Abstract: This research work reports on the influence of process parameters like voltage, spinning electrode speed, distance between electrodes and solution concentration on the morphology and tensile properties of electrospun nanofibrous PVA webs. The electrospinng spinning assignment is carried out with the commercially available Nanospinder machine. The morphological developments are explained on the basis of nanofibre diameter and web density as depicted by the FESEM images. The tensile properties are measured in terms of the mean tensile stress and Young’s modulus. The influence of individual parameters in isolation is studied herewith for assessing their individual influence and accordingly can be put together in order to control the process for proper development of nano webs morphology. The correlation of morphological developments and tensile properties is brought out for deciding the effective combination of process parameters in order to obtain stronger nanofibrous webs.
INTRODUCTION
Electrospinning produces continuous fibrous web having diameters ranging from submicrons to nanometers. This is a simplified technique for development of porous structures with very high surface area. Electrospinning involves the application of electrical force to charge and extrude polymer solutions to obtain nanofibres (Salem, 2007; Frenot and Chronakis, 2003; Sundaray, 2006). Electrospinning consists of four operational stages; a charged cone jet is initiated by applying electric field on the droplet of the polymer solution, then the developed stable jet is accelerated and stretched smoothly in a straight line away from the spinning electrode, subsequently the jet becomes unstable and splits to form fibre structure and finally charged nanofibrous web are formed and collected as a nonwoven web (Bazbouz, 2009).
Electrospun nano-structured web has the potential to be applied in wide range of areas. The high surface area of the web can provide reactive sites for chemicals, hence supporting towards chemical protection (Hsieh et al., 2004). Nano and micro pores of nanoweb provides good moisture and vapor transport properties, making it suitable for breathable sportswear fabrics. Small fibres with large volume of microscopic pores provide many potential applications like insulating fabric, biomaterials and value-added textile fabrics (Hsieh et al., 2004; Ying, 2004; Senecal et al., 2008).
Nanofibres formation and its morphological development by electrospinning is affected by various process parameters; solution properties and concentration, electric potential at the capillary tip, the tip-to-collector distance and spinning conditions (Deitzel et al., 2001; Wang, 2007). In the electrospinning process, controlling the morphology (web density and diameter) is big challenge and difficult task, due to high rate of throughput of polymeric material from spinning head.
Polyvinyl Alcohol (PVA) is a semi-crystalline and hydrophilic polymer with good chemical and thermal stability and biodegradable in physiological environments. PVA is a non-hazardous material, has no negative effects on animals and does not cause any injuries to the skin upon contact. The non-hazardness of PVA polymer with added advantages of its ease of spinning makes it’s the best possible material for development of nanofibrous webs for various applications. Considering these facts, the research endeaviour was taken herewith for preparation of PVA nanofibrous webs by various process parameters such solution concentration, voltage, rotation of spinning electrode and distance between spinning electrodes. The developed webs were subjected for evaluation of morphological and mechanical properties in order to understand the influence of process parameters on the above mentioned properties.
MATERIALS AND METHODS
Sample preparation: Polyvinyl Alcohol (PVA) of hydrolyzed grade was dissolved in distilled water at 80°C with magnetic stirring for at least 3-4 h for obtaining the solution. The PVA solution was electrospun through Nanospider machine for preparation of the webs in accordance with the experimental trials as mentioned in Table 1. The prepared webs were air dried for 20 min to remove residual water molecules.
CHARACTERIZATION
Surface morphology and diameter characterization: Morphology of electrospun fibres were examined by using the field emission scanning electron microscopy (Carlzeiss EV 050). The diameters of nanofibres were estimated by Image J software using the captured FESEM images. The average fibre diameter was determined from 30 mm of the random fibres taken from different areas of the scanned images.
Tensile properties: The tensile properties (Young’s modulus and tensile stress) of the fibrous web were measured on universal tensile testing machine at a crosshead speed of 10 mm min-1 with a specified sample size (length = 20 mm and width = 10 mm). In order to prevent the grips from direct contact with the fibrous web, cushion double sides sticky paper tape were used. Five random samples were tested and mean values are reported (Bazbouz, 2009; Bazbouz and Stylios, 2010).
| Table 1: | Experimental process parameters design matrix of electro spinning set up |
RESULTS AND DISCUSSION
Morphology: The effect of process parameters; voltage, rotation of electrode, distance between electrodes and solution concentration on nanofibre morphology was investigated in terms of Mean Fibre Diameter (MFD) and are presented in Fig. 1, 2, 3 and 4, respectively.
Voltage: Voltage is considered as the most essential parameter in electrospinning, because of the fact that it initiates the jetting and causes instabilities, which subsequently stretch and elongates in the form of nanofibrous web. It is observed from Table 2 and Fig. 1 that the increase in the applied voltage with keeping the other parameters constant, the MFD decreases. The applied voltage may be affecting some factors such as the mass of polymer fed from spinning electrode, the elongation level of the jet by an electrical force and the morphology of the jet single/multiple jets (Huang et al., 2003; Medeiros et al., 2008). The higher voltage may be caused an increase in an electric field and higher electrical forces resulting in a thinner fibre (Huang et al., 2003). It was evident from Fig. 1 that on increasing the voltage, web density was increased with lower size of pore between nanofibres.
Rotation of electrode: It was observed from Table 2 and Fig. 2 that as electrode speed increases, the fibres get thicker. At higher speed a higher mass of solution was ejected from spinning electrode, hence the effective stress on polymeric mass was decreased.
| Table 2: | Mean fiber diameter (MFD) of various experimental trials |
| Fig. 1(a-b): | Effect of applied voltage on fibre diameter, (a) 40 kV and (b) 80 kV (scale 10 μm) |
| Fig. 2(a-b): | Effect of spinning electrode speed on fibre diameter, (a) 6 rpm and (b) 14 rpm (scale 2 μm) |
| Fig. 3(a-b): | Effect of spinning electrode distance on fibre diameter, (a) 110 mm and (b) 150 mm (scale 200 μm) |
Due to decrease in effective stress on mass, jet containing fibrous droplets resulted in less elongation, which is yielded in thicker nanofibrous web (Huang et al., 2003; Medeiros et al., 2008; Rodoplu and Mutlu, 2012; Linh et al., 2010; Zhang et al., 2009). The surface morphology of the nano fibrous web gets denser with visible defects in decreasing electrode speed.
Distance between electrodes: Table 2 and Fig. 3 indicated that the increase in the electrode distance the nanofibres become thicker. The voltage flux defined as a ratio of voltage to distance between the spinning and collecting electrode decreases with the increase in the distance, hence the lower voltage flux yielded in thicker nanofibre diameter with larger pore sizes. It is also observed that there is defects on the nanofibrous webs produced at 110 mm distance because of lower opportunity for the webs to evaporate the solvent before take up at the collecting electrode. Figure 3 revealed and followed the opposite trend of surface morphology as notice with voltage influence, as shown in Fig. 1.
| Table 3: | Mechanical properties of various experimental trials |
| Fig. 4(a-b): | Effect of solution concentration on fibre diameter, (a) 3 w/v% and (b) 6 w/v% (scale 2 μm) |
Solution concentration: It was revealed that the increase in solution concentration, diameter of nanofibre was increased (Table 2 and Fig. 4). At concentrations 3%, the electrospinning process generated a mixture of fibres and droplets (spraying as in the form of beads). These beads disappear as the fibre diameter is increased with increasing polymer concentration to 6%. The appearance of beads at low concentration may be due to instability of droplets at the spinning electrode (Bazbouz, 2009; Huang et al., 2003). Briefly, lower mass of polymeric jet (lesser surface tension), may not be able to withstand effective forces and stress induced by other process variables and thereby, instead of elongation of jet it starts spaying (drops of polymer over the substrate), which is appeared as in the form of beads.
Tensile properties: The tensile properties of the Electrospun nanofibers of PVA nanofibrous web were evaluated to correlate with morphological properties of web and an interpretation has been done with FESEM images. The results of the tensile properties are summarized in Table 3. Tensile properties showed that nanofibrous web having denser morphological structure (higher web density/fibre packing) is having higher tensile stress and Young’s modulus. The denser or compacted fibrous web leads to uniform distribution of the stress, hence result in higher tensile stress and modulus.
CONCULSION
The influence of process parameters on the morphology and tensile properties of electrospun PVA nanofibrous web was studied. The increase in applied voltage increases the nano web density with formation of finer fibres and small size pores. The spinning electrode speed decides the mass throughput rate and higher speed leads to formation of low density webs with thicker fibres. Higher electrode distance leads to formation of coarser fibres and very low distance causes defects in the webs. The influence of the applied voltage and electrode distance follow opposite trend in deciding the web density and fibre diameter. The solution concentration is the major influential factor as the viscosity of solvent deciding the spreading behaviour of the merging jets. Lower solvent concentration leads to major defects in the nano webs. There is very good relationship between the nanofibrous web density and its tensile properties. Higher web density leads to higher mass stress and Young’s modulus.
The process parameters are very crucial in deciding the morphology and tensile properties of the webs. The optimum combination of these parameters will facilitate in the preparation of high web density webs with smaller size pores and can make the webs the strong contender for filter applications.