Page 72 - Carbon Nanotube Fibres and Yarns

64 Carbon Nanotube Fibers and Yarns

a substantial increase of the Young’s modulus, with values up to 40 GPa, in
addition to a significant improvement of the tensile strength.
Badaire et al. [6] studied the influence of stretching and heat treatment
on the properties of SWNT fibers prepared according to Vigolo’s spinning
method. The fibers were dried, rewetted under tensile load, and redried to
improve the alignment. The PVA polymer was removed by annealing the
fiber in hydrogen at 1000°C. The FWHM measured from X-ray scattering
decreased linearly from 27.5 degree in the initial extruded fiber to 14.5
degree after stretching by 80%. Both electrical and thermal conductivity of
neat or composite fibers were improved upon the alignment of the carbon
nanotubes. However, the relative improvement due to further alignment
was modest, by a factor of 3–4 for electrical conductivity for fibers having
the same chemical nature. In sharp contrast, annealing the fibers to remove
insulating polymers resulted in neat nanotube fibers with a significantly
increased conductivity, by several orders of magnitude.
Dalton et al. [7–9] modified the method initially used by Vigolo
et al. and produced a nanotube gel fiber that was then converted into
a 100-m-long solid nanotube composite fiber. Lithium dodecyl sulfate
(LDS) was used as a surfactant in preparing the CNT solution. The solu-
tion was injected into the center of a cylindrical pipe in which the PVA
coagulation solution flows. The resulting fibers were about 50 μm in di-
ameter and contained around 60 wt% SWNTs and 40 wt% PVA. These
fibers showed a tensile strength up to 1.8 GPa and a Young’s modulus up to
80 GPa. Although PVA chains in the CNT fiber enhanced the load transfer
efficiency between CNTs and consequently improved the fiber mechani-
cal performance, they resulted in lower electrical and thermal conductivity
due to the high loading of PVA.
Kozlov et al. [10] described another flocculation-based wet-spinning
process that resulted in polymer-free carbon nanotube fibers. Like the
polymer-based coagulant spinning method used by Vigolo et al. [4], the
polymer-free spinning process utilized dilute, low-viscosity dispersions of
carbon nanotubes (about 0.6 wt% or lower SWNT content). The spinning
solution used was essentially the same lithium-dodecyl-sulfate-stabilized
(LDS-stabilized) aqueous dispersion employed for spinning SWNT/PVA
composite fiber used by Dalton et al. [7]. HiPco SWNTs of 0.6 wt% were
dispersed using a horn sonicator in an aqueous solution of 1.2 wt% LDS
surfactant. A narrow jet of this spinning solution was injected into the floc-
culation bath containing 37% hydrochloric acid, which rotated at 33 rpm.
Flocculation of nanotubes in the spinning solution to form a gel fiber