BASIC CONCEPTS AND AN OVERVIEW - Foundations of Nanotechnology: Pore Size in Carbon-Based Nano-Adsorbents - page 39

they are formed in a chemical vapor deposition reactor. These methods as well as
the twisting of SWCNT film are reviewed. Recently, some researchers reported
the properties of epoxy/CNT fiber composites, which are similar to composites
reinforced with commercial carbon fibers. The composites were formed by the
diffusion of uncured epoxy into an array of aligned fibers of CNTs. The tensile
and compressive properties were measured. The results demonstrated significant
potential of CNT fiber reinforced composites. The work also highlights the issue
in defining the cross-section of CNT fibers and other CNT macro-assemblies for
mechanical property evaluation. Mora et al. noted that the volumetric density of
CNT fibers in epoxy composites was found to be much higher than the density of
the as-spun fiber obtained from gravimetric and diameter measurements. This dif-
ference is related to the volume of free space inside a bundle of fibers. This space
is infiltrated by the epoxy and ultimately increases the CNT/polymer interface
area. The catalyst particles must be eliminated from the fiber; drawing conditions
must be optimized to eliminate entanglements between CNTs; and the fiber needs
to be pulled at the rate at which nanotubes are growing so the growth of an indi-
vidual nanotube is not terminated.
1.1.6.7.4 GEL-SPINNING OF CNT/POLYMER FIBERS
CNTs can act as a nucleation agent for polymer crystallization and as a template
for polymer orientation. SWCNTs with their small diameter and long length can
act as ideal nucleating agents. Study works suggested that the next-generation
carbon fibers will likely be processed not from polyacrylo nitrile alone but from
its composites with CNTs. Furthermore, continuous carbon fibers with perfect
structure, low density, and tensile strength close to the theoretical value may be
feasible if processing conditions can be developed such that CNT ends, catalyst
particles, voids, and entanglements are eliminated. Such a CNT fiber could have
ten times the specific strength of the strongest commercial fiber available today.
In the current manufacture process of carbon fibers, polymer solution is extruded
directly into a coagulation bath. However, higher strength and modulus PAN and
PAN/SWCNT based fibers can be made through gel spinning. In gel spinning,
the fiber coming out of the spinneret typically goes into a cold medium where
it forms a gel. These gel fibers can be drawn to very high draw ratios. Gel-spun
fibers in the gel bath are mostly unoriented and they are drawn anywhere from 10
to 50 times after gelation. Structure of these fibers is formed during this drawing
process. The gel-spinning process, invented around 1980 has been commercially
practiced for polyethylene to process high-performance fibers such as Spectra TM
and Dyneema TM . Although small diameter PAN fibers (10 nm to 1 mL diameter)
can be processed by electro spinning, the molecular orientation and hence the re-
sulting tensile modulus achieved is rather low, and processing continuous fiber by
electro spinning has been challenging. Others have used island-in-a-sea bi-com-
ponent geometry along with gel-spinning to process PAN and PAN/CNT fibers to
make carbon fibers with effective diameters less than 1 mL. Small-diameter fibers