Batch preparation of nanofibers containing nanoparticles by an electrospinning device with multiple air inlets

EMAI process and batch preparation of nanofibers

Figure 2 shows that, in the EMAI process, most of the jets were ejected from the edge of the spinneret, the edge of the groove, and the air inlets. This was because the edge part would produce higher electric field intensity due to the tip effect, while the air inlets would produce more jets due to the auxiliary effect of air flow. When the air flow rate was 150 m3/h (Figure 2a), because of the excessive air flow, a large number of bubbles were generated on the solution surface, and many droplets appeared on the receiving drum, which greatly affected the spinning effect. When the air flow rate was 100 m3/h (Figure 2b), there were still many bubbles on the solution surface and some droplets on the receiving drum, illustrating that the air flow was still too large. When the air flow rate was 50 m3/h (Figure 2c), the whole solution surface became convex, and no obvious droplets appeared between the nanofibers on the receiving drum. When there was no air flow (0 m3/h) (Figure 2d), the number of jets obviously decreased and almost all of them were concentrated at the edge of the spinneret. Therefore, the air flow rate of 50 m3/h was more conducive to spinning.

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Figure 2: EMAI spinning processes at different air flow rates (150 m3/h (a), 100 m3/h (b), 50 m3/h (c), and 0 m3/h (d)) and voltages (40 kV (e), 50 kV (f), and 60 kV (g)).

Based on the above results, the spinning effects of this device under different applied voltages at the optimal air flow of 50 m3/h were studied. It was found that, when the applied voltage was 40 kV (Figure 2e), less jet formation and low spinning efficiency were achieved. When the voltage was 60 kV (Figure 2f), more but unstable jets were formed due to too high electric field intensity, which would result in coarser fibers and fiber bundles. Therefore, the optimal applied voltage value was 50 kV (Figure 2g), due to the stable jets generated under this voltage and the obtained fibers with smaller diameter and more uniform particle distribution.

Figure 1a shows the force analysis of point A on the free surface of spinning solution and point B on the jet formed during the EMAI spinning process when ignoring air resistance and environmental interference. The spinning solution was assumed to be an incompressible ideal fluid [22]. According to previous studies [23], under a high voltage electric field, in point A there is the joint action of mass force (Ph) caused by the fluctuation height of spinning solution, surface tension (Ps) of the spinning solution, and electric field force (PE) produced by the applied voltage. These forces determine whether a jet could be formed at point A. The polymer fluid surface tension would make the liquid surface shrink and bend as much as possible, and the change of external electric field would interact with the accumulated charges on the polymer fluid surface, making the charge density on the charged fluid surface uneven. Accordingly, the formulas to calculate these three forces are as follows:

[2190-4286-14-15-i1](1) [2190-4286-14-15-i2](2) [2190-4286-14-15-i3](3)

where ρ is the density of spinning solution (kg/m3), g is the gravitational acceleration (m/s2), h is the fluctuation height of the polymer spinning solution (m), γ is the surface tension coefficient of the spinning solution (N/m), ε0 is vacuum dielectric constant, E0 is the edge electric field intensity (V/m), Ep is the electric field intensity of the thin liquid surface (V/m), εα is the dielectric constant of the polymer, and k is the amount of radial fluctuations on the spinning solution surface.

In addition, the centripetal force F1 at point B is generated by the horizontal component of the viscous force (τ), which could weaken the instability of the jets. The resultant vertical upward force F2 generated by the applied electric field force FE and the vertical component of viscous force would push the jets upward. The formulas of the resultant forces (F1 and F2) could be written as follows [24]:

[2190-4286-14-15-i4](4) [2190-4286-14-15-i5](5)

where α is the angle between the viscous force and the vertical direction, EB is the electric field intensity at point B, and qB is the charge determined by the spinning solution properties at point B, which is generally an integer multiple of the primary charge (1.60 × 10−19 C).

On the basis of the spinning effects under different voltages (Figure 2e–g) and force analysis, it could be found that the distribution of the electric field intensity plays a very important role in the EMAI process for producing more and stable jets on the whole spinning surface. In the following, the nanofibers fabricated by EMAI are characterized and the electric field distribution in the EMAI process is simulated to illustrate the influences of electric field distribution on the spinning process and nanofiber quality.

Characterization of nanofibers

Figure 3a–d shows the morphology and the corresponding diameter distribution of nanofibers obtained at different air flow rates. When the air flow rate was 150 m3/h (Figure 3a), due to the excessive air flow, the fibers easily adhered to each other, making the average diameter of the fibers larger (746.86 ± 129.12 nm) and leading to serious agglomeration of nanoparticles in the fibers. When the air flow rate was 100 m3/h (Figure 3b), the adhesion between fibers was weakened, resulting in the decrease of average fiber diameter (719.28 ± 108.43 nm) and a reduction of nanoparticle agglomeration in the fibers. When the air flow rate was 50 m3/h (Figure 3c), there was almost no adhesion between fibers. The average diameter of fibers was smaller (596 ± 127.02 nm), and the nanoparticles in the fibers were not agglomerated and evenly distributed. When there was no air flow (Figure 3d), the particles in the fibers were not only very few, but also agglomerated. This might be due to the sinking down of the nanoparticles in the spinning solution without the assistance of air flow. Therefore, the morphology of the fibers obtained using an air flow rate of 50 m3/h was the best, in which the nanoparticles were evenly distributed without agglomeration. This result is consistent with the analysis result in Figure 2a–d.

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Figure 3: Morphology and corresponding diameter distribution of nanofibers obtained by EMAI at different air flow rates (150 m3/h (a), 100 m3/h (b), 50 m3/h (c), and 0 m3/h (d)).

In addition, the morphology and corresponding diameter distribution of nanofibers produced at different applied voltages with the optimal air flow rate of 50 m3/h were investigated. As shown in Figure 4a, when the spinning voltage was 40 kV, the electrospun nanofibers had fewer particles, and the average nanofiber diameter was 491.41 ± 93.75 nm. When the spinning voltage was 50 kV (Figure 4b), the nanoparticles in the nanofibers were not only abundant but also evenly distributed, and the average nanofiber diameter was the smallest (497.63 ± 87.02 nm). When the spinning voltage was 60 kV (Figure 4c), the nanofibers were mostly in the form of fiber bundles, the particle agglomeration was serious, and the average nanofiber diameter was the coarsest (563.08 ± 104.89 nm). This was because the higher the voltage, the greater the electric field force applied to the jet, and the more the jet was stretched, making the fiber diameter smaller. However, when the voltage was too high, the electric field force was too large, so that the spinning speed was too fast, and the jets were unstable and could not be fully stretched, resulting in an increase in fiber diameter and the easy formation of fiber bundles. This result is also consistent with that in Figure 2e–g. Furthermore, it could be clearly seen from Figure 4d that the agglomeration of nanoparticles in the fibers prepared by the SSFSE at 50 kV was very obvious (see the red circle), proving the advantages of EMAI.

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Figure 4: Morphology and corresponding diameter distribution of nanofibers obtained by EMAI at different voltages (40 kV (a), 50 kV (b), and 60 kV (c)) and by SSFSE (d).

Figure 5a displayed the yields of ZnO/PAN nanofibers obtained with different spinning voltages at an air flow rate of 50 m3/h, illustrating that the higher the voltage, the higher the nanofiber yield, which further proved the force analysis in Figure 1a and the analysis results in Figure 2e–g. The EDS analysis result of nanofibers exhibited in Figure 4b showed that the sample contained four elements C, N, O and Zn, indicating that ZnO was successfully loaded on the PAN nanofibers. Zn was uniformly distributed, as shown in Figure 5b.

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Figure 5: The yields of nanofibers obtained with different spinning voltages at the air flow rates of 50 m3/h (a) and EDS spectrum of nanofibers (b).

Electric field simulation

Figure 6 shows the cloud diagrams (Figure 6a1, Figure 6b1, and Figure 6c1) and the corresponding vector diagrams (Figure 6a2, Figure 6b2, and Figure 6c2) of the electric field intensity near the porous spinneret under different voltages. It can be seen that the electric field intensity value at the uppermost edge of the porous spinneret was the largest. In the vertical and horizontal directions of the spinneret, the electric field intensity gets smaller farther away from the spinneret. According to the force analysis in point A in Figure 1a, when the electric field force (PE) acting on the spinning solution surface was greater than the solution surface tension (Ps), unstable fluctuation on the free surface of solution easily occurs, thus significantly increasing the probability of multijet formation [25,26].

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Figure 6: Electric field distribution at the porous spinneret at different voltages (40 kV (a), 50 kV (b), and 60 kV (c)).

In order to study further the electric field forces on the spinning solution surface under different voltages, the electric field intensity distributions on the surface were analyzed, as shown in Figure 7, which further proved the above conclusions obtained from Figure 6. The radial and axial electric field distributions of the porous spinneret under different voltages are illustrated in Figure 8. Figure 8a shows that in the radial direction, the electric field intensity dropped at 5 mm, which was caused by a round hole with a radius of 1 mm. At 20 mm, the electric field intensity increased greatly due to the boundary between the middle circular platform and the groove, where one side was a copper conductor and the other side was the solution, forming a large potential difference resulting in the increase of electric field intensity [27]. Similarly, the electric field intensity dropped sharply and afterwards increased sharply at the junction of the groove and the spinneret edge (30 mm). At the edge of the porous spinneret (30–35 mm), the electric field intensity was further improved greatly due to the tip effect [28], and the electric field intensity at the outermost edge of the spinneret reached the maximum value. This was consistent with the actual spinning situation, which also explained that most of the jets were produced on the edges and in the vicinity of the edges, as shown in Figure 2. Simultaneously, it could be seen from Figure 8b that in the axial direction the electric field intensity decreased continuously with the increase of spinning distance. The above simulation results explain the spinning effects of EMAI under different voltages, indicating that as the spinning voltage is increased, the number of jets generated on the spinning solution surface increases due to the increase of electric field intensity.

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Figure 7: Distribution of electric field intensity at the top of spinneret under different voltages (40 kV (a), 50 kV (b), and 60 kV (c)).

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Figure 8: Radial (a) and axial (b) electric field distributions of the porous spinneret at different voltages.

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