Studies On Electrospun Ultrafine Fibers Of Poly(Lactic Acid) And Its Hybrid Membranes

Electrospinning is a process that produces polymer fibers with diameter in therange of nanometer to micrometer under the external electric field imposed onpolymer solutions or melts. Non-woven membranes generated by this method havehigh porosity. Due to special properties, such as thin diameter and high surface area,these kinds of fibrous membranes have shown potential applications in many areassuch as filters, sensors, wound dressings, controlled release carriers and tissueengineering scaffolds.Firstly, in this work, ultrafine fibers of poly(lactic acid) (PLA) was prepared byelectrospinning and influential factors on fiber morphology were studiedsystematically. Scanning electron microscope and atomic force microscope were usedto investigate fiber morphology. Results showed that the solvent was the criticalfactor to determine the formation of the electrospun PLA fibers. Compared withacetone, N,N-dimethylformamide (DMF) was a better solvent for PLA to electrospin.Entrance of an organic salt, triethylbenzylammonium chloride, led to a great increaseof the conductivity of PLA/DMF solutions, so that the average fiber diameter of theelectrospun PLA fibers decreased dramatically from 500 nm to 100-200 nm. Theaddition of surfactant, Span-80, did not improve the fiber morphology but formedbeaded fiber web. The increase of solution concentration favored the smooth andthick fibers. The effect of tip-target distance on fiber morphology was related tosolution concentration and for 20% PLA solution, the increase of tip-target distancefavored thin and smooth fibers. At 5-cm tip-target distance, the increase of voltagefavored the formation of ultrafine fibers. When solution concentration is as high as25%, fiber morphology was sensitive to flow rate. As the increase of flow rate,interconnective fibrous net with thick fiber diameter was formed and the number ofbeaded fibers increased significantly.By electrospinning, ultrafine fibers of poly(vinyl alcohol)/polyethylenimine(PVA/PEI) were also prepared. SEM, Fourier Transformed infra-red spectroscopy(FT-IR) and X-ray diffraction (XRD) were used to observe and analyze the fibermembranes. Results showed that PEI could not be electrospun alone and ultrafinefibers containing PEI could be produced from PVA/PEI blend solutions byelectrospinning. Mass ratio 75/25 of PVA/PEI and solution concentration of 8% wereoptimal conditions for electrospinning of PVA/PEI solution and average fiberdiameter produced from that solution is 223±45 nm. Addition of surfactant, Tween-20,decreased the surface tension of polymer solutions greatly and solution drop on thefibrous membrane disappeared, but beaded fiber formed. FT-IR and XRD analysessuggested the existence of PVA and PEI in blended fibers.After electrospinning of PLA and PVA/PEI respectively, electrospinning setupfor preparing hybrid ultrafine fibers was designed. SEM, FT-IR, XRD and X-rayphotoelectron spectroscopy (XPS) were used to analyze the blend fibrous membranes.Electrospinning setup with multi-jet improved the spinning speed significantly. Byusing the multi-jet electrospinning setup, PLA and PVA/PEI could be electrospunsimultaneously into uniformly hybrid fibrous membranes of PLA and PVA/PEI. Theanalyses of FT-IR,XRD and XPS indicated that PLA and PVA/PEI coexisted in thehybrid fibrous membranes.