Preparation and characterization of polysulfone amide nanoyarns by the dynamic rotating electrospinning method

Abstract

In this study, a novel electrospinning method and the related spinning device were developed to fabricate a new type of polysulfone amide yarn assembled with nanoscale fiber. The electrospun nanofibers extruded and stretched under the electric field force were assembled and twisted with a rotating receiver, so the freshly prepared nanofibers could be bundled into yarns continuously. The effects of solid concentration and the rotating speed of the collector on the yarn structures and properties were investigated systematically. The three-dimensional electric field was modeled and calculated theoretically to simulate the electric field distribution of the electrospinning system. The experimental results showed that the morphology, mechanical strength, and thermal and wicking properties were obviously affected by solid concentration and the rotating speed of the collector. It was found that the nanoyarn could achieve a better performance in morphology, strength, heat resistance, moisture absorption at 12 wt% and 40 r/min collector rotating speed. Therefore, the proposed advanced electrospinning technique in this paper could be used to produce multi-functional nanoyarns, which may be useful for the application of nanoyarns in various industrial fields.

Keywords

electrospinning nanoyarns polysulfone amide thermal property wicking property

Nanoyarn could be defined as one kind of nanofiber assembly, which is a continuous fiber bundle with interlocked structure, which has been proposed to improve the mechanical property of fibers and develop three-dimensional (3D) fibrous materials.^1–3 As the oriented nanofiber bundle has special properties in comparison with electrospun nanofiber mats,^4,5 it has been used widely in biomedical,^6,7 sensor,^8–11 electrolyte carrier,^12–14 and other numerous fields.^15–18 It is worth noting that electrospinning has been recognized to be the most straightforward method to produce these kind of continuous nanofibers and nanoyarns. Moreover, electrospinning technology has high potential to improve the properties of high-performance fibers through the control of the morphology and structure of the resultant fibers at nanoscale, depending on the spinning conditions and spinneret configuration. However, more improvements are still needed regarding the nanoyarn manufacturing with the utilization of various electrospinning methods, comparing with the traditional yarn production methods, such as ring spinning,¹⁹ rotor spinning,²⁰ and jet spinning.²¹

Although several methods used for the preparation of nanoyarns have been reported recently,^22–24 the mass production of nanoyarn still seems to be a great challenge for industrial applications. These previously reported nanoyarn preparation approaches are generally classified as one-step and two-step methods. For the one-step method, various shapes of receivers were used to collect the nanofibers together and twist them by rotating the collectors to form nanoyarns. As for the two-step method, the nanofibers were gathered to form a bundle first, and then the bundle was twisted by using water,²⁵ air,²⁶ or other external forces to fabricate nanoyarns. Our previous researches have also reported the preparation of nanowrapping yarns with the one-step method by a self-developed electrospinning and wrapping system.²⁷

Most of the yarn electrospinning setups include four main parts: the spinneret, the collector, the twisting device, and the high-voltage power. Many efforts have been made to improve the collector and twisting device to obtain better yarn performance. The concept of fabricating nanoyarn was first presented by Ko et al.²⁸ Ko’s research group invented a new approach to prepare continuous carbon nanotube-filled yarns by using an equipment with ventilation, orientation, attenuation, and a twisting device. Lin et al.²⁹ employed different devices, such as cylinder and coil electrospinning systems and a novel ring collector² to prepare the nanoyarn; their works have greatly contributed to the one-step method of nanoyarn fabricating. Bazbouz and Stylios³⁰ designed a new electromechanical mechanism with a so-called “twist-disk” and “tick-up disk” for spinning “core electrospun nanoyarn.” Wu et al.³¹ used the dynamic liquid electrospinning method to manufacture silk fibroin/poly(L-lactide-co-caprolactone) nanoyarn scaffolds. Ali et al.¹ used an online stretching method to improve the fiber alignment and molecular orientation within the yarn and increase yarn tensile strength, whereas the fiber/yarn diameter could be reduced. Wu et al.³² also modified the nanoyarn electrospinning method to study the effect of spinning parameters on the mechanical properties of nanoyarns. However, the effects of spinning parameters on the performance of electrospinning nanoyarn should be studied systematically in terms of the nanoyarn application, for the purpose of exploring wide applications.

In the present study, a dynamic rotating electrospinning method and related electrospinning equipment were self-developed to fabricate continuous nanoyarns. Polysulfone amide (PSA) fiber has some excellent properties, such as heat resistance, thermal stability, flame retardancy, etc, therefore, it can be applied to develop protective textile materials used in aerospace, high-temperature environments, and civil fields with flame retardant requirements.³³ As a new kind of high-performance synthetic fiber, PSA was selected as the spinning polymer to produce the functional nanoyarns in this research. The effects of solid concentration and the collector rotating speed on yarn diameter, mechanical strength, and the thermal and wicking properties were investigated systematically. Moreover, the 3D electric field distribution of the electrospinning configuration was simulated by using the finite element method (FEM).

Experiments and simulation

Material preparation

The PSA (C₂₀H₁₄O₄N₂S, degree of polymerization ≥ 396, average molecular weight is 150,000–200,000, 12.5 wt%, Shanghai Tanlon Fiber Co., Ltd, China) and the N,N-dimethylacetamide (DMAc, analytical grade, CH₃CON(CH₃)₂, molecular weight: 87.12, Sinopharm Chemical Reagent Co., Ltd, China) were used as received.

Homogeneous PSA solutions with the concentrations of 8, 9, 10, 11, and 12 wt%, respectively, were prepared by dissolving the PSA in DMAc. The solution was prepared by an ultra-high-speed mixer (I25, IKN, China) for 1 h, and an ultrasonic pump (JP-303B, JieMeng, China) for 2 h. All the experiments were conducted under the condition of 20℃, 40–60% relative humidity (RH).

Experimental setup

In this study, a self-made electrospinning device was used to fabricate PSA nanoyarns, illustrated in Figure 1. The device includes a funnel-shaped collector (a metal funnel, outer diameter of 110 mm, inner diameter of 40 mm, length of 85 mm), a high-voltage power supply (ES60 20 W, Gamma, USA), two spinnerets (a three-tube stainless steel needle, blunt-tip type, 0.8 mm outer diameter, 0.5 mm inner diameter, 13 mm length), two syringe pumps, and a winding roller. The spinnerets are connected to the high-voltage power supply and symmetrically distributed 200 mm away from the collector. The polymer solution was extruded from syringe via a syringe pump. The extruding flow rate of the solution was set to be 0.5 ml/h, and the voltage applied to the spinnerets was set to be 30 kV.

Figure 1.

Schematics of the dynamic rotating electrospinning system.

The solution located at the needle tips was stretched by the electric field force and then formed into a charged jet flow. After the jet flowed away from the spinneret, it was drawn into nanoscale fibers and shifted to the collector at high speed. Then the funnel-shaped collector driven by a motor was designed as a negative electrode and gathered nanofibers to form a fiber bundle. During the collector rotation, the funnel-shaped collector exerted a twist on the nanofiber bundles to form a stable yarn, and then the produced yarns were winded on the yarn rolling device with the traction of the guide roller.

Characterization

The yarn morphology was characterized by a scanning electron microscope (SEM; SU8010, Japan). The diameters of the yarn and fiber were calculated based on SEM images using image-analysis software (Image-pro Plus 6.0, USA). The yarn mechanical property was determined by an electronic single fiber strength tester (XS (08) XT-3, China). Thermogravimetric analysis (TGA) were used for further characterization of the thermal resistance of the electrospun PSA nanoyarns. A TGA 4000 (Perkin Elmer, USA) with nitrogen flow rate at 20 ml/min was used in our experiments. The electrospinning PSA nanoyarns’ wicking phenomenon was measured by a textile capillary effect tester (YG871, China).

Electric field simulation

The 3D electric fields of the experimental setups were simulated by COMSOL Multiphysics (5.2 a version, COMSOL Inc., Sweden) software using the FEM. The electric field strengths and distribution were calculated by the electrostatics models embedded in COMSOL software. The physical geometries of the experimental setups (e.g., needles and the collector) were defined according to the practical dimensions, locations, and properties before the calculation. Voltage of 30 kV was exerted to the needles, and the funnel-shaped collector was linked to the boundary ground.

Results and discussion

Simulated electric field

The electric field intensity and distributions simulated by the COMSOL Multiphysics software for the dynamic rotating electrospinning system are illustrated in Figure 2. The two spinnerets with high applied voltage and the grounded collector were included in the simulation model. It is obvious that the electric field is inhomogeneous with an extremely high electric field concentrated on the surrounding area of the needles; the electric field intensity at the receiver is rather weak compared with the spinneret, as shown in Figure 2(a). The electric field intensity at the edge of the receiver is higher than at other places of the collector, because the sharp opening of the funnel is close to the needles and it is easier to produce an induced electric field. Figure 2(b) shows the electric field distribution of the yarn electrospinning setup. The arrows indicate the direction of the electric field, and their length is proportional to the strength at that point. It can be found that the electric field direction is from the needle points to the funnel, and is concentrated at the edge of the funnel. It would be easy to understand how the nanofibers gathered at the edge of the receiver, and then twisted to form a yarn, from the Figure 2(b).

Figure 2.

The electric field intensity and distribution of the electrospinning setups: (a) electric field norm of the center plane; (b) electric field distribution of the center plane.

Effect of rotating speed

The twisting cone angle

During the spinning process, a cone-like membrane could be formed on the funnel opening, as shown in Figure 3. The rotating and winding speed are the two main factors to determine the size of the twisting cone angle; the number of twists and the pitch were controlled by the rotating speed and the winding speed, respectively, as displayed in equations (1) and (2) ³⁴

tan α = \frac{π d}{h} = \frac{π d T}{100}

(1)

T = \frac{ω_{r}}{ω_{w}}

(2)

where α is the half twisting cone angle, d (mm) is the diameter of the yarn, h (mm) is the pitch, T is the twist, ω_r (r/min) is the rotating speed, and ω_w (r/min) is the winding speed.

Figure 3.

Formation of nanoyarn with different rotating speeds: (a) 20 r/min; (b) 30 r/min; (c) 40 r/min; (d) 50 r/min; (e) 60 r/min.

To investigate the effects of rotation speed on the yarn formation and properties, PSA nanoyarns were prepared at five different rotation speeds of 20, 30, 40, 50, 60 r/min, respectively, and the winding speed was 10 r/min. When the winding speed was set to be constant, the twisting cone angle was measured accordingly. It was observed that the twisting cone angle was increased with the rotation speed of the funnel-shaped collector, as depicted in Figure 3. As the winding speed remained constant, the twist of the electrospun nanoyarn also increased with the improvement of rotating speed.

Yarn diameter and fiber diameter

Figure 4 shows the morphology of the electrospun nanoyarn and fibers, as well as the fiber diameters. It was found that fiber diameter was decreased with the increasing of rotating speed (Figure 4), and the rotating speed has no obvious effects on the diameter of the PSA nanoyarn when the rotating speed was higher than 30 r/min (Figure 5). The high rotating speed provides a relatively high drafting force to form nanofibers, which could be assembled into nanoyarns with small diameter. It could be found that a slight decrease of yarn diameter occurred at 60 r/min, which indicates that the rotating speed of 60 r/min is too high and not suitable for PSA nanofiber yarn manufacturing. When the rotating speed was set to be 20 r/min, the speed was not fast enough to twist the fibers completely and the fibers were loose with many interspaces existing among them, so the diameter of the nanoyarn was relatively larger, and it can be observed that the fiber distribution on the surface of the yarn was sparse. As illustrated in Figures 4(a)–(e), with the increasing of rotating speed, the triangle region of twisting tends to be larger and more floating fibers appear to be aligned on the yarn surface; therefore, more loosely curved fibers could be observed on the yarn surface.

Figure 4.

Scanning electron microscope images of the nanoyarns produced under different rotating speeds: (a) 20 r/min; (b) 30 r/min; (c) 40 r/min; (d) 50 r/min; (e) 60 r/min, and histogram of individual fiber diameters into the respective yarns produced under different rotating speeds: (f) 20 r/min; (g) 30 r/min; (h) 40 r/min; (i) 50 r/min; (j) 60 r/min.

Figure 5.

Yarn diameter versus rotating speed.

During the yarn manufacturing process, the diameter of PSA fibers decreased with the rotating speed increasing. The reason behind this phenomenon is that the fiber twist could be increased through the increasing of the rotating speed; simultaneously, a secondary drawing of the fibers was also exerted, and meanwhile the drawing of the fibers in the electric field during the process of fiber forming is namely defined as the first drawing.

Mechanical properties

Mechanical properties were investigated as one of the most important properties for the characterization of nanoyarns. As illustrated in Figure 6, the mechanical strength was significantly affected by the rotating speed. It was found that when the speed was increased from 20 to 40 r/min, the breaking strength was dramatically improved. However, the breaking strength was reduced drastically when the speed was more than 40 r/min, and the increasing of the twist could lead to the reduction of the effective component of each fiber and the effective utilization of fiber strength; therefore, the yarn breaking strength was decreased, and the value of 40 r/min was a critical value that was similar to the critical twist factor.

Figure 6.

Tensile cures (a) and average tenacities (b) of polysulfone amide nanoyarns at different rotating speeds.

Wicking property

The capillary rise rate refers to the property of yarn to absorb moisture from aqueous solution (distilled water); it is an important indicator of physical properties. The electrospun PSA nanoyarns have an excellent wicking property because of the unique nanofibrous structures. The yarns were immersed in aqueous solution for 10, 30, 60 min, respectively, to investigate the wicking property. The resultant yarns spun at 40 r/min achieve the best wicking property measured at all the three observing moments, as shown in Figure 7. When the rotating speed was increased from 20 to 40 r/min, the capillary rise rate for yarns was increased because the increasing of rotating speed makes the fiber assembles twisted tightly. When the rotating speed was increased from 40 to 60 r/min, the excessive rotating speed led to the increasing of the yarn tightness, resulting in a poor water absorbing effect, which reduces the height of the capillary rise. In this experiment, when the rotating speed was set to be 40 r/min, the nanoyarns achieved the best wicking property.

Figure 7.

Height of the hygroscopic climb for yarns prepared at various rotating speeds (in each panel, from left to right: 20, 30, 40, 50, 60 r/min) for different testing times: (a) 10 min; (b) 30 min; (c) 60 min.

Thermal resistance

TGA refers to a thermal analysis technique that measures the sample quality and variation of temperature, and the change of temperature was programmed. Figure 8 shows TG and derivative thermogravimetric analysis (DTG) curves of PSA nanoyarns prepared at different rotating speeds. As shown in the process of increasing temperature from room temperature to about 150℃, the volatilization of solvent and the water bound among the molecules led to the reduction of material weight. The phase between 150℃ and 400℃ is mainly due to the evaporation of small molecules, such as additives. When the temperature increases to about 480℃, the movement of the macromolecular chain happens more intensely and many breaks occurred (for example: -C-S-, -C-N-, etc.); small molecules were released as gas, such as NH₃, SO₂, and CO₂.

Figure 8.

Thermogravimetric (a) and derivative thermogravimetric analysis (b) curves of polysulfone amide nanoyarns preparation at different rotating speeds.

During the weight loss process, the residue when the rotating speed was over 40 r/min is higher than that less than 40 r/min, and the final residues were 30%, 33%, 40%, 39%, and 38%, respectively. This result shows that the effect of speed on the PSA nanoyarn thermal resistance is limited, especially with high rotating speeds. A high rotating speed has effects on the crystalline and orientation of PSA nanoyarns, which can be validated by the testing results of mechanical properties.

Effect of solution concentration

Yarn diameter and fiber diameter

Figure 9 shows the SEM image of the PSA nanoyarns and histograms of nanofiber diameter distribution in the respective yarns with different solution concentrations ((a) 8 wt%, (b) 9 wt %, (c) 10 wt %, (d) 11 wt %, (e) 12 wt %), and Figure 10 shows the effect of solution concentration on yarn diameter. The yarn diameter was increased with the increasing of the solution concentration, as can be seen from Figures 9(a)–(e) and Figure 10. At a higher solution concentration, more PSA macromolecules could be obtained after the solvent evaporated, and they formed nanoyarns with a larger yarn diameter. In contrast, the diameter of the nanofibers was decreased slightly from about 559 to 355 nm with the increasing of the solution concentration, as illustrated in Figures 9(f)–(j). The nanoyarn diameter tends to be increased with the increasing of the solution concentration, but the nanofiber diameter tends to be decreased with increasing of the concentration, which means there are more nanofibers produced by the PSA solution with higher concentration.

Figure 9.

Scanning electron microscope images of the nanoyarns produced under different concentrations: (a) 8 wt%; (b) 9 wt%; (c) 10 wt%; (d) 11 wt%; (e) 12 wt%, and histograms of individual fiber diameter into the respective yarns produced under different concentrations: (f) 8 wt%; (g) 9 wt%; (h) 10 wt%; (i)11 wt%; (j) 12 wt%.

Figure 10.

Effect of solution concentration on yarn diameter.

Mechanical properties

Figure 11 shows the mechanical properties of the PSA nanoyarns with different solution concentrations. It was found that the solution concentration had a positive correlation with the tenacity of the PSA nanoyarn. There was a significant change when the concentration was over 10 wt%. This is because a higher solution concentration could be used to produce more fibers compared with a low solution concentration.

Figure 11.

Tensile cures (a) and average tenacities (b) of polysulfone amide nanoyarns with different solution concentrations.

Wicking property

For the yarn wicking property, the concentration of spinning solution affects the yarn’s structure and the number of fibers in the yarn. As shown in Figure 12, the height of the nanoyarns’ capillary rise was increased with the increasing of solution concentration. This is because the PSA content should be increased with the solution concentration increasing; the PSA nanoyarn spun using higher concentration consists of more PSA fibers as analyzed above, thus producing more air clearance, which could obtain a better bibulous effect.

Figure 12.

Capillary rise height of the yarns prepared with various solution concentrations (in each panel, from left to right: 8 wt%, 9 wt%, 10 wt%, 11 wt%, 12 wt%) for different testing times: (a) 10 min; (b) 30 min; (c) 60 min.

Thermal resistance

Figure 13 presents the TG and DTG curves of PSA nanoyarns prepared with different solution concentrations. As shown in Figure 8, during the whole heating process, the trend is similar to the trend of the curve under different rotating speeds. The decomposition temperatures were similar for these five concentrations; the low residue at 150℃ for 8 and 9 wt% is due to the higher solvent percentage in solution concentration. With the increase of the solution concentration, the residue of the samples has also been increased when the tests were finished; the more the PSA contained in the spinning solution, the better the thermal resistance of the nanoyarns.

Figure 13.

Thermogravimetric (a) and derivative thermogravimetric analysis (b) curves of polysulfone amide nanoyarns prepared with different solution concentrations.

Conclusions

In this study, a series of PSA nanoyarns were prepared by changing the solution concentration and the rotating speed of the receiver. Morphology analysis, mechanical properties, wicking properties, and thermal resistance of the proposed nanoyarns were investigated systematically. The rotation speed of the funnel-shaped collector affects the twist of nanoyarns. Upon increasing the rotation speed from 20 to 60 r/min, the twist of nanoyarns was increased significantly; the mechanical properties, thermal resistance, and infiltration of the proposed nanoyarns illustrated a decreasing trend after the increase, and a maximum at the speed of 40 r/min was observed. Therefore, the optimized spinning rotation speed could be set to be 40 r/min in our experiments. The increase of solution concentration leads to the variation of yarn diameter. The nanoyarns have good appearance and spinnability when the solution concentration is 12 wt%, and mechanical properties, thermal properties, and wicking properties of nanoyarns are better than those spun with solution concentration lower than 12 wt%. Therefore, the optimized spinning solid concentration could be set to 12 wt% in our experiments.

Footnotes

Declaration of conflicting interests

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

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the National Natural Science Foundation of China (Grant No. 11702169), Scientific Research Staring Foundation of Shanghai University of Engineering Science (Grant No. E3-0501-17-01019), and 2017 Talents Action Program of Shanghai University of Engineering Science (Grant No. E3-0507-17-03055) to Dr. Y. Zheng. This work was also supported by 2017 Talents Action Program of Shanghai University of Engineering Science (Grant No. E3-0507-17-03046) to Dr. B. Xin.

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