Page 71 - Carbon Nanotube Fibres and Yarns

Solution-spun carbon nanotube fibers 63

At the right concentrations of SDS, the SWNTs were homogeneously
dispersed to form a single-phase solution with a viscosity similar to that of
pure water. An optimum was found at about 0.35 wt% of nanotubes and
1.0 wt% of SDS. This corresponds to the maximum amount of SWNTs
achievable in homogeneous dispersions.
The surfactant-stabilized SWNT suspension was then injected through
a cylindrical spinneret in the stream of a polyvinyl alcohol (PVA) solution
(5 wt%), which induced the coagulation of the nanotubes through bridging
flocculation. As a result, a mesh could be obtained, which was then washed
several times with pure water to remove most of the surfactant and poly-
mer. The mesh was finally pulled out of the water and collapsed into a fiber
consisting of a densely interconnected nanotube network making use of the
capillary effect at drying.
X-ray scattering analysis showed that the fibers were composed of
SWCNT bundles, PVA chains, graphitic objects, and catalyst particles.
The nanotubes, graphitic objects, and catalyst came from the synthesis
of the raw nanotubes, whereas the PVA chains were introduced during
the elaboration process, where they were adsorbed onto the nanotube
bundles. The diameter of the resulting CNT fibers varied from several
to 100 μm depending on the processing conditions, such as the diame-
ter of the syringe needle, the flow rate of the injected solution, and the
co-flowing polymer solution.
The tensile strength and Young’s modulus of the resulting fibers were
around 300 MPa and 40 GPa, respectively. The electrical conductivity at
room temperature was about 10 S/cm, and a nonmetallic behavior was
observed when the temperature was decreased. Posttreatments, such as
hot-drawing, could enhance nanotube alignment and hence improved the
mechanical performance of the CNT fibers.
Vigolo et al. [5] later applied a stretching treatment to improve the
alignment of the single-wall carbon nanotubes in the fibers. The fibers
were rewetted, swollen, and redried vertically under a tensile load with a
weight attached to the fiber end. Once rewetted and swollen, they could be
stretched up to 160%. The alignment of the nanotubes was studied by X-ray
scattering and characterized by the full-width at half-maximum (FWHM)
of the azimuthal intensity distribution. A smaller FWHM value indicates a
higher level of CNT alignment. The FWHM varied from 75 to 80 degree
for raw fibers to values smaller than 50 degree for stretched fibers, show-
ing a substantial improvement of the SWNTs orientations after stretching
treatment. As a result of the improvement in SWNT orientation, there was