Page 99 - Carbon Nanotube Fibres and Yarns

Carbon nanotube-reinforced polymer nanocomposite fibers 91

Kevlar may be reinforced in the same way. Kim et al. used in-situ po-
lymerization to synthesize PPTA/CNT nanocomposite solution and found
a huge improvement of electrical conductivity [78]. Sainsbury et al. grafted
PPTA onto MWNTs to achieve a better dispersion than directly mixing
unmodified MWNTs with PPTA [79]. PPTA was dissolved in fuming sul-
furic acid (H 2 SO 4 ) to protonate SWNTs and obtain exfoliated SWNT [32].
Cao et al. blended PPTA/H 2 SO 4 solution with SWNTs and studied the
rheology of the blend. When the SWNT concentration was higher than
0.2 wt%, single-phase nematic liquid crystals were formed at 85°C, which
could be used to produce superior-performance fibers [80]. Deng et al.
compared the structures and tensile properties of PPTA and PPTA/SWNT
fibers, and observed that the CNT orientation was lower than PPTA chains
in the nanocomposite fibers. The measured tensile curves suggested that
reinforcement only occurred at low draw ratios (~2) and could become
detrimental to the fiber at a draw ratio higher than 4 [81]. Instead of mixing
CNT with polymers, O’Connor et al. developed a new approach to pro-
duce PPTA/CNT fibers by swelling Kevlar fibers in CNT suspensions. The
fiber strength increased from 3.9 to 4.8 GPa and the modulus from 120 to
130 GPa when 1 wt% CNT was absorbed by Kevlar [82]. CNTs could also
be used as a bridging agent to enhance the interfacial interaction between
Kevlar [83] or PBO [84] fibers and epoxy resins with significantly improved
performances for the resulting composites.

5.4 Challenges
CNTs have exceptional mechanical properties. However, in most of the re-
ported cases, the properties of CNTs are not fully utilized due to a number
of reasons.
5.4.1 CNT dispersion

Fig. 5.11A shows the dependence of specific surface area of CNTs on
the wall number of the CNTs. With the increase of CNT wall num-
ber, the specific surface area of the CNTs dramatically decreases. For
the reinforcement of polymer fibers, SWNT is theoretically superior to
MWNT. Since CNT would always be damaged during processing, there
are experimental evidences that double-walled and few-walled CNTs
(DWNT and FWNT) are preferred over SWNT for retaining mechan-
ical properties of CNTs in the final nanocomposites. Fig. 5.11B shows
the specific surface area of a SWNT bundle as a function of the number