Page 83 - Carbon Nanotube Fibres and Yarns

76 Carbon Nanotube Fibers and Yarns

Herman’s orientation of crystalline polymers increased from 0.4 to 0.7 for
PE fiber containing 1 wt% SWNT [15], from 0.73 to 0.88 for PP fiber con-
taining 1 wt% SWNT [17], and from 0.52 to 0.62 for PAN fiber containing
5 wt% SWNT [25]. In these examples, both neat polymers and nanocom-
posite fibers were prepared under the same processing conditions.
Due to the nucleation and templating effects of CNTs on polymer
chains, the structures and properties of the polymer chains near the CNTs
could be distinctly different from the bulk polymers. Many studies have
found that polymers in the vicinity of CNT exhibited enhanced properties
in comparison with bulk polymers; thus, this particular region is defined as
interphase. Ding et al. fractured MWNT/polycarbonate (PC) nanocom-
posite and examined the PC sheath formed on MWNT surface through
AFM (Fig. 5.2C) [21]. Coleman et al. found a crystalline coating of PVA
around MWNT which improved interfacial stress transfer between poly-
mer matrix and MWNT [26]. Some TEM observations have verified that
polymer chains, such as PP [12a] and PAN [27], in the vicinity of a CNT
are highly oriented along the CNT axial direction. X-ray diffraction of
CNT-reinforced carbon fibers showed a highly ordered phase which does
not exist in the unreinforced carbon fiber control sample [28]. Sandler et al.
observed that in highly aligned PP/CNT nanocomposite films, PP crys-
tallized in the vicinity of the CNT filler has a more compact packing and
higher crystallinity [12a]. For gel-spun SWNT/PAN fiber, highly ordered
PAN crystalline structures with a lattice spacing of 0.52 nm was observed in
the regions surrounding SWNTs (Fig. 5.3A) [27]. WAXD meridional scans
of CNT-containing and pristine PAN fibers revealed that the peak position
was shifted to a low angle with the inclusion of CNTs, suggesting that
CNT causes extended PAN chain conformation along the CNT axis [25b,
27]. For gel-spun SWNT/PVA fiber, the crystal size along the fiber axis is
larger than 40 nm (from HR-TEM), which is much larger than the average
crystal size of PVA (010) planes (8.4–14.8 nm by WAXD). This suggests that
extended chain crystallization occurs in the presence of CNT [30]. Barber
et al. pulled CNT out from PE-butene matrixes by AFM tip to measure
interfacial strength, and found that the polymer chains surrounding CNTs
had a higher yielding stress (47 MPa) than the bulk polymer [31]. Molecular
dynamic simulation of SWNT/PE composites exhibited an aligned discrete
PE adsorption layers in the vicinity of SWNT, which enhanced the me-
chanical modulus and yielding stress [32]. The interphase polymer is highly
oriented and thus has improved mechanical properties over the noninter-
phase polymer. In our previous studies, we have observed that the fracture