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Posted: November 2, 2009

Rice scientists argue nanotubes can be treated like polymers

(Nanowerk News) Maybe we should thank Marie Antoinette for inspiring a new paper by a team of Rice scientists and engineers who remind us, as she once did, that there's nothing new except what's been forgotten.
While the queen of France knew nothing of buckyballs, she knew something about human nature. Wade Adams, Matteo Pasquali, Micah Green and Natnael Behabtu at Rice pick up that thread in their discussion of what we know -- or think we know -- about carbon nanotubes.
Their review in the journal Polymer ("Nanotubes as polymers") makes the argument that single-walled carbon nanotubes (SWNTs) are polymers and should be treated as such.
The point is to remind the nano community that decades of research into polymers can be applied to their work and hasten the development of novel materials for all kinds of uses.
"In one of his earliest lectures about nanotubes, (late Rice professor and Nobel laureate) Rick Smalley said they're the ultimate polymer molecule, with every atom in its place, just like a polymer chain would have," said Adams, director of the Richard E. Smalley Institute for Nanoscale Science and Technology, who focused on polymers for many years at the Air Force Research Laboratory. "I really didn't believe him initially."
There's little doubt that carbon nanotubes -- spawn of the fullerene buckyball discovered at Rice in the '80s -- are a miracle material with potentially great benefits in the realms of health, energy, the environment and beyond. But scientists have often thought of them as being in a class by themselves.
"This article serves to clarify the fact that nanotubes are polymers," Adams said. "You can apply a lot of the wisdom of polymer science and engineering to working with nanotubes. At the same time, you can still apply all the wondrous things we can do with nanomaterials."
Adams said the goal is to change the mindset of a generation of scientists who have come to think of carbon nanotubes as special when, in a very important way, they're not special at all.
"We were seeing a lot of literature out there about nanocomposites that were totally ignorant of the 15-, 20- and 30-year-old literature that explored a lot of these areas and had already clarified some of the things you need to think about if you're going to use these materials," he said.
A polymer is a large molecule that contains patterns of smaller molecules linked by covalent chemical bonds. Polymers are typically thought of as plastics, but they are in fact used in a wide range of synthetic and natural materials. Even DNA -- the stuff of life -- is considered a polymer.
Like polymers, nanotubes are simple, repeating patterns of linked atoms. One key difference is that polymers can be a linear chain and nanotubes are, well, tubes. Breaking them is a lot harder; instead of only one covalent bond to snap, there are 10 or more. And bonds between carbon atoms are among the most tenacious.
As such, SWNTs are the strongest "rigid rods" known to science. They potentially can add incredible strength, stiffness and electrical conductivity to all kinds of composite materials (the paper references current uses in baseball bats and bicycles), and like polymers, SWNTs can be spun into fibers that should be remarkably robust and have near-superconductive capabilities.
The authors note the polymer community has solved some problems that still bedevil SWNT researchers. Nanotubes tend to tangle as they grow, and separating them can be a problem. In the polymer world, manufacturers use strong acids to dissolve clumps of polymer molecules, separating them so they can be spun into fibers.
The same techniques may apply to SWNTs. Pasquali, Adams and their colleagues found that "superacids" have the power to dissolve entangled nanotubes into discrete rigid rods within minutes. Those rods can then be spun into fibers by adapting methods common in long-proven polymer production.
"All that stuff, modeled by (Nobel Prize-winning chemist) Paul Flory in the '60s and '70s, was done in the '70s and '80s -- the practical experience of mixing a rod polymer and a coil polymer in a solvent, how you process it and how you achieve molecular dispersion," Adams said. "All of this work will help us think more correctly about how to make nanotubes into things."
Adams noted the "zillion" references listed at the end of the paper. "Go back and read the literature, people," he said. "It may help you figure it out."
Pasquali cautioned that excessive reliance on the literature can also hamper progress.
“For several years, we were slowed down because our observations on nanotube systems did not fully fit the accepted framework of rod-like polymers. For example, Flory’s models could not explain our experiments. We finally understood and explained the difference between nanotubes and polymers in a recent article in the Journal of Chemical Physics ("Modeling the phase behavior of polydisperse rigid rods with attractive interactions with applications to single-walled carbon nanotubes in superacids"; co-authored by Green, Behabtu and former graduate student Nick Parra-Vasquez). So we need to continue to rely on the literature, but we also need to keep our eyes and minds open for new phenomena.”
Source: Rice University
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