Stretch a rubber band and it gets thinner in the middle. This is what intuition leads us to expect of any elastic material. But scientists in the US and Brazil have found that the sheets of carbon nanotubes known as buckypapers can display some very odd behaviour when stretched or uniformly compressed.
When a sheet of specially designed buckypaper is stretched, it actually increases in width as it lengthens. This may be counterintuitive, but the mechanical properties of buckypapers created by University of Texas at Austin physicist Ray Baughman and his colleagues could be exploited for applications such as artificial muscles, gaskets, mechanical sensors and special composites.
The lateral contraction of a stretched material can be quantified by its Poisson’s ratio, which is defined as the relative contraction strain divided by the relative extension strain. To picture this, think of a wine cork, which has a near zero but positive Poisson’s ratio. This makes it difficult to insert the cork into a bottle, but relatively easy to remove. If the cork had a negative Poisson’s ratio the reverse would be true.
Baughman and his fellow researchers made their buckypapers by drying a fibrous slurry containing a mixture of single-walled and multi-walled carbon nanotubes. When they increased the proportion of multi-walled nanotubes, they found that the paper moved sharply from a small, positive Poisson’s ratio of around 0.06 to a large negative value of –0.20.
The explanation for this discontinuity in Poisson’s ratio lies in changes to the deformation modes of the buckypaper structure, which Baughman likens to a collapsible wine rack. Increase the percentage of multi-walled nanotubes in the structure and the rack is blocked so that its struts can no longer collapse, but can still be stretched. This leads to a negative value of the material’s Poisson’s ratio.
“This abrupt switching of the sign of Poisson’s ratio is so surprising, and the structure of the nanotube sheets so complicated, that we initially believed that a quantitative explanation was impossible using state-of-art theoretical capabilities,” says Baughman. The answer in the form of the wine rack model came through the Texas team’s Internet-mediated collaboration with colleagues in São Paulo and Minas Gerais.
As well as the sharp shift in Poisson’s ratio, the researchers found that buckypapers containing both single-walled and multi-walled carbon nanotubes had a 1.6 times higher strength-to-weight ratio, a 1.4 times higher modulus-to-weight ratio, and a 2.4 times higher toughness than sheets made from single types of nanotube. Baughman adds that the mixing of forms is likely to have the same effect on sheets with other kinds of nanotube arrays. Here Baughman cites the twisted yarns he and his colleagues invented in 2005.
“The demonstrated ability to continuously tune strength, modulus, toughness, electrical conductivity and density by mixing different types of nanotubes is quite surprising,” says Baughman. “Even more surprising is our discovery that we can simultaneously tune Poisson’s ratios from positive to negative values.”
As well as the applications identified above, shaped composites could be formed from thick nanotube sheets made to wrap around a concave, convex or saddle-shaped surface, depending on the sign of the Poisson’s ratio. And if the ratio were tuned to a value of zero, cantilevers for mechanical sensors could be created that do not distort during bending.
“This phenomenon belongs to the class of ‘mesoscopic mechanics’, where the property of a seemingly uniform chunk of material is in fact determined by its nanoscale texture,” says Rice University materials scientist Boris Yakobson. “It is fascinating to reveal how the internal architecture of those filaments defines the overall response of the sheets to mechanical deformations.”
Further reading: “Sign Change of Poisson’s Ratio for Carbon Nanotube Sheets”, Hall et al., Science 320, 504 (2008).
Figure: Atomic force microscope image of multiwalled carbon nanotubes in buckypaper (source: University of Texas at Austin).
Article first published in Nanomaterials News.