Assembly of Well-Aligned Multiwalled Carbon Nanotubes in Confined Polyacrylonitrile Environments:  Electrospun Composite Nanofiber Sheets

Abstract

Highly oriented, large area continuous composite nanofiber sheets made from surface-oxidized multiwalled carbon nanotubes (MWNTs) and polyacrylonitrile (PAN) were successfully developed using electrospinning. The preferred orientation of surface-oxidized MWNTs along the fiber axis was determined with transmission electron microscopy and electron diffraction. The surface morphology and height profile of the composite nanofibers were also investigated using an atomic force microscope in tapping mode. For the first time, it was observed that the orientation of the carbon nanotubes within the nanofibers was much higher than that of the PAN polymer crystal matrix as detected by two-dimensional wide-angle X-ray diffraction experiments. This suggests that not only surface tension and jet elongation but also the slow relaxation of the carbon nanotubes in the nanofibers are determining factors in the orientation of carbon nanotubes. The extensive fine absorption structure detected via UV/vis spectroscopy indicated that charge-transfer complexes formed between the surface-oxidized nanotubes and negatively charged (-CN[triple bond]N:) functional groups in PAN during electrospinning, leading to a strong interfacial bonding between the nanotubes and surrounding polymer chains. As a result of the highly anisotropic orientation and the formation of complexes, the composite nanofiber sheets possessed enhanced electrical conductivity, mechanical properties, thermal deformation temperature, thermal stability, and dimensional stability. The electrical conductivity of the PAN/MWNT composite nanofibers containing 20 wt % nanotubes was enhanced to approximately 1 S/cm. The tensile modulus values of the compressed composite nanofiber sheets were improved significantly to 10.9 and 14.5 GPa along the fiber winding direction at the MWNT loading of 10 and 20 wt %, respectively. The thermal deformation temperature increased with increased MWNT loading. The thermal expansion coefficient of the composite nanofiber sheets was also reduced by more than an order of magnitude to 13 x 10(-6)/ degrees C along the axis of aligned nanofibers containing 20 wt % MWNTs.

Development and characterization of PEEK/carbon nanotube composites

“…This suggests that an increase in the SWCNT content enhances the thermal stability of the composite. Analogous behaviour has been reported in the literature for various polymer/CNT composites [34,35]. The observed stabilization may be attributed to the barrier effect of CNTs, which hinders the diffusion of the degradation products from the bulk of the polymer to the gas phase.…”

Polymer Composites with Carbon Nanotubes in Alignment

“…The improved mechanical and electrical properties of above aligned nanotube/polymer composite have been realized through the direct use of nanotube arrays, sheets, and fibers. In the case of other approaches to align nanotubes (Baji et al, 2010;Ge et al, 2004;Go et al, www.intechopen.com 2004; Haggenmueller et al, 2003;Jang et al, 2009;Ji et al, 2009;Kang et al, 2009;Razal et al, 2007;Vigolo et al, 2000Vigolo et al, , 2002, similar improvements were also observed. For instance, aligned nanotube/polyacrylonitrile fibers after hot-stretching treatment showed a significant enhancement of tensile strength by 320.7% (Ji et al, 2009), while aligned nanotube/epoxy arrays induced by electric field could improve electrical conductivities by four orders of magnitude in the direction of alignment .…”

“…For instance, high-quality nanotube/polydiacetylene composite fibers were produced by the physical attachment of diacetylenic monomers followed by topochemical polymerization of diacetylenic moieties under UV light . Another widely explored method to aligned nanotube/polymer fibers is electrospinning, an electrostatically induced self-assembly process where nanoscale fibers were typically produced (Baji et al, 2010;Ge et al, 2004;Go et al, 2004;Kang et al, 2009). The surface tension, jet elongation, and slow relaxation of nanotubes contribute to orientation of nanotubes.…”

“…However, possibly due to the relatively low degree of alignment, both strengths and conductivities were much lower compared with the direct use of aligned nanotube materials, e.g., strength below 325 MPa and conductivity of 10 -10 10 -8 S/cm at room temperature for the discussed nanotube/polyacrylonitrile fibers and nanotube/epoxy arrays, respectively. Orientation factor had been already used to quantitatively characterize the alignment degree of nanotubes through Herman's equation of f = (3<cos 2 >-1)/2, where  is the average angle between the nanotube and the aligned direction determined by Raman spectroscopy (Ge et al, 2004). However, the orientation factors were only available in very limited studies.…”

Carbon Nanotubes Reinforced Electrospun Polymer Nanofibres

“…There have been several studies on controlling the spatial orientation of electrospun CNT/polymer composite nanofibres [109] [137] [138] [139] [140]. Modification of electrospinning setup has been a common approach to produce aligned electrospun composite nanofibres.…”