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Discharge behaviors and jet profiles during electrospinning of Poly(vinyl alcohol).

INTRODUCTION

Electrospinning is a process to produce nonwoven fabrics of nanofibers from a polymer solution or a melt by applying a high voltage between a tip of needle and a grounded collector. This process enables us to fabricate ultra thin fibers more easily than the conventional fiber spinning processes based on solution and melt spinnings. The electrospun fibers have quite smaller diameters and larger surface areas than those obtained by the conventional fiber spinning processes. These unique characteristics bring us possibilities of a number of applications (1), (2), for example, filtration for nanosized materials, tissue scaffolding (3), separator (4), and other biomedical applications (5). Influences of various conditions on electrospun fibers have been investigated experimentally and theoretically by many researches as described in a review article (1). It has been known that important parameters in electrospinning are not only sample parameters such as molecular weight (6-8), concentration (9-11), conductivity (10), (12), surface tension (10), (13), topology of polymer chain (11), and molecular weight distribution (8) but also system parameters such as distance between the tip of needle and the collector (9), (14), applied voltage (9), (15), (16), ambient conditions (6), (9), (16), (17), diameter of needle (18), and flow rate of extruded sample from the tip of needle (9), (12). One of the most serious problems in applications of electrospinning is the formation of beads on the fibers. Generally, the formation of beads depresses functions of products when fabricating nonwoven. Ma et al. (19), however, have reported that a nonwoven which contains beaded fibers has higher hydrophobic property than that made of beads-less fibers with a same diameter. Therefore, it is necessary to control the beads formation and to clarify the beads formation mechanism.

It is reported that the beaded fibers are produced when developing the Rayleigh instability (20). Yu et al. (21) reported that high elasticity of polymer solutions suppresses the Rayleigh instability on jet. They defined Deborah number as the ratio of polymer fluid relaxation time to Rayleigh instability growth time to evaluate the beads formation parameters. According to their reports, high Deborah number makes bead-less fibers. In their works, however, it is not clear about the variation of elongational viscosity of jet along the spinning line by changing the sample parameters. The elongational viscosity of jet depends on the relaxation time of polymer fluids as reported by Feng (22), (23). It is interesting to clarify the relation between elongational viscosity of jet in electrospinning and the relaxation time of polymer fluids. Thus, our first focus is to clarify the variation of elongational viscosity of jet along the spinning line by changing Deborah number as defined by Yu et al.

When a high voltage is applied to the system, an electric discharge usually takes place during a process of electrospinning. According to the previous work done by Baumgarten (9), the electrospinning process could not be realized when the strong spark discharge was observed. It is also reported that the beads formation depends on the applied voltages (10), (15), (16), (24). From the view point of discharge, it is considered that the beads formation correlates with increasing the electric current (16). The evidence of electric discharge in electrospinning process has never been clarified so far, however. It is important to find the discharge behavior in an electrospinning process. Therefore, our second focus is to clarify the electric discharge behavior in electrospinning process.

EXPERIMENTAL

Preparation and Characterization of Sample

The sample used in the present work is Poly(vinyl alcohol) (PVA) aqueous solutions. Four types of PVA (weight average molecular weights; [M.sub.w] = 28,000, 42,000, 160,000, and 200,000 g/mol) are purchased from Sigma Aldrich (Japan). Three series of PVA solutions are prepared by dissolving PVA in distilled water in various concentrations as shown in Table 1. Series A is different in solution concentration with same molecular weight. Series B has various molecular weight and their concentrations are adjusted so as to show same zero-shear viscosity. Another series C is bimodal blends in which low ([M.sub.w] = 28,000 g/mol) and high ([M.sub.w] = 140,000 g/mol) molecular weight samples are blended in various proportions of 9/1, 8/2, 7/3, and 6/4, respectively. The zero-shear viscosities for these solutions are measured by a rheometer (ARES; TA Inst.) at room temperature with using a couette fixture. [M.sub.w] and the number average molecular weight ([M.sub.n]) are measured by a gel permeation chromatography (GPC SYSTEM 21H; SHOWADENKO K.K.). Deborah number (De) used in this study is defined by the ratio of fluid relaxation time to Rayleigh instability growth time as reported by Yu et al. (21) as following equation:
TABLE 1. Characterization of PVA aqueous solutions.

Sample     [M.sub.w]  Concentration  [M.sub.w]/  [[eta].sub.o]   De
           (g/mol)    (wt%)          [M.sub.n]     (Pa S)

A1            42,000       15           3.2           0.14       0.22
A2            42,000       20           3.2           0.57       1.2
A3            42,000       25           3.2           1.9        4.5
A4            42,000       30           3.2           7.1       15
Bl            28,000       31           3.2           1.6        3.9
B2            42,000       25           3.2           1.9        4.5
B3           140,000       13           3.0           2.1        4.9
B4           200,000        7           3.3           1.7        7.5
C1 (10/0)     28,000       20           3.2           0.16       0.19
C2 (9/l)      40,000       18.7         4.5           0.16       0.31
C3 (8/2)      45,000       17.4         5.1           0.16       0.22
C4 (7/3)      57,000       16.1         5.7           0.15       0.18
C5 (6/4)      65,000       14.8         5.9           0.14       0.24
C6 (0/10)    140,000          7         3.0           0.1        0.17


De = [[lambda].sub.p][[omega].sub.max]/t* (1)

where [[lambda].sub.p] is the relaxation time of fluid, [[omega].sub.max] is the dimensionless maximum growth rate of Rayleigh instability, and t* is the characteristic time. The [[lambda].sub.p] is obtained by capillary breakup elongational rheometer (CaBERl; Thermo Haake). The CaBER measures the time dependent diameter of the stretched fluid filament at the mid point after the fluid stretching. The time evolution of the diameter ([D.sub.mid](t)) of a fluid can be modeled by the following equation (25):

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2)

where [D.sub.1] is the initial mid point diameter just after stretching, G is the elastic modulus, and [sigma] is the surface tension. The characteristic time is expressed by t* = [r.sub.0.sup.2]/v where v = [[micro].sub.s]/([S.sub.p]) is characteristic viscosity, [[micro].sub.s] is the solvent viscosity, S is the retardation number (S = 1/(1 + G[[lambda].sub.p]/[[micro].sub.s])), and [rho] is the density. The characterizations of all samples are shown in Table 1.

Electrospinning Systems and Conditions

Electrospinning system used in this study is shown in Fig. 1a. The detail of electrospinning system is described in our previous article (16). The electrospinning is performed in horizontal mode to avoid dropping of the polymer solutions on the grounded collector. To analyze the jet profiles, the cone and jet shape is observed by CCD camera to which a high magnification lens (UWZ210; Union Optical) is attached. As shown in Fig. 1a, the light source is placed in front of CCD camera to record the shadow of cone shape and jet.

[FIGURE 1 OMITTED]

The observation system of discharge behavior in electrospinning process is shown in Fig. 1b. The discharge emission during electrospinning is observed by a highspeed camera (Fastcam-ultimate SE, Photoron) with image intensifier. To evaluate the discharge during electrospinning, electric currents are measured by oscilloscope (TDS5104B, Tektronix) through a voltage drop across a 100 k[ohm] resistor placed between grounded collector and ground. By measuring the voltage drop across the resistance, the electric current is calculated from the Ohm's law. The electric current measurement is synchronized with above observation system of discharge behavior. All the other experiments except discharge observation are done by constant spinning conditions as follows; applied voltage is 12 kV, distance between tip of needle and the collector is 10 cm, and relative humidity is 50%, respectively. Only for the discharge observation, the voltage is changed from 9 to 24 kV under constant humidity and tip-collector distance as well as other experiments.

Calculation Method of Elongational Viscosity of Jet

Elongational viscosity for electrospun jet drawn from the needle to the collector is calculated based on a variation of jet diameter. The jet diameter is measured as a function of position x along the spinning line by using an image analysis software (Winroof; Mitani) as shown in Fig. 2. The influence of gravity is vanishingly small because the jet mass is very small. When the jet direction is not horizontal due to the effect of electric field, to measure the jet diameter easily, the jet direction of obtained image is rotated so as to be horizontal. The jet diameter is measured every 10 pixels along the spinning line. We determine the position as x = 0 where jet diameter is 200 [micro]m along the spinning line (x axis in Fig. 3), because the jet diameter decreases significantly from ~200 [micro]m in diameter for all samples used in this study. To obtain the variation of the elongational viscosity of jet as a function of position, it is necessary to know the tensile stress ([sigma](x)) and strain rate ([epsilon]) of jet. Assuming the same tensile force at each position, the stresses of jet at each position along the spinning line are calculated from Eq. 3.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

[sigma](x) = T/A(x) (3)

where A(x) is the cross section area of jet calculated by A(x) = ([pi][[d(x)].sub.2])/4, d(x) is jet diameter at each position, and T is the tensile force obtained by T = 3[A.sub.0][[eta].sub.0][[epsilon].sub.0], [A.sub.0] is the cross section area of jet just after extruded from the needle, [[eta].sub.0] is the zero-shear viscosity of sample, and [[epsilon].sub.0] is the strain rate just after extruded from the needle. It has been known that the elongational viscosity of Newtonian fluids is equivalent to three times of the zero shear viscosity (26). It is considered that the extruded sample from the needle is Newtonian fluids because all samples behave like Newtonian fluids from results of frequency sweep test in this study. The strain rate is obtained from Eq. 4.

[epsilon](x) = dv(x)/dx (4)

Assuming the solvent docs not evaporates from the jet in observation region, v(x) is jet velocity at each position along the spinning line

v(x) = 4Q/[pi][[d(x).sub.2]]

where Q is flow rate of polymer solution. From the calculated strain rates and stresses, the elongational viscosity of jet at each position along the spinning line is led as follows:

[[eta].sub.E](x) = [sigma](x)/[epsilon](x). (6)

In the electrospinning, ejected jet from the needle is extended to some extent towards the spinning line before the onset of bending, and then undergoes bending path. The jet diameter along the spinning line is measured till onset of bending instability in this study.

Morphology of Electrospun Fibers

Morphology of electrospun fibers is observed by scanning electron microscope (SEM). The electrospun samples for SEM observations (JSM-5310, JEOL) are dried under vacuum at 30[degrees]C for 1 day.

RESULTS AND DISCUSSION

Elongational Viscosity of Jet and Beads Formation

It has been known that high concentration and high molecular weight lead to the long relaxation time. As shown in Table 1, Deborah number (De) becomes large with increasing concentration and average molecular weight. The variation of jet diameter for series A with different concentrations is shown in Fig. 3. The jet diameter decreases significantly from 200 [micro]m to ~10 [micro]m. When the jet diameter is smaller than 10 [micro]m, the slope of variation of jet diameter becomes gentle via the transition zone (27) from cone to jet. The right end of the plot shows the beginning of bending instability. As the concentration increases, the variation of jet diameter before the transition zone is gentle. In series B, the slope of jet diameter also decreases with increasing the molecular weight and the transition zone shifts to the right, though the zero-shear viscosity of sample is almost same. This result shows that the jet deformation becomes slow with increasing the relaxation time as reported by Feng (23). Figure 4 shows the variation of elongational viscosity of jet in series A. Elongational viscosity of jet increases as the jet travels along the spinning line. The elongational viscosity of jet at the onset of bending instability slightly increases with increasing the concentrations. Under an elongational flow in concentrated polymer solution system, it is reported that the elongational viscosity increases as the Hencky strain increases (25), (28). In our results, it is confirmed that the Hencky strain becomes more than 6 for all samples. At the large Hencky strain, the polymer chains are stretched significantly, resulting in strong increase of the elongational viscosity of jet. In case of sample series B, the elongational viscosity of jet at the onset of the bending instability also increases as the molecular weight becomes higher. Figure 5a and b show the morphology of electrospun fibers for PVA solutions with different concentrations (series A) and different molecular weight (series B), respectively. The beads are easily formed at low concentration and low molecular weight. It is, however, found that the beads formation is suppressed with increasing De as reported by Yu et al. (21). Over 4 of De, the bead-less fibers are obtained as described in Fig. 5a and b. The high De is due to much longer relaxation time than Rayleigh instability growth time. Thus, the beads formation depends on De. Here, it is noted that the elongational viscosity of jet at the onset of the bending instability increases as De increases (see Fig. 4). leading to the beads-less uniform fiber.

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

Figure 6 shows the variation of elongational viscosity of jet as a function of position along the spinning line for bimodal blends (series C). The elongational viscosity of jet at the onset of bending instability is increased with increasing the blend ratio of high molecular weight component. Figure 7 shows the resultant electrospun fibers at different blend ratios of low and high molecular weight, 10/0, 90/10, 80/20, 70/30, 60/40, and 0/10, respectively. In case of blend ratio of 10/0, the beads shape is spherical and fibers are not observed virtually. On the other hand, in the samples of 6/4 and 0/10, the uniform fibers without beads are obtained. Although De among the samples is almost same and less than 1 (see Table 1), the beads formation apparently depends on the blend ratios. The beads formation is suppressed with increasing the high molecular weight component. Here, let us discuss that from the elongational viscosity point of view. The beads formation tends to be suppressed as the elongational viscosity of PVA solution just before the flight of jet increases. The high viscosity makes resistance to growth of the Rayleigh instability (29). Thus, it is considered that the high elongational viscosity of jet exerts the suppression effect of beads formation. Actually, it is observed that there are large differences in the spinning behavior between 10/0 and 0/10. For 10/0, the spinning process is randomly intermitted with audible spark, which is triggered by high electric field strength. On the contrary, in case of 0/10, the jet is ejected continuously without audible spark. The electric discharge behavior during electrospinning is discussed in the next section.

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

Discharge Behavior During Electrospinning Process

To clarify the discharge behavior during electrospinning process, in this section, the electrospinning is performed by changing the applied voltages from 9 to 24 kV. The sample used in this section is A1 which has lowest concentration and De among the series A. Figure 8 shows the variation of elongational viscosity of jet at different voltages, 9, 12, 18, and 24 kV, respectively. The elongational viscosity of jet at 9 kV is highest in comparison with those at 12-24 kV, because the solution at 9 kV shows steep decrease in jet diameter. As the applied voltage increases, the elongational viscosity of jet at the onset of bending instability is decreased. Figure 9 shows the electrospun PVA fibers at different voltages. In ease of 9 kV, the beads-less fibers are obtained, though De is lower than 1. On the other hand, over the 12 kV, many spherical beads can be observed. In addition, the audible sparks due to the generation of electric discharge are observed over the 12 kV. Over the applied voltage of 12 kV, the electric currents during electrospinning show the pulse form coming from electric discharge as shown in Fig. 10. It is clear that the electric discharge during electrospinning starts to be generated more than 12 kV. When the form of electric current shows the spiky pulse-like wave, the baseline of currents shows two steps (see Fig. 10c). The baseline of electric current with the pulse-like wave in Region I is higher than that of Region II. The transition process from Region I to Region II in Fig. 10c is enlarged in Fig. 11. Each high-speed photograph A-E (Fig. 11a) corresponds to alphabet symbols in the graph (Fig. 11b). When the electric current shows the pulse-like wave, the ramified electric discharge spark from the apex of cone is observed at the point of A and B (see Fig. 11a). At the time of C, the corona discharge is observed in whole of the cone. In Region I, it is clear that the corona discharge generates always in whole of the cone and ramified electric discharge extends from apex of the cone at the moment of steep increase of the electric current. With decreasing the baseline of electric current, the corona discharge at the cone disappears. Finally, the corona discharge is not observed as the transition region from Region I to Region II has finished. It is clear that the electric discharge is not generated in Region II. Figure 12 shows the jet behavior during the electrospinning in transition region from Region I to Region II in Fig. 10d. At the time of A and B, the ejection of jet is not observed, though the cone is formed at the tip of needle (see Fig. 12a(A and B)]. It is observed that the apex of the cone is oscillated speedily at random in Region I. This oscillation of apex of the cone is similar to spindle mode as reported by Jaworek and Krupa (30). The oscillation of the apex of cone by using water drop in the electric field is reported by Sugimoto et al. (31). In their reports, the pulse group of current appears during the oscillation of cone. The jet is ejected gradually when the electric current starts to decrease from Region I to Region II [see Fig. 12a(C and D)]. Eventually, in Region II, the jet is ejected continuously. These results show obviously that the jet elongation is suppressed by corona discharge in Region I. When the corona discharge is generated, the space charges are increased. The existence of space charges suppresses the electric field strength partially and then the jet elongation is avoided by generating of corona discharge. Consequently, the beads formation is enhanced by generations of the corona discharges with increasing the applied voltages. It is suggested that the beads formation can be suppressed by decreasing the corona discharge, even if De of sample is lower than 1.

[FIGURE 8 OMITTED]

[FIGURE 9 OMITTED]

[FIGURE 10 OMITTED]

[FIGURE 11 OMITTED]

[FIGURE 12 OMITTED]

Mechanism of Discharge During Electrospinning Process

What is the origin of the electric discharge during the electrospinning? The mechanism of discharge during electrospinning is investigated by using distilled water, because the water does not elongate like the polymer solutions. Figure 13 shows the time dependence of discharge behavior of waterdrop. At the time of A, the waterdrop extends towards an upper left direction without electric discharge. The elongated waterdrop deforms with time and then break up. At the time of C, the ramified electric discharge spark is observed from the apex of the cone and the electric current has a pulse-like peak. Immediately after the breakup of waterdrop, the apex of cone becomes sharp and the electric field concentrates at the apex of cone. As a result, the electric discharge generates from the apex of cone. Thus, the ramified electric discharge spark in Fig. 11a shows not only the charged jet but also the evidence of breakup of jet. The breakup of jet causes the formation of spherical beads by surface tension as shown in Figs. 7 and 9. From all results in this study, the conditions of beads formation is shown in Fig. 14 as a function of De and Trouton ratio (26), the ratio of elongational viscosity of jet at the onset of bending instability to zero-shear viscosity. De is dominant for beads formation in the high De. In the low De region, the beads formation strongly depends on the electric discharge. When the electric discharge (corona discharge) generates, Trouton ratio becomes small due to the suppression of jet elongation. On the other hand, when the electrospinning is performed without electric discharge, the jet elongates significantly under high Trouton ratio. It is strongly suggested that the beads formation is controlled by changing the electric discharge independently of the De.

[FIGURE 13 OMITTED]

[FIGURE 14 OMITTED]

CONCLUSION

In this study, we investigated the variation of elongational viscosity of jet by changing the three series of samples which are changed concentration, molecular weight with same zero-shear viscosity, and blend ratio of low and high molecular weight. As the results, the beads formation is suppressed as the elongational viscosity of jet at the onset of bending instability is increased with increasing the concentration and molecular weight. It is found that the beads formation is suppressed as the blend ratio of high molecular weight component increases, though the De among the sample is almost same. In addition, the electric discharge behavior during electrospinning and the relation between the electric discharge and beads formation by changing the voltages are investigated. We found that the electric discharge is generated from the apex of the cone after the jet break up. The beads-less fibers are obtained without electric discharge. On the other hand, the spherical beads arc formed with electric discharge in high voltage region. From all of the results in this study, it is clear that the high elongational viscosity of jet at the onset of bending instability makes the beads-less fibers. In addition, the beads-less fibers are obtained without electric discharge independently of the De. These results strongly suggest that the beads formation is controlled by changing the condition of discharge independently of the De.

ACKNOWLEDGMENTS

The authors thank EKO Inst. Co., Ltd. for sample characterization by CaBER measurement.

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Yosuke Kadomae, (1) Masataka Sugimoto, (1) Takashi Taniguchi, (1), (2) Kiyohito Koyama (1)

(1) Graduate School of Science and Engineering, Yamagata University, Yonezawa 992-8510, Japan

(2) Japan Science and Technology Agency, CREST, 4-1-8, Honmachi, Kawaguchi-shi, Saitama, 332-0012

Correspondence to: Kiyohito Koyama: e-mail: koyama@yamagata-u.ac.jp Contract grant sponsor: KAKENHI (Grant-in-Aid for Scientific Research) on Priority Area "Soft Matter Physics" from the Ministry of Education, Culture, Sports, Science and Technology of Japan.

DOI 10.1002/pen.21713

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Author:Kadomae, Yosuke; Sugimoto, Masataka; Taniguchi, Takashi; Koyama, Kiyohito
Publication:Polymer Engineering and Science
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Date:Sep 1, 2010
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