Direct Observation and Quantitative Analysis of the Fiber Formation

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Direct observation and quantitative analysis of the fiber formation process during electrospinning by a high-speed camera Ikuo Uematsu, Kenya Uchida, Yasutada Nakagawa, and Hidetoshi Matsumoto Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b02352 • Publication Date (Web): 21 Aug 2018 Downloaded from http://pubs.acs.org on August 28, 2018

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Industrial & Engineering Chemistry Research

Direct observation and quantitative analysis of the fiber formation process during electrospinning by a highspeed camera Ikuo Uematsu†, ‡, *, Kenya Uchida‡, Yasutada Nakagawa‡, and Hidetoshi Matsumoto†,*

†

Department of Materials Science and Engineering, Tokyo Institute of Technology, 2-12-1 Ookayama,

Meguro-ku, Tokyo 152-8552, Japan

‡

Corporate Manufacturing Engineering Center, Toshiba Corporation, 33 Shin-Isogo-Cho, Isogo-ku,

Yokohama 235-0017, Japan

*Address correspondence to [email protected] and [email protected]

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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

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ABSTRACT. Electrospinning is a versatile and straightforward method for the formation of continuous thin fibers and is based on an electrohydrodynamic process from polymer solutions and melts. To improve the throughput of electrospinning and the quality of the electrospun thin fibers, an in-depth understanding of the fiber formation process is required. In the present study, we attempted to quantitatively analyze the bending instability of an electrified thin jet during electrospinning. The electrified-jet flying phenomenon from the spinneret to the substrate was investigated by high-speed camera observation and electromagnetic and kinetic analyses, i.e., the flying velocity of the electrified jet was measured, and the electric potential was obtained from finite element method analysis of an electric field. The charge density and diameter of the electrified jet in the bending instability region during electrospinning was determined by solving the equation of motion.

KEYWORDS

electrospinning, nanofiber, high-speed camera, equation of motion, electromagnetic field analysis, finite element method


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INTRODUCTION

Electrospinning is a straightforward and versatile method for the formation of continuous thin fibers based on an electrohydrodynamic process. This method has the following advantages: (i) it is applicable to a broad spectrum of molecules, such as inorganic molecules, synthetic polymers, proteins and DNA, and (ii) it has the ability to produce thin fibers with diameters in the micrometer and nanometer ranges. Electrospun fibers or fibrous membranes, with large surface-to-volume ratios, have recently attracted much attention in applications such as high-performance filter media, protective clothes, composites, battery separator and electrode materials, drug delivery systems and biomaterial scaffolds for tissue engineering.1-5

There have been an enormous number of reports regarding the fabrication of electrospun nanofibers and their functions.1-5 In addition, processing innovations that not only improve the morphology control but also enhance the production capacity of the electrospun nanofibers and nanofibrous membranes are evolving. In particular, the high-throughput electrospinning systems have been undergoing continuous development (e.g., multineedle spinning6, needleless spinning7, and a combination of air-blowing processes8,9). The realization of a large-scale production system with high throughput and high quality (i.e., well-defined fiber morphology) is required. For this purpose, an indepth understanding of the fiber formation process is essential.


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Electrospinning depends on a highly complex interplay of viscoelastic rheology; inertial, capillary, and aerodynamic forces; heat and mass transfer; phase behavior; and dielectric and coulombic effects. 1, 10-15

The process consist of the steady cone jet region and the bending instability (or whipping

instability) region. Several researchers have reported the direct observation of the formation of thin fibers during electrospinning by using a high-speed camera.16-18 However, reports on the quantitative analysis of the bending instability of the electrified jet during electrospinning based on the direct observation and theoretical models1, 10-14 are limited, particularly regarding a quantitative evaluation of the charge density and the size of the flying jet. Herein, we focused on the quantitative analysis of the charge density and the size of the flying jet in the bending instability. We believe that the charge density of the jet is the most crucial factor, which not only determines the fiber diameter including the distribution but also determines the productivity.

In the present study, we attempted a first-approximation theoretical analysis of the fiber formation process in the bending instability region during electrospinning. The electrified jet flying phenomenon from the spinneret to the substrate was observed by a high-speed camera, and the electric field was analyzed by a finite element method (FEM). The charge density and diameter of the electrified jet in the bending instability region during electrospinning was determined by solving the equation of motion. For simplification, we used the vertical velocity of the jet and the inclination of the jet axis to determine the charge density and diameter of the electrified jet.


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EXPERIMENTAL SECTION Materials. Poly(styrene) (PS, Mw=218,000), and cyclohexanone (CHN, extra pure grade) were purchased from Denka, Japan and Wako, Japan, respectively. Lithium bromide (LiBr, extra pure grade) was purchased from Wako, Japan. These reagents were used without further purification. For the preparation of the spinning solution, PS was dissolved in CHN and then stirred at room temperature for 3 days to give an 18 wt% solution. LiBr was used for tuning the electrical conductivity of the spinning solution.

Electrospinning and high-speed camera observation. The experimental setup is shown in Figure 1. The PS/CHN solution (18 wt%) was electrospun using a commercial device (NF-103, MECC, Japan). A stainless steel nozzle (0.3 mm internal diameter, Unicontrols, Japan), connected to a high-voltage regulated DC power supply, was used as the spinneret. The grounded substrate used as the counter electrode was an aluminum plate (250 × 200 mm). The applied voltage was 25 kV, the distance between the nozzle tip and the substrate was 250 mm, and the flow rate was 1 mL/h. All spinnings were carried out at 28-32 °C and 30-40 % relative humidity. The high-speed camera (MEMRECAM HX-3, Nac Image Technologies, Japan) was used for the direct observation of the electrospinning process. A continuous light illuminated the spinning area by a beam spot from a 250 W metal-halide lamp (MID25FC, Kyowa, Japan), which enabled the acquisition of clear images with a short exposure time of the high-speed camera. The frame rate was 2,000 fps (see Videos S1 and S2).


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The morphology of the as-spun PS nanofibers was observed using field-effect scanning electron microscopy (FE-SEM, S-5500, Hitachi High Technologies, Japan) operated at 0.5 kV. The average fiber diameter was determined by SEM image analysis.

Figure 1. Experimental setup for the visual observation of electrospinning.

Measuring the vertical jet velocity and inclination of the jet axis. The electrified jet path involved in the first bending instability region is complicated, and the velocities of the different jet segments varied in both magnitude and direction. Herein, the vertical velocity and the flying path inclination were determined from the video images for simplification of the analysis (see Figure S1, detailed description is included in the Supporting Information). The lower part of the first bending instability was divided ACS Paragon Plus Environment

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into four regions (From Areas 1 to 4), as shown in Figure 2. The angle of inclination of the jet axis to the horizontal substrate (i.e., counter electrode) θ was determined by image analysis. All measurements were carried out at five or more different points in each area.

Figure 2. Direct observation of the electrospinning process using a high-speed camera.

Electromagnetic field simulation. The three-dimensional (3D) configuration of the electrospinning process used for electromagnetic field simulation is shown in Figure S2. This configuration is similar to our experimental configuration (the distance between the nozzle tip and the substrate was 250 mm, and the size of the collector was 200 × 250 mm). The electric potential distribution was determined by


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solving Poisson’s equation using a nonlinear finite elements analysis software (Marc, MSC Software Corporation, USA). The electric potentials of the nozzle, substrate, and surface of the cabinet wall were given as 25, 0, and 0 kV, respectively, and were used as the boundary conditions.

RESULTS AND DISCUSSION

Direct observation of the fiber formation process during electrospinning by a high-speed camera. The vertical jet velocity (vz) and inclination of the jet axis (θ) to the horizontal substrate in each observation area measured by the high-speed camera video images are summarized in Table 1. When the electrified jet flew from Area 1 to 4, the vertical jet velocity, vz, decreased from 1.09 to 0.76 m/s, and the inclination of the jet axis, θ, also decreased from 15.4 to 8.8 °. These results reflect the conical jet trajectory in the bending instability region during electrospinning, i.e., the spatial breadth of the jet in the x-y plane increased with a decrease in vz.

Table 1. Vertical jet velocity, inclination of the jet axis to the horizontal substrate, and diameter of the jet in each observation area. Distance from substrate

Vertical jet velocity*, vz

Inclination of jet axis*, θ

Diameter of jet**, d

[mm]

[m/s]

[°]

[nm]

Area 1

75-100

1.09

15.4

674

Area 2

50-75

0.96

14.7

642

Area 3

25-50

0.86

12.5

600

Area 4

0-25

0.76

8.8

500

*

Measured values

**

Calculated values using Eq. (2). The average fiber diameter deposited on the substrate from SEM image analysis was

used as the diameter of jet in Area 4, d4 in Eq. (2). ACS Paragon Plus Environment

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Theoretical analysis of the fiber formation process. The diameter of the flying jet in the lower part of the bending instability region was calculated by a theoretical analysis, because the resolution of the high-speed camera was not sufficient for the direct image analysis of the thin fibers with diameters less than 1 µm. The equation of motion for the spinning trajectory during fiber formation (i.e., flying jet during electrospinning) was solved for this purpose.1,13,14 The equation of jet motion is composed of coulombic forces, gravity, and the air resistance of the flying jet as the first, second, and third terms, respectively, of the right side of the following equation:

݉

ௗమ ௭౤ ௗ௧ మ

ଵ

= ‫ܧݍ‬୬ + ݉݃ − ଶ ߩ‫୸ݒ‬,୬ ଶ ܵ୬ ܿ‫ߠݏ݋‬୬ ‫ܥ‬ୈ

(1)

where m is the mass of the jet [kg]; t is the time [s]; q is the charge amount [C]; En is the electric field in the area n [V/m]; g is the gravitational acceleration (= 9.8 m2/s); ρ is the density of air [kg/m3]; vz,n is the vertical jet velocity in the area n [m/s]; Sn is the surface area of the jet [m3]; θn is the inclination of the jet axis to the horizontal substrate in the area n [°]; and CD is the resistance coefficient for air. The charge amount of the electrified jet, q, is regarded as a constant from the viewpoint of energy balance during electrospinning. Eq. (1) was can be rewritten as follows:

݉

ௗమ ௭౤ ௗ௧ మ

= ‫ݍ‬ቀ

గௗ೙ మ ସ

ቁ ‫ܧܮ‬௡ + ߩ ቀ

గௗ೙ మ ସ

ଵ

ቁ ‫ ܮ‬− ଶ ߩ‫ݒ‬௭,௡ ଶ ݀௡ ‫ߠݏ݋ܿܮ‬௡ ‫ܥ‬஽

(2)


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where dn is the diameter of the flying jet [m] and L is the length of the flying jet [m]. As the first step, the charge amount q was calculated using Eq. (2), and then the diameter of flying jet d in each area was calculated. Under the assumption that the jet diameter in Area 4 is the same as the fiber diameter deposited on the substrate, the average diameter analyzed by SEM observations (~500 × 10-9 m, see Figure S2) was used as d4 in Eq. (2). The measured values of 0.76 m/s and 8.82° listed in Table 1 were used as the v4 and θ4, respectively. The electric potential was calculated by solving Poisson’s equation using the FEM for a 3-D configuration (see Figures S3 and S4). The E4 value obtained from the electromagnetic analysis was substituted into Eq. (2), and the charge amount of the electrified jet, q (=charge density Q × jet volume π(d4/2)2 [m3]), was calculated. To simplify the calculation, we adopted the approximation of d2z/dt2=0 for the short jet (L can be deleted in the right side of Eq. (2)). Finally, we obtained the Q value for a jet of 25.5 C/m3. Subsequently, we can calculate the values from d1 to d3 by the following equation using the vz,n and θ values in each area from the highspeed camera image analysis and the En value in each area from the electromagnetic field analysis with the charge amount q obtained previously:

݀௡ =

ଶఘ௩೥,೙ మ ௖௢௦ఏ೙ ஼ವ గሺ௤ா೙ ାఘሻ

(3)

The calculated jet diameters in all areas are listed in Table 1; these values clearly indicate that the electrostatic repulsive force elongated the flying jet during the fiber formation process.12 The decrease


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in the vz value near the substrate was explained by the following two contributions: (i) an increase in the air resistance to the flying jet due to an increase in the projected area of the jet along flying trajectory and (ii) a decrease in the electrostatic contribution due to a decrease in the electric field intensity near to the substrate (see Figure S4). The estimated jet diameter in Areas 2 and 3 in Table 1 agreed well with the diameter measured from SEM image analysis for the sample directly collected from the flying jet, supporting the validity of our theoretical analysis (we could not measure the diameter of the jet in Area 1 due to the large amount of remaining solvent). To verify our theoretical analysis, the mass balance of jet in all areas was investigated. Since the nanofiber-deposited area on the substrate spread in the x-y plane, the spatial breadth in Area 4 can be measured from the SEM images, and the average spatial breadth in Areas 1-3 can be measured from high-speed camera images (see Figure S5). The obtained values are summarized in Table S1. By using the other data from the high-speed camera observations, namely, vz, θ, and the jet diameter in Table 1, we can estimate the total volume of the jet in each area (see Table 2). As the distance to the substrate decreases, the turn number of the jet decreases, and the spatial breadth becomes wider. The estimated jet volume also clearly indicates that the mass balance is almost conserved among all areas in the bending instability region, which supports the validity of our theoretical analysis. Herein, the lower part of the first bending instability region was divided into four regions. We believe that a division into smaller regions enables a more precise analysis of the bending instability of the electrified jet.


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Table 2. Analysis of the bending instability of the jet frying behavior during electrospinning. Distance from substrate Jet diametera

Turn number

Apparent jet length

Apparent jet volume

[mm]

d [nm]

in unit areab

in unit areac [mm]

in unit aread [mm3]

Area 1

75-100

674

4.5

2374

8.47×10-4

Area 2

50-75

642

4

3517

1.14×10-3

Area 3

25-50

600

3.5

4308

1.12×10-3

Area 4

0-25

500

3

4748

9.32×10-4

a

Calculated values using Eq.(2). The average fiber diameter deposited on the substrate from SEM image analysis was

used as d4 in Eq. (2). b Determined from the high-speed camera image c Circumference of a single turn is calculated by the spatial breadth in the x-y plane at the vertical center position of each area. d Calculated by the jet diameter multiplied by the jet length

Effects of electrospinning parameters on the jet behavior in the bending instability region. By using the abovementioned theoretical approach, we investigated the effects of the applied voltage (from 25 to 40 kV) and the addition of an electrolyte to the spinning solution on the jet behavior in the bending instability region during electrospinning. Here, we observed the corona discharge during electrospinning by using an image intensifier. The corona discharge was not observed at the applied voltage ranging from 25 to 40 kV, but it was confirmed near the spinneret at an applied voltage of 55 kV. Therefore, the influence of the corona discharge is negligible in the present work. The analytical results from direct observation by the high-speed camera are summarized in Tables 3 and 4 (For the direct observation for the spinning solution containing LiBr, see Video S2). In Table 3, the vz value increased with an increase in the electric field induced by the applied voltage as expected. The d4 and Q4 values, however, did not show significant changes with an increase in the electric field induced by the applied voltage. This behavior is probably due to a saturation of the


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charge density of the electrified jet at an applied voltage above 25 kV (23.3-25.5 C/m3). Therefore, the vertical charge transport amount per unit time increased with an increase in the applied voltage. This finding agrees with the results in our previous work. The electrostatically-ejected amount per unit time from the spinneret increased with an increase in the applied voltage. 19 In Table 4, the vz, Q4 and d4 values increased with an increase in the electrical conductivity of the spinning solution by the addition of LiBr. Under a constant electric field (=18 kV/m), an increase in the charge density of the jet increases the vz value, but an increase in the vz value decreases the flying time of the jet. The reduction in flying time of the jet will prevent the sufficient electrostatic elongation of the jet, which consequently increases the d4 value． Thus, we can quantitatively analyze the charge state and size of the electrified jet during electrospinning based on a theoretical analysis of the direct observations.

Table 3. Effect of the applied voltage on the jet behavior in the bending instability region during electrospinning from an 18 wt% PS/CHN solution. Applied voltage

Electric field *

Vertical jet velocity, *

Charge density, *

3

Diameter of jet, d4* [nm]

[kV]

E4 [kV/m]

vz, 4 [m/s]

Q4 [C/m ]

25

18

0.76

25.5

500

30

22

0.84

25.1

503

35

25

0.86

23.6

515

40

29

0.92

23.3

508

*

Values in Area 4


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Table 4. Effect of the electrical conductivity of the spinning solution on the jet behavior in the bending instability region during electrospinning from 18 wt% PS/CHN solutions with and without LiBr at an applied voltage of 25 kV. LiBr in spinning solution

Electrical conductivity

Vertical jet velocity,

Charge density, *

Diameter of jet*

[µS/cm]

vz, 4 [m/s]

Q4 [C/m ]

d4* [nm]

0

0.05

0.76

25.5

500

0.05

4.3

1.13

57.1

590

1

8.2

1.50

101.1

608

[wt%]

*

3

*

Values in Area 4

CONCLUSION In the present study, the jet behavior in the bending instability during the electrospinning of a PS/CHN solution was directly observed by a high-speed camera and analyzed by an electrostatic field analysis and equation of jet motion. As a first step, the constant charge amount of the electrified liquid jet q was determined by solving the equation of motion for the electrified jet based on the vertical jet velocity vz, the inclination of the jet axis θ (from direct observation), and the electric field E (from the electric field analysis using the FEM). Then, the diameter d of the electrified jet was determined by solving the equation of jet motion under a constant charge amount, q. The diameter of the electrified jet obtained from our first-approximation analysis agreed well with the experimental value. This analytical approach was also verified by the establishment of a material balance among all areas in the observed bending instability. By using this approach, we first succeeded in quantitatively discussing the effects of the applied voltage and the addition of an electrolyte to the spinning solution on the jet trajectory in the bending instability during electrospinning. The insights provided by this approach will be useful for the precise design of high-throughput and high-quality electrospinning processes. In addition, we believe that this approach can also be applied to recently developed electrospinning processes such as needleless electrospinning 7 and electrospinning with air blowing. 8, 9 ACS Paragon Plus Environment

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SUPPORTING INFORMATION PARAGRAPH High-speed camera videos for the jet during electrospinning from poly(styrene)/cyclohexanone solutions with and without electrolytes, how to determine the vertical velocity of the jet, how to determine d4 in Eq. (2), the 3D configuration of the electrospinning device the for electromagnetic field simulation, a typical result of the electromagnetic field simulation, and the spatial breadth of the jet in the x-y plane. This material is available free of charge via the Internet at http://pubs.acs.org.

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SYNOPSIS TOC.


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Direct Observation and Quantitative Analysis of the Fiber Formation

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