| Electrospun nanofibers have been widely used in filtration,biomedicine,sensors,energy,and other fields due to their high specific surface area,high porosity,and excellent flexibility.In recent years,there has been a substantial increase in the usage of ultrafine nanofibers in a variety of academic and industrial research fields.The spinneret for the traditional single needle electrospinning device is hollow metal capillary tube.The electric field force on the electrode tip increases as the voltage rises,while the liquid column at the needle tip continuously consumption.Once the voltage reaches to a critical value,the liquid column at the needle’s tip disappears and the remaining electric field strength is strongest at the edge of the metal ring.The spinning process is unstable at this point because it is easy to produce more than two jets at the needle tip tube’s edge,and the number,location,and shared charge density of the jets are changing.Because of the instability and unpredictability of these jets,distribution of nanofiber diameters was uneven.The average fiber diameter rises as more jets spread the electric field force at the same time.Thus,for traditional single needle spinning,higher spinning voltage will make the spinning process unstable and have negative impacts on the spinning process.Hence,it is challenging to produce nanofibers with lower diameters.Based on the tip discharge theory and in response to the aforementioned issues,this study developed a solid needle electrode as electrospinning spinneret.The electrode features are:(1)The curvature at the tip of the solid needle electrode is large,resulting in a comparatively strong electric field intensity at the same voltage;(2)Adding a plastic sleeve around the electrode is conducive to the transmission of spinning solution and reducing solvent volatilization;(3)spinning solution with micron-scale thickness tightly covers the electrode surface,effectively reducing charge dissipation,and significantly improving the charge density of the spinning solution.When the voltage reaches a certain threshold,a spinning jet forms at the tip of the solid needle electrode;(4)Due to the special shape of the solid needle electrode(high curvature),when the voltage increases at a certain critical value,a single jet was generated at the high curvature of the electrode,and the spinning process can still proceed stably,indicating the direction for the preparation of polymer nanofibers with smaller diameters and the related application research.The main research contents are as follows:1.Design of single solid needle electrode and analysis of jet formation process.Based on the analysis of the reported spinning methods,the single high curvature solid needle electrode spinning method was proposed,and the electrode structure parameters were optimized.Then,the process of carrying spinning liquid on the electrode surface and the process of jet formation are analyzed.Finally,the spinnability under low voltage and the spinnability of different polymers were verified.The results show that the vertical downward pattern of the solid needle electrode is conducive to producing a spinning jet at the position where the electric field intensity is strongest.Under the synergistic action of gravity,propulsion,and viscous forces,a micron-scale liquid film is formed on the surface of the electrode.After applying a voltage,due to the smaller thickness of the liquid film,the macromolecules in the liquid film are more likely to twist and rearrange to produce a polarization effect,resulting in a larger surface charge on the liquid film.Therefore,only a lower voltage is required to generate a charged jet(at voltage of6 k V,polyvinylidene fluoride(PVDF)and polyvinylidene fluoride-hexafluoropropylene(PVDF-HFP)nanofibers with smooth surfaces and uniform diameters are obtained).In addition,when the applied voltage is 12 k V,different types of polymer nanofibers with uniform diameter distribution can be obtained,demonstrating the wide applicability of the single solid needle electrode spinning device.2.Effect of spinning parameters on electric field strength and nanofiber diameter of solid needle electrode.Through numerical simulation and experimental methods,the effects of different electrode curvature and spinning process parameters on the electrode electric field strength and nanofiber morphology were systematically studied.The results of numerical simulation showed that under the same spinning conditions,the electric field strength of the solid needle electrode increases as the angle of the solid needle tip decreases.The diameter of PVDF-HFP nanofibers increases from 89.39 ± 19.70 nm to 116.68 ± 33.80 nm when the electrode angle rises from 60° to 120°,and the diameter distribution range of the fibers widens.The experimental results show that the spinning process can proceed stably when the solid needle electrode was 60°,the voltage was 12 k V,the receiving distance was 15 cm,and the spinning liquid flow rate was 0.8 m L/h.The obtained polymer PVDF-HFP nanofibers also had the smallest diameter and diameter distribution ranges,which are 89.39 ± 19.70 nm and 40-140 nm,respectively.3.The preparation of polymer ultrafine nanofibers using solid needle electrode tuning and comparison with traditional single needle spinning.Finally,various polymer ultrafine nanofibers were prepared by adjusting the electrospinning parameters of the solid needle electrode.PVDF-HFP nanofibers with diameter of 89.39 ± 19.70 nm were produced using the optimized spinning conditions.The PVDF fiber diameter is 98.51 ± 11.38 nm with a coefficient of variation of about 10% when the voltage is 12 k V and the spinning solution concentration is 2 wt%.Moreover,under optimized conditions,PAN nanofibers with diameter of 86.45 ± 19.06 nm were produced.Using numerical simulation and experimental methods,the effects of solid needle electrode and traditional single needle spinning on the nanofiber morphology were studied.The findings show that,under same spinning circumstances,the solid needle electrode’s electric field intensity is 1.58 times greater than that of the traditional single needle electrode.The diameter of PVDF nanofibers produced with solid needle electrode also shrank from 174.47 ± 30.12 nm to 102.37 ± 10.50 nm,and the coefficient of variation dropped from 17.26% to 10.17% when compared to traditional single needle spinning.As compared to traditional tubular needles,the average diameter of PAN nanofibers produced with solid needle electrodes is roughly 40 nm smaller,and the coefficient of variation of fiber diameter is decreased from 26.67% to 16.29%.It is demonstrated that ultrafine nanofibers with a consistent diameter distribution may be prepared using a solid needle electrode.From the above results,it can be seen that the solid needle electrode tip can induce stronger electric field intensity.This spinning method can generate spinning jet at lower voltage,and can prepare varieties of polymer ultrafine nanofibers with a narrow diameter distribution.Thus,this study provides new ideas for the preparation of smaller diameter nanofibers and their applications such as filtration and separation,biomedicine,etc. |
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