Page 73 - Carbon Nanotube Fibres and Yarns

Solution-spun carbon nanotube fibers 65

occurred very close to the point of contact of the spinning solution and the
acid in the bath. This gel fiber (containing 90 wt% volatilizable liquid) was
washed in methanol to remove the hydrochloric acid. The fiber was then
pulled from the wash bath, stretched over a frame, and dried under tension.
After annealing at 1000°C in argon to remove possible residual impurities, a
pure SWNT fiber was obtained. The mechanical properties of the fiber were
3
relatively low, with a specific breaking stress of 65 MPa/(g/cm ), Young's
3
modulus of 12 GPa/(g/cm ), and a strain-to-failure of about 1%. However, it
had an electrical conductivity of 140 S/cm, which was much higher than the
SWNT-PVA composite fibers obtained by Dalton et al. [8,9].

4.3 Spinning from acid solutions

Strong acids such as fuming sulfuric acid are used in the commercial pro-
duction of high-performance synthetic fibers composed of rod-like poly-
mers. SWNTs behave as rigid rods when dissolved in superacids [3]. Acid
solvents have the unique ability to form liquid-crystalline dopes with a
high concentration of SWNTs (Fig. 4.2). The protonation of single-walled
carbon nanotubes in superacids allows them to be dispersed at high concen-
tration, more than an order of magnitude higher than typical concentrations
achieved in surfactants or organic solvents [11]. The protonation is also fully
reversible. At a high enough concentration (>4 wt%), the SWNTs coalesce
and form ordered domains, behaving similarly to nematic liquid crystalline
rod-like polymers. The ensuing electrostatic repulsion counteracts the at-
tractive van der Waals interaction between CNTs.

Fig. 4.2 Microscopy images under cross polars (rotated by 0 and 45 degree, respec-
tively) of SWNT (8 wt%) dissolved in sulfuric acid, showing the typical birefringent
texture of liquid-crystalline solutions [2]. (Reprinted with permission from N. Behabtua,
M.J. Greena, M. Pasquali. Review: carbon nanotube-based neat fibers. Nano Today 3(5–6)
(2008) 24–34.)