Sign Change of Poisson's Ratio for Carbon Nanotube Sheets
Lee J. Hall,1
Vitor R. Coluci,2
Douglas S. Galvão,2
Mikhail E. Kozlov,1
Mei Zhang,1*
Sócrates O. Dantas,3
Ray H. Baughman1
Most materials shrink laterally like a rubber band when stretched,
so their Poisson's ratios are positive. Likewise, most materials
contract in all directions when hydrostatically compressed and
decrease density when stretched, so they have positive linear
compressibilities. We found that the in-plane Poisson's ratio
of carbon nanotube sheets (buckypaper) can be tuned from positive
to negative by mixing single-walled and multiwalled nanotubes.
Density-normalized sheet toughness, strength, and modulus were
substantially increased by this mixing. A simple model predicts
the sign and magnitude of Poisson's ratio for buckypaper from
the relative ease of nanofiber bending and stretch, and explains
why the Poisson's ratios of ordinary writing paper are positive
and much larger. Theory also explains why the negative in-plane
Poisson's ratio is associated with a large positive Poisson's
ratio for the sheet thickness, and predicts that hydrostatic
compression can produce biaxial sheet expansion. This tunability
of Poisson's ratio can be exploited in the design of sheet-derived
composites, artificial muscles, gaskets, and chemical and mechanical
sensors.
2 Instituto de Física "Gleb Wataghin," Universidade Estadual de Campinas, Unicamp 13083-970, Campinas, São Paulo, Brazil.
3 Departamento de Física, Universidade Federal de Juiz de Fora, UFJF 36036-330, Juiz de Fora, Minas Gerais, Brazil.
* Present address: Department of Industrial Engineering, Florida State University, Tallahassee, FL 32306, USA.
To whom correspondence should be addressed. E-mail: ray.baughman{at}utdallas.edu
