Exploratory Research On The Formation Of Wet Electrospinning

Electrospinning, with its simplified operation and wide applications, is an effective process used to prepare ultrafine fibers from various polymer solutions. Based on the distinction of the collecting media, electrospinning from solution can be divided into dry and dry-jet-wet electrospinning. Till now, however, the real wet electrospinning has not been reported yet.Poly (3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) copolyester is a thermoplastic bio-polymer synthesized by microbial fermentation, presenting the advantages of good biodegradability, histocompatibility and antithrombogenicity. It has a wide range of applications in industry, agriculture, biomedicine and other fields. According to its rich source in nature and no need of complex polymerization equipment, PHBV is alternative to general resins, and has attracted many domestic and foreign researchers’attention in recent years.A Novel wet electrospinning technology was studied in this thesis from PHBV/CHCl3 system using insulating liquids, simethicone and CC14 as coagulation and media. The needle was immersed into the coagulations so that the solidification was completed in liquid media, leading to the fiber formation in the high voltage. Finally, the non-woven fibers were obtained successfully.The solution properties, the condition parameters and their effects to the electrospinning process were discussed. The electrospinnable concentration of the solution was found between 4wt% and 16wt%. With the accretion of the concentration, the shearing viscosity and electrical conductivity increased simultaneously, while the surface tension changed irregularly, and the average diameters of fibers increased. Since the interface tension between solution and coagulation was much lower than the surface tension, the jet critical voltage was also much lower than it was in the other electrospinning processes. Meanwhile, the jet whipping speed of wet electrospinning was much slower than common electrospinning process. In wet electrospinning process, positive and negative high voltage supplies were utilized to avoid the limitation of the collecting distance. Pumping rate was proved to be a more explicit effect, and charge density inside jet was also higher. Besides, both coagulation and solvent in the system can be easily recycled.The fiber morphology was studied by SEM characterization. Porous morphology with skin-core structure in the single fiber was frequently observed. Ribbon-shaped fibers were prepared in wet electrospinning from simethicone media, while fibers with uneven surface mophology were obtained from the CCl4 coagulation due to the collapse. Performances study of the wet-electrospun fibers were studied via DSC characterization, and were compared to the traditional electrospun fibers. The wettabilities of PHBV membranes changed with formation technologies, and the fabric wet electrospun from CCl4 showed super hydrophobic surface. The crystalline regularity and the degree of crystallinity of the wet-electrospun fiber were lower because of its inner porous structure. In mechanical properties study, electrospun mats from simethicone were proved to reach higher break elongations.In the paper, continuous single wet electrospun fiber was successfully collected with diameter of 1μm. However, due to the charge intergration, the whipping and guiding of the fiber was instable, and the voltage can hardly reach a high level, which limited the jet thinning. Thus, continuous single fiber in nano-scale was difficult to obtain.The fiber formation process of wet electrospinning was also elaborated. Solvent and non-solvent concentration radial distributions of jet were discussed by the simulation of system double diffusion, which showed that the diffusion reached the balance in a very short period, and the jet regularly presented in a gradient distribution from the center to the surface. Meanwhile, phase separation behavior of PHBV/CHCl3 into CCl4 was studied, including the calculation of interact parameters among the three components, bimodal line, spinodal line and critical point, and then the ternary phase diagram was ploted and analyzed. The phase transformation mechanism indicated that fiber formation of wet electrospinning followed the nucleation-growth transition. Because of the double-diffusion effects, those around the surface solidified into dense skin structure in seconds. In this situation, double diffusion of the inner part was retarded. This led to the different densification, and than resulted in skin-core structure. Thinning mechanism of jet was studied, which showed that the main fact of jet thinning was the stretching of electrostatic force, rather than the double diffusion.