The Formation Mechanism Of Core/shell Nanofibers Prepared By Coaxial Electrospinning

Coaxial electrospinning is a simple but effective way, which is widely utilized to prepare core/shell nanofibers. The theoretical research of coaxial electrospinning is still in the primary stage, because the spinning mechanism is more complex than that of traditional electrospinning.Therefore, in order to develop the theoretical research, the further reseatched of coaxial electrospinning mechanism needs to be investigated, which lays a good foundation for the perfect theory of coaxial electrospinning mechanism.In this paper, a method of combinng numerical simulation and experimental characterization was used to analyze the mechanism of coaxial electrospinning in this paper. In numerical simulation part, ANSYS finite element analysis software was employed to establish the calculation models based on coaxial electrospinning with different processing parameters. These models were applied to analyze and discuss the uniformity of electric field intensity. They were helpful for optimization of processing parameters during coaxial electrospinning. The numerical simulation results showed that electric field intensity distribution was uniform in the same distance under low working voltage(12 kV~15 kV). When the distance was between 14 cm and 15 cm with the same operation voltage, electric field intensity distribution was more uniform.ANSYS finite element analysis method was used to establish the model of coaxial electrospinning with different composite coaxial nozzle structures, aiming to research the effects on the electric field distribution in coaxial electeospinning process given by different composite nozzle structures. Furthermore, it was applied to optimize the coaxial electrospinning apparatus. The numerical simulation results showed that the same electric field intensity attenuation trend appeared at the tip of concave type nozzle and flush type nozzle. Besides, the electric field intensity distribution of concave type nozzle and flush type nozzle was more uniform than that of convex type nozzle. The electric field intensity distribution presented significant changes at both ends of the collector. The electric field intensity vector sum was smaller than other type coaxial nozzles, and the distribution of electric field intensity was more uniform. The morphology of composite Taylor cone at the coaxial nozzle tip was observed by high-speed camera. The sectional images of electrospun nanofibers showed that the nanofibers obtained by using concave nozzle was superior to other two types of coaxial nozzles in the integrity and stability of the core/shell structure. Besides, under the same spinning parameters, electric field intensity vector sum and direction angle changed significantly within the range of 3 mm at the coaxial nozzle tip. Not all the electric field directions of the outer nozzle of coaxial nozzles within the collecting distance were oriented to the axis direction of coaxial nozzle. However, some of the directions pointed to the outside of the outer nozzle. The electric field intensity vector sum remained low value and the direction of electric field paralleled to the coaxial nozzle axial direction.The rheological properties of core solution and shell solution were tested by torque rheometer.The Matlab software was used to analyze the test data. The experimental results showed that both core solution and shell solution were similar to Newtonian fluids. When the flow rate was in the range from 0.001 meter per second to 5 meter per second, the Reynolds numbers of core fluid and shell fluid were calculated to determine flow state. The calculation results showed that the flow state of core fluid and shell fluid belonged to laminar flow motion. The flow region model of multiphasewas established by Gambit software in coaxial electrospinning. Three different kinds of solution property parameters were set in Fluent flow analysis software with the VOF multiphase model. From the volume fraction of air and core solution results, it could be seen that when viscosity value difference between the core solution and the shell solution was in the range of 0.25 ~ 2.95 kg / m ? s, the coaxial Taylor cone was obtained at the nozzle tip. Besides, the same phenomenon was occurred in the experimental images observed by high-speed camera.The flow states of core solution and shell solution in different flow rate ratio were simulated by numberical simulation method. Through the analysis on axial static pressure and volume fraction focusing on different groups of multiphase flow, it could be found that the when the flow velocity of shell solution was less than 0.5 mL/h, the flow velocity ratio was less than 2, the structure of Taylor cone formed by core and shell solution was stable. When the flow velocity of shell solution was more than 0.5 mL/h, the flow velocity ratio was less than 2, the structure of Taylor cone was stable as well. However, for the latter situation, the flow would be influenced by gravity because of larger quantity of flow. Thus, the continuity of Taylor cone would be adversely affected. The same rule could be noticed in the high-speed real time observation.