Mechanical instabilities in carbon nanotubes

We managed to observe consistent negative stiffness when multi – walled carbon nanotubes (MWCNTs) and multi – walled carbon nanofibers (MWCNFs) were axially compressed. Negative stiffness is a form of mechanical instability and when constrained, offers novel and enhanced viscoelastic properties to a system or composite. Lakes et al. showed that if composites comprise of a small percentage of negative stiffness inclusions, the overall viscoelastic damping increases dramatically under torsion. Most recently, they also showed that the viscoelastic or dynamic modulus of composites with negative stiffness inclusions exceeded that of diamond on several occasions. We postulate that the observed negative stiffness in MWCNT/MWCNFs is a result of shell buckling. This is based upon the observation that the experimentally determined axial critical buckling forces match well with predictions from elastic continuum models. These MWCNT/MWCNFs are compressed with an AFM probe. In a simplistic way, the AFM probe - MWCNT/F configuration could be viewed as a “composite”. We then performed linear viscoelastic measurements on these “composites”. It turned out that under cyclic axial compression, they exhibited enhanced viscoelastic damping too, not unlike what Lakes et al. observed on the larger scale