When most materials are stretched in one direction, they get thinner in the other directions — like a rubber band. However, researchers in the US have discovered that some types of buckypaper — sheets made of woven carbon nanotubes — increase in width when they are stretched. This unexpected property could come in handy for making composite materials, artificial muscles, gaskets and sensors, say the researchers.

Conventional stretching is quantified by a positive Poisson’s ratio (the percent lateral contraction to the percent applied stretch). However, in the past hundred years or so physicists have become aware of a small but growing number of materials with negative Poisson’s ratios. These “auxetic” materials get wider when stretched and include some rocks and living tissue. The structure of these materials tends to include scissor-like struts much like a collapsible wine rack.

Now Ray Baughman and colleagues at the University of Texas at Dallas along with researchers in Brazil have discovered that some types of buckypaper are auxetic (Science 320 504).

The team made their buckypaper in the same way that ordinary writing paper is made — by drying a slurry of nanotube fibres. Carbon nanotubes are sheets of carbon one atom thick that are rolled up into tubes that are only several nanometres in diameter.

SWNTs and MWNTs

The slurry consists of a mixture of carbon single-walled nanotubes (SWNTs) and multiwalled nanotubes (MWNTs). A MWNT comprises several concentric nanotubes. The researchers found that increasing the amount of MWNTs in the paper produces a sharp transition from a positive Poisson’s ratio of about 0.06 to a much larger negative value of around -0.20.

According to the researchers, this transition can be understood using a “wine-rack” model of buckypaper. If two neighbouring nanotube layers are coupled like the struts in a compressible wine rack, the Poisson’s ratio is positive and the rack becomes narrower when stretched. In contrast, if the rack is blocked so that it can no longer be collapsed (but the struts are stretchable), increases in strut length produce a negative Poisson’s ratio.

The team found that the nanotube sheets containing both single-walled and multiwalled nanotubes had a 1.6 times higher strength-to-weight ratio, 1.4 times higher modulus-to-weight ratio and a 2.4 times higher toughness than sheets made of just SWNTs or MWNTs alone. That the properties of buckypaper can be enhanced by mixing nanotube types could also apply to other nanotube arrays, like nanotube yarns, say the researchers.

Artificial muscles

The ability to tune the Poisson’s ratio could be exploited for creating special sheets that wrap around objects with concave, convex, or saddle-shaped surfaces – something that could be used to make structures with a wide range of shapes. The team also believe that tuned materials could be used to make gaskets, artificial muscles and stress/strain sensors in which exposure to certain chemicals induces mechanical stress.

“By choosing a suitable ratio of SWNTs and MWNTs, the Poisson ratio can also be adjusted to zero, which is useful for designing cantilevers for sensing that do not distort in the width direction during bending,” explained Baughman. This he claims, could improve the sensitivity of such sensors.