Page 80 - Carbon Nanotube Fibres and Yarns

Carbon nanotube-reinforced polymer nanocomposite fibers 73

gained great interest from researchers and manufacturers. CNTs have a
2
structure of rolled sp covalently bonded carbon atoms in the shape of a
long cylinder. To a certain degree, CNTs can be regarded as nano-sized
fibers [4]. The anisotropic properties of CNTs make them especially suit-
able for the reinforcements of polymer fiber, which is also anisotropic in
both geometry and properties. Polymer/CNT nanocomposites have been
intensively studied in the past two or three decades, and the resulting nano-
composites show much improved performances than their neat polymers
[5]. CNTs have remarkable mechanical properties, electronic conductivity,
and thermal conductivity [6]. CNT-containing fibers may exhibit multiple
functions, such as electronic conductivity [7], optical property [8], elec-
trochemical activity, and piezoelectricity [9]. Additionally, the diameter of
a CNT could be as small as 1 nm, which makes it possible to influence
and change the surrounding polymer chain structures. Incorporating CNTs
into polymer materials could be considered as a bottom-up method to tune
the fine structures of polymer fibers. This chapter discusses how the incor-
poration of CNTs in polymer fibers affects the polymer chain structures,
improves the performances and functionalities of the fiber, especially tensile
properties. Technical challenges and future perspectives are discussed at the
end of this chapter.

5.2 Unique properties for CNT-reinforced nanocomposites

Although CNT has superlative mechanical properties, the advantages of in-
corporating CNTs into polymer fibers have not been fully utilized mainly
due to the following reasons: (1) CNTs, especially small diameter nanotubes,
tend to aggregate into bundles due to van der Waals interaction. Although
2
CNT has a very high surface area, ~1330 m /g for single-walled CNT
(SWNT), the contacting area between CNTs and polymer matrix is sig-
nificantly reduced once CNTs form rods or bundles rather than stay as ex-
foliated individual tubes in the polymer matrix. It is commonly understood
that the interfacial area plays an important role on the stress transfer between
CNTs and polymer matrices [10]. Poor dispersion of CNTs leads to weak
reinforcement or even detrimental effect to the properties of the resulting
nanocomposite fibers. CNTs inside a bundle can easily slip away from their
neighbors under a load due to the low interfacial interactions and shear
strength of CNT bundles. Thus, CNT dispersion is the key factor for im-
proving the mechanical properties of CNT-reinforced polymer nanocom-
posites [11]. For SWNT-reinforced polymer fibers, in particular, it is very