 % and still showed the potential for further improvement.
A lot of work still needs to be carried out to determine exactly why the synthetic material behaves in this way. Carey, the lead scientist is quoted as saying “We were able to rule out further cross-linking in the polymer as an explanation, 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 applied stress”.
“The use of nanomaterials as a filler increases this interfacial area tremendously for the same amount of filler material added” Professor Ajayan was quoted as saying. “Hence the resulting interfacial effects are amplified as compared with conventional composites.”
Carey drew an analogy between the material and bones. “As long as you’re regularly stressing a bone in the body, it will remain strong, our material is similar in the sense that a static load on the composite doesn’t effect it. You have to dynamically stress it in order to improve it”.
For nanocomposite engineers the work is just beginning as they further explore the physical and chemical dynamics at play here. Professor Ajayan says, “It is a very complicated problem. But fundamentally, it’s important if you’re an engineer of nanocomposites.”


Jonathan Agbenyega