Electrospinning process

Electrospinning, a spinning technique, is a unique approach using electrostatic forces to produce fine fibers from polymer solutions or melts and the fibers thus produced have a thinner diameter (from nanometer to micrometer) and a larger surface area than those obtained from conventional spinning processes. Furthermore, a DC voltage in the range of several tens of kVs is necessary to generate the electrospinning. Various techniques such as electrostatic precipitators and pesticide sprayers work similarly to the electrospinning process and this process, mainly based

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on the principle that strong mutual electrical repulsive forces overcome weaker forces of surface tension in the charged polymer liquid [102]. Currently, there are two standard electrospinning setups, vertical and horizontal. The Experimental setup of electrospinning shown in Figure 2.8.

Figure 2.8: Typical setup of electrospinning device [103].

Basically, an electrospinning system consists of three major components: a high voltage power supply, a spinneret (e.g., a pipette tip) and a grounded collecting plate (usually a metal screen, plate, or rotating mandrel) and utilizes a high voltage source to inject charge of a certain polarity into a polymer solution or melt, which is then accelerated towards a collector of opposite polarity [104,105]. Most of the polymers are dissolved in some solvents before electrospinning, and when it completely dissolves, forms polymer solution. The polymer fluid is then introduced into the capillary tube for electrospinning. However, some polymers may emit unpleasant or even harmful smells, so the processes should be conducted within chambers having a ventilation system [106].

In the electrospinning process, a polymer solution held by its surface tension at the end of a capillary tube is subjected to an electric field and an electric charge is induced on the liquid surface due to this electric field. When the electric field applied reaches a critical value, the repulsive electrical forces overcome the surface tension forces. Eventually, a charged jet of the solution is ejected from the tip of the Taylor cone and an unstable and a rapid whipping of the jet occurs in the space between the capillary tip and collector which leads to evaporation of the solvent, leaving a polymer behind [107-109].

The jet is only stable at the tip of the spinneret and after that instability starts. Thus, the electrospinning process offers a simplified technique for fiber formation [110].

19 2.6.2 Effects of parameters on electrospinning

The electrospinning process is solely governed by many parameters, classified broadly into solution parameters, process parameters, and ambient parameters.

Each of these parameters significantly affect the fibers morphology obtained as a result of electrospinning, and by proper manipulation of these parameters we can get nanofibers of desired morphology and diameters [111].

2.6.3 Solution parameters requirement of optimal viscosity for electrospinning [112]. At very high viscosity polymer solutions usually exhibit longer stress relaxation times, which could prevent the fracturing of the ejected jets during electrospinning. An increase in solution viscosity or concentration gives rise to a larger and more uniform fiber diameter [113]. For solution of low viscosities, surface tension is the dominant factor and just beads or beaded fibers are formed while above a critical concentration, a continuous fibrous structure is obtained and its morphology is affected by the concentration of the solution [114].

2.6.3.2 Surface tension

Surface tension, more likely to be a function of solvent compositions of the solution plays a critical role in the electrospinning process and by reducing the surface tension of a nanofiber solution; fibers can be obtained without beads. Different solvents may contribute different surface tensions. Generally, the high surface tension of a solution inhibits the electrospinning process because of instability of the jets and the generation of sprayed droplets [115]. Basically, surface tension determines the upper and lower boundaries of the electrospinning window if all other variables are held constant [116-118].

20 2.6.3.3 Concentration

There should be an optimum solution concentration for the electrospinning process, as at low concentrations beads are formed instead of fibers and at high concentrations the formation of continuous fibers are prohibited because of the inability to maintain the flow of the solution at the tip of the needle resulting in the formation of larger fibers [119]. Researchers have attempted to find a relationship between solution concentration and fiber diameter and they found a power law relationship, that increasing the concentration of solution, increases the fiber diameter with gelatin electrospinning [120-121].

2.6.3.4 Molecular weight

Molecular weight of the polymer has a significant effect on rheological and electrical properties such as viscosity, surface tension, conductivity and dielectric strength [122]. This is the other important solution parameter that affects the morphology of electrospun fiber and generally high molecular weight polymer solutions have been used in electrospinning as they provide the desired viscosity for the fiber generation.

It has been observed that too low a molecular weight solution tends to form beads rather than fibers and a high molecular weight solution gives fibers with larger average diameters. Molecular weight of the polymer reflects the number of entanglements of polymer chains in a solution, thus solution viscosity. Chain entanglement plays an important role in the processing of electrospinning. It has been observed that high molecular weights are not always essential for the electrospinning process if sufficient intermolecular interactions can provide a substitute for the interchain connectivity obtained through chain entanglements, and using this principle, researchers have prepared oligomer-sized phospholipids from lecithin solutions into nonwoven membranes through electrospinning [123-124].

2.6.3.5 Conductivity/surface charge density

Polymers are mostly nonconductive, with a few exceptions of dielectric materials, and the charged ions in the polymer solution are highly influential in jet formation.

Solution conductivity is mainly determined by the polymer type, solvent used, and the availability of ionisable salts. It has been found that with the increase of electrical conductivity of the solution, there is a significant decrease in the diameter of the electrospun nanofibers whereas with low conductivity of the solution, there results

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insufficient elongation of a jet by electrical force to produce uniform fiber, and beads may also be observed. Researchers have showed that highly conductive solutions are extremely unstable in the presence of strong electric fields, which results in a dramatic bending instability as well as a broad diameter distribution [125].Generally, electrospun nanofibers with the smallest fiber diameter can be obtained with the highest electrical conductivity and it has been found that the there is drop in the size of the fibers is due to the increased electrical conductivity. It was observed that the jet radius varied inversely with the cube root of the electrical conductivity of the solution [126-134].

2.6.4 Processing parameters

Another important parameter that affects the electrospinning process is the various external factors exerting on the electrospinning jet. This includes the voltage supplied, the flow-rate, type of collector, and distance between the needle tip [135].

2.6.4.1 Applied voltage

The high voltage will induce the necessary charges on the solution and together with the external electric field, will initiate the electrospinning process when the electrostatic force in the solution overcomes the surface tension of the solution.

Generally, both high negative or positive voltage of more than 6kV is able to cause the solution drop at the tip of the needle to distort into the shape of a Taylor Cone during jet initiation [136]. Depending on the feedrate of the solution, a higher voltage may be required so that the Taylor Cone is stable. The columbic repulsive force in the jet will then stretch the viscoelastic solution. If the applied voltage is higher, the greater amount of charges will cause the jet to accelerate faster and more volume of solution will be drawn from the tip of the needle. This may result in a smaller and less stable Taylor Cone [137]. When the drawing of the solution to the collection plate is faster than the supply from the source, the Taylor Cone may recede into the needle [138].

Applied voltage has also effects on the morphology and the resultant fibers. Increase in the applied voltage results with a decrease in the fiber diameter. Generally, high voltage results with higher bead formation, but increased jet stretching leads to fewer amounts of beads [139]. At lower voltage, due to the weaker electrostatic force, flight time may last longer. Longer flight time lets the jet to elongate and stretch

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stronger and longer resulting with reduced fiber diameter. Wang and his research team measured both jet diameter and fiber diameter and investigated the effect of voltage difference [140]. They found out that both jet diameter and fiber diameter decreased slightly. Also, better chain orientation within electrospun fibers was seen by an increase in the applied voltage. A proportionality relation between applied voltage and fiber diameter was reached in a study [141].

2.6.4.2 Flow rate

The flow rate of the polymer from the syringe is an important process parameter as it influences the jet velocity and the material transfer rate. A lower feed rate is more desirable as the solvent will get enough time for evaporation [142]. There should always be a minimum flow rate of the spinning solution. It has been observed that the fiber diameter and the pore diameter increases with an increase in the polymer flow rate in the case of polystyrene (PS) fibers and by changing the flow rate, the morphological structure can be slightly changed. Few studies have systematically investigated the relationship between solution feed or flow rate on fiber morphology and size [143-144].

If the flowrate is at the same rate which the solution is carried away by the electrospinning jet, there must be a corresponding increased in charges when the flowrate is increased. Thus there is a corresponding increase in the stretching of the solution which counters the increased diameter due to increased volume. Due to the greater volume of solution drawn from the needle tip, the jet will takes a longer time to dry. As a result, the solvents in the deposited fibers may not have enough time to evaporate given the same flight time. The residual solvents may cause the fibers to fuse together where they make contact forming webs. A lower flowrate is more desirable as the solvent will have more time for evaporation [145].

2.6.4.3 Distance

The distance (the distance from the terminus of the capillary tip to the surface of the collector) defines the strength of the electric field as well as the time available for evaporation of the solvent before the nanofibers reach the collector surface.

Increasing the distance, leaving other parameters constant, generally reduces fiber diameters [146]. Reducing the distance does not affect size and shape of the fibers, but, inhomogeneously distributed beads can be observed [147]. These beads can be

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due to increase in the electric field strength. If the distance between the tip and the collector is longer, solution jet finds more time for the evaporation of the solvent and jet can be stretched sufficiently before it lands to the collecting media. Increasing the working distance enhances both the number of beads and the density of the fibers [148]. Jet diameter dependence on the working distance is studied [149].

2.6.4.4 Effect of collector

In the electrospinning process, usually conductive material is used to cover collecting media. Aluminium foil is one of the most common conductive materials that are used for collection of fibers onto it.

One important aspect of the electrospinning process is the type of collector used. In this process, a collector serves as a conductive substrate where the nanofibers are collected. Generally, aluminium foil is used as a collector but due to difficulty in transferring of collected fibers and with the need for aligned fibers for various applications, other collectors such as, conductive paper, conductive cloth, wire mesh [150], pin [151], parallel or grided bar [152], rotating rod, rotating wheel [153], liquid non solvent such as methanol coagulation bath [154] and others are also common types of collectors nowadays. With less conductive area, there was generation of beaded fibers because of the less surface area. In another study they compared wire screen with aluminium foil and wire screen without aluminium foil in the same conductive area and found that pure wire screen is a better collector for fiber collection because with the use of wire screen the transfer of fibers to other substrates became easy. The fiber alignment is determined by the type of the target/collector and its rotation speed [155].

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25 3. EXPERIMENTAL PART

3.1 Materials

Acrylonitrile (Dow, 99.5>%) vinyl acetate (Innovenne, 99.5>%) were provided by the Aksa Acrylic Factory of Turkey. Ammonium persulfate (APS) was Merck product. Sodium dodecyl benzene sulfonic acid salt was purchased from Sigma Aldrich. Dimethylformamide (DMF), methanol and ethanol were all Merck reagents.

The Pyrrole (Py) was an Aldrich reagent. All these reagents were used as received.

3.2 Synthesis of P(AN-co-VAc) Copolymer

P(AN-co-VAc) latex was prepared by emulsion polymerization technique. The typical process for obtaining latex is as follows: 10 g of a mixture of monomers of AN and VAc (70:30 wt AN:VAc) were dropped into the three-neck flask, the mixture was stirred with distilled water during 20 minutes. Monomers were shown in Figure 3.1. Then, 0.5 g emulsifier sodium dodecyl benzene sulfonic acid (DBSA) was dissolved in 60 mL water. After that, DBSA was added to a stirred aqueous solution. 20 Minutes later, the dissolved 0.92 g APS in 20 mL distilled water was added to the flask. The copolymerization was carried out in a flask equipped with a stirrer (700 rpm) and a condenser and the temperature was adjusted to 70 ^oC and continued for 3 h under stirring. Copolymer structure was shown in Figure 3.2.

Acrylonitrile (AN) Vinyl Acetate (VAc)

Figure 3.1: Monomers used in synthesis of P(AN-co-VAc).

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Figure 3.2: P(AN-co-VAc) structure.

Experimental setup of the polymerization was demonstrated in Figure 3.3. After the copolymerization, the prominent amount of polymer was washed and filtered with distilled water and ethanol, dried at room temperature. Residual emulsion was used for continued polymerization in order to obtain P(AN-co-VAc)/ PPy composites.

Figure 3.3: Experimental setup for synthesis of P(AN-co-VAc).

3.3 Coating of P(AN-co-VAc) Latex Particles with Polypyrrole

After obtaining P(AN-co-VAc) latex, the mixture was cooled for 30 min at room temperature and then, was transferred to 5 round-bottom flasks at equal volume. Four flasks were seperated for the coating processes. One of them is kept for comparison.

Then, pyrrole monomer with different quantities were added dropwisely to the each seperated flasks at room temperature and the reaction continued for 18 hours. The content of the P(AN-co-VAc)/PPy composite was given in Table 3.1. Latex form of

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composites were given in the Figure 3.4. After the 18 hours, reaction was stopped.

Obtained composites in emulsion form were precipitated in methanol. Each product was filtered using filter paper, for the seperation of impurities and oligomers, the resultant products was washed with deionized water and ethanol, dried at room temperature. Composite structure was given in Figure 3.5.

Figure 3.4: Latex form of VAc), VAc)@PPy1, P(AN-co-VAc)@PPy2, P(AN-co-VAc)@PPy3, P(AN-co-VAc)@PPy4, respectively.

Figure 3.5: The structure of composite.

Table 3.1: Feed content of Py and content of PPy in composition.

Sample of Composite Feed Py Monomer Mole

Fraction (mol %)

PPy Mole Fraction (mol %)^a

P(AN-co-VAc)@PPy1 0.030 0.052 P(AN-co-VAc)@PPy2 0.045 0.067 P(AN-co-VAc)@PPy3 0.060 0.082 P(AN-co-VAc)@PPy4 0.075 0.052

a:Determined from¹H-NMR

28 3.4 Preparation of Electrospinning Solutions

Five series of P(AN-co-VAc)/PPy composite solutions with different content of pyrrole dissolved in DMF at room temperature. For each solutions were stirred at room temperature with the speed of 300 rpm at least 4 hour. View of electrospinning solutions were depicted in the Figure 3.6. The polymers were dissolved in DMF and were used to prepare nanofiber mats. The electrospinning conditions are shown in the Table 3.2. The electrospinning apparatus consists of a syringe pump (NE-500 model, New Era Pump Systems, Inc., USA) with feeding rate from 5.5 µL/h to 400 mL/h, high voltage DC power supplier generating positive DC voltage up to 50 kV DC power supply (ES50 model, Gamma High Voltage Inc., USA) and a grounded collector which is covered with aluminum foil. Solution was loaded into a syringe having 12 mm diameter and positive electrode was clipped onto the dispensing needle having 0.8 mm outer diameter. The feeding rate of the polymer solution was controlled by syringe pump.

Table 3.2: Electrospinning parameters for various composites.

Electrospinning Parameters

Solution concentration 10 %

Applied voltage 15 kV

Tip-to-collector distance 15 cm

Feed rate 0.5 mL/h

Figure 3.6: View of electrospinning solutions of (a) co-VAc) and (b)

P(AN-co-VAc)@ PPy4.

a ^b

29 3.5 Electrospinning Process Setup

The metal collector was covered with an aluminum foil. The setup was kept in a plexiglass box for experimenter’s safety. All experiments were carried out under atmospheric pressure and at room temperature. The positive electrode wire was hooked at the metal part of the needle and negative part of the electrode was attached to the metal collector 5 to 60 minutes of operation time was sufficient for the deposition of fibers on aluminum foil. A horizontal setup was chosen for electrospinning process. A picture that was captured during electrospinning is illustrated in Figure 3.7.

Figure 3.7: A representative view of electrospinning devices.

3.6 Characterization of P(AN-co-VAc) Copolymer and P(AN-co-VAc)/PPy Composites

The structure of the copolymers was characterized by means of ¹H NMR spectroscopy and Fourier Transform Infrared-Attenuated Total Reflectance (FTIR-ATR) spectroscopy, UV Visible Spectrophotometry.

 FTIR analysis of P(AN-co-VAc) and P(AN-co-VAc)@PPy1, P(AN-co-VAc)@PPy2, P(AN-co-VAc)@PPy3, P(AN-co-VAc)@PPy4 polymers in powder form and as nanofiber webs were carried out with FTIR-ATR reflectance spectrophotometer (Perkin Elmer, Spectrum One, with a Universal ATR attachment with a diamond and ZnSe crystal) (Figure 3.8).

High Voltage power supply

syringe pump

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Figure 3.8: Perkin Elmer FTIR-ATR spectrophotometer.

 ¹H NMR spectroscopy by using 250 MHz Bruker AC Aspect 3000 NMR spectrometer and 500 MHz Agilent VNMRS Nuclear Magnetic Resonance Spectrometer (Figure 3.9) with deuterated dimethyl sulphoxide, DMSO as a solvent. Values were recorded as ppm relative to internal standard (TMS).

Figure 3.9: Agilent VNMRS 500 MHz Nuclear Magnetic Resonance Spectrometer.

 Viscosity average (Mv) molecular weight of P(AN-co-VAc) was determined in DMF solvent using Ubbelohde equipment. The intrinsic viscosity of the polymer solutions in DMF was determined by Ubbelodhe viscometer at 27.5 for P(AN-co-VAc). The molecular weight was calculated for P(AN-co-VAc) by the Mark–Houwink equations [η] = K×Mvα , K = 2.78×10⁻⁴, α=0.75 [174].

Specific viscosities of P(AN-co-VAc) and P(AN-co-VAc) were calculated by the Ubbelohde viscometer.

 The DSC measurements were carried out on TA Q1000 DSC instrument DSC (Figure 3.10). The measurements of polymers were operated with 4 cycles.

First cycle was heating from -20°C to 250°C, second cycle was cooling from 250°C to -20°C, third cycle was heating from -20°C to 250°C and fourth cycle was heating from -250°C to -30°C under N2 atmosphere. Third cycles were considered for DSC analysis.Each sample was scanned at a heating rate of 10 °C min⁻¹.

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Figure 3.10: TA Q1000 DSC instrument.

 Latex particles and resulting fibers were also characterized as morphological by Scanning Electron Microscope (two diffrent SEM devices are used, Gemini Leo Supra 35 VP and Carl Zeiss EVO MA 10) and by Atomic Force Microscope(Nanosurf EasyScan2 STM, Figure 3.11). The samples for the SEM measurements are prepared by coating of gold (Ion Sputter Metal Coating Device, MCM-100).

Figure 3.11: Nanosurf EasyScan2 AFM.

 Thermal and mechanical properties of molded films obtained by polymer composites were measured by using a TA Q800 Dynamic Mechanical Analyser, DMA (Figure 3.12). Thermal measurements of polymeric films were operated by multi- frequency-strain test method. Samples were heated from the ambient temperature to 150 ^oC by a heating rate of 3 ^oC/min with an applied frequency of 1 Hz. Mechanical measurements were operated by DMA controlled-force test method. The size of the P(AN-co-VAc)@PPy1

film is 10.70 mm, 4.52 mm, 0.054 mm as length, width, thickness respectively. The size of the P(AN-co-VAc)@PPy2 film is 11.00 mm, 5.40 mm, 0.055 mm as length, width, thickness respectively. The size of the P(AN-co-VAc)@PPy3 film is 12.00 mm, 5.40 mm, 0.061 mm as length, width, thickness respectively. The size of the P(AN-co-VAc)@PPy4 film is 14.94 mm, 5.85 mm, 0.055 mm as length, width, thickness respectively.

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Figure 3.12: TA Q800 Dynamic Mechanical Analyser.

 UV analysis of P(AN-co-VAc)@PPy1, P(AN-co-VAc)@PPy2, P(AN-co-VAc)@PPy3, P(AN-co-VAc)@PPy4 nanofibers that were dissolved in DMF

 UV analysis of P(AN-co-VAc)@PPy1, P(AN-co-VAc)@PPy2, P(AN-co-VAc)@PPy3, P(AN-co-VAc)@PPy4 nanofibers that were dissolved in DMF