Mechanism and experimental simulation of non-isothermal melt formation induced by voltage change under auxiliary heating

Abstract

Over the past few decades, it has been universally acknowledged that melt-electrospinning is an attractive solvent-free production process with the aim of alleviating the solvent-related problems generated by traditional electrospinning techniques. Up to now, the temperature and applied voltage have been considered as the two most important factors affecting the melt-electrospinning process and electrospun fibers. In this paper, an auxiliary heating setup was applied to the melt electrospinning system to deeply understand the effect of heat distribution on the melt-electrospun fibers. Jet motions, morphologies, mechanical properties and inner structures of the fibers obtained under different applied voltage conditions with and without auxiliary heating were investigated. The temperature distribution in melt-electrospinning system was simulated by COMSOL Multiphysics software. The experimental results showed that a higher applied voltage results in a smaller fiber diameter, more disordered fiber mat and lower degree of strength with auxiliary heating. The simulation results of the temperature distribution in the spinneret-collector region are in good agreement with the measured values of the infrared images, which proves the feasibility and accuracy of the model.

Keywords

Melt-electrospinning auxiliary heating simulation jet motion crystallinity

It is well known that electrospinning has become a widely used technology for the production of microfibers and nanofibers. Characterized by the nano-scale diameters, fibers have been successfully prepared by drawing support from the solution electrospinning technique, which has become a booming research area over the past two decades.^1–4 However, certain problems generated during the solution electrospinning process, such as the environmental pollution caused by volatile solvents and the difficultly of cleaning of waste liquid, are unavoidable. Melt-electrospinning, which features a cleaner fabrication process, higher productivity and almost no solvent evaporation, exhibits preferable advantages in three-dimensional (3D) biological scaffolds, drug loading and release, air filtration and other fields.^5–10

However, this spinning method also brings the problem of unideal fiber diameter, which has become the focus of a great deal of research in recent years. Mayadeo et al.¹¹ demonstrated that the downstream volume heating of the electrospun fibers in the melt-electrospinning process could lead to a significant reduction in fiber diameter through modeling and experimental methods. In addition, the model was used to take advantage of ambient temperature, which affects fiber refinement through surface rather than volume interactions. Morikawa et al.¹² took metal wire with polymer coating at the tip as the polymer source, melted it by Joule heating of the source line and extracted it to the target by electrostatic force. This method relies on limiting the Taylor cone and reducing the initial jet diameter by concentrating the electrostatic field as the means to reduce the fiber diameter. In addition, they also improved the spinneret to achieve the goal of fiber refinement.¹³

In the melt-electrospinning process, the molten polymer is gradually formed into a filament as the spinneret is stretched by the electrostatic force, and finally deposits it on the collector in the form of a disordered fiber mat. The temperature in the spinning system decreases significantly within the region of 10 mm away from the spinneret and the fiber is formed rapidly due to jet cooling, which is not conducive to the stretching and refinement of the fiber. The application of an auxiliary heating device is capable of changing the temperature distribution in the spinning system to slow down the cooling rate of the jet, thus affecting the jet path and the molding process. In view of this, the secondary drafting and refinement can be carried out in the electrostatic field, with more ideal effects.¹⁴ Previous studies¹⁵ have shown that the temperature distribution in the falling-down section of spinning can be effectively improved by means of the application of auxiliary heating (such as laser heating and thermals). Applied voltage is an important factor affecting fiber formation and refinement in the melt-electrospinning process. No matter whether a high-voltage electric field or auxiliary heating device are applied, certain measures can be taken in a reliable manner to achieve the objective of the fibers with smaller diameters and better properties. Absolutely, the application of an auxiliary heating system is prone to slow down the solidification process of the melt. According to the study conducted by Shabani et al.,¹⁶ jet thinning occurs in the liquid phase region before the solidification, which is mainly affected by the electric field force. Based on this, the application of a high voltage will contribute to further fiber refinement. Besides, the intensity and direction of the electric field have a direct effect on the jet path, fiber fineness, mechanical properties and some other properties.^17–20 Focusing on theoretical aspects of jet instability, the early research and development of melt-electrospinning was carried out. As of today, the research and development of spinning theory has become relatively mature. Also, changing the auxiliary conditions or improving the spinning equipment will be considered as the focus in further research.

The influence of temperature distribution and applied voltage on the jet path, fiber morphologies and mechanical properties was studied by applying an auxiliary heating device and adjusting the applied voltage. At the same time, high-speed photography was used to observe the jet motion. COMSOL Multiphysics software was adopted to calculate the temperature distribution in the melt-electrospinning system and, also, the measured temperature was used to verify the simulation results.

Experimental details

Materials

Pellet-like PP (polypropylene, (C₃H₆) _n , Mv = 210,000 g/mol, melt index = 16, block copolymer PP) was purchased from the Shandong Yousuo Chemical Technology Co., Ltd, China. Pellet-like PP was extruded into a strip state by an extruding machine (Dynisco LME230, China) at a 200°C temperature, and subsequently placed through the inlet, which was fixed on the electrospinning machine.

Experimental setup

The melt-electrospinning setup used in this study consists of a polymer feeding and spinneret system, a high-voltage system, an auxiliary heating system and a collecting system, as shown in Figure l. The feeding system is composed of a 0.2 mm in-built spinneret and a heating unit. PP strips with uniform diameter (2 mm) were fed into the heating unit at a constant rate by a feeding unit, and then sprayed through the spinneret. In this process, the melt outflow rate was controlled by the feeding device. In addition, the polymer feeding and spinneret system was placed on a track, which could move horizontally during the spinning process at a constant speed of 35 cm/min. The high-voltage system included a power supply (RS60P-20 W, Gamma High Voltage Research, USA) with operating voltage ranging from 0 to 50 kV. The high voltage was applied to the spinneret and the collector was grounded. An infrared heating lamp, located 5 cm away from the central axis of the spinning area, was used as an auxiliary heating system. The heating temperature of the infrared lamp in the central area can reach up to 120°C, and the average temperature in the radiation area with a radius of 5 cm can reach up to 80°C. Coated with a layer of aluminum foil, a metal rotating drum was adopted as the collector in this study. Composed of two 150 W lamps (Model-100LED TWIN, China), the heater, which is designed on top of the melt-electrospinning system, was utilized to ensure stable ambient temperature and humidity.

The electrospinning distance was set to 7 cm, which was deemed as the shortest and safest value, and the heating temperature of the spinneret was set to 300°C. The feeding rate, rotating speed of the collector and spinning time were set to 2 mm/min, 4 rpm and 2 h, respectively, intended for collecting continuous and uniform fibers. Drawing support from the auxiliary heating system, two groups of experiments were carried out, with processing parameters displayed in Table 1. All the experiments were conducted under the conditions of the temperature of 50 ± 0.5°C and relative humidity of 20 ± 1%.

Table 1.

Experimental parameters

Group no.	Auxiliary heating temperature (°C)	Spinneret temperature (°C)	Applied voltage (kV)
1	/	300	20, 30, 40
2	75	300	20, 30, 40

Characterization

Under different experimental parameters, the fiber morphologies were observed by a metallographic microscope (Bh200m, China) and the fiber diameters were measured by ImageJ software (NIH Image, Bethesda, Maryland, USA). The crystallization properties of the fiber membrane were measured using X-ray diffractometry (XRD; D/max 2550 PC, Rigaku Corporation, Japan) and a differential scanning calorimetry (DSC; DSC 4000, PerkinElmer, China). The specific parameters of the XRD test are as follows: copper target (test voltage 40 kV, 150 mA); angle (2θ) between the incident X-ray and diffraction line of 5–60°; ambient temperature of 25°C. In addition, the mechanical properties of the fibers were tested by a single-fiber tester (YG006, China) at a crosshead speed of 100 mm/min at 20°C. The jet motion images were captured by a high-speed camera (Photron Fastcam Mini AX200, Japan).

Numerical simulation

During the process of melt-electrospinning, the temperature distribution after applying the auxiliary heating was simulated by the COMSOL Multiphysics software, in order to better understand the influence of auxiliary heating on the temperature distribution. Before the simulation, the physical geometries of the melt-electrospinning setup (e.g., spinneret and collector) were established according to their actual dimensions and locations. A two-dimensional (2D) axisymmetric geometry was used in this study, as follows: the geometry of the nozzle was modeled as a thin single-hole cone wall and a heat source was applied to the spinneret. A solid–fluid heat transfer interface was selected from the COMSOL software to calculate the temperature distribution of the melt-electrospinning system.

Results and discussion

Jet motion

Figure 2 shows the high-speed photographic images of the jet motions without and with auxiliary heating at different applied voltages. It can be seen from the figure that the polymer jets whip after ejecting from the nozzle. A nonuniform jet can be observed when the electrospinning setup is running without auxiliary heating. Under the condition of applied voltage of 20 kV with an auxiliary heating system, small polymer droplets can be observed (Figure 2(d2)) in the spinning line. Since changes in temperature near the spinneret have a great impact on the viscosity of the melt, the addition of the auxiliary heating device results in a decrease of the melt viscosity. With the stretching effect of the electrostatic force, part of the jet breaks.²¹ However, the surface tension of the polymer melt turned part of the fracture jet into a sphere-shaped droplet. Therefore, the images captured by the camera show that there are both discontinuous jets and spherical droplets.

Fiber morphology

Figure 3 shows the morphologies and diameter distributions of the fibers prepared at different applied voltages without auxiliary heating. With the increasing of applied voltage, the fiber membrane gradually changes from a compact arc shape to a loose irregular small circle shape, as shown in Figure 3. At the same time, the fiber diameter becomes smaller under larger applied voltage, and the structure fiber membrane becomes looser. The main reason was that more residual charge accumulated at the fiber surface resulted from the higher applied voltage, and the repellency of residual charges leads to the loose structure of the fiber membrane.²² Furthermore, a strong electric field force is helpful to increase the frequency of jet whipping and decrease the fiber diameter, which also causes the loose fiber membrane structure.

The morphologies and diameter distributions of the fibers prepared at different applied voltages with auxiliary heating are as shown in Figure 4. Beaded fibers can be observed when the applied voltage is set to 20 kV (Figure 4(a1)). This is mainly because the viscosity of the polymer jet decreases due to the application of the auxiliary heating system and the low voltage fails to fully stretch the molecular chain of the Taylor cone and polymer jet and, as a result, beaded fibers with larger diameter are formed. The fiber membrane becomes looser and the fiber diameter becomes smaller with the increasing of applied voltage, which is the same as that collected without an auxiliary heating system. It is worth mentioning that the diameter of the fiber produced by melt-electrospinning with an auxiliary heating system is smaller than that fabricated without an auxiliary heating system, under the circumstances of 30 kV and 40 kV applied voltage, respectively, while the average diameter of fiber produced by melt-electrospinning with an auxiliary heating system is very similar to that of the fiber without an auxiliary heating system while the applied voltage was 20 kV. This may be because the extra heating slows down the solidification process of the polymer jet, so that the jet is prone to be stretched by the strong electric field force. When the applied voltage was 20 kV, the low polymer viscosity resulted in unstable polymer jets and small polymer droplets, thus leading to large fiber average diameters and high irregularity of fiber diameters (σ in Figures 3 and 4) after employing the auxiliary heating system.

Mechanical property

The stress–strain curves of the melt-electrospun fibers fabricated at various applied voltages without auxiliary heating are displayed in Figure 5(a). As illustrated in Figure 5(a), the mechanical strength is significantly affected by the applied voltage. The breaking strength dramatically decreases from 1.16 to 0.39 MPa when the applied voltage increases from 20 to 40 kV. However, when the auxiliary heating system is applied to the melt-electrospinning setup, the breaking strength decreases from 0.94 to 0.36 MPa as the applied voltage increases from 20 to 40 kV, as shown in Figure 5(b). The breaking strength can be obtained through the following formula

σ = \frac{P}{S}

(1)

In this formula, σ is the fiber breaking stress, P is the fiber strength and S is the fiber cross-sectional area. It can be seen from these above-mentioned data that the maximum stress of the fiber will decrease with the increasing of applied voltage, regardless of the application of an auxiliary heating system. When the spinneret temperature reaches up to 300°C, the melt is in a viscous fluid state and the arrangement of the molecular chain becomes more regular correspondingly under the action of electrostatic force during the melting process. What is more, the increasing of applied voltage leads to a high density of charge distribution on the fiber surface, which results in poor jet stability. The small diameter of the fiber also reduces the uniformity of the molecular chain arrangement. In addition, the high stretching rate of the viscous polymer jet caused by the high voltage results in rapid cooling of the jet, thus increasing the uneven crystal size and then weakening the fiber strength.²³

By comparison of the two stress–strain curves at 20 kV in Figure 5, it is not difficult to find that the maximum strain value of the fiber with the auxiliary heating condition is much smaller than that of the fiber without such condition. However, at 40 kV, the situation is completely reversed. The reason for the above-described phenomenon may be that the auxiliary heating system at a low applied voltage in the melt-electrospinning process slows down the cooling rate of the jet. At the same time, the stretching effect of the electrostatic force on the jet becomes weak and the fiber crystallization becomes ununiform, which leads to the occurrence of multiple fracture points in the jet during the stretching process. Therefore, the maximum stress value of the fiber produced with auxiliary heating at 20 kV applied voltage is high and the maximum strain value is small, whereas the main body of jet is fully stretched at the high voltage (40 kV), where macromolecular chains tend to be arranged in a regular manner. This is the main reason for the increasing of maximum strain value of the fiber. It is worth mentioning that the fibers present the maximum strain values at 30 kV under both conditions, indicating that too high or too low applied voltage will limit the strength and mechanical tensile properties of the fibers.

Crystallization properties

DSC thermal analysis diagrams of the fibers manufactured by changing the applied voltage, without and with auxiliary heating, are as shown in Figures 6(a) and (b). In Figure 6(a), the fiber crystallization can be realized at the temperatures of 164.23°C, 162.56°C and 161.91°C, corresponding to the applied voltages of 20, 30 and 40 kV, respectively. However, with the auxiliary heating system, the fiber crystallization temperatures of 163.12°C, 160.81°C and 161.09°C, respectively, can be observed in Figure 6(b). What can be seen from the aforesaid data is that there is only a small difference in the fiber crystallization temperature between before and after the application of the auxiliary heating system, indicating that the auxiliary heating system has no significant effects on the PP performance.

Figures 6(c) and (d) shows the XRD crystallinity curves of melt-electrospun fibers fabricated at applied voltages of 20, 30 and 40 kV without and with auxiliary heating, respectively. It can be observed that the XRD pattern of the melt-electrospun PP fiber shows the strongest diffraction peak at around 2θ = 15.19°. The electrospun PP fibers is more on the crystalline phase rather than the amorphous phase. The crystalline peak shown corresponds to carbon. Besides, there is a weak hill at around 2θ = 21.35° that determines the amorphous structure of PP fibers. The intensity of the peak is lower compared to the carbon peak at 2θ = 15.19°. The experiment showed that the PP fibers melt electrospun with 30 kV, in the absence of auxiliary heating conditions, possessed the highest crystallinity (41.71%), while the PP fibers melt electrospun with 20 kV, under the condition of auxiliary heating, possessed the highest crystallinity (50.63%).

Simulation and verification of the temperature field

Figure 7(a) shows the simulation results of the temperature distribution of the melt-electrospinning configuration with auxiliary heating. Combined with the temperature field distribution without auxiliary heating in the simulation diagram,²⁴ it can be seen that the temperature field diffusion degree in the radiation area around the spinneret increases significantly after the application of the auxiliary heating system. Also, the average temperature within the radiation area increases from about room temperature to 100°C or so.

To verify the temperature simulation results, an infrared camera was utilized to collect the images of the heat distribution in the spinning process, with its maximum temperature limited to 300°C, and the temperature measurement results are shown in Figure 7(b). It can be seen that the high temperature is concentrated around the spinneret, and there is a tendency for the temperature to decrease rapidly to ambient temperature under the spinneret. The simulation and measurement results are shown to be in good agreement with each other, demonstrating the effectiveness of this method.

The simulation results along spinning line with and without an auxiliary heating system are shown in Figure 8. The temperature gradually drops to room temperature at about 30 mm from the spinneret without an auxiliary heating system, while after adding an auxiliary heating system, the temperature drops slowly to room temperature almost at the collector. This indicates that the auxiliary heating can improve the temperature in the jet stretching area, which will slow down the solidification of the jet. It is worth mentioning that the measured values near the surface of the collector show a slight temperature increase that is different from the simulation results, mainly because of the heat reflection from the smooth surface of the collector.

Conclusions

In this paper, the effects of the most two important factors (temperature and applied voltage) on the melt-electrospinning process and resultant fibers were investigated. The jet motion, fiber membrane morphology, fiber diameter and properties of the melt-electrospun PP fiber produced with different applied voltages were studied, and an auxiliary heating system was added to the melt-electrospinning setup to understand the effect of temperature distribution on electrospinning process and fiber properties. The results indicated that higher applied voltage results in a smaller fiber diameter, more disordered fiber mat and lower degree of strength. It is worth mentioning that under the same voltage condition, the diameter of the fiber with auxiliary heating is finer than that without auxiliary heating. In order to better understand the experimental results, the temperature distribution of the melt-electrospinning configuration was calculated. The simulation results of the temperature distribution in the spinneret-collector region are compared with the measured values of the infrared camera image, and the results are in good agreement, which proves the feasibility and accuracy of the model. This study shows that the auxiliary heating system facilitates the refining of the fiber in a high-voltage environment, and the subtle change of temperature is closely related to the fiber diameter. The simulation results of the temperature distribution and experimental data will provide the reference for the fiber to be further refined.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship and/or publication of this articleThis work was supported by the National Natural Science Foundation of China (Grant No. 11702169), to Dr. Y. Zheng and Shanghai Local Capacity Building Project (Grant No. 19030501200) to Dr. B. Xin.

ORCID iDs

Yuansheng Zheng

Binjie Xin

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