Electrospun Fibers And Their Post-treatment Of PVDF And PVDF/PAN

The fiber morphology of electrospinning has a close relationship with solution properties, especially the degree of polymer-polymer chain entanglement. Uniform ultrafine fibers could be formed with strong chain entanglement easily. In the electrospinning process of bicomponent polymers, the lower viscosity phase tended to migrate to the walls of electrostatic jets by higher shear rate, while the higher viscosity phase located at the center belong to lower shear rate. Then, ultrafine fibers with core/sheath or co-continuous structures could be produced. The electrospun membrane composed of above fiber structures could show the more potential applications in biomedical materials, high-efficiency filters, and so on.In this study, poly(vinyl fluoride) (PVDF) solutions and PVDF/polyacrylonitrile (PAN) blend solutions were studied. From the viscosity measurement of the solutions, the polymer chain entanglement as well as the electrospun fiber morphology caused by polymer concentrations, relative molecular mass, solvent mixture and PVDF/PAN mass ratio was analyzed by scanning electron microscopy, respectively. Results suggested that higher solution concentration, higher relative molecular mass and the solvent volatility improved formation of uniform ultrafine fibers. Both Core/Sheath and Co-continuous structures of electrospun PVDF75/PAN25 (75:25, wt/wt) fibers were observed under a transition electron microscopy. In general, the PAN phase was located at the center of the ultrafine fibers and the PVDF phase was at the wall. The shelf time of the polymer solution could improve the formation of the uniform core/sheath structure. When increasing the PAN fraction, the difficulty of polymer phase separation process because of the higher viscosity of the polymer solution made it hard to form the uniform core/sheath fiber structure.The non-woven membrane with high porosity and small fiber diameter generally exhibited lower mechanical properties. In this study, post hot-press treatment and fiber-alignment by high speed collection drum were used to enhance tensile strengths of the electrosun PVDF75/PAN25 membranes. With the hot-press temperature increased, the porosity, the liquid absorption, the crystallinity of PVDF phase as well as the elongation of the membranes decreased, while the tensile modulus and the tensile strength showed an increase trend. Tensile tests in revolving direction of the fiber-aligned membranes suggested that the tensile modulus and the tensile strength were higher than the non-aligned membranes; Hot-press at 135oC, the tensile strength of revolving direction of the fiber-aligned membranes is 55.1±3.7MPa. But the elongation was lower than the later one, whereas the non-aligned membranes in the cross direction showed lower tensile modulus and tensile strengths, but a higher elongation.