The study of electrospinning and the physical properties of electrospun nanofibers.

摘要

Electrospinning is a straightforward method to produce polymer fibers, from polymer solutions, with diameters in the range around 100 nm. Nanofibers of polymers were electrospun by creating an electrically charged jet of polymer solution at a pendant droplet. After the electrospinning jet flowed away from the droplet, the jet bent and followed a complex trajectory, during which electrical forces stretched and thinned it by very large ratios. After the solvent evaporated, nanofibers were left. Well-defined equipment was designed to study the electrospinning process. The samples employed were polyethylene oxide (PEO) aqueous solutions. Jet initiation and bending instability growth were carefully photographed. The shape of jet meniscus, where the single jet ejected from, has been found to be very important to the stability of the jet. Jet current and spinning voltage follow a second power relationship when the gap distance is fixed. The variation of jet length, jet diameter, solution flow rate and fiber diameter were also investigated. Solution viscosity, solution surface tension coefficient, solution resistivity and net charge density carried by the electrospinning jet play important roles in the electrospinning process.; Electrospun fibers sometimes have beads in regular arrays. The viscoelasticity of the solution, net charge density carried by the jet, and the surface tension of the solution are key factors that influence the formation of the beaded fibers.; Nanofibers of a commercial styrene-butadiene-styrene tri-block copolymer were electrospun from solution, and collected either as a nonwoven elastomeric fabric, or on a layer of graphite that was evaporated onto a glass microscope slide. The resulting nanofibers were elastic, birefringent, and most had diameters around 100 nm. A few thin beaded fibers were found among the smooth nanofibers. The diameter of the fibers between the beads was as small as three nanometers. After staining with osmium tetroxide, the nanofibers were examined using transmission electron microscopy. Separated phases of styrene and butadiene blocks were observed. The single-phase domains were irregular in shape, but elongated along the axis of the fiber. Wide-angle X-ray diffraction patterns showed a weak indication of molecular orientation along the fiber axis, and the birefringence confirmed that such orientation was present. The single-phase domains grew larger in nanofibers that were held at room temperature (∼25°C) for several days. Annealing at temperature a 70°C greatly accelerated the growth of the single-phase domains. The nanofibers softened and flattened on the evaporated graphite during annealing.; Carbon nanofibers were produced from polyacrylonitrile (PAN), mesophase pitch and poly(vinyl alcohol) (PVA). Stabilization and carbonization processes were used to convert as-spun nanofibers; to carbon fibers. The diameters of typical carbon nanofibers were in the range from 100 nanometer to a few microns. The carbon nanofibers were observed by polarized optical microscopy, scanning electron microscopy, transmission electron microscopy, and wide angle x-ray diffraction. The resulting carbon nanofibers have physical properties that range from highly crystalline, strong fibers to very porous, large surface area and poorly crystallized fibers.