Orientation of nanotubes

Ko et al. (2003) electrospun composite nanofibers of PAN with dispersed SWCNTs from DMF solutions and reported their orientation in the axial direction in the fiber. Using an AFM-based indentation technique (see Chapter 5), the modulus of the composite PAN fibers (as opposed to that of fiber mats) was measured. The modulus of the nanofibers increased linearly with the volume fraction of CNTs incorporated. Interestingly, the increases were also higher (by a factor of more than two) than that expected on the basis of the mle of mixtures calculated assuming a value of 1 TPa for 

The utility of the technique is illustrated by the Raman spectrum of PAN/MWCNT composite nanofibers shown in Fig. 6.5 (Hou et al. 2005). Strong Raman scattering (disorder-induced scatter) by MWCNTs in the matrix gives rise to D, G, and D signals. The peak at about 2300cm is due to the -CN group of the polymer. Raman spectroscopy also has been used to study CNTs in composite nanofibers of sUk (Ayutsede et al. 2005, 2006), carbon fibers (Chung et al. 2005), and PAN (Hou et al. 2005). 

As already suggested in Chapter 5, X-ray methods are also particularly useful in studying nanocomposites. The WAXD spectrum of the nanocomposites yields nonoverlapping peaks characteristic of the crystallinity inherent in the PAN polymer as well as that associated with CNTs (Hou et al. 2005). 

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Studies from the composite deformation mechanism and interfacial bonding between nanofillers and the polymer matrix have been performed [46-48]. In these reports, the authors performed straining studies to determine the load transfer between carbon nanotubes and the polymer and observed the phenomena of crack propagation and polymer debonding. In some cases, the mechanical deformation processes were followed over the electrospun composite fibers. Microscopic images revealed information on the dispersion and orientation of nanotubes within the fiber and their impact in the mechanical performance regarding strain at break and stress concentration at the pores of the nanotubes. 

Raman spectroscopy has been used to probe interactions occurring in PAni nanotube [23-24] composites, the orientation of nanotube bundles within a matrix [25, 26], and the efficiency of load transfer from the host matrix to SWCNTs [27,28]. Unlike X-ray diffraction (XRD) methods [12], Raman spectroscopy can detect very low concentrations of SWCNTs in a polymer matrix [29,30]. The degree of orientation of aligned nanotubes can be estimated by polarized Raman spectroscopy due to the presence of a strong resonance Raman scattering effect [31,32]. Polarized Raman spectroscopy in combination with a mathematical model [33] has been employed to characterize the orientational order of nanotubes in polymers [34]. Using this model, the polarized Raman intensity of nanotubes is correlated with the orientation order parameters of SWCNTs in a utuaxially oriented system. An orientation distribution function can then be obtained. 

Inspired by experimental observations on bundles of carbon nanotubes, calculations of the electronic structure have also been carried out on arrays of (6,6) armchair nanotubes to determine the crystalline structure of the arrays, the relative orientation of adjacent nanotubes, and the optimal spacing between them. Figure 5 shows one tetragonal and two hexagonal arrays that were considered, with space group symmetries P42/mmc P6/mmni Dh,), and P6/mcc... 

Vigolo, B., Penicaud, A., Coulon, C., Sauder, C., Pailler, R., Journet, C., Bernier, P. and Poulin, P. (2000) Macroscopic fibers and ribbons of oriented carbon nanotubes. Science, 290, 1331-1334. 

Yonemura, H., Yamamoto, Y, Yamada, S., Fujiwara, Y. and Tanimoto, Y. (2008) Magnetic orientation of single-walled carbon nanotubes or their composites using polymer wrapping. Sci. Technol. Adv. Mater., 9 (024213), 1-6. 

The structure of carbon nanotubes depends upon the orientation of the hexagons in the cylinder with respect to the tubule axis. The limiting orientations are zigzag and arm chair forms, Fig. 8B. In between there are a number of chiral forms in which the carbon hexagons are oriented along a screw axis, Fig. 8B. The formal topology of these nanotube structures has been described [89]. Carbon nanotubes have attracted a lot of interest because they are essentially onedimensional periodic structures with electronic properties (metallic or semiconducting) that depend upon their diameter and chirality [90,91]. (Note. After this section was written a book devoted to carbon nanotubes has been published [92], see also [58].)... 

Ti02 nanotubes were used to support M0O3 observing a spontaneous dispersion of molybdenum-oxide on the surface of nanotubes, which was different from that observed on titania particles.Supporting tungsten oxides a preferential orientation of the (002) planes was observed. Vanadium-oxide in the form of nanorods could be prepared using the titania nanotube as structure-directing template under hydrothermal... 

Three examples of particular structures of SWCNTs, depending on the orientation of the hexagons related to the tube axis, (a) armchair-type tube (0 = 30°), (b) zigzag type tube (0 - 0°), and chiral tube (0 < 0 < 30°). Reprint from Carbon, vol. 33, No. 7, Dresselhaus M.S., Dresselhaus G., Saito R., Physics of carbon nanotubes, pages 883-891, Copyright (1995) with permission from Elsevier. 

Small molecules can undergo self-organization into a wide variety of orientations besides nanotubes. For example, in 2003, Percec designed a system of self-assembled dendritic benzamides based on methyl gallate and trimethoxyben-zene that exhibited a liquid crystalline phase both as monomers and as polymers (Fig. 7.7) [52],... 

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