Illustration of the oppositely directed lateral bending on forming a nanotube sheet strip into a ring, when the Poisson’s ratio is positive (left) or negative (right). No lateral bending occurs when the Poisson’s ratio is zero (middle).
The ability to tune Poisson’s ratio could be exploited for designing sheet-derived composites, artificial muscles, gaskets, stress and strain sensors and chemical sensors.
A thick nanotube sheet could also be made to wrap around a concave, convex, or saddle shaped surface depending on the sign of Poisson’s ratio — something that could come in useful for forming shaped composites.
By choosing the ratio of SWNTs and MWNTs, the Poisson ratio can be adjusted to zero, which is useful for designing cantilevers for sensing that do not distort in width during bending. Tensile sensors can provide a sensitivity that is proportional to the volume change produced by stretch, and this volume change can be increased by the team’s discovery of the tunability of Poisson’s ratio.
This transition can be understood by relating the deformation modes of the nanotube sheets to those of a collapsible wine rack. If two neighboring nanotube layers are coupled like the struts in a compressible wine rack, 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.
Baughman and his team subsequently found that the nanotube sheets containing both single-walled and multi-walled 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 SWNTs or MWNTs alone.
A 2006 paper on using nanotubes for artifical muscles