Page 93 - Carbon Nanotube Fibres and Yarns

86 Carbon Nanotube Fibers and Yarns

It is found that the gel-spun PAN-based carbon fibers exhibit both high
strength (5.5–5.8 GPa) and high modulus (353–375 GPa) over conventional
wet-spun PAN-based carbon fibers [63].
Incorporating a small amount of CNTs (0.25–1 wt%) in gel-spun PAN
fiber increases the tensile strength of the precursor from 0.90 to 1.07 GPa
and the Young’s modulus from 22.1 to 28.7 GPa [27]. The addition of CNTs
also extends the PAN molecular chains along their axis [25b, 27] mainly
due to CNT-nitrile interactions [64], facilitates the formation of conjugated
nitrile and reduces the formation of β-amino nitrile [25b, 33c, 65] during
stabilization, and leads to highly ordered graphitic structures after carbon-
ization [35b, 66]. 1 wt% CNT increases the tensile strength of the final
carbon fiber from 1.9 to 3.4 GPa and Young’s modulus from 286 to 425 GPa
[33c]. TEM images in Fig. 5.8B shows that the PAN molecules in the in-
terphase regions are better aligned than in the polymer matrix [27]. The
highly ordered PAN structure develops into an annular graphitic structure
after stabilization and carbonization (Fig. 5.8D) that did not exist in pure
PAN-based carbon fiber control sample (Fig. 5.8C). The formation of an-
nular graphitic phase in carbonized CNT/PAN fiber has also been verified
by TEM [67], Raman [33c], and WAXD [28]. Processing tension [33c] and
temperature [66b, 67a] are known to affect the formation of graphitic phase.
Higher tension during stabilization and carbonization improves the quality
of carbon fiber [66b], and the addition of CNT increases the fiber strength
required to bear higher processing tension [28].
With the addition of CNTs and the formation of annular graphitic
structures, the electrical conductivity and thermal conductivity of the re-
sulting carbon fibers are significantly improved [35a]. The 1 wt% CNT-
containing PAN-based carbon fiber showed 146% and 103% higher
electrical conductivity and 37% and 25% higher thermal conductiv-
ity than the pure PAN-based carbon fibers T300 and IM7, respectively
(Fig. 5.9A and B). These improvements pointed to high-performance
multifunctional carbon fibers.
The strength of a fiber is limited by the existence of defects in the fi-
ber [1]. With the decrease of fiber diameter, the defect number inside a unit
length of a fiber decreases, resulting in an increase of the fiber tensile strength
while other physical characteristics of the fiber are kept unchanged. Electro-
spinning produces fibers with diameters in the range from 40 to 800 nm.
However, polymer chains relax very fast in solution state, which can result
in poor chain orientation and low tensile properties of the electro-spun fi-
ber. Island-in-a-sea bicomponent spinning method has also been used to