Enhancement of Coil Electrospinning Using Two-Level Coil Structure

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Enhancement of Coil Electrospinning using Two-level Coil Structure Haitao Niu, Hua Zhou, Guilong Yan, Hongxia Wang, Sida Fu, Xueting Zhao, Hao Shao, and Tong Lin Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b04145 • Publication Date (Web): 17 Oct 2018 Downloaded from http://pubs.acs.org on October 21, 2018

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Industrial & Engineering Chemistry Research

A helical coil spinneret with secondary coil structure on surface shows increased nanofiber production rate without changing fiber diameter and morphology, because of the increase in both electric field intensity and fiber generating area. 82x29mm (300 x 300 DPI)

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Enhancement of Coil Electrospinning using Twolevel Coil Structure Haitao Niu, Hua Zhou, Guilong Yan, Hongxia Wang, Sida Fu, Xueting Zhao, Hao Shao, Tong Lin* Institute for Frontier Materials, Deakin University, Victoria 3216, Australia

ABSTRACT: Coil based electrospinning is among the most-efficient needleless electrospinning technologies. Conventional coil electrospinning typically uses a coil with smooth wire surface as spinneret. The effect of wire surface morphology on coil electrospinning and nanofiber production was scarcely reported. Herein, we report a novel coil electrospinning that has a secondary coil structure on coil spinneret. The secondary coil was found to have a great effect on coil electrospinning process. It reduced the voltage for jet initiation by 3 kV and increased nanofiber production rate by over 170%, but had little influence on fiber diameter. The finite element analysis indicated that the secondary coil structure increased electric field intensity on coil surface and fiber generating areas. These novel understandings may be useful for designing high-efficiency electrospinning spinnerets for nanofiber mass production.

1. INTRODUCTION Electrospun nanofibers have shown great potential for applications in areas as diverse as tissue engineering, drug delivery, catalysis, sensors, energy conversion and storage, reinforcement, and environmental protection, owning to their high air/liquid permeability, large surface-to-volume

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ratio, tunable fiber diameter and ease of functionalization1-3. Needleless electrospinning has received much attention because of the potential for mass electrospinning of nanofibers. Needleless electrospinning is featured as electrospinning of nanofibers directly from an open liquid surface4. Different from needle electrospinning, needleless electrospinning generates jets without constrained by capillary effect which occurs in needle nozzle. As a result, the adjacent jets have minimal interference with electric field and electrospinning process. Needleless electrospinning is simple to set up, easy to operate, and adaptable to process a variety of spinning solutions. Commonly reported needleless electrospinning spinnerets include balls, cylinders, wires and coils. Although varied in spinneret geometry, they share a similar electrospinning mechanism that a solution is electrospun from a free solution surface5. However, these needleless spinnerets vary in nanofiber production rate, diameter and morphology, because the geometric shapes determine electric field distribution and area for fiber generation6-7. Considerable researches have been devoted to the development of novel needleless electrospinning spinnerets with high nanofiber production rate and fiber quality. Yarin and Zussman4 reported a magnetic field induced needleless electrospinning process in which a twolayer fluid containing upper polymer solution layer and bottom ferromagnetic suspension layer. Jirsak at al.8 used a rotating metal roller as spinneret. The roller was partially immersed in a solution tray. When the solution was charged with high voltage, nanofibers were produced from top roller surface. This setup has been commercialized by Elmarco with the brand name NanospiderTM that can utilize either roller electrode, single wire electrode, or parallel wire electrodes for electrospinning9-10. Liu et al.11 used air bubbles to initiate electrospinning from an open solution surface. Lin et al.12 developed a spiral coil spinneret for needleless electrospinning. Jiang et al.13 reported a pyramid-step fiber-generator, the sharp edges of which generated high



electric field improving electrospinning performances. Yan et al.14 developed a curved slot generating uniform electric field along the slot length direction, which improved electrospinning stability and fiber qualities. Apart from novel spinneret designs, fundamental understandings about needleless electrospinning were also conducted. Lukas et al.5 elucidated jet initiation process from free liquid surface using a cleft spinneret. They indicated that in a high electric field stationary waves selforganized and fast grew from free liquid surface and jets were initiated from the fluid waves when the electric field was sufficient. Niu et al.6 and Wang et al.15 from our group separately used a finite element method (FEM) to analyze the electric field intensity profiles of cylinder, disc and coil spinnerets. They suggested that the spinneret with larger curvature could form higher electric field on coil surface, which resulting in large nanofiber productivity and fine nanofibers with narrow diameter distribution12, 16. Several researchers have reported on the increase of spinneret curvature by introducing an auxiliary structure onto needleless spinnerets, such as Von Koch curve fractal structure17, needles on disk or helix slice18-19, barbed roller20, probed cylinder21 and threaded rod22. However, uneven increase in local curvature can adversely lead to corona discharge and breakdown of electric field2324.

Corona discharge not only increases the consumption of electrical energy, but also reduces

electrospinning efficiency and nanofiber production rate25. It might result in damage of the equipment as well. It remains a challenge to design a needleless spinneret that has both large curvature in the electrospinning area and large fiber generation area. In our previous studies, we have elucidated that coil electrospinning using a rotating helical coil as spinneret can generate centralized, high electric field along the coil wire15. The helical coil spinneret has low jet initiation voltage and high nanofiber productivity. However, using a


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secondary large curvature structure for improving the electrospinning ability of coil spinnerets has not been demonstrated. In this study, we developed a novel needleless electrospinning spinneret with two level coil structure on spinneret. The coil spinnerets were made by winding a copper wire around the primary helical coil. The presence of the secondary coil structure was found to obviously reduce jet initiation voltage and improve nanofiber production rate, however not apparently change fiber diameter. The secondary coil pitch and wire diameter showed effects on coil electrospinning process and nanofiber production rate. The finite element analysis indicated that the secondary coil increased both surface electric field intensity and fiber generating area, facilitating to increase fiber production.

2.

MATERIAL AND METHODS

2.1 Materials Polyvinyl alcohol (PVA, average molecular weight 85,000-124,000, 87% hydrolyzed) was purchased from Aldrich-Sigma and used as received. Electrospinning solutions were prepared by dissolving PVA powder in deionized water at 92 °C under vigorous stirring for 8 hours. The normal coils (coil diameter = 70 mm, wire diameter = 3 mm, coil pitch = 50 mm) (also referred to as “Ncoils” in this study) were obtained directly from a commercial source (local spring manufacturer). The coils were manufactured with consistent diameter, pitch and wire diameter. 2.2 Coil electrospinning The electrospinning experiments were performed on a purposed-built needleless electrospinning machine with temperature and humidity controlled at 30 °C and 40%, respectively. Figure 1a depicts the setup which contains a helical coil, a high voltage DC power supply (ES100P, 4 ACS Paragon Plus Environment


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Gamma High Voltage Research), a metal belt collector, and polymer solution held by a plastic container. The helical coil was partially immersed in the polymer solution. It can rotate along the axil driven by a rotating system. A two-level coil (also referred to as “T-coil”) spinneret was prepared by manually enwinding a copper wire onto the N-coil. To ensure the secondary coil have the consistent coil pitch, the wire winding was assisted with a spacer template. Different spacer widths were employed to prepare the T-coils with different secondary coil pitches. An image to illustrate the T-coil winding is shown in the Supporting Information (Figure S1). The solution was charged by the DC power supply via inserting a steel electrode into the solution through the plastic bath. During electrospinning, the polymer solution inside the bath was loaded onto the coil surface by slow rotation of the coil. The coil rotation speed, applied voltage, collecting distance, and PVA concentration were set at 5 rpm, 80 kV, 20 cm, and 9 wt%, respectively, for all experiments unless otherwise specified. The electrospun nanofibers were collected by the metal belt collector covered with nonwoven mats. The collecting distance for N-coil electrospinning was defined as the distance from the coil top to the collector bottom surface. For the T-coil electrospinning, the collecting distance was the distance from the top of primary coil to bottom surface of the collector because of the much smaller wire diameter (< 1 mm) when compared with the distance between the primary coil top and the bottom surface of the collector (20 cm). The parameters of T-coils used for electrospinning experiments are listed in Table 1. The schematic drawing of these coils are shown in Figure S1. For comparison, N-coil spinnerets were also used for electrospinning. Table 1. Parameters of T-coil and N-coil T-coil 5 ACS Paragon Plus Environment

N-coil

Second ary oil

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Primary coil

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Material Wire diameter (mm) Coil length (cm) Coil pitch (cm) Coil diameter (cm) Material Wire diameter (mm) Coil pitch (mm)

Stainless steel 3 18 5 7 Copper 0.5, 0.7, 0.9 0, 2, 4, 6, 8

Stainless steel 3 and 4 18 5 7 No No No

2.3 Electric field analysis The electric field generated by the coils was calculated by the finite element method (FEM) using the program package COMSOL3.5. The calculation was based on the equation 𝐸 = ―∆𝑉 (where E is the electric field intensity and V is the potential). First, all the parts in the electrospinning setup (i.e. spinneret, solution bath, polymer solution and collector) were drawn using Solidworks software according to their real dimension and location, and the 3D structures were then imported into COMSOL. The physical properties of these parts were set according to their real values. The metal collector was set as zero potential. 80 kV high voltage was applied to the electrode at the solution bottom. All other boundaries were set as continuity. The meshing and solving processes were performed by the software using default settings, and the electric field intensity values and profiles were obtained. 2.4 Characterizations Nanofiber productivities were calculated by measuring the nonwoven mats before and after collecting nanofibers. To remove trace solvent from the as-spun fibers, the fiber samples were dried in a vacuum oven at 40 ℃ overnight. Fiber morphology was observed using the scanning electron microscope (SEM, SUPRA-55VP). The average fiber diameter was calculated based on SEM images using image analysis software (ImagePro+4.5). Each fiber diameter was calculated from over 100 fibers in at least 5 SEM images taken from nanofiber samples prepared in different 6 ACS Paragon Plus Environment


batches. Crystal structure of nanofibers was examined by X-ray diffraction analysis (XRD) using a diffractometer (Panalytical X’Pert Powder XRD) with Cu radiation of 1.54 Å. The nanofiber samples were scanned in the 2θ range of 5 ~ 60° with the step size of 0.025°. Tensile properties were measured on an Instron Tensile Tester at controlled environment temperature 20 ± 2 °C and relative humidity 65 ± 2 %. PVA nanofiber mats were cut into 5 mm wide and 50 mm long strips. The thickness of specimens was measured using a thicknesses tester (Digimatic Indicator, Mitutoyo). The specimens were stretched at a constant speed of 5 mm/s.

3. RESULTS AND DISCUSSION Figure 1a illustrates the setup for T-coil electrospinning. The T-coil spinneret was prepared by winding a copper wire around the wire of a stainless steel coil. The stainless steel coil functioned like a primary coil while the copper wire formed the secondary coil. The coil spinneret was partially immersed in the polymer solution. The T-coil was fitted into a lab-use electrospinning machine (Figure 1b), which had an air-conditioned chamber with automatic control systems for humidity, applied voltage and collecting distance. Electrospinning was performed by rotating the primary coil around its axis that led to the loading of polymer solution onto coil surface, and by applying a high voltage between polymer solution and fiber collector. Figure 1c shows the T-coil electrospinning process. When the applied voltage exceeded a critical value, numerous solution jets were generated from coil surface. Once generated, the jets flied towards the grounded collector. Because of charge repulsion within filaments and interaction between the charged jet and the external electric field, the jet filaments stretched themselves intensively. Meanwhile, solvent evaporation from the filaments resulted in solid fibers subsequently depositing on the collector. It was found that the applied voltage in the


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range of 50-85 kV, when the collecting distance was lower than 20 cm, wetted nanofiber were collected due to insufficient water evaporation from fibers. In this case, the fibers tended to fuse together in some parts to form highly interconnected nanofibers. However, when the collecting distance was over 20 cm, dry separated fibers were produced. This setup was able to produce a nanofiber mat of size around 30 cm × 100 cm (see a photo Figure 1e). The fibers collected looked uniform and had a randomly orientated nonwoven structure (see SEM image in Figure 1d). The low magnification SEM images show that all T-coils produced uniform nanofibers (Figure S3).

Figure 1. a) Schematic illustration of the two-level coil electrospinning setup (inset image is the magnified view of the T-coil), b) purpose-built electrospinning machine, c) photo of electrospinning process, d) SEM image of electrospun PVA nanofibers, e) photo of a nanofiber mat collected.

In our previous study15, we have examined the effect of the single level coil on electrospinning process, nanofiber diameter, and nanofiber production rate. An optimized coil spinneret with 8 ACS Paragon Plus Environment


smooth wire surface has been used for nanofiber production. In this study, we adjusted the dimension of the secondary coil but kept the dimension of the primary coil unchanged, to examine the effect of the secondary coil on electrospinning performance. All electrospinning experiments were performed at the applied voltage and collection distance of 80 kV and 20 cm, respectively. Five properties were examined including jet initiation voltage, fiber morphology, fiber diameter, and fiber production rate. T-coils with secondary coil pitch ranging from 0 mm to 8 mm were examined. They all produced uniform nanofibers with separated fibrous structure. However, these nanofibers did not show noticeable change in morphology (Figure S2). Jet initiation was identified by observing the jet generated from coil surface, and the lowest voltage to start electrospinning was defined as jet initiation voltage. A spinneret with smaller jet initiation voltage suggests lower energy required to initiate electrospinning. It was noted that coil the distance between the secondary coils showed influence on jet initiation voltage. When the coil pitch changed from 0 mm to 8 mm, the jet initiation voltage declined initially until the coil pitch reached 4 mm, and then increased. The smallest jet initiation voltage was 51 kV (at coil pitch 4 mm) (Figure 2d). This could be explained by the change of electric field intensity on T-coils. Stable electrospinning (i.e. continuous electrospinning without corona discharge) usually occurs at higher applied voltage than the jet initiation voltage. For T-coils, the applied voltage to run stable electrospinning was around 60 kV. The electrospinning area, where the fibers were generated from, were also affected by the secondary coil pitch. When the secondary coil pitch was 0 mm and 2 mm, nanofibers were mainly generated from the secondary coils, whereas at the coil pitch over 4 mm, nanofibers were produced from the top surface of both primary and secondary coils. With increasing the secondary coil pitch from 0 mm to 4 mm, the fiber production rate


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increased from 2.65 g/hr to 4.21 g/hr. Further increasing the coil pitch to 8 mm led to decrease of nanofiber production rate to 3.23 g/hr. The effect of secondary coil wire diameter on electrospinning and fiber production was examined. When the secondary coil wire increased from 0.5 mm to 0.9 mm in diameter, the jet initiation voltage had a slight decline, while the nanofiber productivity increased to 8.1 g/hr (Figure 2e). The average fiber diameter reduced slightly when the secondary coil wire diameter increased from 0.5 mm to 0.9 mm (Figure 2f). This is accompanied with slightly narrowed diameter distribution (i.e. standard deviation of the diameter). For the secondary coil with wire diameter 0.9 mm, the effect of its coil pitch on electrospinning showed a similar trend to that for the T-coil with secondary coil wire diameter of 0.5 mm. When the wire diameter was over 0.9 mm, it was difficult to enwind the secondary coil uniformly along the primary coil. For this reason, 0.9 mm was set the upper diameter limit of the secondary coil. When a finer wire (diameter 0.25 mm) was used for making the secondary coil, lower nanofiber production rate (2.6 g/hr) resulted. This is probably because the fine wire has little contribution to the increase of local curvature of the primary coil.



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Figure 2. a)-c) SEM images of PVA nanofibers prepared by the T-coils with different secondary coil wire diameters, a) 0.5 mm, b) 0.7 mm, c) 0.9 mm (scale bar = 2 µm, primary coil wire diameter = 3mm, secondary coil pitch = 4 mm); d) & e) effects of d) secondary coil pitch (primary coil wire diameter = 3mm, secondary coil wire diameter = 0.5 mm) and e) secondary coil wire diameter (primary coil wire diameter = 3mm, secondary coil pitch = 4 mm) on nanofiber productivity and jet initiation voltage; f) fiber diameter of PVA nanofibers produced by T-coils with different secondary coil distances and wire diameters (primary coil wire diameter = 3mm).

For comparison, we also used N-coils as spinnerets for electrospinning. Two N-coils were used, which had different wire diameters, 3 mm and 4 mm. The 3 mm N-coil had a jet initiation voltage of 54 kV and the jet initiation voltage for the 4 mm N-coil spinneret was 56 kV (Table 2). Both were higher than that of the T-coils with secondary coil pitch in the range of 2-8 mm. The nanofibers produced by the N-coils had a similar morphology to those produced by the T-coils (Figure S5). Moreover, the N-coils had smaller nanofiber productivities, 2.94 g/hr and 2.58 g/hr for 3 mm and 4 mm wires, respectively. The nanofiber productivity for the T-coil (secondary coil pitch 4 mm, wire diameter 0.9 mm) was over 170% larger than that of the N-coil (coil diameter 3 mm). Table 2. Nanofiber productivity and jet initiation voltage

N-coil T-coil*

Wire diameter (mm)

Productivity (g/hr)

Jet initiation voltage (kV)

3

2.94 ± 0.09

54

4 0.9

2.58 ± 0.1 8.1 ± 0.1

56 50

* N-coil: coil pitch = 5 cm; T-coil: primary coil wire diameter = 3 mm, primary coil pitch = 5 cm, secondary coil pitch = 4 mm, secondary coil diameter = 0.9 mm


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To evaluate the nanofiber production ability of the T-coil electrospinning, we further compared our T-coils with other needleless electrospinning spinnerets. As listed in Table S1, the T-coil is superior to the other needleless spinnerets in terms of fiber production rate and nanofiber diameter. Although it showed slightly lower production rate than the cylinder, the T-coil generated much finer fibers. To probe the effect of polymer concentration and applied voltage on T-coil electrospinning, we used PVA solutions of 8%-12% for electrospinning (secondary coil pitch 4 mm, wire diameter 0.9 mm). When PVA concentration was 8%, highly interconnected nanofibers were produced, because of the high water content in the solution jets (Figure S6), and only a small amount of nanofibers were produced. When PVA concentration increased from 9% to 12%, nanofiber productivity increased from 5.31 g/hr to 9.72 g/hr (Figure 3a), while the average fiber diameter increased gradually (Figure 3b). In the concentration range studied, nanofibers showed a uniform morphology (Figure S6). For a higher PVA concentration solution, e.g. 13%, the solution was too viscous to be electrospun, resulting in very low fiber productivity. At the same condition, the T-coil had higher nanofiber productivities than the N-coil regardless of the polymer concentration. The effect of polymer concentration on nanofiber production rate and fiber diameter for the N-coil electrospinning followed similar trends to the Tcoil electrospinning. An obvious difference was found in the concentration range of 8%~10%, in which the increase rate of fiber productivity for the T-coil was larger than that of the N-coil. Increasing the applied voltage usually leads to increase in fiber productivity but decrease in fiber diameter 6. At the same applied voltage condition in the range of 65 kV - 85 kV, the T-coil always had higher nanofiber productivity than the N-coil, though the fiber diameter was very



similar (Figure 3c & d). With increasing the applied voltage, both spinnerets showed an increased production rate and decreased fiber diameter. When the applied voltage was in the range of 65 kV - 85 kV, uniform nanofibers were always produced by the T-coil (Figure S7).

Figure 3. Effects of a) polymer concentration on nanofiber productivity, b) polymer concentration on fiber diameter, c) applied voltage on nanofiber productivity, d) applied voltage on fiber diameter (N-coil: wire diameter = 3 mm; T-coil: primary coil wire diameter = 3 mm, secondary coil wire diameter = 0.9 mm, secondary coil pitch = 4 mm).

To find out the effect of secondary coil on coil electrospinning, we performed a FEM analysis. Figures 4a shows electric field intensity profiles of the coil spinnerets. It was interesting to note that the T-coil showed different electric field intensity profiles to the N-coil. The high electric field 13 ACS Paragon Plus Environment

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for the T-coil predominantly centralized on the secondary coil, whereas the high electric field for the N-coil predominantly centralized on the primary coil. This will enable the T-coils to easily electrospin nanofibers from the secondary coil.

Figure 4. a) Electric field intensity profiles of T-coil and N-coil (Red colour and blue colour indicate large intensity and small intensity, respectively), b) electric field intensities of T-coils with different secondary coil pitches, and c) electric field intensities of T-coils with different secondary 14 ACS Paragon Plus Environment


coil wire diameters, d) electric field intensities along the coil wire direction, and e) electric field intensities along the primary coil axis direction (Applied voltage = 80 kV, electrospinning distance = 20 cm. N-coil: wire diameter = 3 mm; T-coil: primary coil wire diameter = 3 mm, secondary coil pitch = 4 mm, secondary coil wire diameter = 0.9 mm).

The electric field intensity profile along the coil surface offers valuable information of electrospinning. Figure 4b shows the effect of secondary coil pitch on the electric field intensity profile. When the pitch was 0 mm, the electric field of the T-coil showed a similar profile to that of the N-coil. With increasing secondary coil pitch, the electric field intensity increased, whereas the electric field intensity in the areas between the two secondary coil wires was lower. Figure 4b also shows that electric field intensities for the T-coils with the secondary coil pitches 4, 6, 8 mm are comparable, except that the 4 mm pitch secondary coil was more compact than the other two. These could be the reasons why the T-coil with secondary coil pitch of 4 mm had a larger nanofiber production rate. Figure 4c shows the effect of secondary coil wire diameter on electric field. When a larger diameter wire was used for making the secondary coil, the intensity divergence between the high electric field and low electric field became larger. The larger wire circumference of secondary coil had larger surface area and resulted in larger fiber generation surface, hence larger nanofiber production rate. In contrast, the N-coil generated uniform electric field along the wire axis direction (Figure 4d). The electric field intensity on the secondary coil of T-coil was larger than that on the N-coil surface. However, the intensity electric field between the secondary coils was lower. Along the primary coil length direction, the electric field intensity on the T-coil was larger than that of the N-coil as well (Figure 4e). From coil surface to the collector, all the coil spinnerets showed a


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rapid electric field intensity decay in the first few centimeters away from the coil, and slow decline towards the collector (Figure S8). The above results have indicated that the secondary coil not only enhances electric field in the fiber generating area both also provides additional surface for jet generation. The high electric field will lead to reduction in jet initiation voltage, while the large coil surface allows more nanofibers to be produced. In addition, the secondary coil helps to form a corrugated solution layer on the primary coil surface. The solution curve crest centralizes electric field force and facilitates jet initiation. In addition, XRD patterns and tensile properties of PVA nanofibers electrospun from both N- and T-coils were measured. All samples had two typical peaks, 2θ = 19.4° and 38.9° (Figure 5a), respectively, corresponding to (101) and (200) reflections of PVA26, regardless of the spinneret used. The nanofiber mats prepared by the T-coil had similar tensile properties to those prepared by the N-coil (Figure 5b).

Figure 5. a) XRD patterns and b) strength-strain curves of electrospun PVA nanofibers (N-coil: wire diameter = 3 mm; T-coil: primary coil wire diameter = 3 mm, secondary coil wire diameter = 0.5 mm, secondary coil pitch = 4 mm).



4. CONCLUSIONS We have shown that a secondary coil on helical coil spinneret can greatly improve the electrospinning performance. The presence of the secondary coil reduces jet initiation voltage by 3 kV and increases electrospinning yield by over 170%. However, the secondary coil shows little influence on the fiber diameter. Such a two-level coil spinneret is able to electrospin polymer solutions of different concentrations. The finite element analysis results indicate that secondary coil enhances electric field in the fiber generating area and provides additional surface for jet generation. These novel understandings may be useful for developing highly efficient electrospinning spinnerets for mass production of nanofibers.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Figures S1−S7, Table S1 reporting Supporting Information about schematically drawing of T-coils, SEM images of prepared nanofibers, effect of secondary coil pitch on electrospinning performances, electric field intensity from the coil spinneret to the collector, electrospinning performance comparison of different needleless spinnerets.

AUTHOR INFORMATION Corresponding Author *Tel: +61-3-52271245. E-mail: [email protected]. Author contributions 17 ACS Paragon Plus Environment

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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Funding Funding support from Australian Research Council (ARC) through a Discovery Project Scheme (DP180101161) is acknowledged.

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was performed in part at the Victorian node of the Australian National Fabrication Facility, a company established under the National Collaborative Research Infrastructure Strategy to provide nano and micro-fabrication facilities for Australia’s researchers.

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Enhancement of Coil Electrospinning Using Two-Level Coil Structure

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