A Study of the Effects of Solution and Process Parameters on the Electrospinning Process and Nanofibre Morphology
Angammana, Chitral Jayasanka
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Nanofibres have been the subject of recent intensive research due to their unique properties, especially their large surface-area-to-volume ratio, which is about one thousand times higher than that of a human hair. They also have several other remarkable characteristics, such as flexibility in surface functionality, superior mechanical properties such as stiffness and tensile strength, their capacity to be formed into a variety of shapes, and the fact that they can be produced from a wide range of organic and inorganic polymers. These outstanding properties make polymer nanofibres the optimal candidates for providing significant improvement in current technology and for opening the door to novel applications in many research areas. Electrospinning is a straightforward and inexpensive process that produces continuous nanofibres from submicron diameters down to nanometre diameters. Many researchers have successfully electrospun a variety of polymer solutions into nanofibres. However, electrospinning any polymer solution directly is not straightforward or simple because of the number of parameters that influence the electrospinning process. The characteristics of the electrospun jet and the morphology of the resultant fibres are highly dependent on the properties of the polymer solution. In addition, what are favourable processing conditions for one polymer solution may not be suitable for another solution. A literature review revealed that there is no clear understanding of the behaviour of the electrospun jet and the way in which fibre morphology varies with variations in influential parameters. In addition, reported results contain significant inconsistencies and very little research has examined the effects of electrical parameters such as the electric field, the polarity of the electrode, and the conductivity and permittivity of the solution. Furthermore, no research has been conducted with respect to optimizing the electrospinning process. In this thesis, a comprehensive study was carried out by giving a special attention to the effects of electric field that have not been thoroughly investigated in the past. The electric field between the needle tip and the collector plate was altered by varying the applied voltage, distance between the needle tip and the collector plate, the inner diameter of the needle, and polarity of the voltage. Based on the experimental work, it was found that the behavior of Taylor cone, the length of the straight jet portion, and whipping jet region is highly influenced by the distribution of the electric field between the needle tip and the collector plate. Based on the stability of the Taylor cone, it was concluded that the stable operating region of the electrospun jet is a very narrow region and it is between 0.9 – 1.1kV/mm for the range of experiments that were carried out in this study. The length of the straight jet portion of the electrospun jet shows a linear relationship to the applied electric field at the tip of the fluid droplet and the whipping jet region is influenced by both the electric field at the tip of the fluid droplet and the distance between the needle and the collector plate. A confirmation were made that there must be enough distance between the needle tip and the collector plate (>200mm) to operate over the complete range of voltages without affecting drying of nanofibres. It was also concluded that the morphology and diameter of the collected nanofibres depend significantly on both the length of the straight jet portion and size of the whipping region. The effects of polarity of the applied voltage on the electrospinning process and nanofibre morphology were investigated using the positive, negative, and AC voltages. However, it was found that the electrospinning can not be achieved with the application of 60Hz AC voltage. It was observed that the behavior of Taylor cone, the straight jet portion, and the whipping jet region depend on the polarity of the applied voltage. During the study, it was accomplished that the reason for this different behavior is the disparity of ionization in the polymer solution with the application of positive and negative high voltages. In this thesis, the effects of multi-needle arrangements on the electrospinning process and fibre morphology were also explained. Finite element method (FEM) simulation results revealed that the local electric field strength around each needle tip weakens significantly in the case of multi-needle schemes due to the mutual influence of other needles in the arrangement compared to the single-needle system. The spacing between the needles was varied, and the effects of the needle spacing were examined. The experimental and simulation results were concealed the correlation between the degree of field distortion and the variation in the measured vertical angle of the straight jet portion for different needle spacing. It was concluded that the local field deterioration at the needle tips in multi-needle schemes degrades the electrospinning process significantly and produces considerable variation in the fibre morphology even though the influence of needle spacing on the average jet current and the fibre diameter are not very significant. In this work, the effects of conductivity and ionic carriers on the process of electrospinning and hence on the morphology of nanofibres were studied using polyethylene oxide (PEO) and polyacrylic acid (PAA) aqueous solutions. Different salts including lithium chloride (LiCl), sodium chloride (NaCl), sodium fluoride (NaF), sodium bicarbonate (NaHCO3), potassium chloride (KCl), and cesium chloride (CsCl) were added in different concentrations to the polymer solutions for introducing different ionic carriers into the solution. The results showed that the average fiber diameter decreases with increase in the conductivity of the solution. In addition, it was discovered that the formation of Taylor cone highly depends on the conductivity in the polymer solution. Formation of multi-jets at the fluid droplet when the conductivity of the polymer solution is increased during the electrospinning was also observed. These behaviors were completely explained using the distribution of the surface charge around the electrospun jet and the variation in the tangential electric field along the surface of the fluid droplet. The stretching of the polymer jet can be related to the amount of ionic carries and the size and mobility of positive and negative ions. The increasing amount of ionic carriers and smaller size positive ions enhance the stretching of the electrospun jet. In contrast, the lesser diameter negative ions decrease the stretching of the electrospun jet. The morphology of electrospun nanofibres can also be varied by altering the type of ionic carriers. A charge modifier, which is a container that is used to hold a solvent surrounding the needle tip during the electrospinning, was introduced to facilitate the electrospinning of insulating and high conductivity polymer solutions. The co-axial flow of the filled solvent on the outer surface of the polymer solution helps to induce enough surface charges during electrospinning and it also keeps the electric field tangential to the fluid surface during the process. Therefore, the introduction of charge modifier greatly enhanced the electrospinning behavior of highly insulating and conductive polymer solutions and liquids. The developed charge modifier method was verified by using sodium alginate which is a biopolymer that cannot electrospin alone due to its high electrical conductivity and silicone rubber which is an insulating liquid polymer at room temperature. One of the most commonly used theoretical model of the electrospinning process was modified to incorporate the non-uniform characteristics of the electric field at the tip of the needle. The non-uniform electric field between the needle tip (spinneret) and the collector plate was calculated based on the charge simulation technique (CST). It gives a better representation of the true electrospinning environment compared to the uniform field calculation in the existing model. In addition, a localized approximation was used to calculate the bending electric force acting on the electrospinning jet segments. It was also introduced a constant velocity to initiate the electrospinning jet during simulation. The incorporated modifications gave better results that closely match with the real electrospinning jet. The modified electrospinning model was used to understand the effects of parameters on the electrospinning process and fibre morphology.