Electrospinning and wet-spinning of elastic fibers

Official URL: http://risc01.sabanciuniv.edu/record=b1078884 (Table of Contents)

In this thesis, two fiber spinning processes were designed and studied; conventional wet spinning and electrospinning which is relatively novel technique. Several process parameters were identified and characterized for both techniques. Diameter, surface roughness, surface elasticity and surface morphology of fibers were characterized using optical, atomic force, and scanning electron microscopes. The electrospinning process produces nanoscale fibers by applying electrical force to a fiber forming polymer solution. A charged liquid jet was ejected from polymer solution to the grounded conductive sheet. After the solvent evaporation, a nonwoven mat with a porous structure composed of unusually thin fibers was left on the sheet. The effect of the solution and instrumental characteristics on fiber morphology including viscosity, conductivity and applied electrical field strength were investigated. Polyurethane and polyacrylonitrile based polymers were electrospun succesfully within a viscosity controlled interval. The diameter of ultrathin fibers was found to depend mainly on viscosity with a power-law relationship. High viscous polyurethane based polymers exhibited curly, wavy and straight structures whereas fibers obtained from low viscous solutions demonstrated beads on strings morphology. Additionally, the nanofibers were not uniform in diameter. Macroscale fibers were produced by using the wet spinning technique. The polyurethane based polymer solution was extruded into a water coagulation bath through a nozzle. After coagulation, the single elastic filament was dried with air blow and wound up. The effect of two process variables, the rate of drawing and the rate of extrusion, on the fiber diameter were investigated. The rate of drawing was inversely proportional whereas the rate of extrusion was directly proportional to fiber diameter. AFM characterization has shown that the surface of fibers was heterogeneous in nature including disordered, fibrilliar, and flat structures. The morphology exhibited on fiber surface did not depend on the two process variables. Fiber diameters in the range of 7 nm to 150 um were succesfully spun from polyurethane based polymer with electrospinning and wet spinning, respectively. In the electrospinning process, nanoscale diameter fibers were obtained, and these fibers provided high surface area to volume ratios. Furthermore, it was found that nanofibers obtained from polyurethane solutions have rougher surface than the wet-spun fibers. In order to compare surface elasticity of two fibers, AFM sensitivity of reference materials (Glass slide, teflon film and parafilm) were examined. Polyurethane based polymer solution was processed with electrospinning, wet spinning and film casting. Parafilm was found to be the softest material and glass slide was the hardest. Elasticities of the materials processed, were found to be between that of glass and parafilm. Electrospun fibers were harder than the film of the same polymer. The stiffness difference between film and electrospun fibers can be explained by the orientation of electrospun fibers due to the electrical force.