Abstract:

Objective The sizes and morphologies of microbeads on beaded fibers will lead to great differences in the overall performance of electrospinning fiber membranes, and thus fiber membranes with specific functions can be prepared in a directional manner by adjusting the size and morphology of beads on beaded fibers. As an important factor affecting the electrospinning process, electric field has not been used as the main research object in the process of preparing beaded fibers up to date. Therefore, it is necessary to investigate the influence of applied voltage on the structure and morphology of microbeads during electrospinning.

Method Beaded fibers were prepared by electrospinning using polystyrene (PS). Light microscope and scanning electron microscope (SEM) were adopted to observe the morphology of fibers, and the influence of applied voltage on the morphology of microbeads was studied. By observing the beaded fibers intercepted in different areas of the electrospinning jet, the evolution of beading morphology during jet whipping was explored. At the same time, the finite element simulation software COMSOL was adopted to simulate the three-dimensional electric field, bead morphology and beaded fiber movement speed. The simulation results were compared with the experimental results for model validation.

Results Based on the simulation results of the magnitude and direction of the electric field, it is evident that the strong electric field is mainly concentrated near the spinneret. As the spinning voltage increases, greater electric field strength in the spinning area leads to stronger tensile effect of the jet. This results in a beaded shape that is approximately circular at an applied voltage of 15 kV and the beaded fibers are bent disorderedly. When the applied voltage is 25 kV, the shape of the beads is close to the spindle shape, and the fibers between the beads are more regular and orderly. As the spinning voltage increases, the speed of the beaded fibers gradually increases. When the voltage is in the range of 20 and 25 kV, the speed of the beaded fibers increases more significantly, because at larger spinning voltages, the solution in the jet is more polarized, the charge on the jet surface is higher, and the force of the electric field is stronger. Owing to the larger surface area of the bead, more charge is generated by the electric field polarization of the jet in the beaded part, which is associated to stronger the electric field drafting, resulting in a greater speed in the beaded part than the speed of the fiber part. It is clearly shown that only the Shish-Kebab (string crystal) structure exists in the linear segment of the jet. This is because the strong electric field is concentrated near the spinneret and the polymer is strongly sheared or stretched. Gradual formation of a beaded structure is observed at the end of the straight segment, because the large tensile force of the electric field causes the jet to break, the broken jet is directly drawn from the Taylor cone, and the droplet tends to contract to form beaded fibers. The result also shows that the closer is the fiber morphology intercepted in the spinning direction to the collector, the longer the electric field of the jet is subjected to and the longer the whipping disturbance action time. The shape of the beads gradually tends to spindle shape under the action of stretching.

Conclusion In this paper, PS was used as raw material to prepare beaded fiber membranes by electrospinning method, and the evolution process of solution electro spun PS beaded fibers and the influence of spinning voltage on beaded morphology and fiber speed were investigated. The experimental results provide a theoretical basis for the controllable preparation of beaded fibers. 1) In the process of electrospinning, as the applied voltage increases, the stretching effect of the jet is more obvious, the beaded shape gradually changes from an approximate sphere to a spindle shape, and the movement speed of the beaded fiber also increases with the increase of the applied voltage. 2) The beaded structure is not formed on the linear segment of the jet, and gradually forms in the whipping zone, and the jet zone is made closer to the collector, the jet is subjected to the electric field and whipping action for a longer time, and the stretching effect of the beaded fiber is more obvious.

Key words: electrospinning, beaded fiber, forming process, spinning voltage, finite element simulation