Page 91 - Carbon Nanotube Fibres and Yarns

84 Carbon Nanotube Fibers and Yarns

content, the conductivity of the nanocomposite with slightly anisotropic
CNT orientations was higher than that with isotropic CNT orientations,
however highly oriented CNTs led to a significantly reduced conductivity
[54]. Potschke et al. reported that PC/MWNT isotropic film was conduc-
tive but highly drawn melt-spun PC/MWNT fiber lost its conductivity
[55]. During CNT/polymer fiber drawing process, electrical conductivity
of CNT-containing fiber decreases up to several orders of magnitude with
increase of draw ratio [7b, 56]. For an isotropic nanocomposite, the CNT
percolation threshold concentration is normally lower than 0.1 wt% [5a],
and even as low as 0.045 vol% in PVC [57]. By comparison, the percolation
threshold in a polymer/CNT fiber is much higher, for example, 0.5–2 wt%
for PMMA/SWNT fiber [51, 54] and 1 wt% for PVA/MWNT [56a]. This
is because the CNT conductive networks in an isotropic composite are
disrupted by the alignment of CNTs during drawing. Whereas, if a highly
drawn CNT/polymer nanocomposite fiber is further annealed at a tem-
perature above its glass transition or melting point, the relaxation of ori-
ented polymer chains will distort the orientation of CNTs and promotes
the formation of CNT conductive paths, therefore improves the electrical
conductivity. Peijs et al. annealed highly aligned PP/MWNT tape with a
CNT concentration of 5.4 wt%, and observed that the conductivity was in
the order: drawn and annealed films > isotropic film > drawn films [7f]. The
increase in conductivity was ascribed to the thermal relaxation of aniso-
tropic CNT bundles which reconstructed the CNT conductive networks.
Similar phenomenon was also observed for CNT-PE/PP fiber [7b]. Since
the percolation status of the CNT conductive network in CNT-polymer
nanocomposites could change during deformation, many researchers pro-
posed to use CNT-polymer nanocomposites as strain sensors [9b, 58].
CNT-containing polymer nanocomposite fibers have been found to have
better solvent resistance than pure polymer fibers. The CNT-containing fi-
bers can remain intact in a chemical solvent at a higher temperature and
for a longer time than their pristine polymeric fibers, such as PVA [30] and
PAN [27].
The addition of CNTs in polymer fibers is expected to improve fi-
ber thermal conductivity, since individual CNTs have unusually high ther-
mal conductivity, for example, 6600 W/mK for SWNT [59] and 3000 W/
mK for MWNT [60] although the high tube-tube and tube-polymer re-
sistance can greatly hinder the thermal transfer [61]. Moderate improve-
ments of thermal conductivity have been observed in many polymer/CNT
nanocomposite films and bulk materials, but the thermal conductivity of