|M.Sc Student||Tsaroom Adi|
|Subject||Core-Sheath Nanofibers from Electrospun Polymer/Metal-|
|Department||Department of Materials Science and Engineering||Supervisor||Professor Michael Silverstein|
|Full Thesis text|
Polymer fibers with diameters in the nanometer range can be produced using electrospinning. Ceramic nanofibers can be produced through the pyrolysis of polymer precursor nanofibers that contain metal salts. In electrospinning, a high-voltage electric field is applied to a polymer solution being pumped through a syringe. Core-sheath nanofibers have been produced through phase separation in polymer blend solutions on evaporation of solvent during the electrospinning process. In this research, nanofibers were produced by electrospinning solutions of commercial triblocks with poly(ethylene oxide) (PEO) endblocks and a poly(propylene oxide) (PPO) midblock, homopolymers, and metal salts. The objectives of this research were to investigate nanofibers from precursor solutions of homopolymers and block copolymers combined with metal-salts, to characterize the effects of solution composition and electrospinning conditions on nanofiber morphology, and to describe the influence of various parameters on the mechanisms of core-sheath formation. The nanofibers were characterized using a combination of microscopy, spectroscopy, scattering, and thermal analysis.
Some nanofibers, both those with and without block copolymer, exhibited core-sheath structures with a metal-atom rich core and a metal-atom poor sheath. The possible mechanisms for core-sheath formation investigated include: PEO crystallization that begins at the surface due to solvent evaporation; viscosity differences within the solution due to the formation of metal-ion complexes; preferential attraction of the metal ions to the collector; and repulsion of the positive ions by the electric field used in electrospinning. The effects on the core-sheath structure produced by the different mechanisms were investigated through: replacing PEO with PAAc, an amorphous polymer, to remove the effects of crystallization; replacing the multivalent ions with a monovalent ion, to reduce the effect of complex formation and to reduce the preferential attraction to the collector; using reverse polarity, to change the surface charge from positive to negative; and varying the homopolymer content to investigate the effects of viscosity. Metal salts, polymer crystallization and a minimal crystalline polymer content, seems to be necessary for core-sheath formation; phase separation within the block copolymer or between the block copolymer and PEO is not responsible for core-sheath formation; metal ion charge and process polarity play a part in core-sheath formation; and higher solutions viscosities can limit diffusion and prevent core-sheath formation. In summary, core-sheath nanofibers are formed from solutions containing metal salts and a sufficient amount of crystalline polymer whose viscosity is low enough to enable metal ion diffusion.