Composite nanofiber sheets of well-aligned polyacrylonitrile nanofibers (PAN) containing multiwall carbon nanotubes (MWCNTs) were prepared by electrospinning a MWCNT-suspended solution of PAN in dimethyl formamide using a moving collector. Scanning electron microscopy, atomic force microscopy, transmission electron microscopy (TEM), IR spectroscopy, Raman spectroscopy, X-ray scattering, and the Instron test were used to characterize the nanofiber sheets. TEM observation showed the MWCNTs were parallel and oriented along the axes of the nanofibers. The mechanical properties of the composite nanofibers were reinforced by MWCNT fillers. Carbonization processes showed that a higher concentration of MWCNTs effectively resisted heat shrinkage of the composite nanofiber sheet.
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.
In natural and synthetic materials having non-racemic chiral centers, chirality and structural ordering each play a distinct role in the formation of ordered states. Configurational chirality can be extended to morphological chirality when the phase structures possess low liquid crystalline order. In the crystalline states the crystallization process suppresses the chiral helical morphology due to strong ordering interactions. In this Letter, we report the first observation of helical single lamellar crystals of synthetic non-racemic chiral polymers. Experimental evidence shows that the molecular chains twist along both the long and short axes of the helical lamellar crystals, which is the first time a double-twist molecular orientation in a helical crystal has been observed.PACS numbers: 61.41. + e Biological materials and their fascinating functions in the human genome have been extensively explored and continue to stimulate new research directions in materials science. Synthetic polymers are similar to proteins and deoxyribonucleic acids (DNA) with respect to their long chain nature, but synthesizing polymers which possess properties similar to biomaterials require, at the very least, an introduction of chirality. The chirality effect on the material properties and structures of small molecular liquid crystals indicates that a series of new phases exist through the introduction of chiral centers which have interesting electro-optical behaviors [1][2][3][4][5]. A helical morphology with a pitch length of several micrometers is typical of chiral liquid crystalline (LC) phases, but exists only in low ordered LC phases. In highly ordered smectic crystal phases, the helical morphology is suppressed by the crystallization process, leading to the traditional parallel close packing scheme in three-dimensional space [1]. We expect that by directionally connecting small LC molecules with covalent bonds to form main-chain nonracemic chiral LC polymers will lead to an enhancement of the conformational chirality strength. The chirality strength should be strong enough to compete with the parallel close packing scheme during crystallization and stabilize the helical morphology in a crystalline state.The material studied is a main-chain chiral polyester synthesized from ͑R͒-͑2͒-4 0 -͕v-͓2-͑ p-hydroxy-o-nitrophenyloxy͒-1-propyloxy͔-1-nonyloxy͖-4-biphenyl carboxylic acid. The polymer has a spacer of nine methylene units, and is abbreviated as PET͑R ء ͒-9, This polymer was specifically synthesized by an A-B type of condensation to ensure strict head-to-tail connections between adjacent repeating units [6,7]. The polymer possesses right-handed chiral centers ͒ء͑ along the mainchain backbone. The specific rotation of the monomer is 228.5 ± . The molecular weight of PET͑R ء ͒-9 is approximately 16 000 g͞mol with a polydispersity of 2 after fractionation, as measured by gel permeation chromatography based on polystyrene standards.Polymer thin films (approximate thickness of 50-100 nm) were prepared by casting a 0.05 (wt) % te...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.