Page 89 - Carbon Nanotube Fibres and Yarns

82 Carbon Nanotube Fibers and Yarns

from 103°C to 116°C, and to 143°C when CNT content was increased
from 0 to 5 wt%, and to 10 wt%, respectively [43]. The upshift of glass tran-
sition temperature was also observed in many other CNT-containing nano-
composite fibers, such as PVA [44], polymethyl methacrylate (PMMA) [45],
PE [46], and poly(ether ketone) (PEK) [47]. The reduced chain mobility also
leads to higher chain activity energy. The glass transition activation energy
increased from 544 kJ/mol for pure PAN fiber to 717 kJ/mol for 0.5 wt%
CNT-containing fiber, and to 809 kJ/mol for 1 wt% CNT-containing fiber
[27]. Similar increase was also observed in other CNT-reinforced nanocom-
posite fibers, for example, PET [48] and PVA [44b].
Oriented polymer chains tend to recoil at a temperature above their glass
transition points, which lead to a shrinkage of fiber length. CNT maintains
its orientation and restricts the entropic shrinkage of polymer chains in
many polymeric fibers, such as PP [17], PVA [30], and PAN [25, 49]. Even
when environmental temperature is higher than the polymer melting tem-
perature, the existence of oriented CNTs in the fiber could retain polymer
chain orientation and reduce fiber shrinkage [17]. Fig. 5.7A1 shows the
thermal shrinkages of PAN composite fibers containing various types of
CNTs [25a]. The addition of CNT nano-fillers in PAN fibers reduce their
thermal shrinkages. When various types of nano-filler were compared, it
is clear that nano-fillers with larger surface area have greater effects on re-
ducing the shrinkages of PAN fibers (Fig. 5.7A2). The scheme in Fig. 5.7B
explains how the addition of CNTs restrains fiber shrinkage. Aligned CNT
orients polymer chains in its vicinity above the glass transition temperature.
The CNT has been found to preserve polymer chain orientation even after
the polymer melts. When an oriented pure PET film is heated to a tempera-
ture higher than its melting point and is subsequently slowly cooled down,
the neat PET shrinks and losses its alignment, whereas the CNT-containing
sample remained oriented [24]. The well-oriented CNTs could retain the
PET chain orientation even in the melting state.
Electrical conductivity of polymer fibers [7, 50] can be greatly improved
by the addition of CNTs regardless whether the polymers are semicon-
ductive or insulating as a result of the percolation behavior of the CNTs,
i.e., the CNTs form an interconnected conductive network in the fibers.
Therefore, the CNT-reinforced polymer fibers are multifunctional and
have broad applications. In comparison to isotropic bulk materials, polymer
chains and CNTs in nanocomposite fibers are oriented along the fiber axis,
and thus their electrical conductivity is anisotropic. Winey et al. found that
a PMMA nanocomposite containing well-aligned SWNTs has an electrical