Effects of Electrospinning Process Parameters on the Morphology of Polymeric Nanofiber Membranes

In this study, polymeric nanofiber membranes were prepared using electrospinning technique with biocompatible polymers such as polycaprolactone (PCL), polyvinyl alcohol (PVA), and polyacrylonitrile (PAN). The morphological effects on nanofiber membranes obtained by maintaining the electrospinning technique parameters of voltage, working distance, and feed flow rate constant between each other were investigated. The morphological (SEM-Scanning Electron Microscope) properties of the produced polymeric membranes were determined. The morphological characterization of the obtained polymeric nanofiber membranes aims to investigate the effects on electrospinning working parameters and the use of membranes resulting from these effects in sectors such as healthcare, filtration, defense, textiles, and agriculture. The concept of "nanotechnology" is a combination of the words nano and technology. Technology is the body of knowledge used to produce useful products and design new products. Nano means "dwarf" in Greek. Expressions defined by nano represent one billionth of any measurement. For example; nanometer expresses one billionth of a meter (1 nm=10⁻⁹ m). When nanoscale structures are viewed in terms of length, they correspond to approximately 10-100 atom (10⁻⁹ meter) systems. For materials within nanotechnology, any dimension (length, width, or thickness) between 100 and 1 nanometer (nm) (1/10 millionth meter to 1/1 billionth meter) is expressed.

Nanofibers can be defined as fibers with a diameter of one micron or less. Today, nanofibers can be produced from many ceramic and polymer materials using various production methods. Nanofibers are fibers produced from polymer solution and polymers and melted at dimensions below micrometers using various methods [1,2].

Today, there are many nanofiber production methods. These include; • Meltblowing, • Fibrillation, • Bicomponent, • Spunbond, • Electrospinning method. The electrospinning method is the process of converting a liquid into nano-sized fibers by applying kV voltage to viscous liquids at very low flow rates. In its application, polymer solution or melt is used as the viscous liquid. The solution coming with uniform flow to the tip of a medical syringe forms a spherical droplet under the effect of surface tensions and thins under the effect of electric forces (Taylor cone) and is transferred as nanofibers to a grounded collector at a specific distance [3]. Figure 1.1 shows the working principle of the electrospinning process. [caption id="attachment_124712" align="aligncenter"] Figure 1.1. Working principle of electrospinning process [4][/caption]  The electrospinning method can be influenced by a series of variables. These parameters can be classified as solution properties, controllable variables, and environmental parameters. Solution properties include polymer viscosity, conductivity, surface tension, and molecular weight. It is difficult to ignore the effect of solution properties. Because changing one parameter usually affects other solution properties. Controllable variables include flow rate, electric field strength, distance between needle tip and collector plate, needle tip design, and collector plate arrangement and geometry. Environmental parameters are temperature, humidity, and air velocity. The concentration of the polymer solution plays a major role in fiber formation during the electrospinning process. Four critical concentrations from low to high should be considered.

• When concentration is very low, polymeric nanoparticles will be obtained. Meanwhile, electrospray occurs instead of electrospinning due to low solution viscosity and high surface tensions.
• When concentration is somewhat higher, bead structure and fiber mixture will be obtained.
• When concentration is appropriate, smooth nanofibers can be obtained.
• When concentration is very high, spiral-shaped microribbons can be observed instead of nano-scale fibers.

The molecular weight of the polymer also has a significant effect on fiber morphologies obtained through the electrospinning process. In principle, molecular weight reflects the entanglement of polymer chains in solutions. By increasing molecular weight, smooth fibers can be obtained. When molecular weight is increased further, microribbons can be observed. However, for electrospinning, molecular weight is not always effective if intermolecular interactions are supported by oligomers. Solution viscosity is an important factor in calculating fiber morphology. It has been proven that continuous and smooth fibers cannot be obtained at very low viscosity and very high viscosity causes difficulties in solution spraying. Therefore, having an appropriate viscosity for electrostatic spinning is a requirement. Generally, solution viscosity can be adjusted by changing the polymer concentration of the solution. Viscosity, polymer concentration, and polymeric molecular weight are interrelated parameters.

Surface tension is a very important factor in electrospinning as a function of the solvent mixtures of the solution. According to research, it has been observed that different solvents produce different surface tensions. By improving concentration, reducing the surface tension of the solution can lead to the transformation of droplet fibers into smooth fibers. Basically, surface tension determines the upper and lower limits of electrospinning when all other conditions are idealized.

Increasing solution conductivity or charge density can be used to produce smoother fibers with fewer beads. In general, natural polymers are usually polyelectrolytes, and ions increase the charge-carrying capability of the polymer jet, causing high tension under the electric field. Applied voltage is one of the most studied parameters among controllable variables. Only when applied voltage is higher than the threshold voltage do charged jets begin to pass through the Taylor cone. At low voltages, the drop hangs on the needle tip and the Taylor cone produces a spheroid-free jet return. As voltage increases, the volume of the drop at the needle tip decreases and causes the Taylor cone to recede. When voltage is increased somewhat further, the jet begins to circulate at the edge of the needle tip without a visible Taylor cone. Multiple particulate structures can be observed under these conditions. As a result, the effect of tension on the diameters of the resulting fibers can be mentioned.

The flow rate of the polymer solution in the syringe is another important process parameter. In general, a low flow rate is recommended for the polymer solution to reach polarization in sufficient time. When flow rate is too high, coarse-diameter particulate fibers will form instead of fine-diameter smooth fibers. This is because of the short drying time and low stretching force before reaching the collector plate.

During electrostatic spinning, collector plates usually serve as a conductive substrate to collect charged fibers. Various plates developed along with the need to transfer fibers. These plates include; a) wire mesh, b) pins, c) grid, d) parallel or grid rods, e) rotating rods or cylinders, f) liquid bath and the like. It is known that the distance between the collector plate and syringe affects fiber diameter and morphology. In short, when the distance is short, the fibers will not have sufficient time to solidify before reaching the collector plate. If the distance is too large, particulate fibers can be obtained. One of the important physical properties of fibers obtained by electrostatic spinning is solvent-induced dryness. Therefore, it is important to set the optimal distance.

Environmental parameters such as humidity and temperature can also affect fiber diameters and morphology. An increase in temperature causes fibers to produce lower diameter. This decrease in diameter results from a decrease in the viscosity of the polymer solution at high temperature. Low humidity can cause complete drying of the solvent and an increase in the solvent evaporation rate. Increased humidity causes the formation of small circular pores on the fiber surface. Further increase in humidity causes the pores to merge.

When the fiber concept is generally considered, the term "nano" refers to the size of the fiber diameter. One of the most important techniques developed for fiber formation is the electrospinning method. With this production method, it is possible to obtain nano-sized, low-weight, and high mechanical strength biocompatible materials. The electrospinning technique is the process of converting liquid into nano-scale fibers by applying kV voltage to viscous liquids at very small flow rates [5]. The effects of working parameters on nanofiber and bead formation in the electrospinning technique are shown in Figure 1.2.   Applying high voltage to the polymer solution is the key point of the electrospinning process. When voltage is applied, the resulting electric field affects the stretching and acceleration of the jet. The distance between the needle tip and collector plate will determine the distance at which the jet will form and will affect the intensity of electrostatic forces. Fiber formation occurs at this distance, the solvent is removed and the fiber solidifies, meanwhile the storage time of the fiber on the collector is realized. Feed rate is an important parameter affecting jet velocity and material transfer rate [7]. Figure 1.3 shows the Taylor cone formation in the electrospinning technique. [caption id="attachment_124714" align="aligncenter"] Figure 1.3. Taylor cone formation in electrospinning technique [8][/caption]  In this study, electrospinning working parameters were applied using various polymers and it was aimed to investigate the effects on the morphology of polymeric membranes.

2. Material and Method

2.1. Materials

Polycaprolactone (PCL) polymers with Mw: 80,000 g/mol, polyvinyl alcohol (PVA) with Mw: 85,000-124,000 g/mol and commercial polyacrylonitrile (PAN), distilled water, dimethylformamide (DMF), chloroform, and parchment paper are used. Sigma-Aldrich brand was preferred as reference.

2.2. Method 2.2.1. Preparation of Nanofiber Membrane Solutions and Nanofiber Membrane Production with Electrospinning Process

Solutions were prepared according to the values in Table 2.1 prior to the electrospinning process using PCL, PVA, and PAN polymers. Nanofibers were produced by applying the electrospinning process working parameters in Table 2.2 to the prepared solutions. In the electrospinning process, working parameters were applied with two parameters kept constant and one parameter variable between each other to ensure production. [caption id="attachment_124715" align="aligncenter"] Table 2.2. Application values of electrospinning process working procedures for nanofiber membrane production[/caption]   [caption id="attachment_124716" align="aligncenter"] Table 2.1. Parameters for preparation of polymeric solutions for nanofiber membrane production[/caption]  

2.2.2. Characterization Studies

Fabrics placed in holders were examined and photographed with a ZEISS EVO SEM microscope. During the examination of the diameter and dimensions of the produced composite nanofibers, x6000 magnification images were examined for SEM analysis at 7 kV potential. The average fiber diameter thickness of nanofibers was measured on high-resolution SEM photographs using Image j (National Health Organization) software.

3. Results and Discussion 3.1. SEM Analysis

Nanofiber formation was observed in all samples. PCL, PVA, and PAN polymers were used interchangeably, and one of the working parameters of the electrospinning process was variable. For this reason, when nanofiber membranes were examined, the finest nanofiber structure in the study appeared in the 10% PVA sample [9-15]. Finer nanofibers and homogeneous morphology were obtained only by reducing the voltage value. Figure 3.1 contains SEM morphological images of nanofiber membranes. [caption id="attachment_124717" align="aligncenter"] Figure 3.1. a. Voltage Value Changed-10% PCL, b. Working Distance Value Changed-10% PCL, c. Feed Flow Value Changed-10% PCL, d. Voltage Value Changed-10% PVA, e. Working Distance Value Changed-10% PVA, f. Feed Flow Value Changed-10% PVA, g. Voltage Value Changed-10% PAN, h. Working Distance Value Changed-10% PAN, i. Feed Flow Value Changed-10% PAN[/caption]   In this study, targeted membrane production was carried out using PCL, PVA, and PAN polymers by the electrospinning method. Nanofibers were obtained by applying two fixed variable values among tension, flow rate, and working distance between PCL, PVA, and PAN polymers. Tension, flow rate, and working distance cause significant changes in fiber morphology. The finest nanofibers in the study were determined as a result of changing the feed flow value in the 10% PVA sample. This can be seen in the SEM images. The obtained polymeric membranes are intended for use in sectors such as healthcare, filtration, defense, textiles, and agriculture.  

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  Erdi Buluş Nanotechnology-Materials Technology Specialist Metallurgy and Materials Engineer İstanbul Arel University ArelPOTKAM Gülseren Sakarya Buluş Health Sciences-Expert Nurse Silivri District Health Directorate Silivri Dr. Mehmet Akkaş Engineering and Architecture Faculty Mechanical Engineering Department/Material-Nanotechnology-Biotechnology Kastamonu University