Page 203 - Carbon Nanotube Fibres and Yarns

Mechanics modeling of carbon nanotube yarns 193

the collapsed structure after the pressure was removed. For example, the
16 × (23,0) bundle (a simulation box containing 16 CNTs) can be fully
collapsed at 0.05 GPa, and the 16 × RC bundle at 0.2 GPa. MD simulations
showed that the sliding friction between neighboring CNTs could increase
by a factor of 1.5–4 from the un-collapsed to the collapsed configurations,
as illustrated in Fig. 8.5. For the 16 × (23,0) bundle, the commensurate
intertube contacts resulted in a high depinning force (static friction) of
≈0.1 meV/Å per carbon atom. After being collapsed, the intertube friction
increased up to 0.4–0.45 meV/Å. In the RC case, although the friction
force is two orders of magnitude smaller, the sliding friction force could
still increase by more than 50% from 0.0017 to 0.0026 meV/Å. The tube
collapse induced a change of the intertube structure. A calculation of the
partial pair distribution function showed that the friction enhancement was
ascribable to the graphite-like stacking of CNT walls.
In order to enhance the intertube friction, polymer molecules are often
introduced between CNTs. These polymers include epoxy [97], polyim-
ide [31], bismaleimide [32,34], polyethylenimine conjugated with catechol
groups [98], poly-dopamine [99], and the most widely used poly(vinyl
alcohol) (PVA) [3,7,30,32,97,100]. The introduction of these long-chain

Fig. 8.5 For a commensurate CNT bundles, the sliding friction increased from ≈0.1
to ≈0.4 meV/Å after the structural collapse. Although the friction force is small,
<0.002 meV/Å, for a bundle containing random chiralities, the friction also increased
by a factor of ≈1.5 after the collapse. (Reproduced with permission from X. Zhang, Q. Li,
Enhancement of friction between carbon nanotubes: an efficient strategy to strengthen
fibres. ACS Nano 4 (1) (2010) 312–316.)