A New Technology, Bubble Electrospinning, For Producing Nanofibers

Electrospinning has become one of the most important and basic method for fabricating nanofibers since it was patented. Because of their excellent properties such as high specific surface area, unique netted texture and porosity, the nanofibers produced by electrospinning have been widely applied to many fields, e.g., textile industry, environment engineering, bioscience and biotechnology, medicine and health, energy storage, military and anti-terrorism security.In this thesis, we first gave a brief introduction to the history, development and the current research focus of the electrospinning technology, and then analyzed various electrospinning apparatus, illustrating their demerits and merits of each apparatus, finally we pointed out that the inefficiency and low output of electrospinning became the bottleneck that affected its development.In Chapter 2, some of the most commonly used electrospinning apparatus were used as examples to examine the shortcomings affecting the output, and revealed possible solutions to overcome the shortcomings. We suggested a novel patented method, called the bubble electrospinning, both theoretical and experimental analyses were carried out. The mechanism of the bubble electrospinning is deceptively simple and its throughout depends on sizes and numbers of produced bubbles, which are similar to Taylor cones in traditional electrospinning process. When the bubble electrospinning process began, multiple jets were ejected from bubbles to the collector. The evaporation of the solvent from the charged jet occurred during the spinning process, and eventually resulted in needed nanofibers collected on the collector. The spinning process can be easily controllable by adjusting the size and number of bubbles. A polymer solution, 12wt% polyacrylonitrile (PAN)/dimethylformamide, was used to bubble-electrospin PAN nanofibers. The obtained mat was analyzed using a scanning electron microscope (SEM) and the average diameter of nanofibers was about 200nm. The throughout was extremely higher than that of traditional electrospinning.In order to investigate the actual mechanism of bubble electrospinning, we analyzed theoretically the dynamics of bubbles without or under electric field, respectively, in the third Chapter. A one dimensional steady mathematical model of bubble dynamics was established, and a mathematical model of charged bubble dynamics was also obtained to derive a formula of the critical electric field and the critical angle of cone. The results showed that the half cone angle was about 23.4°for Polyvinylpyrrolidone(PVP)/ ethanol-water solution, which was less than the half cone angle, 49.3°, of the traditional electrospinning process. A high speed camera was employed to observe the motion of the jet in bubble electrospinning. The photos showed that almost all the jets moved in straight line, different from the instable motion in traditional electrospinning.In Chapters 4 and 5, the effect of the process parameters and the properties of polymer solution on the diameters and morphologies were, respectively, investigated theoretically and experimentally. Both results showed that the average diameter of nanofibers would increase as the applied voltage increased and the relationship was approximately linear. In addition, the shape of beads in the products changed from sphere to spindle and disappeared completely with the increase of the applied voltage, furthermore, the average diameter of nanofibers depended linearly upon the spinning distance measured from the liquid surface to the collector. In this thesis, the effect of some important parameters such as the concentration, the surface tension and the conductivity on the diameters and morphologies of nanofibers were examined. The theoretical analysis showed the relationship between the concentration (C) and theaverage diameter of nanofibers (d) followed the scaling law, d∝C^β, whereβwas the component depending on the solution properties, and our theoretical prediction agreed very well with the above scaling law, for PVP/ ethanol-water solution, we predictedβ=4.5. It showed that the average diameter of nanofibers wasincreased with the increasing concentration. Additionally, we found that the concentration of solution played a determinative effect on the morphologies of the product in the bubble electrospinning process using the different solvents and different concentrations, especially the number and size of the beads would decrease when the concentration of solution increased. The relationship between the surfacetension of solution (F_s) and the average diameter of nanofibers (r) was also obtainedtheoretically as well, which read (?), where F_e was the electric force andC was material constant. An experiment was made to verify the theoretical prediction, and the results showed excellent agreement. The effect of another important property of solution, conductivity, on the diameters and morphologies was considered systematically under different concentrations of potassium chloride (KCl). The experimental results showed that the average diameter of nanofibers decreased with the increase of the conductivity of solution (κ). The empirical formula obtained in this experiment was r∝κ^0.43 for PVP/ ethanol-water solution. The number of beads decreased with the conductivity of the PVP/ ethanol-water solution. Finally, the effects of both the surface tension and the conductivity of solution on the diameters of nanofibers were studied by adding an ionic surfactant, SDS, in the electrospun polymer solution. The additive of SDS could decrease the surface tension of solution and increase the conductivity of solution simultaneously. The results showed that the added additive could decrease the average diameter of nanofibers within threshold value.Some properties of bubble electrospun nanofiber mat such as the crystallinity, the morphologies and the optical properties of micro surface were examined by X-ray diffractometer, atomic force microscope (AFM) and optical microscope with a rotation auxiliary light source, respectively. The results showed that the crystallinity was higher than that of original polymer power, and a typical porous netted texture of bubble electrospun nanofiber mat observed by AFM. Additionally, with the increase of angle of light incidence, the morphology changed continually under an optical microscope. When the variation of the incident angle of the entering light from 0°to 345°, the morphology of nanofiber mat changed and finally returned to the original. The possible reason for the different textures of nanofiber mat is that the electrospun nanofibers were birefringent. Finally, the TiO₂/PVP composite nanofibers were fabricated successfully by bubble electrospinning.