BASIC CONCEPTS AND AN OVERVIEW - Foundations of Nanotechnology: Pore Size in Carbon-Based Nano-Adsorbents - page 37

on the graphitic surface, the nanotubes dispersion was destabilized and collapsed
to form a fiber. These wet fibers could then be retrieved from the bath, rinsed and
dried. Significant rinsing was used to remove both surfactant and PVA. Shear
forces during the flow lead to nanotube alignment. These fibers displayed ten-
sile moduli and strength of 9-15 GPa and

150 MPa, respectively. For stretched
CNT/DNA/PVA fibers, the tensile moduli and strength were

∼

125
MPa, respectively. In the meantime, the coagulation-spinning method was further
optimized by others. They injected the SWCNT dispersion into the center of a
co-flowing PVA/water stream in a closed pipe. The wet fiber was then allowed to
flow through the pipe before being wound on a rotating mandrel. Flow in more
controllable and more uniform conditions in the pipe resulted in more stable fi-
bers. Crucially, wet fibers were not rinsed to remove most of PVA (final SWCNT
weight fraction

∼

19 GPa and

∼

60%). This resulted in large increases in Young's modulus and
strength to 80 and 1.8 GPa, respectively. Furthermore, study works have shown
that single- and multi-walled CNTCNT based fibers could be drawn at tempera-
tures above the PVA glass transition temperature (

∼

180°C), resulting in improved
nanotube alignment and polymer crystallinity. These so-called hot-stretched fibers
exhibited values of elastic moduli between 35 and 45 GPa and tensile strengths
between 1.4 and 1.8 GPa, respectively. In an alternative approach, CNT/polymer
solutions have been spun into fibers using a dry-jet wet spinning technique. This
was achieved by extruding a hot CNT/polymer solution through a cylindrical die.
An approximately 1-10 cm air gap was retained between the die orifice and the
distilled water coagulation bath, which was maintained at room temperature. Sig-
nificant mechanical property increases were recorded for the composite fibers
compared with the control samples with no CNT reinforcement. Another method
used recently to form composite-based fibers from solution is electro spinning.
This technique involves electro statically driving a jet of polymer solution out of
a nozzle onto a metallic counter electrode. In 2003, two groups independently de-
scribed electro spinning as a method to fabricate CNT-polymer composite fibers.
Composite dispersions of CNTs in either PAN or PEO in DMF and ethanol/water,
respectively, were initially produced. Electro spinning was carried out using air
pressure of 0.1-0.3 kg/cm 2 to force the solution out of a syringe 0.5 mm in diam-
eter at a voltage difference of 15-25 kV with respect to the collector. Charging
the solvent caused rapid evaporation resulting in the coalescence of the composite
into a fiber, which could be collected from the steel plate. Fibers with diameters
between 10 nm and 1 m could be produced in this fashion.

∼

1.1.6.7.2 TEXTILE ASSEMBLIES OF CNTS
Some researchers have demonstrated the feasibility in processing of nanotube
yarns with high twist spun from nanotube forests, plied nanotube yarn processed
from a number of single nanotube yarns with counter direction twist and 3-D
nanotube braids fabricated from 365-ply yarns. In addition, the plied nanotube
yarns and 3-D braids were used as through-the-thickness (Z) yarns in 3-D weav-