A microscopy technique originally developed to image the molecular-scale topography of surfaces is now helping engineers choose the right materials for a new generation of lightweight high-strength composites based on carbon nanotubes.
Light, conductive and nearly as strong as steel, carbon nanotubes are being combined with lightweight polymers to produce composite materials with properties attractive for use on future space vehicles. But choosing the right polymer for optimal mechanical performance at the nanometer scale requires a lengthy trial-and-error process.
By adapting the tiny cantilever and position measurement systems used in atomic force microscopy (AFM), researchers at the Georgia Institute of Technology are helping their NASA colleagues shorten that process. Using chemical force microscopy, they are producing detailed information about adhesion between single-walled carbon nanotubes (SWNTs) and molecules of candidate polymers with different functional groups.
"Our hypothesis is that the stronger the adhesive interaction between molecules and nanotubes, the more likely it is that the polymer will fully wet the nanotubes, break up aggregations of nanotubes and form a mechanically-sound composite," said Larry Bottomley, a professor in the Georgia Tech School of Chemistry and Biochemistry. "The intent is to come up with two or three chemical groups that will give us the strongest interaction, and then incorporate these onto polymers for further studies."
Details of the research were presented March 23 at the 225th American Chemical Society National Meeting in New Orleans, LA. The Advanced Materials and Processing Branch of NASA's Langley Research Center has supported the work under grant NGT-1-02002.
In a traditional AFM, a gold-coated tip just 20 to 50 nanometers in diameter is placed on the end of a tiny cantilever beam 200 microns long and 40 microns wide. The tip is then lowered onto the surface, which is then moved under the probe.
Molecular-scale elevations in the surface cause small deflections in the cantilever as the surface moves beneath the tip. A laser beam is reflected off the backside of the cantilever onto a position sensitive detector. The voltage from the detector is proportional to the deflection of the cantilever. A computer is used to transform the resulting data into a three-dimensional image of the surface. Instead of mapping a surface, however, the Georgia Tech researchers use the cantilever beam and deflection measurement to study the adhesion force between alkanethiol molecules on the tip and nanotubes on the surface.
The researchers raise a surface composed of nanotube bundles until it contacts the tip. When the nanotubes on the surface contact the alkanethiols on the tip, they adhere to it. When the surface is lowered, the adhesive force between nanotubes and polymer pulls the cantilever down.
"If there are no adhesive interactions between the tip and the sample surface, the cantilever tip just lets go cleanly when you lower the surface," Bottomley explained. "If there is strong adhesive interaction, the adhesive interaction bends the cantilever down until the restoration force of the cantilever exceeds the adhesive force. That provides a direct measurement of the adhesion." The adhesion forces they are measuring with this method are in the nano-Newton range.
From that information, Bottomley and collaborators Mark A. Poggi of Georgia Tech and Peter T. Lillehei of NASA can judge which polymers – and functional groups – provide the best adhesion to the nanotubes.
To properly interpret the data, the researchers must know how the surfaces interact mechanically. For instance, if the tip containing the polymer touches ridges of a nanotube bundle, the adhesion will be less than if the tip contacts a valley in the bundle.
"There is a very strong dependence on the sample topography and the adhesive interactions we measure. Knowing the shape of the tip and knowing where on this surface to find ridge lines, we can extract out the adhesive interaction between specific functional groups on the tip and the nanotube surface," Bottomley explained. "The broadest impact of this work may be on other people doing this type of molecular study using surface force apparatus or atomic force microscopy. They must take into consideration the area of contact."
Instead of a three-dimensional map of the surface, the technique produces a force volume image showing adhesion force variations across a two-dimensional surface.
"We find dramatic differences in the adhesive interactions with subtle changes in the chemistry of the tip," Bottomley said. "You have the strongest interactions in the amine-terminated samples compared to the methyl-terminated, hydroxyl-terminated and carboxyl-acid-terminated composites."
Developed a decade ago, carbon nanotubes possess many attractive properties. But they also tend to clump together into bundles, which can pose problems in composite manufacture. If the polymer does not interact with or "wet" the nanotubes individually, the result is a mechanical defect that will weaken the resulting composite.
"If the polymer doesn't wet the nanotubes properly or if the nanotubes aggregate, you get a composite in which portions are just the standard polymer," Bottomley explained. "The real challenge is distributing the nanotubes throughout the polymer in a proper orientation."
For the future, the researchers plan to test additional polymers and functional groups, and to study the interaction of single nanotubes with the polymer molecules.
A paper describing the work has been submitted to the journal Nano Letters.
Technical contact: Larry Bottomley (404-894-4014); Fax: (404-894-7452); E-mail: (firstname.lastname@example.org).
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