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Electrospinning is a cost - effective technology. An electrospinning machine (or electrospinning device) can utilize the electrostatic attraction between a charged polymer solution and a collector plate to prepare non - woven mats of polymer fibers with diameters on the order of a few nanometers. Electrospun non - woven mats possess excellent mechanical properties, including a high surface - to - volume ratio, high porosity, small pore size, and ease of functionalization. Due to these characteristics, they have extensive applications in protective clothing, filtration, membrane materials, reinforcing fibers in composite materials, optical sensors and electronics, drug delivery systems, biomedical applications, tissue engineering scaffolds, etc., and can also serve as a model for preparing hollow fibers with nanoscale inner diameters.
The diameters of polyamide fibers prepared by traditional methods (such as melt spinning, wet spinning, and dry spinning) range from 10 to 500 μm. In contrast, the diameters of fibers prepared by electrospinning from polymer solutions are typically in the range of tens to hundreds of nanometers. When a larger specific surface area is required, smaller diameters are more suitable. In addition to the nanoscale fiber diameter, the preparation of smooth, uniform, bead - free nanofibers and the absence of discrete polymer beads in the deposited material are crucial for improving the structural quality of electrospun mats, making them suitable for various industrial applications. Traditional electrospinning uses the capillary method in an electrospinning device, and a stable Taylor cone is required to ensure a stable spinning process and control the fiber diameter and morphology. However, the randomness of fiber deposition is one of the challenges faced by this technology. Therefore, many researchers have attempted to control the electrospinning jet by applying an additional magnetic field or a secondary external electric field between the needle and the collector, and have achieved certain results.
This study focuses on the electrospinning of PA11 nanofibers. Due to its long non - polar chain segments, PA11 may have a higher dielectric strength in applications such as filtration, showing potential application value. However, currently, there are relatively few studies on PA11. Moreover, due to its low solubility in polar formic acid, electrospinning of PA11/formic acid solutions faces challenges. The morphology of electrospun fibers from polymer solutions is affected by many factors, including polymer molecular weight, solvent system, solution concentration, viscosity, conductivity, ambient temperature and humidity, and process parameters. In this study, before adding metal rings to a traditional electrospinning device to change the electric field, preliminary studies were first carried out on the solution concentration and process parameters (voltage, tip - to - collector distance, and flow rate) to determine the optimal conditions for electrospinning PA11 solutions.
3.1 Crucial role of solution concentration
The crucial role of solution concentration: The study found that the solution concentration has a significant impact on the formation of PA11 nanofibers. A 6% (w/v) PA11/FA solution cannot form fibers due to its low viscosity and can only produce droplets. This is because the low concentration leads to insufficient polymer chain entanglement, making it impossible to maintain a continuous jet. In contrast, 8% and 10% (w/v) solutions can form fibers, and the fibers prepared from the 10% (w/v) solution are smoother and more uniform. As can be clearly seen from Figure 2(b), the PA11 nanofibers spun from the 10% (w/v) solution are straight and uniform, corroborating the better fiber quality at this concentration.
3.2 Complex influence of voltage
The complex influence of voltage: The influence of voltage on fiber morphology is complex and related to the tip - to - collector distance. Generally, an increase in voltage enhances the jet stretching and makes the fibers thinner, but an excessively high voltage can lead to an increase in fiber diameter. At a distance of 4 cm, when the voltage increases from 15 kV to 20 kV, the fiber diameter decreases, and when it continues to increase to 25 kV, the diameter increases again. This phenomenon is reflected in Figure 3(b), indicating that voltage interacts with other parameters to jointly affect the fiber morphology. These factors need to be considered comprehensively in actual production.
3.3 influence of the tip-to-collector distance
The influence of the tip - to - collector distance: The optimal distance for electrospinning PA11/FA solutions was determined to be 12 cm. When the distance increases from 4 cm to 16 cm, the average fiber diameter decreases. Figure 4 shows the fiber morphology at different distances. At 4 cm, the fibers are flat due to insufficient solvent evaporation time, while at 16 cm, the fibers are circular, with a smaller diameter and better uniformity, indicating that an increase in distance is beneficial for solvent evaporation and improving fiber morphology. Since there is no significant difference in the fiber diameter distribution between 12 cm and 16 cm, 12 cm is determined to be the optimal distance through comprehensive consideration.
3.4 influence of flow rate on fibers
The influence of flow rate on fibers: The flow rate has a significant impact on the fiber diameter and is related to factors such as concentration. Experiments show that at tip - to - collector distances of 8 cm and 12 cm, when the flow rate increases from 0.03 mL/min to 0.06 mL/min, the average fiber diameter decreases significantly. This is different from the general perception that a high flow rate increases the fiber diameter. The article explains that this may be due to the low concentration of the polymer solution. As can be seen from Figure 5, at a flow rate of 0.03 mL/min, the fibers have fusiform beads, while at 0.06 mL/min, the fibers are more uniform and have a smaller diameter, proving that changes in the flow rate can alter the fiber morphology.
3.5 positive effect of the second electric field and the effect of optimized parameters
The positive effect of the second electric field and the effect of optimized parameters: The introduction of the second electric field, especially when using two metal rings with increasing voltage, significantly improves the fiber quality. The fiber uniformity is enhanced, the diameter is reduced, and the average diameter can reach 145.7 ± 23.7 nm, with minimal bead formation. As can be visually observed from Figure 6(a), the PA11 nanofibers prepared according to the optimized parameters are uniformly cylindrical, with a smooth surface and no beads. The optimized parameters are a concentration of 10% (w/v), a distance of 12 cm, a voltage of +20 kV, a flow rate of 0.01 mL/min, and the use of two metal rings with increasing voltage. Under these conditions, nanofibers that meet the ideal requirements can be prepared.
This study investigated the effects of multiple electrospinning process parameters, such as solution concentration, tip - to - collector distance, applied voltage, flow rate, and the introduction of a second electric field, on the morphology of electrospun PA11 fibers. The results show that there are complex interactions among these parameters, which have significant impacts on fiber diameter, uniformity, and bead formation.
Preliminary studies indicate that a 6% (w/v) PA11/FA solution is not suitable for fiber production as its low viscosity leads to droplet formation. Electrospinning at higher concentrations (8% and 10% (w/v)) can form fibers, and the 10% (w/v) solution can produce smoother and more uniform fibers.
The optimal distance for electrospinning PA11/FA solutions was determined to be 12 cm. At this distance, the fiber uniformity is improved, and bead formation is reduced. The influence of voltage on fiber morphology is complex and depends on the tip - to - collector distance. Generally, an increase in voltage makes the fibers thinner due to enhanced jet stretching, but an excessively high voltage can lead to an increase in fiber diameter due to rapid solution ejection and insufficient stretching time. The tip - to - collector distance, applied voltage, and flow rate are interrelated. The balance between the solution delivery rate and the jet withdrawal rate is very important, highlighting the importance of maintaining steady - state electrospinning.
The introduction of the second electric field significantly improves the fiber uniformity and reduces the diameter, especially when using two metal rings with increasing voltage. This setup produces the smallest average fiber diameter (145.7 ± 23.7 nm) with the least bead formation. The presence of the second electric field enhances the electro - stretching effect, resulting in fibers with a smooth surface and a circular cross - section. This result can be attributed to the regulation of the electrostatic field, which can better control the inherent instability of the charged polymer jet during its movement towards the collector.
Overall, the optimized conditions for electrospinning PA11/FA solutions are a concentration of 10% (w/v), a tip - to - collector distance of 12 cm, a voltage of +20 kV, a flow rate of 0.01 mL/min, and the use of two metal rings with increasing voltage in a traditional electrospinning device. These conditions can prepare uniform nanofibers with a reduced average diameter and minimal bead formation. These findings contribute to the understanding of the electrospinning process of PA11 and emphasize the importance of fine - tuning electrospinning parameters to achieve the desired fiber morphology for various applications.
Article source: https://orcid.org/0000-0002-1034-5411
