Tiny combo boosts strength of plastic
Technology has come a long way by blending materials: Concrete is a lot tougher than solo sand, cement or gravel. You don't see many glass canoes, but mix glass and plastics, and presto-chango, you get strong, light fiberglass. And those winning bikes in the Tour de France were made not of pure carbon, but rather from a blend of carbon fibers and polymers.
And now technologists are racing to blend nanotubes and other extremely minute carbon structures with polymers. These structures are measured in billionths of a meter and therefore are called nanostructures.
Nanotubes, the best-studied nanomaterial, is one of the strongest materials known, far stronger than steel in tensile strength. Adding just 1 percent nanotubes by weight can double some of the mechanical properties of plastic. Because the nanotube-polymer blend can conduct electricity and also act as a semi-conductor, the composite material could, perhaps, be used in solar panels or computers.
But nanotubes are not the only carbon nanostructure. Carbon atoms also assemble into gossamer-thin sheets called graphenes, and into nanodiamonds, a fancy term for diamond dust. So what happens when two of these nano-materials are added to polymer?
In experiments reported this week, C.N. R. Rao of the Chemistry and Physics of Materials Unit of the Jawaharlal Nehru Centre for Advanced Scientific Research (Bangalore, India) reported that doubling up the additives produced an "extraordinary" improvement in stiffness and hardness. These two measures of material strength were as much as 400 percent higher, compared to the same polymer reinforced with a single type of nanostructure.
No sin in this synergy!
That increase was more than would be expected if the two reinforcements were working independently, and therefore Rao and colleagues termed it a synergistic improvement.
Although nanotubes do improve polymer strength, eventually, they clump together, cutting their ability to bond to the polymer and the increase ceases. The dual-additive approach may produce the synergistic improvement by blocking this clumping, the authors wrote.
The researchers tested three nanostructures: tubes, graphene sheets, and nanodiamonds.
A combination of nanodiamond and graphene resulted in the greatest synergistic improvement, followed by nanotubes plus graphene. Blending nanodiamonds along with nanotubes and graphene "is quite interesting and the results indicate a dramatic improvement in the mechanical properties beyond simple additive contributions of either nanofiller alone," says L. Catherine Brinson, professor of mechanical engineering at Northwestern University, who also works in nano-polymer combinations.
Brinson adds that, "The authors speculate that the use of the nanodiamonds in conjunction with nanotubes or graphene may be causing significant changes in the dispersion or nano-scale morphology of the nanofillers," and this seems to be on the money. "We have observed consistently that the degree of dispersion of the nanoparticles and the chemical interaction of the nanoparticles with the polymer chains of the matrix are the underlying mechanisms which control the properties of nanocomposites."
"Carbon nanotubes are being used in composites and there is no question that carbon nanotubes will be used in a big way in the future," Rao wrote to us. However, "I believe that it is too early to say exactly where the applications would be. I think more research will have to be done to find best combinations required for specific applications."
Graphene sheets are much cheaper than nanotubes, and they may also be safer. A 2008 report in Nature Nanotechnology showed that nanotubes acted like asbestos, another sharp, small particle that causes a deadly lung cancer. Introducing carbon nanotubes into mice, "results in asbestos-like, length-dependent, pathogenic behavior," wrote a group lead by Ken Donaldson at the University of Edinburgh. "This is of considerable importance, because research and business communities continue to invest heavily in carbon nanotubes for a wide range of products under the assumption that they are no more hazardous than graphite. Our results suggest the need for further research and great caution...."
Rao has plenty of competitors in the race to commercialize a nano-polymer hybrid, which could make strong, lightweight parts for airplanes, spaceships and sports equipment.
We've heard of carbon-fiber reinforced concrete, but no word yet on concrete fortified with nano-stuff.
David J. Tenenbaum
• Extraordinary synergy in the mechanical properties of polymer matrix composites reinforced with two nanocarbons, by K. Eswar Prasad, Barun Das, Urmimala Maitra, U. Ramamurty, and C.N.R. Rao, PNAS, July 27, 2009.