Fabrication of Helical Nanofibers via Co-Electrospinning - American

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Fabrication of Helical Nanofibers via Co-Electrospinning Huihui Wu, Yuansheng Zheng, and Yongchun Zeng Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie504305s • Publication Date (Web): 09 Jan 2015 Downloaded from http://pubs.acs.org on January 12, 2015

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Industrial & Engineering Chemistry Research

Fabrication of Helical Nanofibers via Co-Electrospinning

Huihui Wu1, Yuansheng Zheng1 and Yongchun Zeng*1,2

H.Wu, Y. Zheng, Y. Zeng 1. College of Textiles, Donghua University, Songjiang, Shanghai, P. R. China, 201620 Email: [email protected]; [email protected]

Y. Zeng* 2. Key Laboratory of Textile Science & Technology, Donghua University, Ministry of Education, Shanghai, P.R. China, 201620 *E-mail: [email protected]

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ABSTRACT: Co-electrospinning is a new branch of nanotechnology for producing composite nanofibers with collective functions and special fiber structures. Helical fibers in nanoscale have been of increasing interest because of their unique characteristics. In this work, we report the fabrication of the helical nanofibers with polyurethane and poly (m-phenylene isophthalamide) by the co-electrospinning system with an off-centered core-shell spinneret. High-speed photography and the three-dimensional (3D) electric field simulation are carried out to help understand the formation of the helical structures. The asymmetrical electric field distribution may be a factor to affect the helical fiber formation. We also show that a series of factors such as the applied voltage, the conductivity of the system, and the composite ratio have considerable effects on the morphologies of the produced helical nanofibers. This work can provide a promising technique for producing nanofibrous nonwovens with helical fiber morphology. Keywords: co-electrospinning; nanostructured composites; helical fibers 1. INTRODUCTION Composite nanomaterials with enhanced functions after recombining have attracted extensive attention in the fields of nanoscale sensors, filtration materials, oil sorbents, solar cells, and so on1,4. So far, a lot of methods have been utilized to fabricate composite nanomaterials, such as chemical vapor deposition5, sol–gel6, hydrothermal7, and co-electrospinning8. Compared with the other methods, co-electrospinning is a simple and efficient method for generating composite fibers with diameter at the micro- and nanoscales. Generally, there are two types of co-electrospinning: coaxial and side-by-side systems. Due to its specially designed multi-channel spinneret consisting of two different polymer solutions, co-electrospinning can produce core-shell structure and hollow fibers. Moreover, the involved two phases introduce an interface interaction and the different physical behaviors, which results in additional structures such as porous nanofibers and nanoscale helical fibers. Lin et al.9 reported a polystyrene/polyurethane fiber as a sorbent for oil soak-up via coaxial electrospinning. They showed that the resultant composite PS-TPU fibers randomly orientated in a form of nonwoven mats with nanoporous structure. Zhu et al.10 synthesized p-CuO/n-TiO2 composite nanofibers using side-by-side electrospinning combined with sol-gel process. They found that the configuration of spinneret had obvious effect on the preparation efficiency of the composite fibers. Helical fibers resembling plant tendrils in nanoscale have been of increasing interest because of their unique characteristics. The introduction of helical fibers into the electrospun nanofibrous nonwovens finds potential applications in fields such as nanoscale sensors, filtration materials, and oil sorbents. The three-dimensional (3D) structure of the helical fibers can provide the nonwovens more voids and larger porosity. And the nonwoven mechanical properties in terms of resiliency and flexibility are expected to be improved. Lin et al.11 combined polyurethane (PU) with polyacrylonitrile 2


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(PAN) to produce helical nanofibers by side-by-side electrospinning system. Chen et al.12 provided an effective method of making nanosprings (i.e. nanoscale helical fibers) via combining a flexible thermoplastic elastomer polyurethane (TPU) component and a rigid thermoplastic component, using coaxial, off-centered, and side-by side co-electrospinning systems. Zhang et al.13 also reported that composite polyethylene terephthalate (HSPET) and polytrimethylene terephthalate (PTT) helical nanofibers were generated via side-by-side electrospinning system. The concept the above researchers introduced is that the parallel arrangement of an elastomeric and a stiff component may have the potential to display helical structure of the composite fibers based on the interface interaction of the two phases due to the shrinkage difference. A different concept is based on the individual physical behavior (such as conductivity) of the two phases. Kessick and Tepper14 reported the formation of helical structures from a composite of one conducting polymer poly (aniline sulfonicacid) and one nonconducting polymer poly (ethylene oxide) by conventional electrospinning from a two-component solution. Although the authors suggested that the helical structures were formed due to viscoelastic contraction upon partial charge neutralization of the charged fibers, the resultant helical diameters being in the range of 5- 20 µm indicated that the helical fiber formation can be explained solely on basis of buckling15. More recently, Sun et al.16 fabricated aligned microscale fibers with curled architectures by a reciprocating-type electrospinning setup. And they believed that the formation mechanism of curled structures can be ascribed to electrically driven bending instability and/or mechanical jet buckling when hitting the collector surface. In this work, we report an efficient and simple approach to prepare the composite helical nanofibers with an elastomeric component (polyurethane) and a rigid component (poly (m-phenylene isophthalamide)) by co-electrospinning technique. The system used in this work is a kind of off-centered core-shell electrospinning. The effects of applied voltage, conductivity of the system, and the relative amount of the elastomeric and stiff components on the characteristics of the helical structures are investigated. To study the formation of the structure, high-speed photography is utilized to capture the composite jet path during the electrospinning process. The 3D electric fields are simulated to understand the effect of electric field distribution on fiber structure formation. 2. EXPERIMENTS AND SIMULATION 2.1. Material Preparation. Poly (m-phenylene isophthalamide) (Nomex) chopped fibers made in Korea was kindly obtained from Shanghai Xiangrun Trading Co., Ltd., China. Polyurethane (TPU) (Desmopan DP 2590A) was from Bayer Materials Science, Germany. N,N,-dimethylacetamide (DMAc) (0.938~0.942g/ml at 20°C), N,N-dimethylformamide (DMF) (0.945~0.950g/ml at 20°C), Tetrahydrofuran (THF) (0.887~0.889 g/ml at 20°C), and Lithium chloride anhydrous (LiCl) (Mw = 42.39g/mol) were purchased from 3



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Shanghai Chemical Reagents Co., Ltd, China. All of these materials were used without further purification. Homogeneous Nomex solution was prepared by dissolving Nomex chopped fibers in the mixture of DMAc with LiCl, stirring for 24 h at 100°C; TPU solution was prepared by dissolving the TPU pellets in mixture solvents of DMF/THF (3/1 volume ratio), stirring for 5 h at the ambient temperature. The properties of the different electrospinng solutions are shown in Table1. All experiments were performed at about 25 °C in air at 40 % ~ 60% RH. Table1 The properties of the different electrospinning solutions

TPU

Nomex

Concentration

Amount of LiCl

Surface tension

Viscosity

Conductivity

(wt.-% )

(wt.-% )

(N m-1)

(Pa.s)

(µS cm-1)

15

——

30.61

1.031

——

18

——

34.43

1.456

——

12

1.5

38.57

1.172

1.64

1.8

38.97

1.582

3.05

2

39.23

2.263

3.35

3

39.79

3.027

3.62

3

38.81

2.702

3.34

4

40.05

3.198

3.53

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2.2. Experimental Setup. The schematic of the off-centered co-electrospinning system is shown in Figure 1. The off-centered core-shell spinneret was made by changing the core needle of a coaxial spinneret from center to one side, as shown in Figure 1. The core needle was a blunt-type stainless steel needle with inner and outer diameters of 0.25 and 0.5 mm, respectively. The shell needle was also stainless steel with inner diameter of 0.8 mm. The solutions for the core and shell materials were separately fed into the spinneret via corresponding syringes and pumps (KDS 220, KD Scientific, Inc. USA). A high-voltage supply (ES-60P 10W/DDPM, Gamma High Voltage Research, USA) was applied to the spinneret and the collector, which was a rotating cylinder with a linear velocity of 14.24 cm/s.

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Figure 1. Schematic of the off-centered co-electrospinning system 2.3. Characterization.

The morphology of the resultant core-shell fibers were observed under a Scanning Electron Microscope (SEM) (JSM-5600LV, Japan) after gold coating (coating time is 60 s). A Transmission Electron Microscope (TEM) (JEM-2100, JEOL, Japan) was used to study the internal morphology of the core-shell fibers. The average fiber diameter was calculated from the SEM images using Photoshop Cs 6 (Adobe System Inc., San Jose, CA, USA) software from a collection of 500 fibers. 2.4. Jet path record.

ARedlake HG-100K high-speed camera (Redlake Inc., San Diego, USA) equipped with a Nikon 24-85 mm, f 2.8 zoom lens was employed to record the jet motion during the co-electrospinning process. This camera has the capability of recording images at a frame rate up to 10 0000 frames per second (f/s). The light source was two 2500 W lamps. A Nikon digital camera (Nikon Inc., Japan) equipped with a Nikon 24-85 mm, f 2.8 zoom lens was also employed in the experiment. 2.5. Electric Field Simulation. The 3D electric fields were analyzed by Ansoft Maxwell (ANSYS Inc., USA) software using the finite element method (FEM). The electric field intensities were calculated by Ansoft Maxwell software. Before the calculation, the physical geometries of the co-electrospinning setups (e.g., electrode, spinneret and collector) and polymer solutions were established according to their practical dimensions, locations, and relative permittivities. 5



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3. RESULTS AND DISCUSSION 3.1. Jet path observation. Figure 2a shows the composite droplet formed from the off-centered core-shell spinneret under the conditions of 15-cm working distance and 20-kV applied voltage. The TPU-shell solution was tinted with a commercial dye (0.01 wt %) to improve the imaging contrast. The droplet at the spinneret exit is shown to be a composite with off-centered core and shell solutions. Figure 2b shows that a composite jet emanates from the tip of the droplet. At the beginning stage of the spinning process, the composite jet is drifted from center to one side after it has been issued from the tip of the droplet. From Table 1, we can see that compared to the Nomex solution, the conductivity of the TPU solution is almost null. Due to the different conductivities of the two solutions, almost all the free charges are located at the part of jet surface which contains Nomex solution17. This leads to different electric forces applied on the two solutions. Consequently, the composite jet is deflected sideways. Meanwhile, the composite jet begins to create coils. Unlike the spiraling loops with growing diameters caused by whipping instability in electrospinning, these coils are almost of the same dimension. We believe that the formation of these coils results from the interface interaction of the TPU phase and the Nomex phase due to their non-uniform shrinking behavior. In our previous study18, the jets formed from a coaxial spinneret and a single-needle spinneret of the electrospinning systems have been shown. The deflection of the jet at the beginning stage of the spinning process is not observed in coaxial and single-needle electrospinning. The jet path in the stable stage of the spinning process is shown in Figure 2c. It can be seen that the composite jet goes through the off-axis section followed by whipping instability, which is similar to the jet motion in a single-fluid electrospinning process.

Figure 2. Images of (a) composite droplet formed from the off-centered core-shell spinneret, (b) jet path created at the beginning of the spinning, and (c) stable spinning. (a) was captured with the digital camera; (b) and (c) were captured with high-speed photography at a rate of 10000 fps. 3.2. Electric Field Distribution. 6


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The 3D electric field for the off-centered co-electrospinning system was simulated. For comparison, the electric field simulations for the coaxial and the single-needle electrospinning systems were also carried out. The simulations were under the conditions of 15-cm working distance and 20-kV applied voltage, which were used in the experiments for a stable spinning process. Figure 3 shows the electric field distributions in the x-y plane (i.e. the horizontal plane) at the position close to the spinneret (z = 1 mm) for the three systems. It is obvious that compared to the coaxial and the single-needle configurations, the electric field created by the off-centered configuration is asymmetrical. It is worth noting that the electric field intensifies at the needle edges. This effect that sharp edges generate a strong electric field has been observed by Thoppey et al.19. These analyses can be quantified by the calculated data shown in Figure 4. In the figure, the electric field intensities along the y-axis at the position of z = 1mm for the three systems are compared. Ignoring the edge effect, the single-needle spinneret with TPU solution creates the highest electric field intensity among the three systems. The off-centered and the coaxial configurations create almost the same electric field intensity at the core needles with Nomex solution. For the off-centered configuration, the electric field intensity of the core-needle with Nomex solution is about 1×106 V/m higher than that of the shell-needle with TPU solution. This asymmetrical electric field may be another factor that leads to the deflective jet path shown in Figure 2b.

Figure 3. Comparison of the electric field distributions for (a) off-centered co-electrospinning system, (b) coaxial system, and (c) single-needle system; The upper figures illustrate the electric field distribution at the spinnerets, and the lower figures illustrate the corresponding electric field in the xy-plane at z = 10mm; The working distance is 15 cm and the applied voltage is 20 kV.

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Figure 4. Electric field intensity along x-axis (or y-axis) at z = 1 mm for (a) off-centered co-electrospinning system, (b) coaxial system, and (c) single-needle system. The working distance was 15 cm and the applied voltage was 20 kV. 3.3. Morphology of composite TPU/Nomex fibers. The SEM image of the electrospun nonwoven sheet of TPU/Nomex fibers is shown in Figure 5a, indicating that fibers with helical structures are generated with the off-centered co-electrospinning system. The fibers were generated by electrospinning 12 wt% Nomex in DMAc with 2 wt% LiCl as core and 18 wt% TPU in DMF /THF as shell. The processing conditions used were 15-cm working distance and 20-kV applied voltage. The fiber diameters varied from about 100 to 500 nm. The nanoscale helical fiber has a three-dimensional spiral shape, which is different from the curved structures with larger dimensions of the buckling fibers. Buckling fibers have been widely observed15, and buckling is believed to result primarily from the impinging of the fibers on the collector. The inserted TEM picture in Figure 5a confirms the formation of an off-centered core-shell structure of the helical fibers. Helical and spiral conformations can be found in biological systems such as plant tendril, which is 8


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shown in Figure 5b. We can see that the electrospun helical nanofibers resemble the tendrils in shapes. In tendrils, the coiling appears as a result of the intrinsic curvature, which occurs via asymmetric contraction of the fiber ribbon. The non-uniform shrinkage of the upper and lower sides gives its intrinsic curvature20. Godinbo et al.21 believe that the mechanism for the electrospun fiber curvature should also be provided by such an asymmetric deformation of some nature. In our case, a non-uniform shrinkage of the elastomeric component (TPU) and the stiff component (Nomex) quite obviously contributes to the formation of nanoscale helical fibers. The intrinsic curvature of the fibers, which is responsible for the formation of the nanoscale helical structure, occurs as soon as the composite jet formed from the droplet, which has been observed in Figure 2.

Figure 5. (a) The SEM image of helical TPU/Nomex fibers, (b) plant tendril, (c) a helix with helix radius and helix pitch. These helical structures of the nanofibers are expected to contribute to more voids and larger porosity of the electrospun nanofibrous nonwoven, and better resiliency and flexibility of the nonwoven mechanical properties. To analyze the helical structure qualitatively, we use the curvature of the helix to determine the degree of the helical structures in a nonwoven mat. A larger helix curvature indicates a more tight helical structure. The curvature of the helix is defined as 20

k=

r  p r2 +    2π

(1) 2

where r is the helix radius, p is the helix pitch, as showed in Figure 5(c). These parameters were evaluated from SEM images of the helical nanofibers. The average helix curvature k was calculated from 20 helical fibers and the standard deviation was obtained. 3.4. Effect of parameters on the formation of helical structures. The mechanism of the helical structure formation in electrospun nanofibers may be very complex. We believe that the electric force generated by the electric field and the elastic force generated by the flexible component (TPU) applied on the jet during the spinning process are involved in the structure 9



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formation. Therefore, a series of factors such as the applied voltage, the conductivity of the system, and the relative amount of the elastomeric and the stiff components may influence the curvature formation. Figure 6 shows the SEM images of the fibers electrospun from 12 wt% Nomex with 2 wt% LiCl as core and 15 wt% TPU as shell under various applied voltages. The flow rates for the core solution (Nomex) and the shell solution (TPU) were 0.2 ml/h and 0.1 ml/h, respectively, indicating the composite ratio of elastomeric and stiff components was 1:2. We can see that under the applied voltages of 20 kV and 25 kV, considerable fibers are present in the form of helical structures with an average fiber diameter of around 240~250 nm, indicating the nanoscale helical fibers are yielded. However, much fewer helical fibers are observed under the 15-kV applied voltage. The calculated average helix curvature (k) under 15-kV, 20-kV and 25-kV applied voltages are 1.059×10-2, 3.247×10-2, and 4.202×10-2, respectively. And the curvature variances under the three applied voltages are 0.897, 2.205, and 4.202, respectively. It can be seen that k increases with increasing voltage, indicating that higher voltage creates more tight helical structures. Under the applied voltage of 20 kV, the system can produce helical nanofibers with more uniform distribution of k. Therefore, the applied voltage of 20 kV was used in the following experiments.

Figure 6. SEM images of the fibers electrospun from 12 wt% Nomex with 2 wt% LiCl as core and 15 wt% TPU as shell To analyze the role of the conductivity of the system, off-centered core-shell electrospinning from 12 wt% Nomex with 1.5 wt%, 1.8 wt% and 2 wt% LiCl as core solutions and 18 wt% TPU as shell solution were carried out. Three flow rate ratios of core (Nomex) to shell (TPU) of 2:1, 1:1 and 1:2 were used in the experiments. Figure 7 shows the SEM images of the resultant fibers. The fibers range in diameter around 150~400 nm with helical structures. It is worth noting that there are droplets shown on the nonwoven mats in Figure 6. These small droplets attached to the fibers may be due to the low concentration of the system. In Figure 7, by increasing the TPU concentration from 15 wt% to 18 wt%, the droplets disappear.

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Figure 7. SEM images of the helical TPU/Nomex fibers from 18 wt% TPU as shell and 12 wt% Nomex as core with 1.5%, 1.8%, 2.0% LiCl, respectively, under different core/shell flow rate ratios of 2:1, 1:1, and 1:2. The working distance is 15 cm and the applied voltage is 20 kV. Figure 8 shows the average helix curvature (k) of the helical nanofibers with different flow rate ratios of core to shell solutions. We can see that k increases when the concentration of LiCl added to the Nomex solution increases from 1.5 wt% to 1.8 wt% under the same flow rate ratio of core to shell solution. However, further increasing of the LiCl concentration (2.0 wt%) generates the smallest k among the three cases. Under the 2:1 flow rate ratio of core to shell solutions, the case of 2.0 wt% LiCl concentration leads to a k as small as null, indicating that 2.0 wt% LiCl added to the Nomex solution hardly generates helical structure. The formation of the helical structures may result from the right balance of the electric force and the elastic force exerted on the jet. From Table 1, we can see that with increasing concentration of LiCl, the conductivity of the Nomex solution increases. At the initial stage of the spinning process, larger conductivity leads to larger electric force applied on the side of Nomex component, and consequently to more significant deflection of the composite jet. The deflection of the jet to the Nomex part may increase the elastic force exerted by the TPU part. On the other hand, larger electric force leads a faster drawing of the jet. From this point of view, increasing conductivity of the system results in a smaller time interval for the elastic force applying on the jet, and becomes a negative factor for the curvature formation. The experimental results in Figure 8 also show that the composite ratio of Nomex to TPU solutions 11



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plays a role in affecting the formation of the helical nanofibers. We believe that the effect contributes from the elastic force applied on the jet. A larger flow rate of the elastomeric component (TPU) results in a larger elastic force, and therefore creates a larger curvature of the helix.

Figure 8. Helix curvature of the electrospun helical fibers from 18 wt% TPU as shell and 12 wt% Nomex as core with 1.5%, 1.8%, 2.0%LiCl, respectively, under different core/shell flow rate ratios of 2:1, 1:1, and 1:2. The working distance is 15 cm and the applied voltage is 20 kV. We further increased the concentration of Nomex solution from 12 wt% to 14 wt% and the amount of LiCl to 3 wt% for electrospinning. The SEM images of the resultant fibers are shown in Figure 9. It is interesting that the electrospun fibers appear orientation and with much fewer helical fibers. It is believed that high conductivity of the system leads to stronger electric force exerted on the jet. The jet undergoes more rapid stretching and the fibers are orientated. This result further verifies that the formation of the helical structures depends on the right balance of electric force and elastic force applied on the jet.

Figure 9. The SEM images of the TPU/Nomex fibers from 18 wt% TPU as shell and 14 wt% Nomex with 3.0% LiCl as core under different core/shell flow rate ratios of 2:1, 1:1, and 1:2. The working distance is 15 cm and the applied voltage is 20 KV. 12


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4. CONCLUSIONS Nanoscale helical fibers have been generated via electrospinning Nomex as core solution and TPU as shell solution with an off-centered core-shell co-electrospinning system. An attempt has been made to study the formation of the helical nanofibers by high-speed photography and electric field simulation. The high-speed photographs of the composite jet path show that the intrinsic curvature of the fibers, which is responsible for the formation of the nanoscale helical structure, occurs as soon as the composite jet formed. The electric field distributions of the off-centered co-electrospinning, the coaxial electrospinning, and the single-needle electrospinning were simulated. Compared with other configurations, the off-centered configuration creates an asymmetrical electric field, indicating the composite jet may suffer non-uniform electric force. The effects of the applied voltage, the conductivity of the system, and the relative amount of the elastomeric and stiff components on the morphology of the helical nanofibers were investigated. The results show that these parameters play roles on the formation of the helical nanofibers by affecting the electric force and the elastic force applied on the jet during the spinning process. These results may be helpful for the fabrication of stretchable nanomaterials which have potential applications in the fields such as nanoscale sensors, filtration materials, and oil sorbents. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]. Fax: +86 21 67792627. Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work was financially supported by the National Natural Science Foundation of China (11272088), the National Natural Science Foundation of China (U1432115), the National Natural Science Foundation of China (51303200), the Keygrant Project of Chinese Ministry of Education (113027A), and the China Scholarship Council.

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Fabrication of Helical Nanofibers via Co-Electrospinning - American

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