Measurement and analysis of jet current and jet life in roller electrospinning of polyurethane

Abstract

In this study, jet current and jet life in roller electrospinning of polyurethane (PU) were measured. The relationships between jet current and jet life and number of Taylor cones/m² (NTC/m²), spinning performance (SP), and fiber properties (diameter, non-fibrous area) were analyzed. In addition, the effects of PU and tetraethylammonium bromide (TEAB) concentrations on jet current and jet life were determined. It was observed that jet current increases with PU and TEAB concentration, while jet life decreases. According to the results, NTC/m² and spinning performance increase with jet current and decrease with jet life. Moreover, it was observed that jet current movement gives an idea about jet life, and it was also determined that there is a relationship between jet life and fiber morphology.

Keywords

polyurethane jet current jet life roller electrospinning nanofibers

Electrospun nanofibrous materials have attracted much attention because of their specific properties (e.g. large surface area to volume ratios, small pore size, flexibility in surface functionalities, better mechanical properties).^1–4 When it comes to the textile field at the nanoscale, roller electrospinning is a new and successful method for producing nanofibrous materials on an industrial scale. This method was invented by Jirsak et al. at Technical University of Liberec in 2005.⁵ It was then commercialized under the Nanospider trade name by the Elmarco Company.⁶ To date, few studies have been conducted to understand the roller electrospinning mechanism,^7–11 because this technique is very young.

Solution properties such as viscosity, conductivity and surface tension are one of important process parameters in the roller electrospinning. Cengiz-Callioglu et al. found that surface tension values increased with polyurethane (PU) concentration; however, there was no relationship between surface tension and the tetraethylammonium bromide (TEAB) concentration. They also found that rheological properties, such as viscosity, increased with PU and TEAB concentration. Moreover, the number of Taylor cones/m² (NTC/m²) increased together with viscosity (PU concentration) and conductivity (TEAB concentration).⁹

There are some reports in the literature regarding jet current in needle electrospinning. Fallahi et al. conducted a series of studies on jet electric current measurement during electrospinning of polyacrylonitrile in dimethylformamide (PAN/DMF).^12–14 They studied the effect of applied voltage on jet electric current and found that jet current increased with jet flow rate and applied voltage.¹² In another study, Fallahi et al. reported that jet current increased as solution conductivity increased.¹³ In 2010, they described a new method of measuring jet current during needle electrospinning, and they investigated the effect of applied voltage on the surface and volume charge density of the jet in electrospinning of PAN solutions. According to their results, the volume charge density of the jet decreased as the applied voltage increased.¹⁴ Samatham and Kim used real-time electric current measurement during the needle electrospinning process; they claimed that electric current can be used to control the efficiency of the electrospinning process.¹⁵

Many studies have also focused on the mechanisms of the fluid properties at the tip of the needle. Hohman et al. and Shin et al. developed a theoretical framework for understanding the physical mechanisms of electrospinning.^16–19 Both studies suggested that whipping instability accounted for bending and stretching of the jet. Bhattacharjee et al. investigated the nature of the current in electrospinning and found that the jet current was dependent on voltage, feed rate, and solution conductivity.²⁰ Ying et al. claimed that during electrospinning, current affected fiber morphology and Taylor cone shape affected fiber uniformity.²¹

On the other hand, there are few studies about jet current measurement in needleless electrospinning. The basic studies were carried out by Pokorny et al., who described a measurement method using memory oscilloscope recording, explained with experimental data.^22,23 Cengiz-Callioglu et al. conducted a study using this new method and found that during rod electrospinning, jet current increases with PU and TEAB concentration.²⁴ They also found strong relationships among the electric current of the jet, spinnability, and spinning performance. After that study, Yener et al. measured electric current during needle, rod, and roller electrospinning, using poly(ethylene oxide) (PEO) as the polymer. They found that jet current increased with increasing needle protrusion length, and that it did not depend on relative humidity in needle electrospinning. According to their rod electrospinning results, current per jet does not depend on the number of jets. However, current can be considered to be regularly distributed among the jets present in both rod and roller electrospinning.²⁵

The aim of the current work is to measure the jet current and jet life of roller electrospinning of polyurethane (PU) in real time, and to analyze the relationships between jet current and jet life and NTC/m², spinning performance (SP), and fiber morphology (diameter and non-fibrous area).

Experimental details

Materials

Polyurethane (molecular weight 2000 g/mol, Larithane LS 1086; Novotex, Italy) was used as the polymer, dimethylformamide (DMF, Fluka) was used as the solvent, and TEAB (Sigma Aldrich) was used as the salt. Solutions were prepared with various concentrations of PU (15, 17.5, and 20 wt%) and TEAB (0, 0.4, 0.8, and 1.27 wt%). These values were selected after considering the results of preliminary studies.^7,9 All of the solutions were prepared by stirring the mixture of PU/DMF and TEAB at room temperature for 24 h.

Methods

Electrospinning process and jet current measurement

The electrospun nanofibers were prepared using the roller electrospinning method. Roller electrospinning is a new technique that uses electrical force to spin nanofibers from a free liquid surface toward a collector electrode. A roller, which acts as a fiber generator, is connected to a high voltage supplier, and a grounded collector is positioned at a distance away from the roller. The tank is filled with a polymer solution and the roller body is immersed into the solution in the ratio of 1/3. The rotating roller has a specific speed through the instrument of stepper motor, and as the roller rotates, a thin layer of solution is carried onto the roller surface. A high-voltage supplier is connected to the roller, and when the applied voltage exceeds a critical value, many Taylor cones are created on the liquid surface of the roller. As the solvent evaporates, polymer solution jets move toward the collector (Figure 1), and dry PU nanofibrous layers collect on the collector electrode.

Figure 1.

Schematic diagram of roller electrospinning.

During the spinning process, solution jets and Taylor cones on the roller surface were recorded via digital camera; at the same time, solution jet current was measured using a multimeter fixed on the electrospinning setup. A resistance (R = 9811 Ω) was fixed between the collector and a grounded cable to measure the electrical current flowing in the solution jets. The current was determined by measuring the voltage on the resistor using a digital multimeter (Agilent, 33401 A). At the same time, the voltage data of the solution jets were stored on a computer, and graphs of the current were reflected on the computer screen.

Process parameters (Table 1) were arranged at an optimum level to obtain dry, bead-free nanofiber layers. All of the conditions of the process parameters were kept constant during the spinning process. The roller electrospinning setup (solution tank, roller, and collector) was enclosed by Plexiglas. The constant humidity and temperature were blown into chamber by handmade air humidifier-heater conditioning.

Table 1.

Process parameters of roller electrospinning

Roller length (cm)	Roller speed (rpm)	Supporting material speed (cm/min)	Distance between electrodes (cm)	Voltage (kV)	Humidity (%)	Temperature (℃)
14.5	2	10	11	57	24 ± 1	17 ± 1

Jet life measurement

Jet life can be defined as the period of time from the point that the jet appears to the point that the jet disappears. In the roller electrospinning technique, there are a number of jets during the spinning process, and the lifetime of every jet is different from that of the others.⁸ During the electrospinning process and while the current measurement were taken, a digital camera (Sony Full HD NEX-VG10E Handycam, 14.2 megapixel-E18-200 mm lens) was used to observe the Taylor cones and analyze the jet life on the free liquid surface of the roller. The picture of Taylor cones and solution jets on the roller surface and Taylor cone structure is given in Figure 2.

Figure 2.

(a) Picture of Taylor cones and solution jets on the roller surface; (b) Taylor cone structure.

The initiation-growing and ending periods were observed for 50 different Taylor cones, and the average time was calculated to determine the jet life of the roller electrospinning.

Characterization of nanofiber layers

Nanofiber morphology was observed using a Phenom FEI (Netherlands) digital scanning electron microscope (SEM). First, nanofiber layer samples were dried at room temperature for 24 h. Then, the samples were coated with a 7 nm layer of gold, using a Quorum Q150R ES sputter coater. The fiber diameters and non-fibrous areas were measured using NIS-Elements AR (Nikon) computer software, and the average fiber diameter of 100 different fibers was determined.

Results and discussion

Analysis of jet current results

The graphs of jet current versus time of 15, 17.5, and 20 wt% PU/DMF solutions with various TEAB concentrations are shown in Figures 3 –5. The peaks and fluctuations on the figures relate to sudden changes in NTC on the roller surface. As determined by previous studies, it is not possible to spin nanofibers efficiently from 15 wt% PU/DMF without TEAB salt, due to low conductivity.^7,9,26–28 Only a few insufficient cones appeared on the roller. This behavior was reflected by the multimeter device as low current value, and, more rarely, zero current value (Figure 3(a)).

Figure 3.

Jet current versus time in electrospinning of 15 wt% PU with various TEAB concentrations: (a) 0 wt%, (b) 0.4 wt%, (c) 0.8 wt%, and (d) 1.27 wt%.

Figure 4.

Jet current versus time in electrospinning of 17.5 wt% PU with various TEAB concentrations: (a) 0 wt%, (b) 0.4 wt%, (c) 0.8 wt%, and (d) 1.27 wt%.

Figure 5.

Jet current versus time in electrospinning of 20 wt% PU with various TEAB concentrations: (a) 0 wt%, (b) 0.4 wt%, (c) 0.8 wt%, and (d) 1.27 wt%.

As seen in Figures 3 –5, jet current increases with TEAB concentration. The increased current values indicate an increase in NTC on the roller surface. At first view, it might be thought that the increase in current values seen in Figures 3, 4, and 5 indicate increasing solution conductivity and not NTC. However, as stated in the literature, solution conductivity decreases with PU concentration in the same amount of TEAB concentration, due to low ion movement in the highly concentrated solutions.^7,9,26–28 This phenomenon between solution conductivity and NTC and jet current was explained with another demonstration (Figure 6). This shows that the jet current of the solution increases with PU concentration, while solution conductivity decreases. Thus, Figure 6 proves that the jet current values increased due to the increase in NTC, and not solution conductivity.

Figure 6.

Jet current versus time in electrospinning of 0.8 wt% TEAB with various PU concentrations: (a) 15 wt%, (b) 17.5 wt%, and (c) 20 wt%.

Figure 7 shows the relationship of jet current and number of Taylor cones/m² and spinning performance at various PU and TEAB concentrations.

Figure 7.

Relationship of jet current and NTC/m² (a) and spinning performance (b) at various PU and TEAB concentrations: (+) 0 wt%, (*) 0.4 wt%, (x) 0.8 wt%, and (–) 1.27 wt%.

As seen in Figure 7, there is a strong direct relationship between jet current and NTC/m² and spinning performance. Our previous studies showed that productivity of fabric based on nanofibers depends on NTC on the roller surface.^7,9 Each Taylor cone can be considered as spinneret of the roller electrospinning process, and in all fabric production, high productivity requires more spinnerets. Figure 7 shows that average jet current value estimates not only NTC, but also spinning performance. The current diagram reflected on the computer screen through the multimeter was the result of the estimation that occurred during the roller electrospinning process. Thus, current measurement in the solution jet is a time-saving, versatile method.

Analysis of jet life results

The relationships of jet life with current and spinning performance are shown in Figure 8.

Figure 8.

The relationships between jet life and jet current (a) spinning performance and jet life (b) at various PU and TEAB concentrations: (+) 0 wt%, (*) 0.4 wt%, (x) 0.8 wt%, and (–) 1.27 wt%.

As shown in Figure 8, current and spinning performance increase with PU and TEAB concentration; however, jet life decreases. The PU includes polar groups that increase attractive forces between the macromolecules. NTC/m² and spinning performance increases by adding TEAB to the PU solutions because the functionality of these polar groups increases. This relationships are well known, based on previous studies.^7,11,27,28 On the other hand, there is an opposite relationship between jet life and NTC/m² and spinning performance. It is believed that the main reason for the decline of jet life with PU and TEAB concentration is NTC. The cones are continually needed to deposit polymer solution onto the roller surface; they are used up by the polymer solutions quickly and disappear, and jet life ends when the Taylor cones disappear. Another Taylor cone subsequently appears at another place on the roller, and a new jet forms at the tip of the new Taylor cone.

Eventually, the current-test method presents reliable results, providing an opportunity to read, on the computer screen, whether the NTC is high or low during the roller electrospinning process, and to provide information regarding solution spinnability. The current graphs not only present changes in the NTC per unit area, but also give an idea about the jet life. When the jet current graphs were analyzed in detail, it could be seen that current fluctuations in the graphs produced different behaviors, which correspond to jet life (Figure 9).

Figure 9.

Relationships between jet current behaviors and fiber morphology: (a) 20 wt% PU, (b) 20 wt% PU + 1.27 wt% TEAB.

The jet current fluctuations shown in Figure 9 consist of two behaviors: movement of the horizontal (x-time differences in second) axis and the vertical (y-current differences in µA) axis. Figure 9(a) shows that the solution in the jet current graph includes 20 wt% PU solutions without TEAB. As mentioned previously, a low electrical current value indicates a low NTC. Furthermore, low fluctuations of current indicate long jet life during electrospinning. The long and more or less stable diagram movement of the current on the x-axis (around 10 s; see Figure 9(a)) and narrow diagram movement of the current on the y-axis (broadest diagram movement of current value, 15 µA–25 µA on y-axis; see Figure 9(a)) show a long jet life on the roller surface (also see Figures 3(b), 4(b), and 5(a)). On the other hand, Figure 9(b) shows that the short and unstable diagram movement of the current on the x-axis (around 2.5 s; see Figure 9(b)) and wide diagram movement of the current on the y-axis (broadest diagram movement of current value, 105 µA–175 µA; see Figure 9(b)) show a short jet life on the roller surface. Similar behavior can be seen in Figures 3(d), 4(d), 5(c), and 5(d).

It can also be seen clearly in Figure 9 that there is a relationship between jet life and fiber morphology. Because, every jet needs time to elongate its shape and evaporate the solvent to obtain dry nanofibers as well as needle electrospinning, jets with shorter lives are not able to reach nanoscale and may sometimes form wet nanofibers, called non-fibrous area; as a result, fiber diameter increases when the jet life decreases. The relationships between current and fiber properties such as diameter and non-fibrous area are shown in Figure 10.

Figure 10.

Relationships between jet current and fiber properties at various PU and TEAB concentrations: (+) 0 wt%, (*) 0.4 wt%, (x) 0.8 wt%, and (–) 1.27 wt%. (a) Fiber diameter (nm), (b) Non-fibrous area (%).

As seen in Figure 10, average fiber diameter and non-fibrous area increase with current, PU, and TEAB concentration, while jet life decreases. The relationships between fiber diameter and PU and TEAB concentrations in this study are similar to those of our previous studies.^7,9 However, the opposite result was observed for non-fibrous areas. This inconsistency might be due to differences in spinning and environmental conditions between this study and previous studies. In addition, there was a high quantity of beads on the fiber surface of the 15 wt% PU solution without salt, due to the low viscosity. Apart from the 15 wt% PU solution without salt, the non-fibrous area increased as PU and TEAB concentrations increased (Figure 10(b)).

Conclusions

Jet current and jet life were measured during the roller electrospinning of PU, and the relationships of those parameters with NTC/m², spinning performance, and fiber properties such as diameter and non-fibrous area were analyzed. The current-test method, which is a new method, was used to determine average jet current during roller electrospinning. Using this method, it is possible to analyze NTC/m², spinnability, jet life, and fiber morphology. Our experimental results showed that NTC/m² and spinning performance increase with jet current and decrease with jet life. In short, there is a direct relationship between jet current, NTC/m² and spinning performance, and an opposite relationship with jet life. On the other hand, jet current increases with PU and TEAB concentration, but jet life decreases. There is also a strong relationship between jet life and fiber morphology.

Footnotes

Funding

This work was supported by the Ministry of Education, Youth and Sports of the Czech Republic (student’s grant competition TUL in specific university research in 2012—Project No. 4866—and 2013—Project No. 48004).

Acknowledgments

Thanks also go to Prof. Dr. Oldrich Jirsak and the Nonwoven Department of the Textile Engineering Faculty, TUL, CZ, for providing the working environment, including laboratory space and equipment.

References

Teo

Ramakrishna

. A review on electrospinning design and nanofibre assemblies. Nanotechnology 2006; 17(14): 89–106.

Gibson

Schreuder-Gibson

Rivin

. Transport properties of porous membranes based on electrospun nanofibers. Colloid Surface A 2001; 187: 469–481.

Lee

Kim

. Stress–strain behavior of the electrospun thermoplastic polyurethane elastomer fiber mats. Macromol Res 2005; 13(5): 441–445.

Pedicini

Farris

. Mechanical behavior of electrospun polyurethane. Polymer 2003; 44(22): 6857–6862.

Jirsak

Sanetrnik

Lukas

. A method of nanofibres production from a polymer solution using electrostatic spinning and a device for carrying out the method, Patent WO,2005,024,101, USA, 2005.

Elmarco Company. Nanospider™ electrospinning technology, 2013. http://www.elmarco.com/electrospinning/electrospinning-technology/ .

Cengiz-Çallıoğlu

. Polyurethane nano fiber production by roller electrospinning method, PhD Thesis, Süleyman Demirel University, Turkey, 2011.

Dao

. The role of rheological properties of polymer solutions in needleless electrostatic spinning, PhD Thesis, Technical University of Liberec, Czech Republic, 2011.

Cengiz-Çallıoğlu

Jirsak

Dayık

. Investigation into the relationships between independent and dependent parameters in roller electrospinning of polyurethane. Text Res J 2013; 83(7): 718–729.

10.

Yener

Jirsak

. Comparison between the needle and roller electrospinning of polyvinylbutyral. J Nanomater 2012: 124–130. doi:10.1155/2012/839317.

11.

Yalçınkaya

. Analysis of Taylor cone structure and jet life on the nanofiber production with needle and needle-less electrospinning methods, MsC Thesis, Süleyman Demirel University, Turkey, 2012.

12.

Fallahi

Rafizadeh

Mohammadi

. Effect of applied voltage on jet electric current and flow rate in electrospinning of polyacrylonitrile solutions. Polym Int 2008; 57(12): 1363–1368.

13.

Fallahi

Rafizadeh

Mohammadi

. Effects of feed rate and solution conductivity on jet current and fiber diameter in electrospinning of polyacrylonitrile solutions. e-Polymers 2009; 104: 1618–7229.

14.

Fallahi

Rafizadeh

Mohammadi

. Effect of applied voltage on surface and volume charge density of the jet in electrospinning of polyacrylonitrile solutions. Polym Eng Sci 2010; 50(7): 1372–1376.

15.

Samatham

Kim

. Electric current as a control variable in the electrospinning process. Polym Eng Sci 2006; 46(7): 954–959.

16.

Hohman

Shin

Rutledge

. Electrospinning and electrically forced jets. I. Stability theory. Phys Fluids 2001; 13(8): 2201–2220.

17.

Hohman

Shin

Rutledge

. Electrospinning and electrically forced jets. II. Stability theory. Phys Fluids 2001; 13(8): 2221–2236.

18.

Shin

Hohman

Brenner

. Electrospinning: A whipping fluid jet generates submicron polymer fibers. Appl Phys Lett 2001; 78(8): 1149–1151.

19.

Shin

Hohman

Brenner

. Experimental characterization of electrospinning: The electrically forced jet and instabilities. Polymer 2001; 42(25): 9955–9967.

20.

Bhattacharjee

Schneider

Brenner

. On the measured current in electrospinning. J Appl Phys 2010; 107(4): 044306–044306.

21.

Ying

Zhidong

Qiang

. Controlling the electrospinning process by jet current and Taylor cone. In: Proceedings of the IEEE annual report conference on electrical insulation and dielectric phenomena, . Nashville, Tennessee, USA, 16–19 October2005, pp. 453–456.

22.

Pokorny

Jirsak

. Measurement of electric current during electrospinning. In: Proceedings of the 7th international conference (TexSci10), Liberec, Czech Republic, 6–8 September, 2010.

23.

Pokorny

Mikes

Lukas

. Measurement of electric current in liquid jet. In: Proceedings of the Nanocon conference, Ostrava, Czech Republic, 12–14 October 2010, pp. 282–286.

24.

Cengiz-Çallıoğlu

Jirsak

Dayık

. Electric current in polymer solution jet and spinnability in the needleless electrospinning process. Fiber Polym 2012; 13(10): 1266–1271.

25.

Yener

Yalçınkaya

Jirsak

. On the measured current in needle and needleless electrospinning. J Nanosci Nanotechno 2013; 13(7): 4672–4679.

26.

Cengiz-Çallıoğlu

Jirsak

Dayik

. The influence of non-solvent addition on the independent and dependent parameters in roller electrospinning of polyurethane. J Nanosci Nanotechno 2013; 13(7): 4727–4735.

27.

Yalçınkaya

Yener

Cengiz-Çallıoğlu

. Effect of concentration and salt additive on Taylor cone structure. In: Proceedings of the 4th international Nanocon conference, Czech Republic, 23–25 October 2012, pp. 80–85.

28.

Yalçınkaya

Yener

Cengiz-Çallıoğlu

. Relation between number of Taylor Cone and jet life on the roller electrospinning. In: Proceedings of the international Istanbul textile congress, Istanbul, Turkey, 30–31 June 2013, pp. 213–218.