E-TEAM. Characterisation of Polyamide 6 Nanofibres. European Masters in Textile Engineering. Özgür Ceylan. Promoter: Karen De Clerck.

Transcription

1 Association of Universities for Textiles E-TEAM European Masters in Textile Engineering Characterisation of Polyamide 6 Nanofibres Özgür Ceylan Promoter: Karen De Clerck Ghent University Academic year:

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3 Association of Universities for Textiles E-TEAM European Masters in Textile Engineering Characterisation of Polyamide 6 Nanofibres Özgür Ceylan Promoter: Karen De Clerck Ghent University Academic year:

4 Preface First of all, I would like to express my appreciation to my supervisor, Prof. Dr. Karen De Clerck, for suggesting this topic, excellent guiding and for inspiration throughout this study. I would not be able to accomplish this thesis without her never-ending countenance. I would also like to thank Lieve Van Landuyt for her endless and invaluable efforts to teach me characterisation techniques. I really appreciate every support she ensured through my thesis and every single answer she gave to my endless questions. I should also thank Sander De Vrieze, my advisor in the electrospinning process, for sharing his knowledge and time with me. Special thanks go to my colleagues in Fashion Design Department for their support throughout my studies in abroad and I am very grateful to Anadolu University for the financial support during my E-Team studies. I would like to express my feelings for my friends: Sezgi, Osman, Mevlit and Onur. These two years would not be so fun and easy without you guys. I really feel lucky to know each one of you. Thanks for everything. Finally, this work would not be completed without the inspiration and motivation that my family gave to me. I express my deepest esteem to my family for supporting the every step I take through my ideals. I dedicate this thesis to them. Gent, May 2008 Özgür Ceylan

5 Copyright The author gives permission to make this Master thesis available for consultation and to copy parts of the Master thesis for personal use. All pictures belong to the author unless otherwise stated. Any other uses fall under the copyright limitations, especially with regard to the obligation of mentioning the source explicitly on quoting the results of this Master thesis. ii

6 Characterisation of Polyamide 6 Nanofibres Özgür Ceylan Supervisors: Karen De Clerk, Lieve Van Landuyt, Sander De Vrieze E-Team European Masters in Textile Engineering Academic Year: Abstract Increased interest in nanotechnology has revived a fiber processing technique invented back in the 1930 s. Electrospinning is a straightforward and cost effective method to produce small fibres in the range of from less than 3 nm to over 1 µm. In electrospinning, a droplet of polymer solution is held by its surface tension and charge is induced on its surface by an electrical field. When the electrical force overcomes the surface tension, a charged jet of the solution is ejected. As the jet travels in air, the solvent evaporates and leaves behind a charged polymer fibre. Continuous fibres can be collected, usually in the form of a nonwoven fabric. A wide variety of polymers have been electrospun mostly from solution, although some examples of melt spinning also present. Due to the small fibre diameters, high surface area, tailorable surface morphology, and the creation of an interconnected fibrous network, electrospun fibres have found use in a variety of applications. However, a multitude of parameters directly affect the electrospinning process. Understanding how these parameters affect the fibre properties is crucial for obtaining a better understanding of the electrospinning process. In order for electrospun fibres to be successfully used in commercial applications, the processing parameters and solution characteristics that affect the fibre properties and molecular structure must be fully understood and allow to be iii

7 controlled. In this way the electrospinning process could be used to tailor fibres to meet specific application requirements. In this regard, the aim of this research is to study the effects of two most important processing parameters: applied voltage and solution concentration, on the morphology of the fibres formed to better understand the structure/property/process relationship. For this purpose, PA 6 polymer solutions were electrospun to produce nano-scale fibres and emphasis was given to control these parameters during the electrospinning process. Then, the obtained fibres were characterised by using FE-SEM, DSC, Raman and FT-IR spectroscopy techniques. Keywords: electrospinning, polyamide 6, characterisation iv

8 Concentration Temperature ( C) Characterisation of Polyamide 6 Nanofibres Özgür Ceylan Supervisors: Karen De Clerck, Lieve Van Landuyt, Sander De Vrieze Abstract Electrospinning is a straightforward and cost effective method to produce small fibres in the range of less than 3 nm to over 1 µm. However, a multitude of parameters directly affect the electrospinning process and resultant fibre properties. The aim of this research is to study the effects of the two most important processing parameters, applied voltage and solution concentration, on the morphology of the electrospun fibres to better understand the structure/property/process relationship. Keywords electrospinning, polyamide 6, characterisation I. INTRODUCTION Increased interest in nanotechnology has revived a fibre processing technique invented back in the 1930 s. Electrospinning is a straightforward and cost effective method to produce small fibres in the range of less than 3 nm to over 1 µm. However, a multitude of parameters directly affect the electrospinning process. Understanding how parameters affect fibre properties is crucial for obtaining a better understanding of the electrospinning process. In order for electrospun fibres to be successfully used in commercial applications, the processing parameters and solution characteristics that affect the fibre properties and molecular structure must be fully understood and allow to be controlled. In this way the electrospinning process could be used to tailor fibres to meet specific application requirements. In this regard, the aim of this research is to study the effects of the two most important processing parameters, applied voltage and solution concentration, on the morphology of the electrospun fibres to better understand the structure/ property/ process relationship. For this purpose, PA 6 polymer solutions were electrospun to produce nano-scale fibres and emphasis was given to control these parameters during the electrospinning process. Then, the obtained fibres were characterised by using FE-SEM, DSC, Raman and FT-IR spectroscopy techniques. A. Materials II. MATERIALS AND METHODS Polyamide 6 (PA 6) was dissolved in a formic acid/acetic acid solution (all from Sigma Aldrich). The solutions were slightly stirred with a magnetic stir bar at least 3 hours at room temperature. B. Electrospinning In the electrospinning set up, a 20 ml syringe was used to stock each of the as prepared polyamide 6 solutions. A 15 cm long needle with a diameter of 1 mm was used as a nozzle. The feeding rate of the solution was controlled by a KD Scientific Syringe Pump Series 100. A piece of aluminium sheet was used as a collector. A Glassman High Voltage Series EH power supply was used to charge the PA 6 solution by connecting the emitting electrode to the nozzle and the grounding electrode to the collector. C. Characterisation The diameters of the PA 6 fibres were investigated using a Jeol Quanta 200 F FE-SEM. Each specimen was gold coated by a Balzers Union SKD 030 device before being observed under the SEM. Thermal analysis of electrospun fibres was performed using a differential scanning calorimeter under nitrogen with a flow rate of 50 ml/min. (Model Q2000 DSC, TA Instruments). A Perkin-Elmer Spectrum GX system was used with a Raman beam splitter, a Nd:YAG-laser (1064 nm) and a InGaAs-dedector to obtain Raman spectra of the samples. For the IR spectra a Perkin-Elmer Spectrum GX system from Perkin Elmer, with a DTGS detector and a DRIFT accessory was used. III. RESULTS AND DISCUSSIONS A. Effect of Solution Concentration Solutions of PA 6 in formic acid/acetic acid were prepared at various concentrations, ranging from 10 to 15 wt%. In this particular work, only for concentrations below 13 wt %, it was possible to electrospin with the fixed processing parameters. Above this solution concentration, feeding rate of the solution was increased in order to achieve steady state conditions. Distribution of fibre diameters were calculated from scanning electron microscopic images by using image analysis software. The smallest diameters were obtained for the 10 wt % solution. The diameters of fibres obtained for concentrations at 11, 12 and 13 wt % were similar and around 108 nm. The largest diameters, almost double in comparison to the 10 wt %, were produced with concentrations at 14 and 15 wt %. Figure 2 demonstrates the melting behaviour of PA 6 nanofibres for all solution concentrations. At solutions of higher concentrations the γ-phase melting peak became less distinctive by slightly shifting towards higher temperatures, thus more overlapping with the α-phase melting peak. Or also, when the solution concentration decreased the melting area of the obtained fibres becomes broader which corresponds to an early melting of the γ-phase crystals. Heat Flow (W/g) Figure 1 DSC Thermograms for different concentrations

9 K-M Raman Sh ift / c m cm PA 6 14wt% 17 kv 0.9ml/h PA 6 14wt% 19 kv 1.2 ml/h PA 6 14wt% 21 kv 1.5 ml/h PA 6 14wt% 23 kv 1.5 ml/h PA 6 14wt% 25 kv 1.5 ml/h Raman Sh if t / c m cm-1 Spectra of nanofibre webs obtained for different solution concentration were normalised on the 1080 cm -1 (Gauche conformation 1 ) peak and represented in Figure 1. The variations on the 1123 cm -1 (Trans conformation 1 ) due to concentration differences were observed. PA 6 10 wt % PA 6 11 wt % PA 6 12 wt % PA 6 13 wt % PA 6 14 wt % PA 6 15 wt % Trans Gauche The Raman spectra of all samples were normalised on the 1080 cm -1 (Gauche conformation) peak, Figure 5. The ratios Trans peak (1123 cm -1 ) / Gauche peak (1080 cm -1 ) were calculated. PA 6 14wt% 17 kv 0.9ml/h PA 6 14wt% 19 kv 1.2 ml/h PA 6 14wt% 21 kv 1.5 ml/h PA 6 14wt% 23 kv 1.5 ml/h PA 6 14wt% 25 kv 1.5 ml/h Trans Gauche Figure 2 A selected Raman spectra for different concentrations The average of the peak ratio for these two particular peaks was calculated. In general, it has been observed that with increasing concentration the Trans/Gauche peak ratio is increasing which corresponds to the Trans conformation becoming more favourable in the fibre structure. Thus, crystals become more stable in the nanofibre structures with increasing PA6 concentrations. Figure 3 demonstrates the FT-IR spectra of the nanofibres mats obtained for various concentrations. Concentration γ Figure 3 A selected FT-IR spectra for different concentrations On the spectra the γ-phase 2 (975 cm -1 ) and the α-phase 2 (929 cm -1 ) characteristic peaks can be observed. The average ratios for these two peaks were compared. The results demonstrate that the γ phase crystals are decreasing with increasing concentration. Or also, the γ phase crystals becoming more stable and α like crystals at higher concentrations. This result is in line with the Raman and DSC results. B. Effect of Applied Voltage A series of electrospinning experiments for the 14 wt % solutions were carried out with a variation in applied voltage from 17 to 25 kv. It should be noted that with this particular concentration, it was not possible to produce nanofibres below 21 kv under steady state conditions. Thus, the feeding rate of the solution was varied below 21 kv in order to achieve steady state conditions. For fibres obtained by increasing applied voltage together with feeding rate the measured diameters were similar. For the fibres produced at constant feeding rate, the average diameters decreased from 98 nm to 91 nm with increased applied voltage from 21 kv to 25 kv. Figure 4 shows the effect of applied voltage on the melting behaviour of PA 6 nanofibres measured by DSC. The γ melting peak shifts to higher temperatures (towards the α melting peak) at higher applied voltages. Moreover, for constant feeding rates an increase in applied voltage results in an additional shoulder on the α melting peak which corresponds to a more stable α phase crystal formation. Heat Flow (W/g) γ Temperature ( C) α α Voltage Figure 5 A selected Raman spectra for different applied voltages The results demonstrated that for constant feeding rate, with increasing applied voltage an increase was observed in Trans conformation. These results are in line with the DSC results. Figure 6 shows γ and α phase peaks (975 cm 1 and 929 cm 1 ) in the FT-IR spectra for nanofibres obtained by various applied voltages. Figure 6 A selected FT-IR spectra for different applied voltages On the spectra the γ- (975 cm -1 ) and the α-phase (929 cm -1 ) characteristic peaks were observed and the average peak ratios for these two particular peaks were calculated. The results demonstrated that an increase in applied voltage together with feeding rate had no significant effect on fibre structure. However, when the feeding rate is constant and the applied voltage is increased, the formation of α-like crystals was favoured. These findings are again in line with DSC and Raman results. Voltage IV. CONCLUSIONS Solution concentration especially together with feeding rate has a significant effect on resultant fibre diameter. The variations in molecular structure of electrospun nanofibres due to concentration differences showed very slight differences. The results demonstrate that with increasing concentration the stability of crystals, particularly γ phase crystals, are increasing. The applied voltage together with feeding rate has no significant effect on fibre diameter. However, at constant feeding rate, increasing applied voltage induces a decrease in fibre diameter and favours a more stable molecular structure in the fibre morphology. Although some variations were observed in fibre morphology in line with variations in fibre diameter due to changes in solution concentration and applied voltage, it is important to stress that the variations in morphology are very small compared to the changes in diameter. That is, changing fibre diameter does not result in significant variations in molecular structure of the obtained nanofibres. V. REFERENCES 1. Stephens, J.S., Chase, D.B. & Rabolt, J.F. Effect of the electrospinning process on polymer crystallization chain conformation in nylon-6 and nylon-12. Macromolecules 37, (2004). 2. Wu, Q., Liu, X. & Berglund, L.A. FT-IR spectroscopic study of hydrogen bonding in PA6/clay nanocomposites. Polymer 43, (2002). Figure 4 DSC Thermograms for different applied voltages

10 Table of Contents Prefacce Copyright Characterisation of polyamide 6 nanofibres Abstract Extended abstract Table of contents Abbreviations i ii iii iii v vii x CHAPTER 1: INTRODUCTION Nanotechnology Electrospinning Historical Background of Electrospinning The Electrospinning Process Influence of Processing Parameters on Fibre Formation and Uniformity Polymer Solution Parameters Viscosity / Solution Concentration / Polymer Molecular Weight Solution Conductivity Surface Tension The Dielectric Constant of Solvent Processing Conditions Voltage Feeding Rate Type of Collector Distance between Needle Tip and Collector Ambient Parameters 9 vii

11 1.3 Characterization of Nanofibres Fibre Diameter and Porosity Molecular Structure Mechanical Properties Review of Previous Studies on Electrospinning and Characterization of Polyamide 6 Nanofibres Applications of Electrospun Nanofibres Filtration Application Medical Applications Nanofibres Reinforcement Objectives 18 CHAPTER 2: EXPERIMENTAL Materials Electrospinning Variation in concentration Variation in applied voltage Characterization of Polyamide 6 Fibres FE-SEM DSC Raman Spectroscopy FT-IR Spectroscopy 23 CHAPTER 3: RESULTS AND DISCUSSION Effect of Solution Concentration Fibre Properties Molecular Structure 29 viii

12 DSC Studies Raman Studies FT-IR Studies Conclusion Effect of Applied Voltage Fibre Properties Molecular Structure DSC Studies Raman Studies FT-IR Studies Conclusion 50 CHAPTER 4: CONCLUSION AND FUTURE WORK Conclusion Future Work 53 REFERENCES 54 ix

13 Abbreviations PA 6 Polyamide 6 SEM Scanning Electron Microscopy FE-SEM Field Emission Scanning Electron Microscopy DSC Differential Scanning Calorimetry FT-IR Fourier transform infrared spectroscopy IR Infrared x

14 Chapter 1: Introduction 1.1 Nanotechnology Nanotechnology is nowadays a popular interdisciplinary field that has been booming in many areas including mechanics, electronics, biology, medicine, and material science. Nanotechnology is a field of science to build macro and micro materials and products with atomic or molecular precision. One nanometre is one billionth meter. The term nanotechnology was first used by Eric Drexler, however, it first appeared in a scientific paper by Norio Taniguchi in In his paper Taniguchi introduced nanotechnology in order to define extra high precision and ultra fine dimensions. In fact, Richard Feynman is considered the father of nanotechnology since in a speech given at the American Physical Society meeting in 1959; he mentioned some future possibilities for nanotechnology applications.[1]. Formulating a common definition for nanotechnology is a difficult task but basically anything between 0.1 nm to 1µm would be characterized in the field of nanotechnology. According to the Foresight Institute, molecular nanotechnology will be achieved when it is possible to build things from the atom up, and to rearrange matter with atomic precision. The National Science Foundation, on the other hand, defines nanotechnology as research and technology development at the atomic or molecular levels, in the length scale of approximately nanometer range, to create and use devices and systems that have novel properties because of their small and/or intermediate size [2]. There are two main approaches for nano-scale production: top down and bottom up [2]. The top down method involves manufacturing of nanomaterial from bulk materials. Most of the nanomaterials and devices are currently created using top down approach. In contrast, the bottom up method emphasizes the creation of structures atom by atom or molecule by molecule. For instance, using atomic force microscope to arrange atoms or molecules. 1

15 The importance of nanotechnology comes from the fact that properties of substances significantly change when their size is reduced to the nanometre range. When a bulk material is divided into small size particles in the nanometre range, the individual particles exhibit unexpected properties, different from those of bulk material [3]. The main aim of nanotechnology is to use these properties by gaining control of structures at atomic and molecular level. In order to achieve this goal intensive studies have been initiated across disciplines and industries world wide. Textile industry is one of the early adopters of nanotechnology products and processes. Nanotextile products have already emerged in the market and the number of nano-claimed products or by-products is increasing substantially. 1.2 Electrospinning Electrospinning is a technique in which an electrical potential is applied mostly to a polymer solution or a melt to produce a polymer fibre. The unique difference of the technique from traditional fibre spinning methods is the addition of an electrical field. As such, the generated fibre diameters are much smaller than fibres spun from a melt extrusion process Historical Background of Electrospinning Electrospinning is not a new technology. The phenomenon that a water drop on a dry surface was drawn into a cone when an electrical charge was applied on it, was pointed out about 400 years ago by William Gilbert. This is the beginning of the story of electrospinning. In the late 1800 s Lord Rayleigh investigated the charges that are necessary to overcome the surface tension of a liquid drop. He showed that a charged liquid becomes unstable when the charge reaches a critical value. When the electrostatic force overcomes the surface tension, which acts in the opposite direction of the electrostatic force, liquid is ejected in the form of fine jets [4]. The electrospinning of polymeric materials has been known since the 1930 s. Formhals published a series of patents including different designs for experimental set up, different collector types 2

16 and their applications. In 1966, Simons improved the set up of electrospinning and produce more stable fibres. He also found that it is possible to produce relatively continuous fibres with higher viscous solutions [5]. Academic attention and industrial interest was very small at that time except in Russia where electrospinning was already applied industrially. In 1960 s Taylor analyzed the conditions of the droplet deformed by an electrical force. Theoretically he showed that a conical interface between two fluids can exist in equilibrium in an electric field, but only when the cone has a semi-vertical angle 49.3 o. He also proved experimentally the theory by constructing an apparatus for producing the necessary electrical field and took the photographs of conical interfaces. Results demonstrated that the droplet elongates at the onset of instability and its end forms into a conical shape that has semi-vertical angles close to 49.3 o [6]. In 1971, electrospinning was studied by Baumgarten. He explained the relationship between fibre diameter, solution viscosity, jet length, feeding rate of solution and surrounding gas [7]. The following year, Simm invented a process for the production of composite filters based on electrospun fibres. In his process, polystyrene solution was electrostatically sprayed on a cellulose fleece. The aim was to use the resultant thick, porous fibre fleece as an air filter [8]. Since the 1980 s and especially in recent years, electrospinning gained substantial academic attention due to the surging interest in nanotechnology and as ultra fine fibres of various polymers with diameters down to nanometres can be easily produced with this process The Electrospinning Process Electrospinning is a direct process that produces polymer nanofibers. Electrospinning utilizes an electrical force in order to shape polymer solutions into small diameter fibres. In the process, polymer is dissolved in an appropriate solvent and contained in a syringe with a pump. The pump allows for a constant flow of polymer solution to the tip of the needle. A drop of solution is suspended from the needle tip due to the surface tension of the polymer solution. A high voltage is applied across the needle causing the drop of polymer solution to deform and become conical 3

17 in shape which is known as a Taylor Cone. When the electrical field reaches a critical value at which the electrical forces overcome the surface tension forces, a charged jet of solution is ejected from the tip of the Taylor Cone. This part of the process is known as jet initiation. After the jet appears, the path of the jet is straight for a certain distance. Then, due to the electrical forces, fluid instabilities occur at the bottom of the straight segment of the jet. It is believed that the stable jet is splitting or splaying into multiple other jets due to the charged repulsion caused by the bending instability. Doshi and Reneker indicated that fibre diameters decreased due to the simultaneous stretching of the jet and evaporation of the solvent resulting in the increase of surface area, that is, the charged density. Similar to electrospraying process, the increase of the charge density splits the jet into smaller jets then resulting in the formation of fibres with very small diameter. On the other hand, recent studies have suggested that bending instabilities cause the reduction of the jet diameter. This bending result a series of spiralling loops with growing diameters and allows a large elongation to occur in a small region of space. The trajectory of electrospinning jet gained substantial academic attention in recent years [9-14]. The final stage of the process is the solidification of the jet into nanofibers. The target generally consists of a covered metal sheet. As the jet travels in air, the solvent evaporates, leaving behind a charged polymer fibre. Continuous fibres are collected in the form of a nonwoven web. A typical experimental set up of the electrospinning process is shown in Figure

18 Figure 1. 1 An Experimental Set Up of Electrospinning Process Influence of Processing Parameters on Fibre Formation and Uniformity The parameters affecting electrospinning and the fibre formation may be broadly classified into polymer solution parameters, processing conditions and ambient conditions [5,15] Polymer Solution Parameters The properties of the solution including viscosity, conductivity, surface tension, polymer molecular weight, have an important influence in the electrospinning process and the resultant fibre morphology. However, the effects of solution properties are mostly difficult to isolate since changing one parameter can affect other solution properties. For instance, varying solution concentration will affect the surface tension of the solution. 5

19 Viscosity / Solution Concentration / Polymer Molecular Weight Solution viscosity is controlled by changing polymer solution concentration or molecular weight. Many experiments have shown that solution viscosity in relation with polymer molecular weight and concentration is one of the most important parameters affect the resultant fibre properties, mainly diameter, and microscopical appearance in the electrospinning process [16-26]. It is well known that a minimum viscosity for each polymer solution is necessary to generate fibres without beads. In general, increasing viscosity by means of solution concentration or molecular weight favours formation of uniform, smooth nanofibers with larger diameters in circular shape. There is a maximum viscosity value where the solution can not be electrospun into nanofibres. Higher viscosities can also cause drying out of the droplet at the tip of the needle and prevent electrospinning Solution Conductivity Electrospinning involves stretching of the solution caused by the repulsion of the charges at its surface. Thus, if the conductivity of the solution is increased, more charges can be carried by the electrospinning jet [27]. The conductivity of the solution can be increased by the addition of salt or alcohol. It has been found that increasing solution conductivity results in more uniform fibres with fewer beads. Moreover, higher solution conductivity causes a reduction in fibre diameter and an increase in the deposition area of the fibres on collector [17,19-20,28-32] Surface Tension The electrospinning process starts when the electrical forces overcome the surface tension of the solution. Thus, a lower surface tension of a solvent will be more suitable for electrospinning. The impact of surface tension on fibre properties was investigated by many researchers [16,22,29,33-6

20 37]. It has been found that lower surface tension mostly favours the formation of smooth, uniform fibres with less bead defects. It should be noted that different solvents may contribute to different surface tensions The Dielectric Constant of Solvent The dielectric constant of a solvent has an important effect on the electrospinning process. Relatively fewer studies have been performed to investigate the effects of dielectric constant on generated fibre properties [20,38-41]. Results demonstrated that, bead formation and diameter of obtained fibres are reduced with an increase in dielectric constant of solvent. The decrease in diameter occurs as a result of the increase in bending instability of electrospinning jet. It should be noted that the addition of solvents with higher dielectric constant to polymer solutions does not always have a positive effect on resultant nanofibers due to the possible interactions between polymers and solvents [40] Processing Conditions Various external factors including applied voltage, the feeding rate, type of collector, distance between needle tip and collector have affects on the electrospinning process. Although these parameters are less significant than the solution parameters, they have a certain influence on fibre properties Voltage The effects of applied voltage on generated nanofibers is one of the most studied parameters among the controlled variables of the electrospinning process [21,22,25,30,37,42-47]. Generally, the high voltage, more than 6 kv, is able to deform the solution drop at the tip of the needle into the shape of a Taylor Cone. At relatively low voltages, jet originates from the Taylor Cone and produce bead free fibres. As the voltage is increased, the volume of the drop at the tip of the needle will decrease and result in a smaller and less stable Taylor Cone. In this case, a jet originates from the liquid surface within the tip and an increase in bead formation is observed. If 7

21 the applied voltage is increased further, the Taylor Cone will disappear and many beads will be obtained [42]. In most cases, the higher voltage induces the greater stretching of the solution and results in a reduction in fibre diameters [17,43,45]. However, in some solution systems an increase in fibre diameter was observed with higher voltage. The formation of secondary jets especially at low concentrations is another reason for the resultant finer fibres with higher voltage [47]. High voltage may also affect the crystallinity of polymer fibres. An increase in applied voltage favours more ordered polymer molecules and results in higher crystallinity. However, it should be noted that a minimum time is required for orientation of molecules, thus a sufficient distance from tip to collector is necessary to obtain nanofibers with higher crystallinity [47] Feeding Rate The feeding rate will determine the amount of solution available for electrospinning. Few studies have systematically investigated the relationship between feeding rate and resultant fibre properties [28,31,36,40,44]. In general, an increase in feeding rate of solution favours an increase in fibre diameter; however there is a limit to the increase in the diameter of fibre due to higher feeding rate. Too high feeding rates may result in beading since fibres may not have enough time to evaporate the solution prior to reaching the collector Type of Collector Various materials and geometries have been studied as collector in electrospinning process [34,39,49-56]. In general, conductive materials are preferred. The conductivity, as well as the porosity of the collector has an influence on the packing density of the fibres [35]. Collecting nanofibers with a rotating drum instead of a static collector favours alignment of fibres. In addition, these types of collectors assist evaporation of solvent and make it possible to use solvents with higher boiling points [40]. 8

22 Distance between Needle Tip and Collector The distance between the tip and the collector has been examined by many researchers in order to control the fibre properties since varying this distance will have an influence on the electrical field strength and flight time of the fibre [26,30,37,43,44,47,57,58]. It is well known that a minimum distance is required to finish the evaporation of solvent before reaching the collector. The effect of distance is strongly dependent on the solution system. In some cases, the distance variations do not have an important effect on fibre diameter. However, when the distance is too low, bead formation is observed [43]. In general, a longer distance favours a reduction in fibre diameter, due to the longer flight time for fibres prior to reaching the collector [47,58]. There are also some systems in which longer distance results in an increase in fibre diameter due to the reduction of the electrical strength [27,59] Ambient Parameters Variation of the ambient parameters including atmospheric composition, humidity and pressure may have a significant impact on the electrospinning process. However, the effects of these parameters were investigated by only a few researchers [7,43,60,61]. It is well known that high humidity in an electrospinning environment results in pores on the fibre surfaces. Moreover, different surrounding gases have different effects on the electrospinning process. For instance, Freon 12 atmosphere induce an increase in fibre diameter while helium prevents electrospinning [7]. It was also observed that vacuum atmosphere favours formation of nanofibers with better characteristics. It should be noted that the effect of these parameters can differ for different polymer solution systems. Moreover, it is really a difficult task to isolate the effects of each parameter since they are interrelated. However, it is well known that the variation of parameters in the electrospinning process provides different possibilities for the production of electrospun materials. 9

23 1.3 Characterization of Nanofibres In literature, the focus on electrospinning is to obtain proper fibres without beads and droplets by optimising process parameters. Up to now, not so much effort has been spent on the characterization of electrospun materials except for the fibre diameter. However, such a characterization including fibre diameter and porosity, molecular structure and mechanical properties, is very important for possible end use features of fibres. Various techniques are available for the characterization of nanofibres Fibre Diameter and Porosity Scanning electron microscopy (SEM), field emissions scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM) are used to investigate diameter and porosity characteristics of electrospun nanofibres. The investigation of fibre diameters with SEM is the most common method for the characterization of nanofibres. Gold coated polymer nanofibres with diameters from 200 nm to 1000 nm are observed at around x magnification with 10 to 20 kv acceleration voltages under SEM [27]. By using this technique, the effects of process parameters on nanofibres diameter were studied by many research group for different solution systems such as poly(ethylene oxide)[19,21,28,51,63,64], polysulfone [44,64], poly(vinylalchool) [17,37,60,66-69], poly(acrylonitrile)[70,69], poly(lactide) [19,69], polystyrene[17,21,40,62,72,73], cellulose acetate[35,41,73], polyamide[24,75-77], polypropylene[78-80], natural silk solutions[58,81-83], polyurethane[16,84-86] poly(caprolactane)[38,86]. In general, results demonstrated that fibres become more uniform and thicker in diameter with increasing solution viscosity. At lower concentrations mostly thinner fibres are formed. In order to produce fibres with diameters down to a few nanometres, it is necessary to increase the electrical conductivity of the solution. It is not possible to make a general recommendation about fibre diameters for process and solution parameters since the ideal values of these parameters change according to polymer solvent system. 10

24 When the fibre investigation is conducted at extremely high magnification, it is possible to damage the fibres due to the temperature rise caused by electron bombarding. Thus, in the case of ultra fine fibres with diameters less than 200 nm, it is recommended to use FE-SEM in order to make accurate measurements. The main difference of FE-SEM is that high resolution images can be obtained by using low acceleration voltage [27]. The gold coating of the samples in SEM and FE-SEM has an influence on accuracy of diameter measurements. In order to avoid coating effects on diameter measurements of nanofibres, transmission electron microscopy can be used. It has been used by several research groups to measure the diameter of ultra fine polymer nanofibres [16,88-93]. Pores play an essential role in the physical and chemical properties of electrospun materials. Thus, it is very important to analyze the pore size and its distribution. Pore size can be measured by direct and indirect methods. Direct methods involve the use of techniques such as SEM, TEM and AFM. These techniques provide detailed images of electrospun nanofibres from which pore size and distribution can be determined. Indirect methods include bubble point measurements, solute retention challenge, molecular resolution porosimetry, extrusion porosimetry and intrusion porosimetry [27]. The effect of processing and solution parameters on porosity characteristics of electrospun nanofibres were studied by several researchers [25,43,61,94-96]. The influence of porosity on transport properties nanofibres membranes was also investigated [96,97]. In general, it has been observed that the ambient parameters and solution systems have a significant influence on porosity as well as the transport properties of nanofibres Molecular Structure The molecular structure of nanofibres has been investigated by using Fourier transform infrared (FT-IR), X-ray diffraction (XRD), Raman spectroscopy and differential scanning calorimetry (DSC). In literature, molecular structure studies of electrospun nanofibres focus on three main aspects including size effect, process parameters and solution properties. With respect to size effect, the 11

25 change in molecular structure due to the diameter variations of fibres from micrometer scale down to nanometre scale is investigated. In some cases, the differences between bulk material and electrospun fibres are also emphasized. Another concern is the effect of processing parameters such as applied voltage and collector design on molecular structure of electrospun fibres. The higher applied voltage may lead to higher electrical force which pulls out the polymer jet during electrospinning or a rotating drum collector may lead to higher elongation of the polymer jet. Similarly, solution properties such as molecular weight of polymer or concentration may also affect the crystalline structure of the generated nanofibres by electrospinning. The differences in molecular structures of electrospun nanofibres and bulk materials were investigated by only a few researchers [38,99-103]. In general, results demonstrated that electrospun nanofibres have ordered polymer chains although their degree of crystallinity is not significantly high. It is obvious that the electrical force affects the polymer chain order on nanofibres. The effects of processing parameters on molecular structure of electrospun nanofibres were investigated by different groups [44, ]. Results demonstrated that crystallinity of electrospun nanofibres increase with increasing voltage up to a certain level. The increase in distance between tip and collector also favours higher crystallinity. Since rotating collectors induce the stretching of polymer jets, obtained fibres have more ordered structures. In sum, processing parameters affect the polymer chain order of nanofibres. For production of ordered chain structure in nanofibres, higher voltage and rotating collectors should be considered [27]. The influence of solution parameters on molecular structure of nanofibres was investigated by few researchers [28,65,104,106,107]. Since solution properties are interrelated, it is a difficult task to make clear conclusions about their effects on molecular structure of nanofibres. For poly (vinyl alcohol) solutions, it has been found that higher molecular weights of polymer favours higher crystallinity in resultant fibres. Moreover, solutions with higher conductivity results in more orientation in the fibre structure [59]. 12

26 1.3.3 Mechanical Properties In most of the applications, nanofibres are exposed to mechanical stresses from the surrounding media during their service lifetime. Such stresses can cause deformations of the fibres. Therefore, it is necessary to characterize the mechanical properties of nanofibres for appropriate applications. Mechanical tests of nanofibrous membranes can be performed by using conventional methods. However mechanical characterization of single nanofibres is still a challenge due to the handling difficulty to extract a single fibre from the electrospun nanofibres web and also non availability of accurate testing apparatus. Mechanical characterization of single nanofibres, nanofibres yarn and nanofibres webs were studied by using a nanotensile tester, atomic force microscopy (AFM), three point bending tests and mechanical resonance methods [ ]. It has been found that AFM based nanoindentation systems and the nano tensile tester are useful techniques for accurate measurements. Certain differences in mechanical properties of bulk material and electrospun membranes were observed. The concentration of polymer solution affects the mechanical response of generated nanofibres and a correlation between fibre diameter and mechanical characteristics was observed. Fibres with smaller diameters favours higher strength but lower elongations due to the higher draw ratios of electrospinning process. 1.4 Review of Previous Studies on Electrospinning and Characterization of Polyamide 6 Nanofibres Polyamide 6, also known as Nylon 6 or poly (caprolactam), is one of the most widely used synthetic polymers for fibres and exhibits excellent physical and mechanical properties. Like most of other engineering polymers, nylon 6 has been processed into nanofibres by electrospinning and the properties of resultant nanofibres were investigated. 13

27 Similar to other electrospun polymers, preliminary researches in electrospinning and characterization of polyamide 6 nanofibres focus on the effects of solution and processing parameters on fibre diameter and porosity[18,24,75,119,120]. The effects of such parameters have been found to be similar with general results obtained from electrospinning studies. Additionally, the effects of emitting electrode polarity and solution temperature were investigated. Results demonstrated that fibres produced under negative electrode polarity were larger in diameter and have flat cross section while fibres obtained under positive electrode polarity were smaller in diameter and have a circular cross section [74,75]. An increase in solution temperature during electrospinning results in a decrease in fibre diameter. Wettability of nanofibres in relation to the solution additives and properties of resultant material was also investigated [76]. It has been found that there is no big difference between contact angles of film and fibre membranes obtained from pure polyamide 6, that is surface structure does not play an essential role in wettability of materials. On the other hand, acridine doping into polyamide 6 solutions doubled the contact angles [76]. The influence of rapid structure formation and high elongation flow within the electrospinning process on internal structure of polyamide 6 fibres was investigated by a few groups [ ,108]. For this purpose, the structure of electrospun fibres was compared with melts extruded fibres and bulk materials. The results demonstrated that nanofibres were partially crystalline, with degrees of crystallinity not significantly smaller than that of melt extruded fibres. However, a reduction in crystallinity of material was observed in comparison with bulk material. The polymer crystalline structure of polyamide 6 was altered from an -form to a -crystalline form after electrospinning. This result implies that the high stress is induced during the formation of nanofibres. The annealing of polyamide fibres resulted in a transformation from -crystalline form to -form which is also similar in normal scale fibres. The orientation of the crystals along the fibre was found to be strongly inhomogeneous. It was also found that the electrospinning process was not destroying the chemical architecture of polymers [99]. In addition, thermogravimetric analysis of polyamide 6 pellets and electrospun nanofibres demonstrated that both materials decomposed in two steps, however the decomposition of nanofibres started at higher temperatures [121]. 14

28 The electrospinning set up in production of polyamide 6 nanofibres was modified by some researchers [9,122]. In one of the modified processes, solution was electrified and pushed by air pressure through the walls of the porous polyethylene tube. As a result multiple jets were formed on porous surface and nanoscale fibres were electrospun. The comparison of nanofibres obtained from a single jet and with those from a porous tube showed that the mass production rate for porous tube is about 250 times greater than a typical single jet. Moreover, the fibre diameters were found to be similar in average but the distribution of diameters for the porous tube was broader than that of single jet. In another set up, air flow was used between the capillary source electrode and target collector in order to drive the polymer jet inside the electrostatic field. As a result, a large amount of crimped nanofibres was produced without beads [9]. The variations in molecular structure of electrospun polyamide 6 nanofibres depending on type, physical state and linear velocity of the collector were investigated by two research groups [123,124]. In the first case, aluminium sheet and a water bath with different temperatures and conductivity were used as collectors. An increase in fibre diameter was observed for electrospun fibres collected in the water bath instead of on the aluminium sheet. Similarly the fibre diameter and bonding was increased with the temperature whereas diameters linearly decreased with an increased conductivity of the water bath. Spectroscopic analysis with Raman and FT-IR techniques showed that the crystalline structure of polyamide 6 is depended on the type of the collector. Polyamide 6 electrospun nanofibres display the -crystalline form while they are collected on aluminium sheet and the -crystalline form when collected in a water bath. The extent of transformation from the - to the -phase was found to be linearly increased with temperature and conductivity of the water bath. In the second case, the electrospun nanofibres were collected by rotating drums with different linear velocities and randomly. When polyamide 6 was randomly deposited, it was observed that the parallel and perpendicularly polarized infrared spectra were identical and this means there is no molecular orientation in the electrospun membrane. When the linear velocity of the collector was increased up to a limit, some changes in band intensity were observed, indicating the molecular orientation took place in fibre axis. For higher linear speeds of collector, there was no significant increase in crystallinity of nanofibres according to differential scanning calorimetry results. Moreover, a decrease in the average fibre diameter was observed with higher collector speeds as a result of stress induced by collector in 15

29 the electrospinning process. These studies proved that controlled transformation of polyamide 6 is possible with electrospinning process. 1.5 Applications of Electrospun Nanofibres Electrospinning is a simple and powerful technique for the production of ultra thin fibres from different type of materials. The simplicity of processing, the diversity of material opportunities for electrospinning and the unique properties of electrospun nanofibres make this technique and resultant fibres attractive for various applications Filtration Application The pore structure properties including overall porosity, the average pore size and surface area influence the filtration performance. In this regard, the low density, large surface area to mass ratio, high pore volume, and tight pore size of nanofibres nonwovens make them appropriate for a wide range of filtration applications. Several researches have been conducted to measure and improve the filtering efficiency of nanofibres mats [71,93, ]. It is well known that the addition of nanofibres to conventional fibrous filter media significantly improves the filtering performance. However, in some cases an increase in the pressure drop of filters was observed with additional nanofibres. It has been found that electrical properties of the polymers as well as the fibre diameter and the media structure affects filtration efficiency of nanofibrous mats. For instance, decrease in fibre size improves the overall separation efficiency. The differences between standard and nanofilters were also investigated [97]. Due to their submicron size electrospun fibre mats presented higher performance in moisture diffusion and in filtration of airborne particles than standard products in use. With regards to these observations, it is possible to conclude that the electrospun fibre mats are favourable for protective clothing applications due to their high breath ability, elasticity, and filtration efficiency. 16

30 1.5.2 Medical Applications Nanofibrous polymer systems have several applications in the field of medicine and pharmacy since almost all of the human tissues and organs are deposited in nanofibrous forms or structures. As such, nowadays research in electrospun polymer nanofibres has focused mainly on medical application including tissue engineering, wound healing and drug delivery. The electrospun fibres have been investigated as promising tissue engineering scaffolds since they mimic the nanoscale properties of the extracelluar matrix. Human cells are able to attach and grow only around fibres with diameters smaller than those of the cells. As such, the main interest of tissue engineering is the creation of biocompatible three dimensional scaffolds for repair and replacement procedures [5]. Recently, various reports about building scaffolds from natural or synthetic nanofibres have been published [82, ]. The polymer nanofibres can be used for the treatment of wounds such as burns and abrasions. It has been found that wounds heal faster when they are covered by a thin web of nanofibres [83, ]. Such nanowebs have enough pores size to achieve the exchange of liquid and gases with the environment but they are also able to protect wound from bacteria. Smith and Reneker described a process in which a fibre mat is directly electrospun onto wound. Nowadays, it is proposed in some literature that by using handheld electrospinning devices, it is possible to apply nanofibres directly onto the wounds [8]. In general, smaller dimensions of drug and carrier favour better absorbance of it by the human body. Drug delivery with polymer nanofibres is based on the principle that the dissolution of the drug increases with increasing surface area of drug and its carrier. Even if the drug delivery in the form of nanofibres is in an early stage, researchers are trying to optimise the efficiency of the process [36, ]. 17

31 1.5.3 Nanofibres Reinforcement The main principles for the reinforcement by macroscopic fibres are valid also for the reinforcement by nanofibres. However, nanofibres have several advantages over macroscopic fibres such as high Young s Moduli and impact strength. The majority of the work in literature on nanofibres reinforcement focused on carbon nanotubes. Polymer nanofibres obtained from electrospinning have been used relatively less in reinforcements. Due to the results of research, it is clear that nanofibres reinforcement offers great opportunities for various materials including elastomers, thermoplastics and ceramics [8]. 1.6 Objectives Electrospinning is a versatile method to produce ultra fine fibres with diameters in the micrometer down to nanometres range. Various polymers have been successfully electrospun into ultra fine fibres in recent years mostly in solvent solution and some in melt form. The nanofibres obtained from electrospinning process exhibit several interesting characteristics such as a large surface area to mass ratio, small pore size, and surface functionality. These unique properties make electrospun nanofibres excellent candidates for a great number of applications. So far, much effort has been spent on optimising the production of nanofibres. The observation of fibre diameter and homogeneity by scanning electron microscope is a common method for most of the researches. However, to promote commercial applications of electrospun nanofibres, more research is needed to understand the important intrinsic material characteristics for each specific application. It is well known that the end use properties of nanofibres membranes will be influenced by various characteristics including molecular structure, morphology and mechanical properties. An understanding of these characteristics in relation to the processing parameters will help the optimisation of process, as well as the investigation of true potential for this processing technique for future functional materials. In this perspective, the main objective of this research is to investigate the influence of processing parameters on characteristics of generated nanofibres. For this purpose, electrospinning technique 18

32 was used to produce ultra fine polyamide 6 fibres. The processing parameters including solution concentration, feeding rate of solution and applied voltage were controlled during production. The analysis of obtained nanofibres was performed using SEM, DSC, Raman and FT-IR spectroscopy. 19

33 Chapter 2: Experimental 2.1 Materials Polyamide 6 (PA 6) pellets with an average particle size of 3 mm was supplied from Sigma Aldrich and used as received. Solvent chosen for this research was 98 wt % Formic acid and 99.8 % acetic acid purchased from Sigma-Aldrich and also used as received. Solutions for electrospinning were prepared by dissolving PA 6 pellets in a 1:1 Formic acid: Acetic acid solvent mixture. The solutions were slightly stirred with a magnetic stir bar at least 3 hours at room temperature. Solutions of varying concentrations (10 wt % to 15 wt %) were used to investigate the effects of solution concentration on the characteristics of obtained fibres. It should be noted that it was not possible to produce nanofibres in steady state conditions for 13 wt % and higher concentrations without increasing feeding rate of solutions. 2.2 Electrospinning In the electrospinning set up, a 20 ml syringe was used to stock each of the as prepared polyamide 6 solutions. A 15,24 cm long needle with 1,024 mm diameter was used as a nozzle. The feeding rate of the solution was controlled by KD Scientific Syringe Pump Series 100. A piece of aluminium sheet was used as a collector. A Glassman High Voltage Series EH-bron power supply was used to charge the PA 6 solution by connecting the emitting electrode to the nozzle and the grounding electrode to the collector. A schematic set up of electrospinning process is presented in Figure Variation in concentration Solutions of varying concentrations (10 wt % to 15 wt %) were used to investigate the effects of solution concentration on the characteristics of obtained fibres. It should be noted that it was not possible to produce nanofibres in steady state conditions for 13 wt % and higher concentrations without increasing feeding rate of solutions. 20

34 Figure 2. 1 A schematic set up of electrospinning process and the obtained fibre mat Variation in applied voltage In all experiments the distance between tip and collector was fixed at 6 cm. In order to investigate the effects of applied voltage on characteristics of as-spun PA 6 nanofibres, the applied voltage was varied between 17 and 25 kv. Similar to the electrospinning process of solutions with higher concentrations, it was not possible to achieve steady state conditions with constant feeding rates. Therefore, the feeding rates of the solutions were decreased, when the applied voltage was less than 21 kv. 21

35 2.3 Characterization of Polyamide 6 Fibres FE-SEM The diameters of PA 6 fibres were investigated using a Jeol Quanta 200 F FE-SEM. The samples for these observations were prepared by cutting an aluminium sheet covered by nanofibres and fixing it to a SEM stub. Each specimen was gold coated by a Balzers Union SKD 030 device before being observed under SEM. For each spinning condition, at least 75 measurements for the fibre diameter were recorded DSC Thermal analysis of electrospun fibres was performed using a differential scanning calorimeter under nitrogen with a flow rate of 50 ml/min. (Model Q2000 DSC, TA Instruments, DA, USA). Calibration was performed with, pure indium, tin, gallium and sapphire. Samples weighing 3 ± 0.1 mg were loaded in a DSC aluminium pan and the pan was sealed with a crimping tool. Samples were equilibrated to 25 C for 10 min, and then heated from 25 to 300 C at a heating rate of 10 C/min and then cooled down to 0 C. The heat flow was recorded for all samples. The objective of the first heating was to observe whether any differences occurred due to concentration and applied voltage variations. The first heating was analysed for melting temperature (Tm) and heat of fusion which can be defined as the area under the melting curve. The second heating was used as an intrinsic material reference. The modulated mode of the instrument was used to investigate the glass transition and onset temperatures of the samples. Heat flow of the materials was monitored over the range of 0 C to 300 C with a temperature modulation (+/- 0.2 C every 60 second) superimposed on a 2 C/min. Universal Analysis 2000 software was used for the interpretation of the thermographs. 22

36 2.3.3 Raman Spectroscopy In order to obtain the Raman spectra a Perkin-Elmer Spectrum GX system was used with a Raman beam splitter, a Nd:YAG-laser (1064 nm) and a InGaAs-dedector. The spectra were taken in the back-scattering mode between 400 to 3500 cm -1, with a resolution of 4 cm -1, a data-interval of 1 cm -1 and at least 128 scans. The laser power was 750 mw FT-IR Spectroscopy For the IR spectra a Perkin-Elmer Spectrum GX system from Perkin Elmer, with a DTGS detector and a DRIFT accessory was used. FT-IR spectra were performed by scanning the samples 128 times from 400 to 3500 cm -1 with resolution of 4 cm -1 in the drift accessory. 23

37 Chapter 3: Results and Discussion 3.1 Effect of Solution Concentration Fibre Properties Solutions of all PA 6 pellets in 98 v/v formic acid were prepared at various concentrations, ranging from 10 to 15 wt %. At concentrations below 10 wt %, the electrospinning process would generate a mixture of fibres and droplets. Electrospinning with concentrations higher than 15 wt % was prohibited because of their high viscosity. The higher viscosity solutions proved difficult to force through the syringe needle of the apparatus used in these experiments, making the control of the solution flow rate to the tip unstable. In this particular work, only for concentrations below 13 wt %, it was possible to electrospin with fixed processing parameters. Above this solution concentration, feeding rate of the solution was increased in order to achieve steady state conditions. The electrospinning parameters for each solution concentration is summarised in Table 3.1. Concentration Tip to Collector Applied Voltage Feeding Rate Distance 10 wt % 6 cm 21 kv 1.3 ml/h 11 wt % 6 cm 21 kv 1.3 ml/h 12 wt % 6 cm 21 kv 1.3 ml/h 13 wt % 6 cm 21 kv 1.4 ml/h 14 wt % 6 cm 21 kv 2 ml/h 15 wt % 6 cm 21 kv 2 ml/h Table 3. 1 Electrospinning parameters for different solution concentration 24

38 Figure 3.1 shows a selected series of FE-SEM micrographs to show the effect of concentration on morphological appearance of the obtained fibres. At low concentrations a combination of smooth and beaded fibres were obtained. In addition, the fibres were discontinuous at lower concentrations. However, with an increase of solution concentration, the beads disappeared and only smooth fibres obtained on the collector. 25

39 (a) (b) (c) (d) (e) (f) Figure 3. 1 FE-SEM micrographs of materials obtained from solutions of polyamide 6 at concentrations of a) 10 wt %, b) 11 wt %, c) 12 wt %, d) 13 wt %, e) 14 wt %, f) 15 wt % (magnification = x, scale bar = 1µm, working distance = 8 mm) 26

40 These images demonstrate the two different fibre morphologies observed as a result of varying solution concentration within the processable range. At the low concentrations, the fibres are discontinuous and have an irregular morphology with beads. On the other hand, at the high concentration the nanofibres have a regular morphology without beads. At low concentration, the viscoelastic force (a result of low degree chain entanglements) in a given jet segment was not large enough to counter the higher electrical forces, resulting in the break up of the charged jet into smaller jets, more susceptible to form beads on the fibres. At higher concentrations, the charged jet did not break up into small jets due to increased chain entanglements which are sufficient to prevent the break up of the jet and to allow the electrical stress to further elongate the charged jet during its flight to the collector. However, if the concentration was not high enough (11 wt %) bead formation was still observed but in a lower rate. When the concentration of the solution was increased to 12 and higher wt %, the beads disappeared altogether, leaving only smooth fibres on the target. During the electrospinning process it was observed that the diameter of the deposition area of the nonwoven webs on collector decreased with increasing solution concentration. With increasing concentrations, thus increasing viscoelastic force, the ejected jets are less prone to diameter reduction. Therefore, the charged jet travels along a straight trajectory for a longer distance before undergoing bending instability. As a result, the jet path is reduced and the bending instability spreads over a smaller area. In addition, splitting of the jet into smaller jets was observed at lower concentrations. It is considered that the observed larger diameter of the nonwoven webs is a result of these two phenomena. Distribution of fibre diameters calculated from scanning electron microscopic images using image analysis software indicated that the average fibre diameter increased with increasing concentration. Figure 3.2 represents the standard deviations and the average diameters of all solution concentrations. 27

41 Fibre Diameter Diameter (nm) wt % 11 wt % 12 wt % 13 wt % 14 wt % 15 wt % Concentration (wt %) Figure 3. 2 The average fibre diameter of polyamide 6 as a function of concentration Interestingly, the fibre diameter increased almost double with an increase in concentration and feeding fate. This extreme variation in fibre diameters could be explained by a number of phenomena. For instance, similar to the electrospraying process, at low concentration (10 wt %) the electrical forces may induce the splitting of the charged jet into smaller jets resulting in the formation of fibres with very small diameters. The splitting of the jet may also favour the fast evaporation of solution and induces a stress on dry fibres which may also explain the reason of discontinuous fibres obtained at 10 wt %. However, when the concentration is high enough, it may discourage secondary jets from breaking off from the main jet which may favour the increased fibre diameter. In addition, as mentioned in the first chapter, the feeding rate determines the amount of solution available for electrospinning. Thus, when the feeding rate is increased, there is a corresponding increase in fibre diameter due to the greater volume of the solution that is drawn away from the needle tip. 28

42 3.1.2 Molecular Structure DSC Studies DSC is a thermo analytical technique in which the difference in the amount of heat required to increase the temperature of a sample and reference are measured as a function of temperature. The basic principle underlying this technique is that, when the sample undergoes a physical transformation such as phase transitions, more (or less) heat will need to flow to it than the reference to maintain both at the same temperature. On the other hand, Modulated DSC imposes a sinusoidal temperature ramp over a conventional linear temperature ramp and processes a resulting signal in a DSC curve. Then the curve is separated into reversible and irreversible components. For instance, the glass transition is an example of a reversible component while crystallization during the heating represents an irreversible component. In this particular work, DSC and MDSC were used to measure a number of characteristic properties of the samples including glass transition, melting behaviour and crystalline structure. In order to optimise the reproducibility of the DSC measurements, several tests were performed. First, the amount of sample required for optimum measurements was investigated. It has been found that samples less than 1 mg result in spikes on the thermograms most probably due to the sample movement in the pan. Better results were obtained with an increase in sample mass. For this particular work and type of sample, 3 mg was found to be an optimum value. Over this loading, problems occurred due to the burst of the pan lid. Also different pan types were used and the best results were obtained with T-zero aluminium pans. Finally, the influence of the sample layers in the pan was explored. The results demonstrated that when the sample load is less than 1 mg, an increase in the number of sample layers favours spike formation on thermograms. However, for higher amounts of loading, the number of layers does not have a significant effect on reproducibility of the results. A selected series of thermograms for different trials are shown in Figure

43 6 4 Heat Flow (W/g) 2 Loading 0-2 Exo Up Temperature ( C) Universal V4.2E TA Instrume Figure 3. 3 A selected series of thermograms of different trials for optimising the DSC method The degree of crystallinity and melting temperatures of PA 6 nanofibres obtained from the first heating scan are reported in Table 3.2. The results are the average of at least four different scans of each sample. The degree of crystallinity was calculated using an enthalpy of fusion of 190 J/g for the fully crystalline material [100]. The melting temperature for the α-phase was almost the same for all concentrations. However, a slight variation in melting temperature of the γ-phase was observed with increasing concentrations. For constant feeding rates, the electrospun polyamide 6 fibres obtained from solutions of low concentrations show lower percentage of crystallinity compared with that of higher concentrations. As explained in the previous sections, the process of electrospinning, especially the later stages of electrospinning, can be understood as the rapid solidification process of the stretched macromolecular chains under the high elongation rate. The rapid solidification limited the development of crystallinity because the macromolecular chains had no time to form crystalline registration. In our case, with varying concentrations, the 30

44 lower concentration (10 wt %), results in finer fibres, probably partly due to the splitting of the charged jet into multiple jets. Therefore the solidification occurs faster than the higher concentrations, (12 wt %, with the same feeding rate), resulting in a small percentage of crystallinity. Solution Concentration Melting Temperature o C (γ phase) Melting Temperature o C (α phase) Degree of Crystallinity % 10 wt % wt % wt % wt % wt % wt % Table 3. 2 Melting temperature and crystallinity percentage of electrospun polyamide 6 fibres For the higher concentrations (12-15 wt %), the steady state conditions required an increase in feeding rate. This induced very slight decrease in the crystallinity of the nanofibres. The increase in feeding rate neutralised the larger concentration effect on the percentage crystallinity. Indeed with increasing feeding rate one may expect again a decrease in crystallinity content. 31

45 2 1 Heat Flow (W/g) 0-1 Concentration Temperature ( C) Figure 3. 4 DSC Thermograms for the effect of concentration on melting behaviour of polyamide 6 for concentrations of 10, 11, 12, 13, 14 and 15 wt % Figure 3.4 demonstrates the melting behaviour of PA 6 nanofibres for all solution concentrations. It is obvious that all samples exhibit multiple melting endotherms corresponding to the melt of γ and α-phase crystals. The γ-phase melting peak became less distinctive at solutions of higher concentrations. In addition, when the solution concentration decreased the melting area of the obtained fibres became broader which corresponds to an early melting of γ-phase crystals. It is considered that these results are not due to the transformation between γ-phase crystals and α- phase crystals but rather related to the stability of crystals in polymer structure. We suggest that the decrease in concentration results in an instability of γ-phase crystals. As mentioned before, the decrease in solution concentration favours the formation of multiple jets in electrospinning process which may result in the fast evaporation of solution. Since the crystallisation process requires some time for higher stability, the fast evaporation results in instable crystals in fibre 32

46 structure. In sum, with increasing concentration the stability of crystals in the fibre structure increases. The MDSC thermograms showing the effects of concentration on the glass transition behaviour of polyamide 6 nanofibres is reported in Figure PA 6 10 wt % PA 6 11 wt % PA 6 12 wt % PA 6 13 wt % PA 6 14 wt % PA 6 15 wt % Deriv. Rev Cp (J/(g C²)) Temperature ( C) Figure 3. 5 MDSC Thermograms for the effect of concentration on glass transition behaviour of polyamide 6 for concentrations at 10, 11, 12, 13, 14 and 15 wt % When thermograms were smoothed and put on top of each other, only very slight differences were observed in the glass transition behaviour of electrospun PA 6 nanofibres. Thus, it is considered that concentration differences do not result in significant differences in orientation of amorphous phase of obtained fibres. The average glass transition temperature was measured around 80 o C for all samples. 33

47 Raman Studies Raman spectroscopy measures the vibrational energy levels of materials. The technique is based on a change in the induced dipole moment or polarization to produce Raman scattering. When a beam of light strikes a molecule, it can be either absorbed or scattered. Most of the photons scattered elastically, these are termed as Rayleigh scattering. Some of the photons are scattered inelastically, and these are known as Raman scattering. These Raman scattered photons with different frequencies, constitute the Raman spectrum of the molecule. Due to the high sensitivity of the Raman effect for certain groups like C-C bonds, Raman spectroscopy is used for analysis of the chemical composition and structure of polymers. Moreover, it can also be used for determining the configuration and conformation of polymer chains [139,140]. It is well known that polyamide 6 is a polymorphic material, having more that one energetically favourable structures including - and -phase. In -phase hydrogen bonding occurs between antiparallel chains resulting into a fully extended planar zigzag conformation. In -phase hydrogen bonding between parallel chains results in a mismatch of hydrogen bonding. Actually -phase of PA 6 is a consequences of more trans conformation between C(=O)-CH 2 and NH-CH 2 bond where as the -phase has relatively more gauche conformation between C(=O)-CH 2 and NH-CH 2 and thus being less stable with a lower melting temperature. In the Raman spectra of the polyamide 6 nanofibres generated from different solution concentrations, a number of differences were observed in the C-C stretch region ( cm -1 ). The C-C stretching region is composed of three primary peaks, 1065, 1080 and 1123 cm -1. The 1065 and 1130 cm -1 peaks are indicative of an all trans CC backbone conformation while the 1080 cm -1 peak is attributed to the presence of gauche bonds. The C-NH bending region of the Raman spectrum is also sensitive to the conformation (planar or non-planar) of the amide group. The bands observed in the region of cm -1, and cm -1 are indicative of trans amide group [99]. 34

48 A selected spectrum of nanofibres webs obtained from different solution concentration were normalised on the 1080 cm -1 (gauche conformation) peak and represented in Figure 3 6. The variations on the 1123 cm -1 (trans conformation) due to concentration differences were observed. PA 6 10 wt % PA 6 11 wt % PA 6 12 wt % PA 6 13 wt % PA 6 14 wt % PA 6 15 wt % Trans Gauche Raman Sh ift / c m-1 Figure 3. 6 A selected Raman spectra for different concentrations The average of the peak ratio for these two particular peaks was calculated by the average of at least 3 measurements for each sample. The results and standard deviations are shown in Table

49 Solution Concentration Average Ratio 1123 (Trans)/1080 (Gauche) Standard Deviation 10 wt % wt % wt % wt % wt % wt % Table 3. 3 The average ratio of 1123 (trans)/1080 (gauche) peaks In general, it has been observed that with increasing concentration the trans/gauche peak ratio is slightly increasing which corresponds to the trans conformation becoming more favourable in the fibre structure. Thus, crystals become slightly more stable in nanofibre structures electrospun from increasing concentrations. This result is in line with the diameter measurements and DSC results, as also the γ melting peak shifts slightly to higher temperatures and becomes more α-like FT-IR Studies Infrared spectroscopy, which is a complementary technique to Raman spectroscopy, is one of the most commonly used spectroscopic techniques. In infrared spectroscopy, IR radiation is passed through the sample. Some of the infrared radiation is absorbed by the sample while some of it is passed through. The resulting spectrum represents the molecular absorption and transmission, creating molecular characteristics of the sample. The application areas of IR spectroscopy include, measuring the concentration of end groups, determining the reaction order and chemical processes, investigating the structural changes produced by chemical reactions, characterizing copolymers, measuring stereo regularity, conformation, and branching, characterizing polymer blends, measuring morphological units in polymers, and investigating crystallinity and orientation[141]. 36

50 Band (cm -1 ) Assignment 3300 Hydrogen-bonded NH stretching 3086 CH 2 asymmetric stretching 2931 CH 2 asymmetric stretching 2859 CH 2 asymmetric stretching 1645 Amide I 1544 Amide II 1369 Amide III + CH 2 wagging 1264 Amide III + CH 2 wagging 1236 CH 2 wagging/twisting 1200 CH 2 wagging twist wag vibration 1170 CONH skeletal motion 1119 CC stretching 1078 CC stretching 975 CONH in-plane 929 CONH in-plane Table 3. 4 FT-IR band assignment of polyamide 6 Table 3.4 shows the FT-IR band assignment of polyamide 6[123,142]. The characteristic absorption peaks of the α-phase are 1478 cm -1, 1416 cm -1, 1373 cm -1, 1199 cm -1, 959 and 928 cm - 1. The characteristic peaks of the γ-phase are 1439 cm -1, 1369 cm -1, 1236 cm -1, 976 cm -1 and 712 cm -1. The characteristic peaks of amorphous phase are still not clear in literature [142]. Figure 3.7 demonstrates the FT-IR spectra of nanofibres mats obtained from various concentrations. In order to compare the variations between α-phase and γ-phase due to concentration differences, the spectra was normalised on the 1170 cm -1 band of the PA 6 [143]. 37

51 Concentration γ α K-M cm-1 Figure 3. 7 A selected FT-IR spectra of various concentrations On the resultant spectra the γ- (975 cm -1 ) and the α-phase (929 cm -1 ) characteristic peaks were observed. The average ratios for at least 3 scans of these two peaks and corresponding standard deviations were given in Table

52 Solution Concentration Average Ratio 975 (γ phase) / 929 (α phase) Standard Deviation 10 wt % wt % wt % wt % wt % wt % Table 3. 5 The average ratios for 975 (γ phase) / 929 (α phase) peaks The results demonstrate that the γ phase crystals are decreasing with increasing concentration. In other words, γ phase crystals becoming more stable α like crystal at higher concentrations. This result is in line with Raman and DSC results Conclusion The effect of solution concentration on generated fibre properties was investigated. It has been shown that concentration, especially together with feeding rate, has a significant effect on the resultant fibre diameter. The smallest diameters were obtained for the 10 wt % solution. The diameters of fibres obtained for concentrations at 11, 12 and 13 wt % were similar and around 108 nm. The largest diameters, almost double in comparison to 10 wt %, were produced with concentrations at 14 and 15 wt %. We propose that this extreme variation in fibre diameter is due to the splitting of jet into smaller jets at lower concentrations. In contrast, at higher concentrations the secondary jet formation is prevented by higher viscoelastic forces, in combination with the higher feeding rate. The variations in molecular structure of electrospun nanofibres due to concentration differences were investigated by DSC, Raman and FT-IR. The results were complementary. In general, the 39

53 results showed very slight differences between concentration variations. Therefore, we suggest that these variations are not due to complete transformation of structure from one phase to another, rather due to the stability variations. The results demonstrate that with increasing concentration the stability of crystals, particularly γ phase crystals, are increasing. It is considered that, multiple jet formation at low concentrations results in fast evaporation of solution. Since the crystallisation process requires some time for higher stability, the fast evaporation results in more instable crystals in the fibre structure. 3.2 Effect of Applied Voltage The electrical field intensity depends on both applied voltage and distance between tip and collector. In this particular work, the effect of electrical field intensity was studied by varying the applied voltage at constant tip to collector distance. A series of experiments with nanofibres obtained from concentrations at 14 wt % solutions, were carried out when the applied voltage was varied from17 to 25 kv and the tip to collector distance was held at 6 cm. It should be noted that with this particular concentration, it was not possible to produce nanofibres under 21 kv in steady state conditions. Thus, the feeding rate of solution was varied under 21 kv in order to achieve steady state conditions. The electrospinning processing parameters for each sample are reported in Table 3.6. Concentration Tip to Collector Distance Applied Voltage Feeding Rate 14 wt % 6 cm 17 kv 0.9 ml/h 14 wt % 6 cm 19 kv 1.2 ml/h 14 wt % 6 cm 21 kv 1.5 ml/h 14 wt % 6 cm 23 kv 1.5 ml/h 14 wt % 6 cm 25 kv 1.5 ml/h Table 3. 6 Electrospinning parameters for different samples 40

54 During production of the samples, it has been observed that the shape of the initiating droplet on the tip of the needle is affected by the strength of applied voltage. The balance between the surface tension and the electrical force is critical to determine the initial cone shape. For instance, at 21 kv the diameter of the droplet at the tip of the needle was found to be larger and the jet was initiated from this droplet. It was possible to observe a Taylor Cone with these electrospinning conditions. When the applied voltage was increased, the jet velocity increased and the solution was removed from the tip more quickly. As the volume of the droplet at the needle became smaller, the Taylor Cone shape was deformed at 23 kv. With a further increase in voltage, the droplet at the tip of the needle disappeared and the jet was initiated from the tip. It was observed that the actual jet initiation occurred at some points inside the spinneret at an applied voltage of 25 kv Fibre Properties Figure 3.8 shows a selected series of FE-SEM micrographs to demonstrate the effect of applied voltage on the morphological appearance of the obtained fibres. For samples produced by increasing applied voltage together with feeding rate (17 kv-0.9 ml/h, 19 kv-1.2 ml/h and 21 kv- 1.5 ml/h) similar morphological appearances were observed. In some cases there were beads on the fibre axis. In contrast to this, nanofibres obtained at a constant feeding rate, increasing applied voltage resulted in some variations in the fibre morphology. At low voltage (21 kv), a combination of smooth and beaded fibres was obtained. However, with an increase in applied voltage (23 and 25 kv), the beads disappeared and only smooth fibres were obtained on the collector. An increased stretching of the jet due to the higher voltage favoured less bead formation in the fibre axis. Similar results were reported by Jarusuwannapoom [22]. 41

55 (a) (b) (c) (d) (e) Figure 3. 8 FE-SEM micrographs of materials obtained from electrospinning of polyamide 6 at applied voltages of a) 17 kv, b) 19 kv, c) 21 kv d) 23 KV e) 25 kv(magnification = x, scale bar = 1µm, working distance = 8 mm) 42