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Single wall carbon nanotube composite nanofibres from electrospun polyacrylonitrile copolymer as a potential… Mertens, Joël 2015

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 Single Wall Carbon Nanotube Composite Nanofibres from Electrospun Polyacrylonitrile Copolymer as a Potential Transparent Conductor  by JOËL MERTENS B.A.Sc., The University of British Columbia, 2009  A THESIS SUBMITTED IN PARTRIAL FULFILMENT OF  THE REQUIREMENTS FOR THE DEGREE OF   MASTER OF APPLIED SCIENCE  in  THE FACULTY OF GRADUATE AND POSTDOCTORAL STUDIES (Materials Engineering)  THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver)  August 2015      © Joël Mertens, 2015 ii Abstract Diversifying and securing sources of energy is considered one of the greatest challenges that humanity faces over the next 50 years. Of all the potential energy sources, solar energy is one of the most promising, with costs dropping and capacity increasing at an exponential rate. As new photovoltaic technologies become available, the need to develop new transparent conductor technologies with a range of functionality increases. The potential for using electrospinning as a method to develop a transparent conductor based on carbon nanofibres is explored in this study. Electrospinning has great potential for this type of application due to its ability to create fibres with high aspect ratios and the ability of the process to easily scale up.  A copolymer of polyacrylonitrile (PAN), polyacrylonitrile-co-methyl acrylate (PAN-co-MA) is characterized and explored as a precursor for creating carbon nanofibres. By exploring and specifying the solution properties, future work using PAN-co-MA can be optimized more efficiently. In addition to PAN-co-MA, varying amounts of single wall carbon nanotubes (SWNT) were added to the spinning solution to determine how composite SWNT/carbon nanofibres perform compared to the original carbon nanofibres. Varying carbonization temperatures from 700˚C to 1000˚C were explored and samples containing SWNT showed up to two orders of magnitude better conductivity compared to the benchmark condition for some scenarios. In all conditions the samples with SWNT outperformed those without.  A method to coat the nanofibre membranes with PEDOT:PSS was developed, which has uses both for thin film and bulk functionalized nanofibre uses where conductivity is important.  iii  Thin film samples of composite SWNT/carbon nanofibres were created and characterized with respect to their transparency and sheet resistance. Transparencies over 96% were achieved. Once coated with PEDOT:PSS, the sheet resistance dropped to 414 ohm/sq while maintaining over 93% transparency for some conditions.   iv Preface This dissertation is original, unpublished, independent work by the author, J. Mertens.   v Table of Contents Abstract .................................................................................................................................... ii Preface ..................................................................................................................................... iv Table of Contents .................................................................................................................... v List of Tables ........................................................................................................................ viii List of Figures ......................................................................................................................... ix List of Symbols and Abbreviations ...................................................................................... xi Acknowledgements .............................................................................................................. xiii 1. Introduction and Objectives .......................................................................................... 1 1.1 The Importance of Energy .................................................................................................... 1 1.2 History of Photovoltaic Technologies ................................................................................... 1 1.3 The Growing Role of Solar Energy ...................................................................................... 2 1.4 Research Objectives ............................................................................................................... 4 2. Literature Review and Background Information ........................................................ 5 2.1 Organic Photovoltaics ............................................................................................................ 5 2.2 Transparent Conductors for Organic Photovoltaics .......................................................... 7 2.3 Nanomaterials....................................................................................................................... 10 2.4 Electrospinning..................................................................................................................... 16 2.5 Carbonization of Polyacrylonitrile and Copolymers ........................................................ 20 3. Experimental Methods ................................................................................................. 24 3.1 Materials ............................................................................................................................... 24  vi 3.2 Polymer Solution Preparation ............................................................................................ 24 3.2.1 SWNT Solution Preparation ........................................................................................... 25 3.2.2 Solution Mass Fraction Reporting .................................................................................. 25 3.3 Intrinsic Viscosity Measurement ........................................................................................ 27 3.3.1 Intrinsic Viscosity Polymer Solution Preparation .......................................................... 27 3.3.2 Efflux Time Measurements ............................................................................................ 27 3.4 Electrospinning..................................................................................................................... 29 3.4.1 Parameter Optimization .................................................................................................. 30 3.5 Nanofibre Carbonization ..................................................................................................... 31 3.6 PEDOT:PSS Coatings.......................................................................................................... 33 3.7 Conductivity Measurement ................................................................................................. 34 3.8 Transparency Measurement ............................................................................................... 34 4. Experimental Results .................................................................................................... 36 4.1 Intrinsic Viscosity ................................................................................................................. 36 4.1.1 Viscosity Average Molecular Weight ............................................................................ 39 4.1.2 Fibre diameter vs. Concentration .................................................................................... 40 4.2 Electrospinning Parameters of PAN-co-MA ..................................................................... 44 4.2.1 Response Analysis of Voltage and Polymer Flow Rate on Fibre Diameter ................... 45 4.3 Electrospinning PAN-co-MA and SWNT .......................................................................... 48 4.3.1 Effects of SWNT on Electrospinning Parameters .......................................................... 52 4.4 Carbonization ....................................................................................................................... 52 4.4.1 Microstructure Analysis ................................................................................................. 53 4.4.2 Conductivity ................................................................................................................... 57 4.5 PEDOT:PSS Coatings.......................................................................................................... 59 4.6 Thin Film Development ....................................................................................................... 63  vii 4.6.1 Transparency .................................................................................................................. 63 4.6.2 Sheet Resistivity ............................................................................................................. 64 4.6.3 PEDOT:PSS Coated Thin Films ..................................................................................... 65 5. Discussion of Results ..................................................................................................... 68 5.1 PAN-co-MA Polymer Characterization ............................................................................. 68 5.2 Electrospinning Parameters of PAN-co-MA ..................................................................... 69 5.2.1 Response Analysis .......................................................................................................... 71 5.3 Electrospinning PAN-co-MA with SWNT ......................................................................... 72 5.4 Carbonization ....................................................................................................................... 73 5.5 PEDOT:PSS Coatings.......................................................................................................... 74 5.6 Thin Film Nanofibre Samples ............................................................................................. 74 6. Conclusions and Future Work..................................................................................... 76 6.1 Recommendations for Future Work .................................................................................. 79 References .............................................................................................................................. 81   viii List of Tables Table 1: Total energy required to produce 1m2 of PV module ................................................. 5 Table 2: Energy payback time of various PV technologies ...................................................... 6 Table 3: SWNT Transparent Conducting Films ..................................................................... 15 Table 4: Nanomaterial Transparent Conducting Films ........................................................... 15 Table 5: Calculated Molecular Weight ................................................................................... 39 Table 6: Nanofibres from Response Analysis ........................................................................ 45 Table 7: Significance of Results for Response Analysis ........................................................ 48 Table 8: Electrospun PAN-co-MA w/ SWNT Fibre Distribution .......................................... 51 Table 9: Significance of Results for Electrospun PAN-co-MA w/ SWNT ............................ 52 Table 10: Conductivity of PAN-co-MA / SWNT nanofibre membranes ............................... 57 Table 11: Conductivity Increase due to SWNT in Carbonized Nanofibres............................ 58 Table 12: Transmission at 550 nm wavelength ...................................................................... 63 Table 13: Sheet Resistance of Nanofibre Thin Films ............................................................. 65 Table 14: Sheet Resistance of PEDOT:PSS Coated Samples ................................................ 65 Table 15: Transmission at 550nm for PEDOT:PSS Coated Samples .................................... 66 Table 16: Fibre Diameter Maximum/Minimum Limits by Flowrate, as Influenced by Voltage Range .............................................................................................................................. 70 Table 17: Berry Number Analysis for PAN-co-MA copolymer ............................................ 71    ix List of Figures Figure 1: Worldwide Operational Photovoltaic Capacity (Logarithmic Scale)........................ 3 Figure 2: Types of Organic PV devices .................................................................................... 7 Figure 3: Energy Pathways in Organic Photovoltaic Device.................................................... 8 Figure 4: Carbon nanomaterials by dimensions...................................................................... 10 Figure 5: Carbon Nanotube Unit Vectors ............................................................................... 12 Figure 6: Basic Electrospinning Setup .................................................................................... 17 Figure 7: Beaded Fibre formation ........................................................................................... 19 Figure 8: Molecular Structure of PAN and Copolymers ........................................................ 21 Figure 9: Oxidative Stabilization of Polyacrylonitrile ............................................................ 22 Figure 10: Mass Fraction Example for an Electrospinning Solution with Nanoparticles ...... 26 Figure 11: Ubbelholde Viscometer ......................................................................................... 28 Figure 12: Katotech Nanofiber Electrospinning Unit ............................................................. 29 Figure 13: Thermolyne 79400 Tube Furnace ......................................................................... 31 Figure 14: Sample Holders for Carbonization ........................................................................ 32 Figure 15: Carbonization Process Diagram ............................................................................ 33 Figure 16: Sigma Aldrich Polymer ......................................................................................... 38 Figure 17: SPP Polymer .......................................................................................................... 38 Figure 18: Fibre diameter vs. polymer concentration for Sigma Aldrich PAN-co-MA ......... 41 Figure 19: Fibre diameter vs. polymer concentration for SPP PAN-co-MA .......................... 42 Figure 20: Fibre Diameter and Concentration Relationship ................................................... 43 Figure 21: Response Surface for Electrospinning Parameters ................................................ 46 Figure 22: Electrospun PAN-co-MA with SWNT ................................................................. 50  x Figure 23: Carbonized PAN-co-MA ....................................................................................... 53 Figure 24: Carbonized PAN-co-MA with 1% SWNT ............................................................ 54 Figure 25: Carbonized PAN-co-MA with 5% SWNT ............................................................ 55 Figure 26: Bar Graph of Fibre Size vs. Carbonization Temperature Relationship ................. 57 Figure 27: Nanofibre conductivity vs. Carbonization Temperature ....................................... 58 Figure 28: PEDOT:PSS Coated Nanofibres (top side) ........................................................... 59 Figure 29: PEDOT:PSS Coated Nanofibres (back side) ........................................................ 60 Figure 30: PEDOT:PSS Coated Nanofibres (lifted coating) .................................................. 60 Figure 31: 50% MeOH / 50% PEDOT:PSS Coated Nanofibres ............................................ 61 Figure 32: Fibre Structure beneath MeOH / PEDOT:PSS Peeled Coating ............................ 62 Figure 33: Fibre Structure at edge of MeOH / PEDOT:PSS Coated Nanofibres ................... 62 Figure 34: Optical Transmission of Nanofibre Thin Films .................................................... 64 Figure 35: Optical Transmission of Nanofibre Thin Films with PEDOT:PSS Coatings ....... 66 Figure 36: Theoretical vs. Actual Nanofibre / PEDOT:PSS Optical Transmission ............... 67 Figure 37: Typical Molecular Weight Distribution ................................................................ 69     xi List of Symbols and Abbreviations CdTe Cadmium Telluride COOH Carboxylic acid DMF Dimethylformamide DMSO Dimethylsulfoxide eV Unit of energy, electron volt HOMO Highest occupied molecular orbital IR Infrared ITO Indium tin oxide kV Unit of electrical potential, kilovolt LUMO Lowest unoccupied molecular orbital MeOH Methanol Mw Molecular weight ƞ Coefficient of viscosity PAN Polyacrylonitrile PAN-co-MA Polyacrylonitrile methyl acrylate copolymer PEDOT:PSS Polystyrene sulfonate doped poly(3,4-ethylenedioxythiophene) PV Photovoltaic PVP Polyvinylpyrrolidone RPM Revolutions per minute S/cm Unit of electrical conductivity, siemens per centimeter SEM Scanning electron microscopy SPP Scientific Polymer Products, Inc   xii SWNT Single wall carbon nanotube UV Ultraviolet wt% Percentage by weight Ω/☐ Unit of electrical resistance, ohm per square                   xiii Acknowledgements It is my great pleasure to acknowledge and thank Dr. Frank Ko and Dr. Peyman Servati for their patience, guidance, and support over the course of this research. I would also like to express my gratitude to my co-workers in the Advanced Fibrous Materials Laboratory and the Flexible Electronics Engineering Laboratory as well as the staff within the UBC Materials Engineering Department for providing their expertise and experience along the way.  I would like to acknowledge the funding from the Natural Sciences and Engineering Research Council of Canada along with the Canada Foundation for Innovation and the Asian Office of Aerospace Research and Development.  Finally, I would like to thank my wife Samantha and my family for their continued support and encouragement over the course of this degree. 1 1. Introduction and Objectives 1.1 The Importance of Energy Civilization is dependent on an available supply of energy to provide the level of structure that a complex society requires to function [1]. While traditionally this was provided through enhanced human labour and from renewable energy indirectly supplied by the sun (such as through agriculture), the industrial revolution was a catalyst in the move to fossil fuels as a primary energy source. Since then, our industrial society has relied heavily on these non-renewable resources to provide us with ever increasing improvements in our quality of life. As the most easily accessible conventional fossil fuel supplies are depleted we must turn to unconventional supplies that have a lower energy return on investment [2]. With our free energy supply growing more complex and uncertain, it is important to investigate and identify the means through which we can secure a sustainable and plentiful energy source for the future. Richard Smalley, a prominent Nobel laureate recognized for his discovery of buckminsterfullerene, considered energy to be the number one problem that humanity faces within the next 50 years and issued a challenge to scientists to find a solution [3]. One of the solutions that he advocated for is an expansion in the use of solar technologies to provide for our primary energy needs.  1.2 History of Photovoltaic Technologies The discovery of a relationship between light energy and the electronic properties of materials occurred less than 200 years ago with Alexandre-Edmond Becquerel’s discovery of the photovoltaic effect in 1839 [4]. Just over 100 years ago in 1904 Albert Einstein published his paper on the photoelectric effect, resulting in his Nobel Prize win in 1921 and one of the  2 leading discoveries on the path to the quantum revolution [5]. However, the first modern silicon photovoltaic cell was not created until 1954 [6]. While the initial efficiency was only 6% it demonstrated the possibility of photovoltaics as a means to generate electricity. This is largely considered the first generation of modern photovoltaics. While these first generation solar cells remain popular and have a theoretical efficiency of up to 29% (the Shockley-Queisser limit for single junction silicon solar cells) they are expensive to manufacture. The second-generation solar cells, known as thin film solar cells, tried to address the cost issue by using less materials and having simpler manufacturing steps. These technologies include amorphous silicon technology, solar cells based on Cadmium Telluride (CdTe) materials, and Copper-Indium-Gallium-Diselenide (CIGS) technologies. Third generation photovoltaic technologies include multi-junction solar cells meant to overcome the Shockley–Queisser limit of single bandgap photovoltaics. Frequency conversion, hot-carrier effects, and multiple carrier injection are all areas of research to improve these photovoltaic devices. Another area of photovoltaic research includes organic photovoltaics. This is an area of intense interest due to the potential for low cost and the ability to create flexible solar cells. Organic photovoltaics bring their own set of challenges that need to be overcome, including the difficulty in finding a suitable transparent conductor to enable flexible applications. Indium Tin Oxide (ITO), a semiconductor in wide use as a transparent conductor, has generally been found to be inadequate in this regard [7]. 1.3 The Growing Role of Solar Energy As costs drop and new technologies are developed, the importance and impact of photovoltaics continues to rise. Photovoltaics first found a role within the space industry before establishing itself as an option in the terrestrial off-grid market [8]. Most recently  3 photovoltaics have found on-grid applications. Since 1995, the growth in the global operational photovoltaic capacity has been exponential, passing the 100 Gigawatt (GW) level in 2012 as shown in Figure 1 below [9].  Figure 1: Worldwide Operational Photovoltaic Capacity (Logarithmic Scale) While this demand has been largely stimulated by government subsidies, falling prices have reached a point where unsubsidized photovoltaic installations make economic sense in certain jurisdictions. It is likely that locations that have achieved grid parity for photovoltaic systems will continue to emerge as the price for solar generated electricity falls to an estimated $0.08/kWh by 2020 in areas with high solar irradiance [8]. Given the continued rapid growth within the solar industry, there is a potential risk for supply bottlenecks in some of the raw materials needed. Indium, necessary as a transparent electrode material, is one of the metals that have been identified as potentially of concern [10]. By finding alternative transparent conductors for use in photovoltaic devices this concern can be mitigated.   4 1.4 Research Objectives This work investigates the potential of using an electrospun carbon/single wall carbon nanotube (SWNT) composite material as an alternative to Indium Tin Oxide (ITO) as a transparent electrode, specifically targeted towards applications for organic solar cells. Noting that carbon nanotubes and other one-dimensional nanoscale materials have been proposed as a replacement to ITO [11-13], bulk carbon/SWNT nanofibres will be assessed to determine if the level of bulk conductivity achievable is relatable to thin layers of nanofibres and the future design considerations for electrospun transparent conductors.  Electrospinning creates high aspect ratio nanofibres that can incorporate other nanomaterials into its structure. While the electrospinning of polyacrylonitrile (PAN) as a precursor to a carbon nanofibre is well described in literature, there is limited analysis on using co-polymers as the carbon fibre precursor in electrospinning. As many industrial processes use co-polymers rather than homopolymer PAN, the methyl acrylate co-polymer of PAN is of interest and its electrospinning properties are to be examined.  Polyethylenedioxythiophene with polystyrene sulfonate (PEDOT:PSS) is a commonly used transparent conductor material in organic solar cells. The need to create a coated composite of electrospun carbon/SWNT nanofibres and PEDOT:PSS is of interest to assess the performance of these films for organic photovoltaics and to determine if any synergies or challenges exist by combining these materials.  5 2. Literature Review and Background Information 2.1 Organic Photovoltaics Organic photovoltaics utilize organic semiconductors to directly transform solar energy into electricity. Organic photovoltaics provide several advantages over traditional semiconductor solar cells: the potential for low-cost processing (thereby enabling low cost PV devices), soft mechanical properties to allow for flexible devices and roll-to-roll processing, the ability for efficient light harvesting through materials with large light absorption coefficients, and lightweight photovoltaic devices [14-16]. In addition, life cycle analysis has shown that organic photovoltaics have very attractive energy payback times and upfront energy costs. The data summarized in Table 1 below show the energy required to produce 1 m2 of module for various PV technologies. Table 1: Total energy required to produce 1m2 of PV module Solar Cell Type Total Energy (MJ) Organic PV 45 CdTe 212 Multicrystalline Si 1203 Monocrystalline Si 1376 Sources: [17, 18]  The total energy to produce organic photovoltaics is over twenty five times less than silicon modules of the same area. It should be noted that the efficiencies of organic photovoltaics are not yet comparable to their silicon counterparts, but even accounting for the current  6 limitations on module efficiencies the energy payback period of organic photovoltaics is more attractive than for other photovoltaic technologies. The energy payback period for several types of solar cells is shown in Table 2 below.  Table 2: Energy payback time of various PV technologies Solar Cell Type Energy Payback Time (years) Organic PV 0.29-0.52 CdTe 0.75-2.1 Multicrystalline Si 1.5-2.6 Monocrystalline Si 1.7-2.7 Source: [19]  However, organic semiconductors have a different photovoltaic pathway compared to conventional inorganic semiconductors. Organic semiconductors do not generate free electrons and holes, generating instead an exciton, a tightly bound electron-hole pair. The exciton is an electrically neutral, mobile excited state that can propagate through a material and must be separated before energy can be extracted from the system. This requires a different approach to fabrication than conventional semiconductor photovoltaic devices in order to achieve good efficiency.   The first organic molecule discovered to exhibit photoelectric behaviour was anthracene in 1906 [20]. The first organic solar cell, a single layer device, was demonstrated in 1958 but suffered from poor conversion efficiency [21]. The concept of the heterojunction was first explored in the late 1950’s [22] and was first incorporated into a bilayer organic photovoltaic  7 device in 1986 [23]. While this device performed much better than the single layer devices, it achieved just less than 1% conversion efficiency. Both of these types of devices suffered due to the exciton recombination, where the charges recombine before they can be separated to do any useful work. Charge recombination can be reduced by limiting the distance the exciton needs to travel to an interface capable of separating the charges. Typical exciton diffusion lengths are on the order of 7-10nm [24,25] and the distance to an interface should be on this order of distance. This was achieved by dispersing the electron donor and acceptor materials into a bulk heterojunction as is seen in Figure 2 below. By increasing the surface area of the interface it is more likely that an exciton can reach the boundary and have its charges separated. This effect can further be enhanced by adding additional materials such as single wall carbon nanotubes to the interface [26].  Figure 2: Types of Organic PV devices 2.2 Transparent Conductors for Organic Photovoltaics The development of bulk heterojunction devices address several of the challenges in separating and transporting charges efficiently in organic photovoltaics. Another area with ongoing research is the development of electrodes that allow for more efficient charge collection and charge-carrier mobility. For efficient charge collection, materials must be  8 chosen with its work function in mind. This requirement can be seen visually in an energy diagram for a typical organic photovoltaic device as shown in Figure 3 below.   Figure 3: Energy Pathways in Organic Photovoltaic Device Figure adapted by author from [27] For a traditional device, the donor material transfers the electron from its lowest unoccupied molecular orbital (LUMO) to the LUMO of the acceptor material. This is then collected at a metal electrode with a work function that is lower than the LUMO of the acceptor. The electron hole follows an opposite pathway, being transported to the transparent electrode from the highest occupied molecular orbital (HOMO) of the donor material. In this case, the work function of the transparent electrode should be higher than that of the HOMO of the donor material. Typically, Indium Tin Oxide (ITO) is used as a transparent electrode material in photovoltaics as it possesses relatively low sheet resistance while maintaining a reasonable  9 level of transparency to incoming light. ITO can have sheet resistance of less than 100 ohm per square, often down to less than 10 ohm per square, while maintaining over 85% transmission of light in the visible spectrum. ITO has been measured to have a work function of 4.8 eV [28].  One of the limitations for ITO in organic photovoltaics is that it is brittle, making it unsuitable for use in highly flexible electronics. For this reason, a transparent, conductive polymer is often used instead. The most commonly used is Poly(3,4-ethylenedioxythiophene) doped with Polystyrene sulfonate (PEDOT:PSS). PEDOT:PSS has a work function that can be tuned in the range of 4.8-5.1 eV [29, 30] and optical transmission and sheet resistance can achieve similar levels to that of ITO [31]. Other approaches to developing materials for transparent electrodes are also being researched including using fluorinated tin oxides (FTO), zinc oxides (ZnO), graphene, carbon nanotubes and metal nanowires [32-35]. The approaches using nanomaterials (graphene, nanotubes, and nanowires) all have potential applications in organic photovoltaics as they can be made to into flexible transparent conductors, a concept that will be further explored in the following section of this work.  10 2.3 Nanomaterials Nanotechnology as a term is one that is becoming increasingly common in today’s world. Generally, the accepted definition of nanotechnology is a technology that involves dimensions and/or tolerances of fewer than 100 nanometres. Nanomaterials are therefore materials that are fabricated using nanotechnology techniques and include at least one dimension that is under 100 nanometres. These can be grouped according to how many bulk (larger than 100 nanometres) dimensions they posses, as shown in Figure 4 below.  Figure 4: Carbon nanomaterials by dimensions  Dr. Richard Feynman first proposed the term nanotechnology back in 1959 at a talk where he envisioned a top-down approach to manipulating and using matter at a molecular level [36]. This was followed by Drexler’s alternate proposal in 1981 of forming nanoscale devices using molecular assembly of proteins in a bottom-up approach [37]. Creating nanomaterials was still mostly hypothetical at this point, but developments in manipulating and viewing materials at the molecular level were made possible with equipment such as the first scanning tunneling microscope (STM) in 1981 followed by the first atomic force microscope (AFM) in 1986 [38-40]. One of the first true nanomaterials to be discovered was Buckminsterfullerene, or C60, in 1985 [41]. Buckminsterfullerene represented a new allotrope of carbon that held potential for use in nanodevices as it was extremely round and  11 stable. Further research into carbon allotropes led to the discovery of carbon nanotubes in 1991 [42].   Carbon nanotubes are hollow tubes formed from wrapped sheets of graphene (single layer graphite). Single wall carbon nanotubes consist of only one layer while multiwall carbon nanotubes are concentric rings of carbon nanotubes held together by van der Waals forces. Both single and multiwall carbon nanotubes possess interesting material properties that make them desirable in numerous potential applications. Of particular interest to this work are the electronic properties of carbon nanotubes. Single wall carbon nanotubes are capable of ballistic electrical conductivity [43,44] in armchair configuration while they are electrical semi-conductors in zigzag configuration. These different configurations are based on the unit vectors that the axis of the carbon nanotube wraps itself from the graphene sheet, as can be shown in Figure 5 below.   12  Figure 5: Carbon Nanotube Unit Vectors  The differentiation in these electronic characteristics is due to variances in the alignment of the C-C bonds and their pi orbitals along the length of the carbon nanotube, leading to differences in the allowed electronic states. Metallic single wall nanotubes only have a few quantized energy channels, allowing electrons to flow through without encountering resistance (ballistic transport) [45-47]. On the other hand, in semiconducting single wall nanotubes the electrons must tunnel through a series of close low transmission conduction  13 barriers [48]. The carrier mobility of metallic single wall nanotubes is approximately 10,000 cm2 V−1 s−1 and the maximum current density is 4 × 109 A cm−2, which is over three orders of magnitude higher than the bulk value for typical metals [49]. It should be noted that for multiwall carbon nanotubes, the electrical conductivity is more difficult to predict as the concentric layers of nanotubes can have different chirality. As produced, single wall carbon nanotubes are a mix of semiconducting and metallic varieties, but a number of different separation techniques have been explored including alternating current dielectrophoresis, etching, and adsorption techniques [50-52].  In addition to these unique electronic properties, carbon nanotubes are also exemplary for their tensile strength and stiffness. The stiffness of both single walled and multi walled carbon nanotubes, as measured through their Young’s modulus, can exceed 1 Terapascal (TPa) [53, 54] with some claims reaching over 4 TPa [55]. It has been shown that for single wall carbon nanotubes there is a small dependence on the diameter of the nanotube and its chirality [56]. In multiwall carbon nanotubes the Young’s modulus can vary by up to two orders of magnitude depending on the degree of disorder in the nanotube, with arc-grown multiwall carbon nanotubes performing better than those grown by catalytic methods [57]. The tensile strength of carbon nanotubes is also significant, theoretically calculated to over 100 GPa for single wall nanotubes [58] and over 150 GPa for multiwall nanotubes [59]. The values obtained experimentally can vary significantly, due in part to the difficulty in performing direct mechanical testing due to the size scales involved. Experimental results are still fall in the gigapascal range.  14 Carbon nanotube composites and films have been explored for numerous applications to exploit the desirable attributes of these particles. Due to their high aspect ratio, the percolation threshold for single wall carbon nanotubes is among the lowest known for thin films of 1-D conductors [60]. For polymer-carbon nanotube composites, the electrical conduction depends on a number of parameters, including the nanotube concentration, the aspect ratio of the nanotubes, functional attributes of the nanotubes (including type, surface modifications, etc.), orientation, and alignment of the nanotubes, polymer type, and dispersion method [61]. Other models have suggested that the critical factor that controls the electrical conduction of these polymer/carbon nanotube composites is the electron tunneling between the gaps of neighbouring carbon nanotubes along the network [62, 63]. More aligned carbon nanotube networks can have larger intertube distances for a given volume fraction, leading to higher tunneling barriers between the nanotubes and higher film resistance [64].  Carbon nanotube networks have been investigated for use as the transparent conductor as they can achieve similar transmittance and sheet resistance to ITO films. A table showing the performance of single wall carbon nanotube networks as transparent conductors can be seen in Table 3 below:      15 Table 3: SWNT Transparent Conducting Films Sheet Resistance (Ω/☐) Transmittance Reference 30 85% [65] 200 85% [66] 300 82% [67] 160 87% [68] 200 80% [69] 920 86.7% [70] 60 90.9% [71] 188 91.7% [72]  The results show a large variance due to the different factors that were optimized for each experiment, but are still within an order of magnitude of the performance of ITO transparent conductors. Other nanomaterials evaluated for use as transparent conductors can be seen in Table 4 below. Table 4: Nanomaterial Transparent Conducting Films Technology Sheet Resistance  (Ω/☐) Transmittance Reference Graphene 125 97.4% [73] Graphene 30 90% [73] Graphene 1000 80% [74] Graphene 100 70% [75] Graphene 100,000 - 500,000 85% - 95% [76] Silver Nanowire 182 88% [77] Silver Nanowire 63 87.50% [77] Copper Nanowire 186 90% [78] Copper Nanowire 30 85% [78] Electrospun Copper Nanowire 200 96% [79] Electrospun Copper Nanowire 50 90% [79]   16 It should be noted that many nanomaterials have shown promising results in the laboratory, but achieve lower fill power ratings when integrated into actual photovoltaic devices than similar devices made with ITO films. One issue with the metal nanowires is that the PEDOT:PSS etches them due to its acidity, leading to lower conductivity [68]. 2.4 Electrospinning Electrospinning is a promising method of producing 1-D nanoscale fibres (nanofibres) using simple processing parameters. The origins of nanofibre technology reaches back eighty years to 1934 when Anton Formhals was issued a patent for the process and apparatus for preparing artificial threads [80].  This patent explained how a high voltage power supply could be used to generate silk-like fibres from a polymer solution. The concept of electrospinning remained dormant until 1971 when Baumgarten published a paper on electrostatic spinning of microfibers, detailing some of the parameters that influence the process [81]. Interest in this process remained limited until 1993 when Reneker et al. explained how nanofibres could be obtained from polymer solutions using the electrospinning process [82].  Since that time, interest in electrospinning has grown significantly for a wide array of uses. Electrospun nanofibres have extremely long aspect ratios, greater even than those of nanotubes. They also have a very high surface area ratio, leading to a large range of applications and the fibre diameter can be easily controlled by changes to the polymer solution. Electrospinning has been proposed for biomedical applications, including tissue scaffolding and drug delivery [83 - 85] and has been commercialized for applications such as filtration [86, 87] and waterproof breathable fabrics [88]. Electrospinning is also useful for imparting nanoparticle attributes to a bulk  17 multifunctional material [89]. This can include materials for mechanical strength [90], energy applications, or electromagnetic shielding [91].  Electrospinning has the advantage of being a relatively simple technique to produce nanoscale materials. The basic requirements for an electrospinning setup are:  A high voltage power supply (kV)  A metallic collector  A reservoir for the polymer solution (a syringe, as an example)  A flat tipped needle made of conductive material These can be assembled as shown in Figure 6 below.  Figure 6: Basic Electrospinning Setup  Placing a high voltage across a polymer solution using the metallic needle as one electrode and the collector as the counter electrode causes a jet of polymer solution to be emitted from the needle tip. The applied voltage must be high enough to overcome the critical voltage, the voltage where the repulsive forces overcome the surface tension of the polymer solution.  18 After surpassing the critical voltage, the electrostatic forces cause the polymer to be ejected in a jet. As the jet extends, the instabilities between the electrostatic forces and the viscosity of the solution cause the polymer to whip and extend at high forces drawing the material into thin fibres. As the fibre whips and extends, the surface area becomes very large allowing the solvent to evaporate before the fibres are deposited on the collector.  The final fibre diameter can be controlled by varying the several parameters. Increasing the applied voltage leads to smaller diameter fibres until an optimum threshold is surpassed, at which point the fibre diameters may increase slightly [81]. This can be explained by the increased acceleration of the polymer jet at high voltages. These higher speeds increase the whipping forces but also decrease the flight time of the polymer jet before it reaches the collector. This reduced time leads to less time for the polymer to elongate and form thinner diameter fibres. By controlling the collector distance and flow rate of the polymer, the diameter can also be influenced. For more advanced electrospinning setups a syringe pump is used to control the flow rate of the polymer in the syringe. The effects of these parameters on electrospinning nanofibres is not completely understood but models are being developed and explored to provide better insight into the process to fine tune control [92 – 94].  Solution properties have the largest effect on the final fibre diameter, with the solution viscosity being the critical factor. As solution viscosity is reduced, the whipping forces can draw out the polymer jet into smaller fibres more easily. The thinnest nanofibres can be less than 20 nanometres in diameter [95], which is equivalent to around 4000 molecules crossing at any location of the fibre for a typical polymer with diameter 0.5 nanometres [96]. The  19 solution surface tension is another important parameter as it can influence the solution concentration where smooth fibres transition into beaded fibres (see Figure 7 below) before finally only electrospraying droplets with no continuous fibre formation.  Figure 7: Beaded Fibre formation To understand the solution properties and their relationship to the fibre diameter and fibre morphology, the Berry’s number was proposed. The Berry's number is the product of intrinsic viscosity of the polymer in the specific solvent and the polymer concentration of the solution [97]. The Berry’s number is useful in predicting behaviour of the ability to electrospin a certain polymer/solvent solution based on several easily measured solution properties. Between concepts such as the Berry’s number and models for the process parameters, the fibre diameter can be experimentally controlled, even though our fundamental understanding is not complete.  One limitation of electrospinning is the difficulty in controlling single fibre placement. As the fibres tend to whip and extend in a manner that is difficult to predict, nanofibres have  20 traditionally been produced as a randomly distributed non-woven mat. The deposition of nanofibres on the collector does influence the overall electrical pathway, limiting the fibre mat thickness though providing a means of ensuring more even distribution of nanofibres. There have been various methods used to try and control the deposition of nanofibres with some success. Near-field electrospinning uses a very small gap between the needle tip and the collector to minimize the distance that the fibre can travel [98]. Another approach has been to align nanofibres through the use of a secondary electrostatic field [99]. By switching the direction of the electrostatic field it is possible to create a 3D grid structure. The most common way to create aligned nanofibres is using a drum that is rotating at high speed as the collector [100, 101] though this can affect the performance of the nanofibres if the collector is rotated too quickly. The forces caused by the rapidly rotating drum can cause the nanofibres to break upon deposition and it is important to carefully select a speed that is fast enough to align the nanofibres but not cause breakage. 2.5 Carbonization of Polyacrylonitrile and Copolymers One approach to creating electronically active nanofibres through electrospinning is to choose a polymeric material that can be converted into carbon through pyrolysis. Polyacrylonitrile (PAN) and its copolymers are among the most popular carbon fibre precursor, especially for the electrospinning process. The properties of electrospun polyacrylonitrile have been well studied [81, 102] though less information is available for its copolymers. Polyacrylonitrile has a stable structure of repeating cyanoethylene monomers whilst its copolymers replace some cyanoethylene units with a different monomer.  21  Figure 8: Molecular Structure of PAN and Copolymers To convert the polyacrylonitrile fibres into carbon fibres is a multistep process. Before they can be pyrolized, the fibres must first be stabilized. The stabilization step takes place under oxidative conditions and consists of dehydrogenation, cyclization, and oxidation processes [104]. In the dehydrogenation step, conjugated carbon double bonds are generated by the removal of hydrogen atoms via water molecules formed due to the presence of oxygen. In the cyclization step, intermolecular and intramolecular interactions of the nitrile groups form crosslinked, conjugated C=N bonds and complete the six carbon pyridine ring. The oxidation step consists of adding oxygen atoms, in the form of carbonyl groups, to the polyarylonitrile chains.  22  Figure 9: Oxidative Stabilization of Polyacrylonitrile Figure adapted by author from [103] The stabilization step is carried out in air at temperatures ranging from 200˚C to 300˚C and can take from 30 minutes up to several hours [105, 106]. The stabilization step is the critical step in the production of carbon fibre as it dictates the achievable quality of the final carbon fibres. Successful stabilization can be observed as the polyacrylonitrile changes colours from white to a reddish or dark brown (depending on the stabilization temperature used) [94].   23  Once the stabilization step is complete, the carbonization process can begin. The air environment is switched to one without oxygen; typically nitrogen or argon gas is used. In the absence of oxygen, pyrolysis rather than combustion of the polyacrylonitrile occurs. Once an inert environment has been introduced, the temperature can be ramped up to the desired carbonization temperature. Carbonization occurs between 400˚C and 1500˚C, with higher temperatures providing better quality carbon fibres. As the temperature increases, several different processes take place. At 300˚C there is a sharp exothermic peak resulting from an increase in the cyclization of the nitrile groups of the polyacrylonitrile. This exothermic peak can be reduced through the use of co-polymers. Between 450˚C and 700˚C, off gassing occurs as nitrogen containing compounds like hydrogen cyanide (HCN) and ammonia (NH3) are released, as well as water vapour as the carbonyl groups are driven off. During this stage, there is significant weight loss from the polymer and shrinking of the fibre structures. Above 700˚C, nitrogen gas can evolve, allowing for further elimination of nitrogen from the carbonized structure [107]. Crosslinking between polymer chains also occurs as the crystal structure continues to develop. High temperatures in excess of 1300˚C results in further reductions of the residual nitrogen content to less than 4% [105]. Further temperature increases result in most of the nitrogen being removed, resulting in the formation of anisotropic carbon sheets. Above 2500˚C, graphitization of the carbon occurs, increasing its crystal structure and performance.   24 3. Experimental Methods 3.1 Materials Polyacrylonitrile-co-Methyl Acrylate (PAN-co-MA), 94% Acrylonitrile, Mw=100,000 was purchased from Scientific Polymer Products, Inc. while a second selection was purchased from Sigma-Aldrich. The supplier did not report the molecular weight and composition of this polymer. N,N-Dimethylformamide, Certified ACS Grade (DMF) CAS# 68-12-2 was purchased from Fisher Scientific. Polyvinylpyrrolidone (PVP), Mw=55,000 was purchased from Sigma-Aldrich and –COOH functionalized single wall carbon nanotubes (SWNT) were purchased from cheaptubes.com. The purity was 90% -COOH functionalized SWNT with an outer diameter of 1-2nm. The amorphous carbon content was < 3% and multi-wall carbon nanotube content was < 5%. The reported nanotube length was 5-30 micron and the degree of –COOH functionalization was 2.7%. The electrical conductivity was reported to be > 100 S/cm. Poly(3,4-ethylenedioxythiophene) with polystyrene sulfonate (PEDOT:PSS) was purchased from HC Starck Inc. The specific product purchased was Clevios PH 500, an aqueous dispersion of PEDOT:PSS with 1-1.3% solids content. The reported specific conductivity was 300 S/cm for a dried coating after the addition of 5% dimethyl sulfoxide as a dopant. 3.2 Polymer Solution Preparation All PAN-co-MA solution was prepared by weighing DMF in a 20mL liquid scintillation vial. The corresponding amount of dry PAN-co-MA polymer was subsequently weighed and added to the DMF. A magnetic stir bar was added and the mixture was placed on a hot plate  25 at 80˚C for 4 hours at 500 RPM. The polymer solution was allowed to cool to room temperature before electrospinning. 3.2.1 SWNT Solution Preparation The preparation of the SWNT solutions followed the same basic procedure as those solutions not containing nanotubes. First DMF was weighed in a 20mL liquid scintillation vial. The desired amount of PVP was then weighed and added to the DMF. This was stirred on a vortex mixer until the PVP was dissolved. The desired amount of single wall carbon nanotubes was weighed and added to the DMF/PVP solution. The solution was then sonicated as per the SWNT supplier’s recommended procedure. A Sonics VCX 750 ultrasonicator was operated for 20 minutes at 40% amplitude.   A water/ice bath helped to ensure that the solution remained cool while it was sonicated. Once the sonication was complete, a magnetic stir bar was added and with the appropriate weight of PAN-co-MA polymer. This was then stirred at 80˚C and 500 RPM for 4 hours. The heating time is minimized to allow the polymer to completely dissolve while decreasing the potential for agglomeration of the SWNTs. The polymer solution was allowed to cool to room temperature before electrospinning. 3.2.2 Solution Mass Fraction Reporting Solution concentrations can be calculated in several ways once additional functional materials are added to the spinning solution. This can create difficulties in assessing and comparing against other results. For this work, the mass ratio of solvent to polymer is held steady and the additional functional materials are calculated based on their expected mass fraction in the final nanofibres. The surfactant required to aid in the dispersion of the carbon  26 nanotubes is not included in the calculations of these percentages and is calculated solely on the required mass ratio to disperse the carbon nanotubes. This is considered acceptable as the amount of surfactant is sufficiently small that it should not have a great impact on the electrospinning solution or its ability to be electrospun. Figure 10 below shows the calculated mass requirements for each component in a 5% PAN-co-MA / DMF electrospinning solution containing 5% SWNT and using a SWNT to surfactant ratio of 5:1.  Figure 10: Mass Fraction Example for an Electrospinning Solution with Nanoparticles   27 3.3 Intrinsic Viscosity Measurement The intrinsic viscosity was measured to determine the viscosity average molecular weight for the two PAN-co-MA copolymers. This allows for a better comparison of the polymer properties and characteristics. 3.3.1 Intrinsic Viscosity Polymer Solution Preparation A dilute polymer solution is needed to effectively measure the efflux time and determine the intrinsic viscosity. For both selections of copolymer, 15mL of DMF and 0.3g of PAN-co-MA were added to a 20mL liquid scintillation vials to make a final solution with a polymer concentration of 0.02g/mL. A magnetic stir bar was added and the solution was stirred for 4 hours at 500 RPM on a hotplate. The solution was allowed to cool overnight to ensure that it had reached equilibrium with the room temperature before continuing beginning to measure the intrinsic viscosity. 3.3.2 Efflux Time Measurements A Cannon-Fenske viscometer tube was used to measure kinematic viscosity through the efflux time measurements. The viscometer used had a tube size of 100 and the measuring range was 3-15 centistokes. It was cleaned with water and ethanol to remove any impurities from the instrument. The viscometer was dried in an oven at 80˚C for 24 hours before being allowed to cool to a room temperature of 20˚C overnight. The viscometer was set up then set up vertically on a stand. Ten millilitres of DMF solvent at room temperature was added to the lower reservoir (reservoir A) by introducing it through tube D.   28  Figure 11: Ubbelholde Viscometer Using a suction bulb attached to the top of tube E, the DMF was drawn up the capillary tube (tube F) until it had partially filled the second of the upper reservoirs (reservoir C). Releasing the suction bulb, the liquid was allowed to flow freely and drain back into the main reservoir (A). The sample was timed when the meniscus passed between lines G and H and the efflux time was recorded. This was repeated a minimum of 2 additional times until three data points were obtained for the DMF solvent that agreed within 0.1 seconds or 1%, whichever was greater. Once the data for the solvent was obtained, 2 mL of the intrinsic viscosity solution was added to the lower reservoir A through tube D. The sample was mixed by forcing air through the top of tube E to bubble into reservoir A. This was repeated several times with  29 short breaks in between. The solution was then allowed to rest for 15 minutes to allow for complete mixing and for any dissolved air to leave. Following the same steps as for the pure solvent, the mixture was charged into the upper reservoirs and the efflux times were measured until 3 data points with the desired precision (0.1 seconds or 1%) were obtained. At this point another 2 mL of intrinsic viscosity solution was added and the process was repeated as described above. This was continued until 10mL of intrinsic viscosity solution was mixed into the 10mL of solvent and sufficient data points were obtained. This entire procedure was repeated for both copolymers. 3.4 Electrospinning Electrospinning was performed on a Katotech Nanofiber Electrospinning Unit with drum collector manufactured by KATO TECH CO. LTD.  Figure 12: Katotech Nanofiber Electrospinning Unit   30 The prepared polymer solutions were placed in 10mL Luer-Lok tipped syringes (BD Syringe #309604) with a 1” 20 gauge (0.61-0.69mm ID) needle. The needle was blunted for safety purposes and to provide better polymer bead formation at the tip. The high voltage power supply was connected with positive voltage applied at the needle tip and grounded at the collector. The voltage and polymer flow rates were varied depending on the experiment being conducted while the gap between the needle tip and collector was held constant at 20cm.  The collector was covered with aluminum foil to collect the nanofibres being produced. To gather nanofibres on quartz slides the slide was introduced into the electrospinning path in front of the collector on the end of an insulated collection arm. This arm was cycled in and out of the electrospinning path at approximately 1 Hz (60 cycles per minute). This allowed for an even deposition of the nanofibers onto the quartz slides. 3.4.1 Parameter Optimization To determine the optimum conditions for electrospinning PAN-co-MA copolymer a two-variable response surface analysis was conducted. A 10wt% PAN-co-MA solution was prepared using the copolymer from Sigma-Aldrich. The polymer solution flow rate and electrospinning voltage were varied through several values as part of the analysis. The flow rate was chosen by determining the minimum flow rate where a stable electrospinning jet was observed and measuring multiples of that value to obtain low, medium, and high values. The electrospinning voltages were chosen such that a stable jet was achieved without excessive leakage current. The electrospun nanofibre samples that were obtained were analyzed using scanning electron microscopy (SEM) and image analysis techniques to obtain  31 the fibre diameter distribution. The SEM images were analyzed in ImageJ with fifty data points taken from each image. The data points were randomly selected by further subdividing each image into four quadrants. This technique was used for all fibre diameter analysis. 3.5 Nanofibre Carbonization Carbonization was performed in a 79400 Thermolyne quartz tube furnace as pictured in Figure 13. The as-spun nanofibre membranes were placed in steel frames to provide tension during the stabilization process. The steel frames measured 2 inches wide by three inches long. For the samples collected on quartz slides, Mill grade 40 (400 micron opening) steel mesh was placed on top during stabilization to provide tension. The sample holders can be seen in Figure 14.   Figure 13: Thermolyne 79400 Tube Furnace   32  Figure 14: Sample Holders for Carbonization The samples were heated in an air atmosphere to a stabilization temperature of 250˚C, with a heating rate of 2˚C/min. Once the temperature was achieved it was held steady for 120 minutes to allow the samples to adequately stabilize. At this point, the frames were removed from the samples. Next, the gas flow was switched from air to nitrogen with an additional 30 minute hold at 250˚C to ensure oxygen would be absent during the high temperature carbonization step. The stabilized samples were then heated to the ultimate carbonization temperature (ranging from 700˚C to 1000˚C) at a ramp rate of 5˚C/min. Once the desired carbonization temperature had been achieved it was held for 60 minutes. The resulting samples were then allowed to cool to room temperature over several hours before the nitrogen gas and furnace were shut off. The carbonization procedure described above can be seen in Figure 15.  33  Figure 15: Carbonization Process Diagram 3.6 PEDOT:PSS Coatings The PEDOT:PSS was first doped by weighing out the required amount and adding 5% (by weight) dimethyl sulfoxide (DMSO). The DMSO is required to achieve the full conductivity of the PEDOT:PSS material used. Once added, the solution was mixed on a vortex mixer for two minutes at 1500 RPM. For samples that were further diluted with methanol, a mixture of 50% by mass methanol and 50% by mass doped PEDOT:PSS was prepared. This solution was also mixed for two minutes on the vortex mixer at 1500 RPM.   34 The PEDOT:PSS solutions were then added to the nanofibres. For the nanofibre membranes, the solution was drop cast until the sample area was covered.  For the thin film samples, the solution was placed on the quartz slide and was spin coated at 1000 RPM. All samples were placed in an oven at 140˚C for 10 minutes to cure and allowed to cool to room temperature before any further measurements. 3.7 Conductivity Measurement The nanofibre membranes and thin film samples each pose a different set of challenges in measuring conductivity. The porous nature of the nanofibre membranes means that changes in pressure between measurements can have an impact on the results, while for the thin films the coverage poses difficulties in maintaining good contact. To address these concerns and to ensure consistency of results, electrodes of silver paste were deposited manually onto the samples. The distance between the electrodes was controlled as carefully as possible, with any differences in the gap distance or electrode length captured in the conductivity calculations. The silver paste electrode approach allowed for reproducible results using a two-point measurement technique for both the nanofiber membranes and thin film samples. The contact resistance between the silver paste electrodes and the measurement probes was negligible compared to the sample resistance. A Keithly 2400 Probe Station was initially used to increase the sensitivity of the conductivity results but showed no differences to using a desktop multimeter which was used for all subsequent results.  3.8 Transparency Measurement The transparency of each of the thin film samples was evaluated by measuring the optical power intensity over the photoluminescence spectrum and subtracting a baseline result for a blank quartz slide. A Cornerstone 130 1/8m Monochromator was used to generate light over  35 the spectrum from 350 nanometre to 850 nanometre wavelength, with a Newport Model 842 optical power meter measuring the optical power intensity at each wavelength. The spot size of for each measurement was a 2mm by 2mm square. The transparency of each sample was reported at 550nm wavelength light.   36 4. Experimental Results 4.1 Intrinsic Viscosity The specific and relative viscosities for each PAN-co-MA copolymer at various dilute polymer concentrations was determined by measuring the efflux times in an ubbelholde viscometer and relating it to the efflux time of the pure solvent. The specific and relative viscosities are given by equations (1) and (2)  𝜂𝑟𝑒𝑙 =𝜂𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛𝜂𝑠𝑜𝑙𝑣𝑒𝑛𝑡   (1)   𝜂𝑠𝑝 =𝜂𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛−𝜂𝑠𝑜𝑙𝑣𝑒𝑛𝑡𝜂𝑠𝑜𝑙𝑣𝑒𝑛𝑡   (2)  where 𝜂𝑟𝑒𝑙 and 𝜂𝑠𝑝 are, respectively, the relative and specific viscosities and 𝜂𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛 and 𝜂𝑠𝑜𝑙𝑣𝑒𝑛𝑡 are, respectively, the solution and solvent viscosities. The equations can be related back to the efflux time by noting in equations (3) and (4) that    𝑡𝑠𝑜𝑙𝑣𝑒𝑛𝑡 =𝜂𝑠𝑜𝑙𝑣𝑒𝑛𝑡𝜌𝑠𝑜𝑙𝑣𝑒𝑛𝑡           (3)  𝑡𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛 =𝜂𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛𝜌𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛         (4)  where 𝑡𝑠𝑜𝑙𝑣𝑒𝑛𝑡 and  𝑡𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛 are, respectively, the efflux times of the solvent and the polymer solution and 𝜌𝑠𝑜𝑙𝑣𝑒𝑛𝑡 and 𝜌𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛 are, respectively, the solvent and solution densities. By assuming that the ratio of solvent density to solution density is 𝜌𝑠𝑜𝑙𝑣𝑒𝑛𝑡𝜌𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛= 1 , equations (1) and (2) can be simplified into equations (5) and (6):   37 𝜂𝑟𝑒𝑙 =𝑡𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛𝑡𝑠𝑜𝑙𝑣𝑒𝑛𝑡     (5)  𝜂𝑠𝑝 =𝑡𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛−𝑡𝑠𝑜𝑙𝑣𝑒𝑛𝑡𝑡𝑠𝑜𝑙𝑣𝑒𝑛𝑡      (6)   This assumption is valid for dilute polymer solutions such as the ones that are used in this experiment.  Once the specific viscosity has been determined for a range of polymer concentrations the intrinsic viscosity for that polymer can be found by extrapolating the viscosity back to a concentration of zero. This can be accomplished by plotting the polymer concentration against the specific viscosity over concentration. The two charts below are for the data collected from each of the two polymers analysed for this work.  38   Figure 16: Sigma Aldrich Polymer   Figure 17: SPP Polymer y = 38026x + 174.75 R² = 0.9938 01002003004005006000 0.002 0.004 0.006 0.008 0.01 0.012ηsp/c (mL/g) c (g/mL) ηsp/c y = 33036x + 136.63 R² = 0.9864 01002003004005000 0.002 0.004 0.006 0.008 0.01 0.012ηsp/c  (mL/g) c (g/mL) ηsp/c  39 The intrinsic viscosity of the Sigma Aldrich and SPP PAN-co-MA polymers were 174.75 and 136.63 mL/g, respectively, as taken from the intercepts of Figure 16 and Figure 17. 4.1.1 Viscosity Average Molecular Weight The viscosity average molecular weight can be determined once the intrinsic viscosity is known by using the Mark-Houwink equation, given below:   [𝜂] = 𝑘𝑀𝑎      (7)   According to the Polymer Handbook [108], the parameters for polyacrylonitrile in dimethylformamide at 20˚C are: k = 1.77x10-4 mL/g a = 0.78 Using these parameters and using the previously calculated intrinsic viscosities, the viscosity average molecular weight for each polymer can be found in Table 5 below: Table 5: Calculated Molecular Weight PAN-co-MA molecular weight Supplier Reported Mw Intrinsic Viscosity (mL/g) Viscosity Mw Sigma-Aldrich n/a 174.75 132,154 SPP 100,000 136.63 96,398    40 4.1.2 Fibre diameter vs. Concentration The fibre diameter has been shown to have a power relation with the polymer concentration [109, 110], with the general equation being expressed as  𝑑 ∝ 𝐶𝛿       (8)  where d is the fibre diameter, C is the polymer solution concentration, and δ is the scaling component. The scaling component can differ greatly between different types of polymers and between similar polymers with varying molecular composition. The relationships for the two copolymers were analyzed to help select the appropriate copolymer for additional experimentation. This involved electrospinning each copolymer at several different concentrations to determine how the diameter is influenced by the polymer characteristics. The flowrate was 0.018mL/min and the voltage potential was 15kV as these conditions were found to be optimal, as explained in section 4.2. The resulting fibres from each concentration can be seen in the charts on the following pages. All images shown are at 5000X magnification to make a direct visual comparison between each image possible. The average fibre diameter for each sample is noted under its concentration.   41  2wt% mean = 63 ± 22 nanometres 6wt% mean = 359 ± 32 nanometres   4wt% mean = 196 ± 60 nanometres 8wt% mean = 500 ± 42 nanometres   5wt% mean = 235 ± 29 nanometres 10wt% mean = 745 ± 39 nanometres   Figure 18: Fibre diameter vs. polymer concentration for Sigma Aldrich PAN-co-MA     42  4wt% mean = 92 ± 15 nanometres 10wt% mean = 296 ± 37 nanometres   6wt% mean = 156 ± 42 nanometres 12wt% mean = 363 ± 35 nanometres   8wt% mean = 213 ± 61 nanometres    Figure 19: Fibre diameter vs. polymer concentration for SPP PAN-co-MA   43  Figure 20: Fibre Diameter and Concentration Relationship From the figure we can see that the PAN-co-MA obtained from Sigma Aldrich has a power relationship of approximately 1.5 while the PAN-co-MA obtained from SPP has a power relationship of 1.25.  While the SPP copolymer had smaller fibre diameters at each given concentration, the Sigma-Aldrich copolymer was able to produce smooth fibres with no droplets at as low as 5wt%. Given that there are limits to nanoparticle concentrations to get effective dispersion, a lower polymer concentration should enable more nanoparticles in the final fibres. As such, the Sigma-Aldrich copolymer was selected for further experimentation. y = 22.275x1.5173 R² = 0.995 y = 16.302x1.2507 R² = 0.9986 01002003004005006007008009000 2 4 6 8 10 12 14Fibre Diameter (nm) Polymer Concentration (%) Sigma-Aldrich CopolymerSPS Copolymer 44 4.2 Electrospinning Parameters of PAN-co-MA The electrospinning process parameters for the PAN-co-MA polymer needed to be assessed to determine the optimum conditions. Critically, the most important parameters investigated were the polymer flow rate and the electrospinning voltage. Other parameters were held constant, including:  Needle gauge: 20G  Needle to Collector distance: 20 cm  Temperature: 25˚C  Relative Humidity: 30% The relative humidity was controlled through the use of a dehumidifier placed next to the electrospinning unit. Plastic 10mL syringes were used to hold the polymer solution, 10% PAN-co-MA in DMF.  The voltage and flow rate were varied together to generate a 2-D response surface and allow for analysis of the two factors together. Five different voltages were considered; 13kV, 15kV, 17kV, 19kV, and 21kV. Three flow rates were used; 0.006 mL/min, 0.012 mL/min, and 0.018 mL/min. The lower bound of the electrospinning voltage was selected as it was the lowest voltage above the critical voltage for jet formation that allowed for continuous production of nanofibres. The lower bound for the flow rate was similarly selected as the lowest flow rate that allowed for the continuous nanofibre production.   45 4.2.1 Response Analysis of Voltage and Polymer Flow Rate on Fibre Diameter The resulting nanofibres from each set of conditions was analyzed to find the average fibre diameter, as shown in Table 6. Table 6: Nanofibres from Response Analysis Voltage / Flow Rate 0.006 mL/min 0.012 mL/min 0.018 mL/min 13 kV  909 ± 179 nm  768 ± 40 nm 771 ± 58 nm 15 kV 962 ± 252 nm 790 ± 49 nm 744 ± 39 nm 17 kV 1042 ± 467 nm 777 ± 80 nm 748 ± 40 nm 19 kV 1039 ± 410 nm 812 ± 117 nm 775 ± 49 nm    46 Voltage / Flow Rate 0.006 mL/min 0.012 mL/min 0.018 mL/min 21 kV 934 ± 331 nm 782 ± 93 nm 775 ± 54 nm  The nanofibres themselves appear to be of good quality, with average results varying from a low of 744 nm to a high of 1042 nm. The surface and contour plots in Figure 21 and Figure 22 respectively show that the importance of correct flow rate appears to be a more critical factor than voltage for selecting the optimum conditions. Additionally, as the flow rate is increased the effect of the electrospinning voltage seems to decrease, resulting in a more uniform fibre distribution between voltage conditions.  Figure 21: Response Surface for Electrospinning Parameters   47  Figure 22: Contour Plot for Electrospinning Parameters 4.2.1.1 Response Surface Experimental Analysis While the response analysis suggests that the optimum conditions are 15 kV and 0.0108 mL/min, the significance of these parameters needs to be examined to ensure the correct conclusions are drawn. As a means of assessing this, a series of paired student t-test was used. The student t-test is used to compare whether two populations are significantly different from each other with respect to the measured variable. The distribution of the results for each population is assumed to be normally distributed and continuous; a valid assumption in this scenario.  As the student t-test can only compare two populations at a time, and as we are primarily concerned with determining if the proposed optimum conditions are experimentally better, the fibre diameter distribution for the 15 kV, 0.0108 mL/min conditions was compared to each remaining set of conditions. The populations were treated as independent with equal  48 variance, with two-tailed distributions. The p-values and the experimental significance are reported in Table 7. Table 7: Significance of Results for Response Analysis Sample Fibre Diameter (nm)  p-value Significant at 95% confidence level? Significant at 99% confidence level? 13 kV, 0.006 mL/min 909±179 6.06E-09 Y Y 15 kV, 0.006 mL/min 962±252 2.98E-08 Y Y 17 kV, 0.006 mL/min 1042±467 1.93E-05 Y Y 19 kV, 0.006 mL/min 1039±410 2.04E-06 Y Y 21 kV, 0.006 mL/min 934±331 0.00011 Y Y 13 kV, 0.012 mL/min 768±40 0.00382 Y Y 15 kV, 0.012 mL/min 790±49 1.30E-06 Y Y 17 kV, 0.012 mL/min 777±80 0.01137 Y N 19 kV, 0.012 mL/min 812±117 0.00021 Y Y 21 kV, 0.012 mL/min 782±93 0.00926 Y Y 13 kV, 0.018 mL/min 771±58 0.00819 Y Y 15 kV, 0.018 mL/min 744±39 n/a – optimum conditions being compared against 17 kV, 0.018 mL/min 748±40 0.60679 N N 19 kV, 0.018 mL/min 775±49 0.00098 Y Y 21 kV, 0.018 mL/min 775±54 0.00186 Y Y  4.3 Electrospinning PAN-co-MA and SWNT The electrospinning parameters and solution properties were explored and optimum conditions chosen for this work as described in the previous sections. However, one additional factor to be investigated is the effect of the addition of single wall carbon nanotubes to our solutions. The addition of nanoparticles to electrospun nanofibres increases the functionality of the resulting carbon nanofibres and single wall nanotubes are desirable for our application as they have very high electrical conductivity, as discussed in Section 2.3. While the addition of nanoparticles does have an impact on electrospinning parameters (including solution viscosity, surface tension, solution conductivity), conditions will be  49 maintained as close to the optimum conditions investigated as possible. The effects that the carbon nanotubes had on the electrospinning process will be discussed in Section 4.3.1.  Three conditions were explored in this work; a baseline level with no SWNT, a moderate level of 1% SWNT, and a high concentration of 5% SWNT. Higher levels of SWNT were attempted but did not yield spinnable solutions. The nanofibres obtained can be seen in Figure 23 on the following page.              50  PAN-co-MA, As spun  PAN-co-MA w/ 1% SWNT, As spun  PAN-co-MA w/ 5% SWNT, As spun  Figure 23: Electrospun PAN-co-MA with SWNT   51 The effect of adding the SWNT appears to have little influence on the fibre morphology at moderate concentrations, with more noticeable effects at the 5% SWNT concentration. This can also be seen in the fibre diameter distribution for these samples. Table 8: Electrospun PAN-co-MA w/ SWNT Fibre Distribution   Average Fibre Diameter (nm) Standard Deviation (nm) PAN-co-MA,  As Spun 265 32 PAN-co-MA w/ 1% SWNT, As Spun 253 37 PAN-co-MA w/ 5% SWNT, As Spun 324 66  These results appear surprising as the addition of a moderate amount of SWNT shows a decrease in fibre diameter. However, experimental analysis using the student t-test method conducted similarly to the description in Section 4.2.1.1 and using PAN-co-MA without SWNT as our baseline shows that the fibre diameter distribution between PAN-co-MA without SWNT and with a moderate amount of SWNT is not significantly different.        52 Table 9: Significance of Results for Electrospun PAN-co-MA w/ SWNT   p-value Significant at 95% Confidence Interval? PAN-co-MA,  As spun n/a n/a PAN-co-MA w/ 1% SWNT, As Spun 0.09411 N PAN-co-MA w/ 5% SWNT, As Spun 1.47E-07 Y  4.3.1 Effects of SWNT on Electrospinning Parameters The addition of SWNT to the electrospinning solutions did have an effect on the electrospinning parameters needed for continuous fibre formation that merits mention. While all other parameters were held constant as described in Section 4.2, the polymer flow rate did have to be increased. For the addition of 1% SWNT, the flow rate was altered up to 10% higher while for 5% SWNT the flow rate needed to be moderated up to 30% higher for continuous fibre formation. 4.4 Carbonization Multiple carbonization temperatures were selected to provide a more detailed cross-section of the changes that occur to the PAN-co-MA / SWNT nanofibres during the carbonization process. Three temperature conditions were selected; 700˚C, 850˚C, and 1000˚C.  53 4.4.1 Microstructure Analysis The resulting carbon fibres were analyzed using SEM to determine the microstructure and morphology of the nanofibres. The results for each electrospinning solution can be seen in Figure 24, Figure 25, and Figure 26 on the following pages. As Spun Average Diameter: 265 nm Standard Deviation: 32 nm  700˚C  Average Diameter: 220 nm Standard Deviation: 41 nm  850˚C  Average Diameter: 212 nm Standard Deviation: 48 nm 1000˚C Average Diameter: 200 nm Standard Deviation: 42 nm Figure 24: Carbonized PAN-co-MA    54    As Spun Average Diameter: 253 nm Standard Deviation: 37 nm  700˚C  Average Diameter: 219 nm Standard Deviation: 58 nm  850˚C  Average Diameter: 210 nm Standard Deviation: 50 nm 1000˚C Average Diameter: 201 nm Standard Deviation: 42 nm Figure 25: Carbonized PAN-co-MA with 1% SWNT     55   As Spun Average Diameter: 324 nm Standard Deviation: 66 nm  700˚C  Average Diameter: 254 nm Standard Deviation: 60 nm  850˚C  Average Diameter: 240 nm Standard Deviation: 55 nm 1000˚C Average Diameter: 224 nm Standard Deviation: 79 nm Figure 26: Carbonized PAN-co-MA with 5% SWNT  The SEM imagery and analysis shows that, as expected, there is a sizeable drop in fibre diameter from the as-spun material to those carbonized at 700˚C, with more gradual decreases continued as the carbonization temperature increases. The fibre morphology also  56 changes throughout the carbonization process. The fibres at 700˚C begin to show a rippled fibre appearance, especially pronounced in the samples containing the carbon nanotubes. At 850˚C there is a noticeable fusing of the fibres. Both of these features appear more pronounced in the samples carbonized at 1000˚C.  The change in fibre diameter as a function of carbonization temperature was plotted in a bar graph to analyze for any effects that the carbon nanotubes may have introduced.  From the plot in Figure 27 it is much clearer to see that there are no significant differences between the three conditions with respect to fibre shrinkage caused by the carbonization process. The standard deviation of the average fibre diameter was much greater than the differences between samples containing different amounts of carbon nanotubes.  57  Figure 27: Bar Graph of Fibre Size vs. Carbonization Temperature Relationship 4.4.2 Conductivity For each nanofibre sample, the conductivity was recorded as shown in Table 10 below. Table 10: Conductivity of PAN-co-MA / SWNT nanofibre membranes Conductivity, S/cm   700˚C 850˚C 1000˚C PAN-co-MA 0.04 ± 0.01 1.82 ± 0.40 5.32 ± 0.80 PAN-co-MA w/ 1% SWNT 1.72 ± 0.40 3.98 ± 0.93 7.68 ± 1.52 PAN-co-MA w/ 5% SWNT 5.02 ± 0.90 7.86 ± 1.55 11.26 ± 1.50  The conductivity increased with increasing carbonization temperature as expected. The effect of increasing carbon nanotube content also showed a positive correlation, demonstrating that the carbon nanotubes are contributing to the overall conductivity of the fibre network. This can be seen in Figure 28. 0%20%40%60%80%100%120%140%As-spun 700˚C 850˚C 1000˚C Percentage of initial diameter (%) Carbonization Temperature (˚C) PAN-co-MAPAN-co-MA w/ 1% SWNTPAN-co-MA w/ 5% SWNT 58  Figure 28: Nanofibre conductivity vs. Carbonization Temperature The effect that the carbon nanotubes have on the conductivity of the nanofibres is much more pronounced at the lower carbonization temperatures, showing over an of magnitude improvement for the addition of 1% SWNT and over 2 orders of magnitude improvement for the addition of 5% SWNT for samples carbonized at 700˚C. Table 11: Conductivity Increase due to SWNT in Carbonized Nanofibres Change in Conductivity relative to PAN-co-MA  700˚C 850˚C 1000˚C PAN-co-MA w/ 1% SWNT 3986% 218% 144% PAN-co-MA w/ 5% SWNT 11647% 431% 212%  02468101214700 850 1000Conductivity (S/cm) Carbonization Temperature (˚C) PAN-co-MAPAN-co-MA w/ 1% SWNTPAN-co-MA w/ 5% SWNT 59 4.5 PEDOT:PSS Coatings As organic solar cells often use a PEDOT:PSS layer, it is important to understand how the nanofibres interact with such a coating. Initially, an attempt was made to coat the nanofibres with the PEDOT:PSS solution as delivered, with only 5% DMSO added as a dopant to increase the film conductivity. The solution did not wet into the nanofibres and instead cured as a coating on the outer layer of the nanofibres. Figure 29 shows the coating as it appeared from the top, with Figure 30 showing the back side with no signs of any coating.   Figure 29: PEDOT:PSS Coated Nanofibres (top side)        60 Uncoated Nanofibres PEDOT:PSS Coated Nanofibres (from back)   Figure 30: PEDOT:PSS Coated Nanofibres (back side) To provide a more detailed view of how far the coating penetrated into the fibre layers, the coating was partially lifted and imaged, as shown in Figure 31. The coating can be seen to stick well to the first level of the nanofibres in a continuous layer; however, there is no PEDOT:PSS penetration further into the nanofibre network.  Figure 31: PEDOT:PSS Coated Nanofibres (lifted coating)   61 Methanol was added to the PEDOT:PSS solution to aid in the penetration of the coating into the nanofibre network. A ratio of 50%wt methanol and 50%wt doped PEDOT:PSS was applied in a similar fashion and allowed to cure. The results showed a much better penetration of PEDOT:PSS into the fibrous structure 5000x Zoom 10,000x Zoom   Figure 32: 50% MeOH / 50% PEDOT:PSS Coated Nanofibres When an attempt to peel back the PEDOT:PSS coating is attempted on these samples, the nanofibres cannot be separated and break instead. This suggests that the PEDOT:PSS is forming a connected network that indeed penetrates into the fibre layers.  62  Figure 33: Fibre Structure beneath MeOH / PEDOT:PSS Peeled Coating The view from the edge of the sample, where it has been partially exposed offers another view of the PEDOT:PSS penetrating into the fibre structure. This can be seen in Figure 34 below.  Figure 34: Fibre Structure at edge of MeOH / PEDOT:PSS Coated Nanofibres   63 4.6 Thin Film Development Samples were prepared and carbonized according to the methods listed in Chapter 3. To achieve the best performance, PAN-co-MA with 5% SWNT was used to generate the specimens. Difficulties during the carbonization stage prevented the analysis of 1000˚C carbonized samples. This is likely due to the sensitivity of the PAN-co-MA / SWNT composite nanofibres during the stabilization and carbonization process. For these thin films, small perturbations in conditions such as residues or contaminations can play a significant factor in successfully producing samples. 4.6.1 Transparency The transmission of the films was measured and recorded. Both showed very good results with higher than expected transmission at 550nm wavelength. Table 12: Transmission at 550 nm wavelength   Transmission (%) Standard Deviation Thin Film 700˚C 97.42 0.37% Thin Film 850˚C 97.22 0.38%  The overall light transmission remained high for the visible spectrum. All measurements were above 96% transmission from 390 – 700 nm wavelengths, with values remaining above 94% into the measured UV and IR spectrums.  64  Figure 35: Optical Transmission of Nanofibre Thin Films 4.6.2 Sheet Resistivity The sheet resistivity of each film was measured and recorded in Table 13. The results are below the requirements for use as a transparent conducting layer in organic photovoltaics as the measured sheet resistance is too high by several orders of magnitude. Note that for the sheet resistance measurements, the measurement uncertainty is higher than the standard deviation of the results due to the variability in the edge of the silver paste electrode. As this is a significant source of variability in the sheet resistance calculations, the uncertainty was reported instead. The measurement imprecision was assumed to be 0.5mm for all calculations.  80 82 84 86 88 90 92 94 96 98 100 350 400 450 500 550 600 650 700 750 800 850 Transmissivity (%) Wavelength (nm) 700C 850C  65 Table 13: Sheet Resistance of Nanofibre Thin Films   Sheet Resistance (ohm/sq) Uncertainty (ohm/sq) Thin Film 700˚C 24440 2276 Thin Film 850˚C 8853 911  There does not appear to be a clear correlation between the sheet resistance in the thin film specimens and the bulk conductivity measured for the nanofibre membranes. Comparing the results in Table 10 and Table 13, the conductivity at 850˚C is 57% higher than at 700˚C but the sheet resistance is 276% lower. The optical transmission is similar in both samples, suggesting that there are other variables that may be affecting the sheet resistance of nanofibre thin film transparent conductors. 4.6.3 PEDOT:PSS Coated Thin Films The thin film samples were coated in PEDOT:PSS and characterized for conductivity in the same manner as the uncoated samples. Table 14: Sheet Resistance of PEDOT:PSS Coated Samples   Sheet Resistance (ohm/sq) Uncertainty (ohm/sq) Thin Film 700˚C 996 70 Thin Film 850˚C 414 24 Quartz Slide  (no nanofibres) 680 78  Both the samples at 700˚C and 850˚C saw large drops in sheet resistance; however, this was mostly an effect of the PEDOT:PSS itself. The sample carbonized at 850˚C did moderately outperform the PEDOT:PSS coating without nanofibres. The optical transmission at 550nm for all three samples was measured and is shown in Table 15.  66 Table 15: Transmission at 550nm for PEDOT:PSS Coated Samples   Transmissivity (%) Standard Deviation Thin Film 700˚C 93.39 0.39% Thin Film 850˚C 93.39 0.39% Quartz Slide   (no nanofibres) 96.17 0.37%  The overall light transmission can be seen in Figure 36 below. As expected, there is a drop in transparency between the quartz slide only coated in PEDOT:PSS and those that have the nanofibres and PEDOT:PSS coating. As the nanofibre samples were measured before and after the additional layer of PEDOT:PSS we can compare the overall net effect of adding the coating, shown in Figure 37 to the theoretical value.   Figure 36: Optical Transmission of Nanofibre Thin Films with PEDOT:PSS Coatings  80 82 84 86 88 90 92 94 96 98 100 350 400 450 500 550 600 650 700 750 800 850 Transmissivity (%) Wavelength (nm) 700˚C (w/ PEDOT) 850˚C (w/ PEDOT) Quartz Slide (w/ PEDOT)  67  Figure 37: Theoretical vs. Actual Nanofibre / PEDOT:PSS Optical Transmission The theoretical value is based on no interaction between the nanofibres and the PEDOT:PSS layers and is simply the transmittance of the two layers combined. As is shown, the actual nanofibre / PEDOT:PSS samples show a lesser transmissivity in the visible spectrum than the theoretical value. 85 87 89 91 93 95 97 99 350 400 450 500 550 600 650 700 750 800 850 Transmissivity (%) Wavelength (nm) 700˚C (w/ PEDOT) 850˚C (w/ PEDOT) Theoretical  700˚C Theoretical  850˚C  68 5. Discussion of Results 5.1 PAN-co-MA Polymer Characterization The results of the intrinsic viscosity measurements in Figure 16 and Figure 17 provide a good linear fit for calculating the intrinsic viscosity. These values were used to calculate the viscosity average molecular weight using the Mark-Houwink relationship, with the experimental equation parameters taken from polyacrylonitrile in dimethylformamide. No values for the methyl acrylate copolymer of polyacrylonitrile were found and similar experiments have used the homopolymer parameters [111]. It should also be noted that these parameters show that dimethylformamide is considered a very good solvent for this polymer, as an ‘a’ parameter value of 0.8 is considered to be a good solvent, with typical values ranging from 0.5 to 0.8 for flexible polymers.  While the molecular weight of the copolymer from Sigma-Aldrich was not stated, the calculated viscosity average molecular weight can be compared to the reported weight average molecular weight for the copolymer from Scientific Polymer Products, Inc. as was detailed in Table 5. The viscosity average molecular weight was slightly under the weight average molecular weight reported by the manufacturer. This is in agreement with expectation, as the viscosity average molecular weight should fall between the number average and weight average molecular weights, as shown in Figure 38. As the viscosity and weight average molecular weights are close in value, it suggests that the PAN-co-MA from SPP has a low polydispersity index.  69  Figure 38: Typical Molecular Weight Distribution 5.2 Electrospinning Parameters of PAN-co-MA The analysis of the electrospinning voltage and flowrate, and their relationship to each other yielded some interesting results. Compared to Baumgarten’s work [81] where the effect of solution flowrate was determined to be small, this work shows that a flowrate at the low end of the spinnable range can have an impact on the resultant fibre diameter. This may be partially due to some solvent evaporation from the droplet at the end of the needle changing the effective polymer concentration at the electrospinning jet. However, above this minimum flowrate threshold the effect on fibre diameter was minimal, aligning well with the Baumgarten’s observations. It is also notable that the interaction between flowrate and voltage showed that lower flowrates also make the voltage effects on fibre diameter more significant as seen in Table 16.  70 Table 16: Fibre Diameter Maximum/Minimum Limits by Flowrate, as Influenced by Voltage Range Flow Rate Minimum Measured Average Fibre Diameter (nm) Maximum Measured Average Fibre Diameter (nm) Maximum, as Percentage of Minimum, Average Fibre Diameter (%) 0.006 mL/min 909 1042 115% 0.012 mL/min 768 812 106% 0.018 mL/min 744 775 104%  The analysis of the effect on fibre diameter by varying the polymer concentration yielded expected results. The curves in Figure 20 had a very good fit with power relationships of 1.5 and 1.25 for the two PAN-co-MA copolymers measured. This agrees well with experiments for polyacrylonitrile that show a power relationship of 1 (linear fit) from He et al. [110]. As described in Section 2.4, the Berry number is another technique that can be used to characterize the spinnability of a polymer. The Berry Number results for the two PAN-co-MA polymers analyzed are shown in Table 17.          71 Table 17: Berry Number Analysis for PAN-co-MA copolymer   PAN-co-MA from  Scientific Polymer Products, Inc PAN-co-MA from  Sigma Aldrich Solution Concentration (%) Berry Number Fibre Morphology Berry Number Fibre Morphology 2 2.95 Not Spinnable 3.78 Beaded Nanofibres 4 6.03 Beaded Nanofibres 7.71 Continuous Nanofibre with Some Beading 5 - Not measured 9.74 Continuous Nanofibre 6 9.24 Continuous Nanofibre with Some Beading 11.82 Continuous Nanofibre 8 12.59 Continuous Nanofibre 16.10 Continuous Nanofibre 10 16.08 Continuous Nanofibre 20.57 Continuous Nanofibre 12 19.74 Continuous Nanofibre - Not Measured  The results show that if the Berry number for PAN-co-MA is below 3 it is unspinnable, while between 3 and 7 the nanofibres will be beaded. From 7 to 9.5 the fibre morphology will be a mix of continuous nanofibres and beaded nanofibres. Above 9.5 the resulting morphology is smooth, continuous nanofibres. 5.2.1 Response Surface Analysis The relationship between voltage, flow rate, and fibre diameter has been explored in several works, with results showing a relationship between increasing flow rate and increasing fibre diameter. The relationship between voltage and fibre diameter shows a small decrease in fibre diameter as voltage increases, until an inflection point where further increases in voltage result in the fibre diameter slowly increasing again. The results in this work show good agreement with the voltage and fibre diameter relationship at medium and high flow rates, but the relationship between flow rate and fibre diameter is less clearly demonstrated. The lowest flow rate (0.006 mL/min) had significantly larger fibre diameters and did not  72 follow any expected trends. This result may have been due to unobserved instabilities in the spinning jet, i.e. continuous electrospinning conditions were not achieved despite visual confirmation. Fluid evaporation from the droplet may also have played a role in this result, as fluid evaporation is generally not accounted for in existing models. The general agreement to the fibre diameter relationships with the medium and high flow rates seem to support this conclusion.  5.3 Electrospinning PAN-co-MA with SWNT The addition of single wall carbon nanotubes to the electrospinning solution resulted in some changes to the fibre morphology and the spinnability of the solution itself. As mentioned previously in this work, the addition of the SWNT resulted in the need of a higher flow rate to maintain continuous electrospinning conditions. This is explained by Fridrikh et al. [112], as the electrospinning jet and spinnability of the solution is determined by solution properties such as conductivity, dielectric permittivity, dynamic viscosity, and surface tension. The addition of SWNT into the spinning dope directly influences several of these parameters, most notably the conductivity of the solution. From Ohm’s law, the increased conductivity of the solution results in a higher electric current given the fixed electric field. To balance the effect of the higher electric current, a higher flow rate is required.  The changes in fibre morphology are also easily explained. Adding SWNT limits the ability of the spinning jet to draw out through the whipping process. This, coupled with the increased flow rate, results in slightly larger nanofibres being deposited onto the collector. The fibres also have a less uniform fibre diameter and appearance, more noticeable at high SWNT loading. This may be due to SWNT agglomerates that are encapsulated by the  73 nanofibres. At lower SWNT loading it is easier to prevent the SWNT from agglomerating together. For the moderate single wall carbon nanotube loading of 1%, the fibre morphology remains very similar to the control condition with no carbon nanotubes. 5.4 Carbonization The carbonization of the nanofibre membranes showed good consistency in results. The fibre diameter distribution remained similar through all carbonization temperatures for all initial/final conditions excepting the 5%SWNT composite PAN-co-MA nanofibres carbonized at 1000˚C. For this one condition the fibre distribution did have a substantially larger standard deviation. This can be explained by the possibility of agglomerations of SWNT that did not substantially change diameter as part of the carbonization process.  Another result were further exploring is the fusing of the nanofibres at 850˚C. Fibres that are not fully stabilized have been known to melt and fuse together as part of the carbonization process [113]. While the carbonization processing steps, including stabilization temperature and time, are in agreement with other works involving PAN-co-MA this is an area that could be explored further in the future. For the purposes of this work the fusing was not considered a critical parameter to control, with overall stability of the resulting carbonized fibre network a larger concern.  The conductivity results for the nanofibre membranes were as expected. The effect of the carbon nanotubes was especially pronounced at the lower carbonization temperatures. However, even at higher carbonization temperatures the value of adding carbon nanotubes into the fibre matrix is clearly demonstrable. The composite 5% SWNT/carbon nanofibres  74 show over twice the conductivity as carbon fibres without SWNT at the highest temperature of 1000˚C. Adding 1% SWNT yielded similar nanofibre conductivity at 700˚C as the pure carbon nanofibres had at 850˚C. Increasing the SWNT loading to 5% yielded similar conductivity at 700˚C as the pure carbon nanofibres had at 1000˚C. This is a clear benefit as higher carbonization temperatures result in more brittle nanofibres. For applications where flexibility is a key consideration the addition of SWNT into the PAN-co-MA matrix should be strongly considered. 5.5 PEDOT:PSS Coatings The interaction between the nanofibres and PEDOT:PSS coatings is an important consideration for transparent electrode applications. Metal nanofibres have been considered for this type of application in the past but the sulfonic acid of the PEDOT:PSS etches the fibres and degrades the end performance by over a factor of two [79]. While more of a concern for nanofibre membranes and less significant for thin film applications, the surface tension between the nanofibres and the PEDOT:PSS needs to be minimized to allow for penetration and coating of the individual nanofibres. For  electrospun nanofibres there can be an increase in the contact angle, reducing wettability, compared to larger fibres [114]. While the as-received PEDOT:PSS did not penetrate into the pores of the nanofibre membrane, the addition of methanol did reduce the contact angle enough to allow the sample to be wet out. The dried PEDOT:PSS shows good adhesion to the fibre structure as shown by the SEM images. 5.6 Thin Film Nanofibre Samples Thin film nanofibre samples were prepared to determine the performance and suitability of electrospun nanofibres as a transparent conductor. The previous work with nanofibre  75 membranes was used to select the optimum conditions to create these samples. As sample conductivity is a key parameter, PAN-co-MA with 5% SWNT added was used. Carbonization was similarly controlled, but no samples were able to be generated at 1000˚C. The stabilization of the nanofibres may have played a role in the difficulties obtaining samples at this condition as the small amount of nanofibres makes it extremely sensitive to any fibre loss during the carbonization process. However, for 700˚C and 850˚C good samples were obtained with transparency exceeding expectations. The exceptional transparency is expected in part to be due to the degradation of some of the nanofibres during carbonization resulting in less material remaining on the samples than anticipated.  The sheet resistance of the nanofibre thin film samples was too high for what is required to be considered for most transparent electrode applications. However, once the samples were coated with PEDOT:PSS the sheet resistance was greatly reduced. Most of the improvement was due to the conductivity of the PEDOT:PSS itself, but the coated nanofibre sample carbonized at 850˚C outperformed that of the baseline sample (without nanofibres) by 36%. With a sheet resistance of 414 ohm/sq and a transmittance of over 93%, composite SWNT/carbon nanofibres coated with PEDOT:PSS may be a suitable alternative for some transparent electrode applications. Indeed, the results are in a similar range that has been achieved with SWNT and graphene transparent electrodes [65-76] as shown in Table 3 and Table 4.  76 6. Conclusions and Future Work In this study, PAN-co-MA was characterized and utilized to create carbon nanofibres through the electrospinning process. The effect of adding SWNT was investigated, including changes to spinnability, fibre morphology, and conductivity. Thin film samples were created and characterized for their suitability as a transparent conductor. Further, the thin film samples were coated with PEDOT:PSS and analyzed. The resulting thin film composites with PEDOT:PSS achieved a sheet resistance of just over 400 ohm/sq while maintaining transparency greater than 90%.  For electrospinning applications, homopolymer polyacrylonitrile (PAN) has often been used as a precursor for creating carbon nanofibres. However,  PAN copolymers such as PAN-co-MA are important for industrial carbon fibre production processes. As research has shown that PAN-co-MA may be preferable to PAN to produce superior carbon nanofibres [115] understanding the solution parameters allows for future work to quickly determine optimum conditions. The solution parameters of two commercially available PAN-co-MA polymers were determined and the Berry Number calculated. For smooth nanofibres, the Berry number should be above 9.5 for PAN-co-MA. The relationship between intrinsic viscosity and solution concentration was also explored and found to be quite similar to PAN. Combining these results, it should be possible to replace any electrospinning experiments that use PAN with PAN-co-MA with minimal effort.  The effect of adding single wall carbon nanotubes (SWNT) to the electrospinning solution was also explored. PAN-co-MA samples were spun with different levels of SWNT loading  77 and characterized using SEM. The results show that it is possible to incorporate up to 5% by weight of SWNT into the electrospinning solution. However, with this high loading there was some SWNT agglomeration that resulted in changes to the fibre morphology. However, adding 1% SWNT did not affect the fibre morphology or fibre size at an experimentally significant level. There were some effects of adding SWNT to the electrospinning solution, the most significant being that a higher flow rate was needed to maintain continuous fibre production. This is an expected result due to the increased solution conductivity.  The carbonization process followed similar methodology to that found in literature. As determined using SEM, the results showed that a similar reduction in average fibre diameter for sample containing SWNT and those that did not. At higher SWNT loading the resulting fibre distribution after carbonization was broader than those with moderate or no SWNT. This may be explained by the agglomeration of the SWNT; as they are not expected to be greatly affected by the carbonization process, fibre areas with agglomerations of SWNT would not see as much reduction in fibre diameter. The carbonization results also showed some fusing of the nanofibres at 850˚C which may be indicative of incomplete stabilization. For the PAN-co-MA nanofibres that contained 5% SWNT, the carbonization process gave rise to considerably twisted carbon fibres. While some rippling was seen in the other two conditions it was not as noticeable as for the 5% SWNT loading. If straight nanofibres are required the appropriate SWNT loading should be considered.  The conductivity enhancements of adding SWNT to the PAN-co-MA is considerable, especially at lower carbonization temperatures. At 700˚C, the conductivity can be improved  78 by one to two orders of magnitude by changing the SWNT loading. While less significant at 1000˚C due to more complete carbonization of the PAN-co-MA, the conductivity can still be over twice as high with SWNT than without. For applications where both conductivity and flexibility are important it is possible to use the addition of SWNT to reduce the carbonization temperature but still achieve comparable conductivity. The nanofibres are less brittle when carbonized at lower temperatures and are more easily handled.  Thin film nanofibre samples were created by electrospinning PAN-co-MA with 5% SWNT directly onto quartz slides and carbonizing them. Difficulties were encountered in creating carbonized samples at 1000˚C, but samples at both 700˚C and 850˚C were successfully developed. Due to the low coverage of nanofibres the process is extremely sensitive to any outside variables. The transparency of the samples was exceptional, with transmittance above 96%. However, the sheet resistance of the samples was too high as compared to ITO coatings and was at the high end of other transparent conductors being developed in literature. The effects of coating the nanofibres with PEDOT:PSS were also considered. PEDOT:PSS was found to only coat the top layer of nanofibres and didn’t distribute into the pores of the nanofibre membranes. However, by reducing the contact angle by mixing the PEDOT:PSS with methanol, good coverage throughout the nanofibre membrane structure was possible. This opens up the possibility of another mechanism of improving the conductivity of bulk carbon nanofibres. For thin film samples, coating the samples with PEDOT:PSS greatly reduced the sheet resistance. For samples at 850˚C, the sheet resistance was 36% lower than a reference case of PEDOT:PSS without nanofibres. This strongly suggests that the interaction between the PEDOT:PSS and the carbon nanofibres is a positive one that  79 improves the conductivity of the nanofibre network. With a sheet resistance of 414 ohm/sq and a transmittance of over 93%, this composite structure is within the range for further investigation as a transparent conductor. 6.1 Recommendations for Future Work The results of this work are promising but also create further questions that should be explored and answered. The cause of the fusing of PAN-co-MA nanofibres at 850˚C should be explored. It is recommended that a more detailed study of the stabilization parameters be explored, including temperature ramp rate, stabilization time, and stabilization temperature. Alternative methods of providing stabilization for thin film samples is another area that can be expanded upon as stabilization has a large impact over the quality of the final carbonized samples.  The results of the experiments with PEDOT:PSS and bulk nanofibre membranes also warrant further investigation. For applications where conductivity plays a large role in the functionality, such as those explored by Bayat [90] and Lee [109], the addition of PEDOT:PSS may augment results and improve durability. For these types of applications, the method of PEDOT:PSS deposition should also be improved. A dip coating process may be preferable to drop casting to ensure consistency of PEDOT:PSS over larger samples.   There are a number of opportunities that can be explored to optimize the thin film samples for transparent electrode applications. Further work into generating samples at higher carbonization temperatures should be explored. This may need to be done in conjunction with the work on improving stabilization conditions as mentioned above. Additionally, work  80 on controlling and varying the deposition rate of nanofibres would allow for the determination of conditions where both transparency and conductivity are optimized. Aligning the nanofibres through the use of a secondary electric field during electrospinning may also provide further opportunities of optimizing the nanofibre coverage and network conductivity. Finally, varying concentrations of PEDOT:PSS should be explored to augment the optimization of the transparent conductor.  One further area that was not explored was determining a method to transfer the nanofibre thin films onto a flexible substrate. As flexibility is one of the key performance attributes over ITO, it is necessary to determine a method by which the thin film samples can either be transferred or developed on a flexible surface.     81 References 1. Tainter, Joseph A. 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