| Nanofibers made by electrospinning technique show outstanding features of large specific surface areas and ultrafine diameter ranging from nanometers to micrometers. Carbon nanofibers (CNFs) from electrospun polyacrylonitrile (PAN) precursor fibers have attracted much research attention in recent years. Due to the potential high mechanical, thermal and electrical performances of CNFs, they can be applied promisingly in super-capacitors, composites and batteries etc. This research work includes the following aspects:(1) Polyacrylonitrile (PAN) precursor nanofibers with an average diameter of400nm were synthesized by electrospinning. To obtain optimum stabilization conditions, they were stabilized at250,265and280℃for1,2and3h, followed by carbonization at1000℃to fabricate CNFs with a diameter of200nm. The morphology, thermal properties, and chemical structure of the precursor nanofibers, stabilized fibers and the final CNFs, were characterized by SEM, TEM, DSC, TG, XRD and XPS. It was found that the optimum stabilization conditions involved thermally treating them from30to280℃at a heating rate of2℃/min under a constant load of1kN, and keeping them at280℃for two hours. Under these conditions, dehydrogenation and intra-cyclization reactions were almost completed; the PAN crystal structure was almost completely destroyed, and a thermally-stable ladder-like polymeric structure was formed. Nanofibers carbonized at1000℃were electrically conductive, and the conductivity was highly dependent on the stabilization conditions. The best electrical conductivity of～20.2±1.2S cm-1was obtained under the conditions described.(2) Three-dimension (3-D) hydroxyapatite (HAp) and electrospun carbon nanofiber (CNFs) composites were biomimetically synthesized. CNFs were fabricated from electrospun polyacrylonitrile precursor nanofibers, which were fabricated by stabilization at280℃for2h, followed by carbonization at1000℃. CNFs were treated with sulfuric acid, nitric acid and concentrated NaOH solutions. We found that concentrated NaOH solution treatment was very effective in rending hydrophilicity to the CNFs mat. The morphology and structure of CNFs/HAp composites were characterized by Fourier transtorm mtrared spectroscopy (FT-IR), scanning electron microscope (SEM), transmission electron microscope (TEM), energy dispersive spectroscopy (EDS), X-ray diffraction spectroscopy (XRD), X-ray photoelectron spectroscopy (XPS) and wetting measurements. It was found that surface treatment of CNFs in NaOH solution improved the growth of HAp. Such obtained HAp was calcium deficient and carbonate-containing.(3) Randomly organized and uniaxially aligned polyacrylonitrile (PAN) nanofibers with an average diameter of～400nm were prepared by electrospinning. They were embedded into poly(methylmethacrylate)(PMMA) to fabricate reinforced composite films by solution impregnation technique. Though the PAN nanofibrous mats were white in color, and fiber contents in composites were as much as16wt%, and the refractive index (RI) was different between PAN (RI=1.5187) and PMMA (RI=1.489), the composite films showed excellent optical transparency with as high as90%light transmittance in the visible light range, regardless of the fiber orientation. The mechanical properties of composite films steadily improved with nanofiber contents, and were dependent on the fiber orientation. The tensile strength and Young’s modulus of the composite film containing16%aligned fibers were respective40%and30%more than that with corresponding amount of randomly organized fibers. The embedding of PAN nanofibers displayed no effect on the glass transition temperature of PMMA matrix, but increased the stiffness of the composite films in the glassy and glass-rubber transition state, as demonstrated by the dynamic mechanical analysis. |
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