Page 86 - Carbon Nanotube Fibres and Yarns

Carbon nanotube-reinforced polymer nanocomposite fibers 79

ripple travel to sufficient distances corresponding to the dispersions of CNTs
and the development of interphases.
5.2.2 Development of interphase structures

Since there are always interactions between CNTs and surrounding polymer
chains, the structures and properties of these chains will always be different
from the bulk polymers. In order to significantly enhance the properties
of nanocomposite fibers, highly oriented and crystallized interphase struc-
ture is preferred. Marilyn et al. studied the interaction between PAN and
CNT through solution crystallization and observed that both highly ori-
ented crystalline and amorphous PAN structures could be formed on the
surfaces of CNTs depending on the preparation conditions [33b]. Fig. 5.5A
shows highly crystalized PAN-coated SWNTs. After carbonization, highly
crystallized PAN could be graphitized at a low temperature of 1100°C
in the vicinity of SWNTs (Figs. 5.5B–D) [37], whereas, amorphous PAN
formed mostly turbulent carbon structures. Fig. 5.5C clearly shows the for-
mation of highly graphitized carbon structures. By comparison, without
the addition of CNTs, the graphitization of PAN normally occurs only at a
temperature of 2300°C or higher.
Although the formation and development of the interphase in nano-
composites fibers could lead to much improved structures and properties,
the working mechanisms are still unclear and the dependences of interphase
formation on polymer type, CNT type, processing methods, and condi-
tions need to be further investigated. Recent research suggests that the in-
terphase structure could still develop under external stimulations after the
polymer/CNT nanocomposite had been processed and shaped. Carey et al.
observed that the storage modulus (E′) of polydimethylsiloxane (PDMS)/
CNT nanocomposite film kept increasing during external dynamic elastic
straining [38]. Unlike work hardening of metal or other materials which
happens during large deformation, the self-hardening of nanocomposites
occurs under small dynamic straining. Owuor et al. observed similar stiff-
ening behavior for CNT sphere-reinforced PDMS [39]. Senses et al. also
observed similar stiffening phenomena for poly(methyl methacrylate)/
nano-silica particle nanocomposite [40]. Xu et al. prepared graphene and
graphene oxide-reinforced PDMS nanocomposites and observed that G′
increased by 4.6% after dynamic straining at an amplitude of 0.5% and a fre-
quency of 5 Hz [41]. We found that PAN/CNT nanocomposites exhibited
significant improvements on E′ after dynamic straining at an amplitude of
0.5% and a frequency of 1 Hz (Fig. 5.6A) [42]. By comparison, without the