Composite nanofibers orientation

Wang, Q., Yao, Q., Chang, J., Chen, L., 2012. Enhanced thermoelectric properties of CNT/PANl composite nanofibers by highly orienting the arrangement of polymer chains. J. Mater. Chem. 22,17612-17618. 

Polarized Raman spectra and XRD of the composite nanofibers show obvious anisotropic behavior (Figure 6.28), which verified that the PANl backbone chains were orientated along the CNTs axis due to the strong chemical interactions between PANl and CNTs and the electric field induction, as shown. 

It has been established that electrospinning a polymer solution containing well-dispersed carbon nanotubes leads to nanocomposite fibers with the embedded carbon nanotubes oriented parallel to the nanofiber axis due to the large shear forces in a fast fiber-drawing process. Table 1 lists most of the polymer/CNT composite nanofibers produced by electrospining, along with their fiber diameters and tensile properties. 

The orientation and the properties of the received electrospun nanofibers depend mainly on the design, movement pattern, and composition of the used collector. Smooth nanofibers are obtained upon using different shapes of the collector. However, nanofibers orientation can be controlled according to the geometry and way of movement of the collector (Pham et al. 2006). This was reported upon developing nanofibers using different patterns of collectors with different geometries and movements as shown in Fig. 3 (Zander 2013). 

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... 

Orientation of clay platelets in the axial direction has an impact on the mechanical properties of the nanofiber. In PMMA/MMT clay composite nanofibers, the mechanical deformation process was reported to be somewhat different from the brittle failure of comparable bulk nanocomposites. The deformation in the nanofibers occms via shear flow that involves a nanoscale necking process (Kim, G.-M., et al. 2005a). However, in this instance the interpretation is complicated by the nanoporous morphology of the fiber electrospun from CHCI3. 

The crystalline morphology of composite nanofibers is influenced by the orientation of macromolecules as weU as by the presence of filler particles in the fiber. As with unfilled polymer nanofibers, the percent crystallinity of composite nanofihers also tends to he lower than that of the bulk material. 

MMT is a reinforcing filler in polymers such as poly(urea urethane) (Ge et al. 2000) as evidenced by the very significant increase in mechanical properties of the composite nanofibers. Unfilled polymer nanofiber mats of polyurethane (PU) (M — 150,000 g/mol) were electrospun from llwt% solution in DMAc/THF (7 3wt/wt) into nanofibers with 150nm to 410nm. The tensile properties of these mats are shown in Table 6.4, where the last digit in the nanofiber designation is the weight fraction of MMT in the polymer. Based on the WAXD patterns for the composite nanofibers, MMT appeared to be well dispersed, exfoliated, and oriented in the axial direction of the samples. 

Baji, A., Mai, Y.W., Wong, S.C., Abtahi, M. and Chen, P. 2010. Electrospinning of polymer nanofibers Effects on oriented morphology, structures and tensile properties. Composites ScL Technol. 70 703-718. 

Ward and his coworkers investigated the interlayer adhesion in self-reinforced PP composites modified with different nano- and micron-sized particles [27-29]. They found that the introduction of a small amount of carbon nanofibers (CNFs) led to improved performance of polypropylene single-polymer composites obtained by hot compaction of oriented CNF/ PP tapes. The peel strength of a CNF/PP woven fabric composite was significantly increased. In addition, the authors pointed out that the drawn CNF/PP tapes showed substantial voiding around the fibers which were closed and sealed by the hot compaction process. As a result, the composite density increased to its initial value [27]. 

In a previous work, the same authors also observed that the composite peel strength was significantly increased by introducing interleaved films of the same polymer between the layers of woven oriented tapes [30]. They continued working on PP self-reinforced composites enhanced with the addition of CNT, trying to improve properties by the use of interleaved films [28]. Samples of single-polymer polypropylene composites were produced with and without interleaved films, and with and without carbon nanofibers, located either in the oriented polypropylene tapes, in the interleaved film or in both. Maximum peel strength was observed... 

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