Investigation into the relationships between independent and dependent parameters in roller electrospinning of polyurethane

Abstract

In this study, definitions and explanations of the relationships between selected independent and dependent parameters of roller electrospinning are introduced. We aimed to define and completely analyze new parameters, such as the number of Taylor cones per square meter, the spinning performance for one Taylor cone, total spinning performance, fiber diameter uniformity coefficient and non-fibrous area percentage. Also, new measurement methods were developed and explained to analyze these parameters. According to the experimental results, strong and significant relationships between independent and dependent parameters of roller electrospinning were found. These independent and dependent parameters were affected by varying the concentrations of polyurethane and tetraethylammoniumbromide (TEAB) salt. In particular, the spinnability of the polymer solution, which is the most important factor for the roller electrospinning method, significantly increased with the TEAB concentration. If the spinning performance is positive, a specific solution is spinnable. However, the solution is not spinnable if the spinning performance is zero.

Keywords

Polyurethane tetraethylammoniumbromide polymer solution electrospinning Taylor cone nanofiber

The research and development of nanotechnology has rapidly increased in recent years. Polyurethane (PU) nanofiber production is of interest to many areas of research, as the novel properties of these fibers make them attractive for a wide array of applications.^1–5 Nanofibers exhibit large surface area to volume ratios⁶ and have shown superior mechanical properties (modulus and strength)^7,8 when their diameters are below 500 nanometers.⁹ Several methods exist for the fabrication of nanofibers,¹⁰ such as drawing,¹¹ template synthesis,¹² phase separation,¹³ self-assembly,¹⁴ melt-blowing,¹⁵ bicomponent spinning¹⁶ and electrospinning.^17–21 Electrospinning is a favored technique because of its advantages, for instance, it is multi-purpose and has a simple set-up. Electrospinning drives fiber production by electrostatic forces in high electric fields. Up to the present, two popular ways of producing nanofibers using high voltages have been developed: needle and needle-less electrospinning. Various needle-less electrospinning methods were developed by some researchers^22–26 to prevent problems of the needle electrospinning method, such as needle clogging and low spinning rate. In this study, we used roller electrospinning, which is one of the needle-less electrospinning methods. This method was invented by Jirsak et al.²⁴ from Technical University of Liberec and commercialized by the Elmarco Company under the Nanospider trade name. Roller electrospinning is quite a new technique; therefore, the parameters (independent and dependent) of this method have not yet been fully defined. There are few studies related to this method.^27–31 The parameters differ in some aspects from those of needle electrospinning.³² Independent parameters can be adjusted and controlled and dependent parameters depend on the independent parameters. The type of polymer and solvent, the concentration of the polymer solution, electrical conductivity, surface tension, the dielectric and rheological properties of the solution, distance between the electrodes, applied voltage, type of supporting material and its electric properties and ambient parameters, such as relative humidity and temperature, are independent parameters for roller electrospinning. The number of Taylor cones per unit area, spinning performance per one Taylor cone, total spinning performance, average fiber diameter, diameter distribution and non-fibrous area (NFA) are dependent parameters of this method.

In this article, we demonstrated the relationships between these independent and dependent processes and material parameters of the roller electrospinning method using PU polymer.

Experimental details

Materials

PU, Larithane LS 1086 (an aliphatic elastomer based on 2000 g/mol, linear polycarbonated diol, isophorone diisocyanate and extended isophorone diamine), a product of Novotex, Italy, was used as the polymer, dimethylformamide (DMF) was used as the solvent and tetraethylammoniumbromide (TEAB) was used as the salt. Solutions were prepared at different polymer concentrations, such as 10, 12.5, 15, 17.5 or 20 wt% PU, and each solution included 0, 0.4, 0.8 or 1.27 wt% TEAB salt.

Methods

Analysis of independent parameters

In this section, analysis of independent parameters, such as polymer concentration, type of solvent, electrical conductivity, dielectric constant, surface tension and the rheological properties of the solution, were introduced. Various polymer solutions were prepared with different PU and TEAB concentrations. All solutions were prepared under the same conditions (stirring time, etc.). Then, solution properties, such as conductivity, surface tension, viscosity and complex modulus, were determined. Conductivity and surface tension properties were determined by a conductivity meter (Radelkis, OK-102/1) and the Wilhelmy method (Krüss) using a platinum plate and a highly precise electronic balance, respectively. The dielectric permittivity values were obtained from measurements in a two parallel plate condenser with the sample between the plates. One tip of the plate was connected to a high voltage and the other was connected to the dielectric meter. The rheological properties of the solutions were measured using a Bohlin rheometer at 25℃.

All solutions were electrospun into nanofibers using the roller electrospinning method (Nanospider) (Figure 1).

Figure 1.

Schematic diagram of the roller electrospinning method.

The Nanospider consists of a slowly rotating cylinder and high-voltage power supply to spin fibers directly from the polymer solution.²⁴ A high voltage is connected to the rotating roller. The collector electrode is usually grounded or charged to the charge opposite to that of the spinning electrode. As the cylinder rotates, a thin layer of polymer solution is carried on the roller surface and many Taylor cones³³ are created. The jets of polymer solution are transformed into nanofibers and solid nanofibers are obtained due to rapid solvent evaporation before reaching the collector electrode. The optimum process parameters of the roller electrospinning, which were determined in previous experimental studies,³⁴ were applied during the spinning experiments (Table 1).

Table 1.

Process parameters of roller electrospinning

Roller length (cm)	Roller diameter (cm)	Roller speed (rpm)	Take-up cylinder speed (cm/min)	Distance between the electrodes (cm)	Voltage (kV)	Temp.(℃)	Relative humidity (%)
14.4	2	2	10	11	76.1	22	27

Conditioned air is blown into the spinning device from a suitable air conditioner. The air conditioner is able to keep the relative humidity between 18 and 60% and the temperature between 18 and 30℃. All nanofibers were collected on the polypropylene spunbond nonwoven antistatic material.

Analysis of dependent parameters

After measurements of solution properties, the solutions were electrospun and the spinning process recorded by a camera to observe Taylor cones on the surface of the roller. Normally, for needle electrospinning methods, only one Taylor cone occurs during the spinning process. However, many Taylor cones were observed on the surface of the roller during these spinning experiments (Figure 2).

Figure 2.

Taylor cones on the roller surface during the spinning process.

From the recorded images, the number of Taylor cones on the spinning electrode (NTC) was counted. Then, the density of the Taylor cones (DTC) was calculated using Equation (1):

DTC = \frac{NTC}{A} [m^{- 2}]

(1)

where A is the area of the spinning surface. In the case of the spinning roller, the spinning area A is approximately equal to one third of total roller surface (Equation (2)):

A = \frac{1}{3} \times π \times D \times l [m^{2}]

(2)

where D is the roller diameter

[m]

and l is the length of roller

[m]

. Spinning performance (SP) or polymer throughput is the amount of nanofiber material in grams per minute, produced using a one-meter long roller (grams/minute/meter). This is one of the most important parameters of the roller electrospinning method. The values are calculated from the nanofibrous web obtained from roller electrospinning. The equation is given below:

SP = \frac{V \times W \times M}{l} [g / min / m]

(3)

where

SP

is the spinning performance (g/min/m), V is the take up cylinder speed (m/min), W is the width of the nanofiber web on the collected nonwoven material (m), M is the area weight of the nanofiber web (g/m²) and l is the length of the spinning roller (m).

Spinning performance per one Taylor cone (SPTC) [g/h] can be determined by the amount of polymer passing through one Taylor cone per 60 minutes,²⁸ using the following formula and considering Equation (3) with l = 1 m:

SPTC = \frac{SP \times 60}{NTC} = \frac{SP}{DTC \times A} = \frac{180 \times SP}{π \times D \times DTC}

(4)

Finally, fiber properties were analyzed using scanning electron microscopy (SEM). From the SEM pictures, fiber diameter, diameter uniformity and the percent of NFA were determined with the aid of Lucia 32 G computer software. Average fiber diameter was calculated using 200 different diameter values for each sample. The fiber diameter uniformity coefficient (FDUC) was specified using the number and weight average calculations method. Number average has been used as an arithmetic mean in mathematics science, and the method that was used to calculate the uniformity coefficient has the same principle as the molar mass distribution in macromolecular chemistry. We calculated both of these values using Equations (5) and (6) given below:

A_{n} = \frac{\sum n_{i} d_{i}}{\sum n_{i}} (number average)

(5)

A_{w} = \frac{\sum n_{i} d_{i}^{2}}{\sum n_{i} d_{i}} (weight average)

(6)

where d_i is the fiber diameter and n_i is the fiber number.

The FDUC was determined using Equation (7); the expected value should be very close to 1 for uniform fibers:

FDUC = \frac{A_{w}}{A_{n}}

(7)

The NFA percentage refers to the quality of the spinning process; this was determined using SEM pictures. NFA is the area fraction of NFA in a membrane to the total area of product.²⁸ The equation is given below:

\begin{matrix} NFA = [total area of non - fibrous area / total \\ area of nanofibrous membrane] * 100 \end{matrix}

(8)

Results and discussion

Independent parameters

Firstly, the effect of PU and TEAB concentrations on the solution properties, such as conductivity, dielectric constant, surface tension, viscosity and complex modulus values, was determined. Figure 3 shows the effect of PU and TEAB concentration on the conductivity of the solution.

Figure 3.

The effect of polyurethane (PU) and tetraethylammoniumbromide (TEAB) concentrations on the conductivity of the solution.

The conductivity of the PU solution increased with TEAB concentration. In the literature, similar results were also observed.^35,36 In a static electric field, ions act as charge carriers, which leads to high conductivity of a solution. High values of conductivity of the solutions containing TEAB show a high degree of dissociation of TEAB in DMF. If the solution included TEAB, conductivity decreased with PU concentration. Solution viscosity increased with PU concentration. Therefore, conductivity decreased because the mobility of ions in the solution decreased with increasing viscosity. On the other hand, if the solution did not include TEAB, there was no relationship between conductivity and PU concentration. The effect of PU and TEAB concentration on conductivity was statistically significant (analysis of variance (ANOVA) test).

When we analyzed the effect of PU and TEAB concentration on the dielectric constant of the solution, it was observed that the dielectric constant increased with TEAB (Figure 4).

Figure 4.

The effect of polyurethane (PU) and tetraethylammoniumbromide (TEAB) concentrations on the dielectric constant of the solution.

There is a sudden jump in dielectric constant at 0.4 TEAB due to the presence of a polar compound (salt). If the solution included TEAB salt, the dielectric constant increased with PU; however, the dielectric constant was variable when the solution did not include TEAB.

Thus, by adding TEAB to the PU solution, the dielectric constant increases because of stronger polarity of the functional groups. There are secondary bonds similar to H-bridges between PU and TEAB ions (Scheme 1).

Also, it was observed that the effects of PU and TEAB concentrations on the dielectric constant were statistically significant (ANOVA test). According to the changing surface tension of the solution with PU and TEAB concentrations, surface tension values increased with PU concentration (Figure 5) because the higher polymer concentration induced stronger intermolecular interactions.

Figure 5.

The effect of polyurethane (PU) and tetraethylammoniumbromide (TEAB) concentrations on the surface tension of the solution.

However, there was no relationship between surface tension and the TEAB concentration. This result is consistent with the literature.³⁴ The effect of the PU concentration on surface tension was statistically significant; however, the effect of TEAB was not (ANOVA test). The effect of PU and TEAB concentrations on the rheological properties of the solution is given in Figures 6 and 7.

Figure 6.

The effect of polyurethane (PU) and tetraethylammoniumbromide (TEAB) concentration on the viscosity of the solution (shear rate: 1 s⁻¹).

Figure 7.

The effect of polyurethane (PU) and tetraethylammoniumbromide (TEAB) concentrations on the complex modulus of the solution (shear stress: 30 Pa).

Rheological properties, such as viscosity and complex modulus values, increased with PU and TEAB concentrations.

Higher polymer concentration induced stronger interactions because of the shorter distance between the macromolecules. Therefore, the viscosity and complex modulus increased with PU concentration. Intermolecular interactions are positively influenced by polar groups, so TEAB makes the functional groups of PU solution more polar. The effects of the concentrations of PU and TEAB on viscosity and complex modulus were statistically significant (ANOVA test, P value of TEAB for viscosity: 0.045, for complex modulus: 0.037).

Relations between independent and dependent parameters

Dependent parameters, such as the number of Taylor cones per square meter, spinning performance for one Taylor cone and total spinning performance values, were determined. It was not possible to spin solutions at PU concentrations of 10 and 12.5 wt%. So, the number of Taylor cones per square meter was zero at this point. The number of Taylor cones per square meter increased with PU and TEAB concentration beginning from a PU concentration of 15 wt%. An amount of 15 wt% of PU without TEAB was close to the border between spinnable and non-spinnable (Figure 8).

Figure 8.

The effect of polyurethane (PU) and tetraethylammoniumbromide (TEAB) concentrations on the number of Taylor cones/m².

The spinnability of solutions starts at 15% PU concentration. Therefore, there is a jump at this point. Further increase in the number of Taylor cones is more or less stable. This phenomenon is related to viscosity and corresponding to the degree of entanglement of macromolecules. If the solution viscosity and degree of entanglement is too low, this solution is non-spinnable. The reason for non-spinnability is the low strength of polymer jets, which break in a short time after having been created and interrupt the spinning process. Solution viscosity increases with the PU concentration.

The PU polymer includes polar groups that increase attractive forces between the macromolecules. By adding TEAB to the PU solution, the functionality of these polar groups increases. This is important, particularly for the consistency and life of the jet coming out of the Taylor cone. If the life of the jet is too short, there is only a small number of cones per area unit of spinning surface, which means poor or absent spinning performance. The effect of the PU and TEAB concentrations on the number of Taylor cones per square meter was statistically significant (ANOVA test). As can be seen in Figure 9, there was no relationship between the PU and TEAB concentrations and the spinning performance for one Taylor cone.

Figure 9.

The effect of polyurethane (PU) and tetraethylammoniumbromide (TEAB) concentrations on the spinning performance of one Taylor cone (SPTC).

It was obvious that the maximum values of SPTC (higher than 0.20 g/h) were obtained from PU concentrations of 17.5 and 20 wt% without TEAB. At a high concentration of solutions, the viscosity is too high and deformability of the jets decreases. Too high viscosity retards creating Taylor cones and the spinning process. The effect of the PU concentration on the spinning performance of one Taylor cone was statistically significant; however, this was not significant for the TEAB concentration. In Figure 10, the effect of PU and TEAB concentrations on the total spinning performance of a PU solution is given.

Figure 10.

The effect of polyurethane (PU) and tetraethylammoniumbromide (TEAB) concentrations on spinning performance.

Spinning performance (polymer throughput) values were zero at 10 and 12.5 wt% of PU because of the low PU concentration. At the 15 wt% point of PU, spinning performance (polymer throughput) values increased with PU and TEAB concentrations. Attractive forces between macromolecules increased with increasing PU and TEAB concentrations. Therefore, the consistency and life of the jet coming out of the Taylor cone increased. This led to a high Taylor cone number and spinning performance values. The polarity of PU macromolecules increases with adding TEAB salt, which leads to stronger attractive forces between macromolecules. Therefore, the same result as mentioned above was obtained for the Taylor cone number and spinning performance values. Taylor cones need a specific time interval (T), statistically, to be created. If the life of a jet is shorter than this time T, only a few cones form on the spinning surface. The effect of PU and TEAB concentrations on spinning performance was statistically significant (ANOVA test).

Quality of nanofibers and nanofiber layers

Fiber morphology, including fiber diameter, diameter uniformity and NFA, was analyzed using SEM. The SEM pictures of the series at 1000 × and 15,000× magnification are given in Figures 11–13.

Figure 11.

Scanning electron microscopy (SEM) images of nanofibers with various polyurethane (PU) and tetraethylammoniumbromide (TEAB) concentrations (1000–15,000×). (a) 12.5% PU – 1.27% TEAB; SP: 0.106 (g/min/m). (b) 15% PU – 0.4% TEAB; SP: 0.021 (g/min/m). (c) 15% PU – 0.8% TEAB; SP: 0.248 (g/min/m). (d) 15% PU – 1.27% TEAB; SP: 0.617 (g/min/m).

Figure 12.

Scanning electron microscopy (SEM) images of nanofibers with 17.5% polyurethane (PU) and various tetraethylammoniumbromide (TEAB) concentrations (1000–15,000×). (a) 0% TEAB – SP: 0.121 (g/min/m). (b) 0.4% TEAB – SP: 0.132 (g/min/m). (c) 0.8% TEAB– SP: 0.872 (g/min/m). (d) 1.27% TEAB– SP: 0.884 (g/min/m).

Figure 13.

Scanning electron microscopy (SEM) images of nanofibers with 20% polyurethane (PU) and various tetraethylammoniumbromide (TEAB) concentrations (1000–15,000×). (a) 0% TEAB – SP: 0.170 (g/min/m). (b) 0.4% TEAB – SP: 0.426 (g/min/m). (c) 0.8% TEAB – SP: 0.955 (g/min/m). (d) 1.27% TEAB – SP: 1.314 (g/min/m).

As can be seen in Figure 11(a), low fiber density and high NFA were obtained. This is because of the low PU concentration and consequent low spinning performance (polymer throughput). High fiber density and a uniform nanoweb structure were obtained from 15 wt% PU with 0.4, 0.8 and 1.27 wt% of TEAB.

Uniform nanofibrous webs with high density were obtained from 17.5 wt% PU with 0, 0.4, 0.8 and 1.27 wt% of TEAB. In particular, at 0.8 and 1.27 wt% of TEAB concentration, stickiness begins between the fibers because of the high PU and TEAB concentrations (Figure 12). High fiber stickiness was found in solutions with 0.8 and 1.27% TEAB and 20 wt% PU (Figure 13). Microfibers were obtained from PU solutions with 0.8 and 1.27 wt% of TEAB because of the high PU and TEAB concentrations and interactions between the PU and TEAB macromolecules.

The effect of PU and TEAB concentrations on fiber diameter are shown in Figure 14. According to this figure, fiber diameter increased with the PU and TEAB concentrations. This result is in agreement with the literature.³⁵

Figure 14.

The effect of polyurethane (PU) and tetraethylammoniumbromide (TEAB) concentrations on the fiber diameter.

The effect of the PU concentration on fiber diameter was statistically significant, although the effect of the TEAB concentration was not (ANOVA test). Histograms of the nanofibers shown in Figures 11–13 are given in Figures 15–17. Generally, uniform and unimodal curves were obtained from all histograms.

Figure 15.

Fiber diameter distributions with various polyurethane (PU) and tetraethylammoniumbromide (TEAB) concentrations.

Figure 16.

Fiber diameter distributions with 17.5% polyurethane (PU) and various tetraethylammoniumbromide (TEAB) concentrations.

Figure 17.

Fiber diameter distributions with 20% polyurethane (PU) and various tetraethylammoniumbromide (TEAB) concentrations.

The effect of the PU and TEAB concentrations on the FDUC is given in Figure 18.

Figure 18.

The effect of polyurethane (PU) and tetraethylammoniumbromide (TEAB) concentrations on the fiber diameter uniformity coefficient (FDUC).

There was no relationship between the PU and TEAB concentrations and the uniformity coefficient. The uniformity coefficient close to 1 indicates maximum uniformity. The fibers obtained from the solution containing 15 wt% PU and 1.27 wt% TEAB had the highest FDUC of 1.031. The NFA percentage was considered to be 100 for non-spinnable solutions. As the TEAB concentration increased, the NFA decreased, except with 10 and 12.5 wt% PU (Figure 19).

Figure 19.

The effect of polyurethane (PU) and tetraethylammoniumbromide (TEAB) concentrations on the non-fibrous area of percentage.

Scheme 1.

Chemical interaction between polyurethane (PU) and tetraethylammoniumbromide (TEAB).

The NFA percentage decreases as the PU concentration increases. Also, the effect of the PU concentration on the FDUC and NFA percentage was statistically significant, although the effect of the TEAB concentration was not (ANOVA test).

Conclusions

A solution of PU (Larithane LS 1086, product of Novotex, Italy) in DMF was used for this study. The solvent was modified by adding TEAB salt. The PU concentration was equal to 10, 12.5, 15, 17.5 or 20% by weight. The salt concentration for each solution was established at a level of 0, 0.4, 0.8 and 1.27% by weight.

The study focused on the role of independent variables, namely polymer concentration, type of solvent, electrical conductivity, dielectric constant, surface tension and the rheological properties of the solution, on the properties of the roller (needle-free) electrospinning process.

New measures describing the roller electrospinning process were introduced. These are: Taylor cone density, total spinning performance, spinning performance for one Taylor cone, FDUC and NFA coefficient.

All chosen independent parameters play a significant role in the electrospinning process. PU concentration is the most important parameter. The role of salt is also important; however, it does not have notable influence on solution surface tension, nanofiber production efficiency for one Taylor cone, fiber diameter, diameter uniformity coefficient and NFA percentage.

The study describes the nanofiber fabrication process for the roller (needle-free) electrospinning method that delivers a high mass of nanofibers at specified fiber diameter and nanofiber mass density.

Footnotes

Acknowledgements

The authors would like to thank Mr Tuan Anh Dao and Baturalp Yalçınkaya for their help. The authors would also like to thank the Nonwoven Department of the Textile Engineering Faculty, Technical University of Liberec in the Czech Republic for providing the working environment, including laboratory space and equipment.

Funding

This work was supported by a grant from the Czech Ministry of Industry and Commerce 1 H-PK2/46 and the Unit of Scientific Research Projects of Süleyman Demirel University in Turkey (Project No: 1796-D-09).

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