Page 81 - Carbon Nanotube Fibres and Yarns

74 Carbon Nanotube Fibers and Yarns

rare to achieve exfoliated SWNTs in a nanocomposite; (2) the poor interfa-
cial stress transfer between CNT and polymer restricts CNTs’ reinforcement
efficiency. CNT has an atomic level smooth surface, and it interacts with
polymer chains mainly through van der Waals force that is relatively weak. At
the same time, CNT has a very high modulus, ~1 TPa, which is much higher
than that of common engineering plastic materials (50 MPa ~ 10 GPa), and
is also considerably higher than that of high-performance polymeric fibers,
normally 50–400 GPa. During the deformation of nanocomposites, shear
stress would concentrate at the interface between CNTs and polymer ma-
trix. As the deformation increases, the interfacial stress could become ex-
cessive because of the huge modulus difference between the polymer and
the CNT, leading to the slippage of the CNT from the polymer matrix. An
effective interfacial stress transfer is critical to the efficient utilization of the
reinforcement potentials of CNTs. We now discuss the effects of CNT on
polymer chain structures and properties in detail.

5.2.1 Nucleation/templating effects and interphase structures
CNTs exhibit strong nucleation and templating effects for polymer chains
in well-compounded CNT/polymer composites. The nucleation effect of
CNTs on polymer chains has been found in many polymer systems, such
as polypropylene (PP) [12], PVA [13], polyamide [14], polyethylene (PE)
[15], and poly(ethylene terephthalate) (PET) [16]. For PP, the spherulite
crystallite diameter decreases from 400 μm in neat PP to 20 μm in a CNT-
containing sample [17]. Also, due to CNTs’ strong nucleation effect, the
crystallization of polymer chains would occur at a higher temperature and
a faster rate in CNT-containing nanocomposites than in neat polymers.
The increase of nucleation sites caused by the addition of CNTs is ascribed
to the tremendous interfacial area between CNTs and polymers. Because
CNTs have a very high surface area, a small amount of CNTs is able to
cause a significant increase of nucleation sites. Most studies on CNT crys-
tallization effect adopted a low CNT concentration normally less than or
equal to 1 wt%.
Another important effect is that CNTs align and template polymer
chains in their vicinity. CNT has a nano-sized 1D cylindrical shape with a
high aspect ratio. The anisotropic geometry enables CNT to orient polymer
chains and crystals. For the crystallization of PE and nylon-6,6, it has been
observed that they form periodic lamellar crystals along CNT axis, namely
shish-kebab structure [18]. Highly oriented periodic nano shish-kebab PE
crystals decorated on multiwalled CNT (MWNT) surfaces inside an CNT