ZHENXIN ZHONG ALL RIGHTS RESERVED

Transcription

1 2011 ZHENXIN ZHONG ALL RIGHTS RESERVED

2 MORPHOLOGY AND INTERNAL STRUCTURE OF POLYMERIC AND CARBON NANOFIBERS A Dissertation Presented to The Graduate Faculty of The University of Akron In Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy Zhenxin Zhong May, 2011

3 MORPHOLOGY AND INTERNAL STRUCTURE OF POLYMERIC AND CARBON NANOFIBERS Zhenxin Zhong Dissertation Approved: Advisor Darrell H. Reneker Committee Member Gary R. Hamed Committee Member Stephen Z. D. Cheng Committee Member Shi-Qing Wang Accepted: Department Chair Ali Dhinojwala Dean of the College Stephen Z. D. Cheng Dean of the Graduate School George R. Newkome Date Committee Member George G. Chase ii

4 ABSTRACT Evaporation and the associated solidification are important factors that affect the diameter of electrospun nanofibers. The evaporation and solidification of a charged jet were controlled by varying the partial pressure of water vapor during electrospinning of poly(ethylene oxide) from aqueous solution. As the partial pressure of water vapor increases, the solidification process of the charged jet becomes slower, allowing elongation of the charged jet to continue longer and thereby to form thinner fibers. The morphology and internal structure of electrospun poly(vinylidene fluorides) nanofibers were investigated. Low voltage high resolution scanning electron microscopy was used to study the surface of electrospun nanofibers. Control of electrospinning process produced fibers with various morphological forms. Fibers that were beaded, branched, or split were obtained when different instabilities dominated in the electrospinning process. The high ratio of stretching during electrospinning aligns the polymer molecules along the fiber axis. A rapid evaporation of solvent during electrospinning gives fibers with small and imperfect crystallites. These can be perfected by thermal annealing. Fibers annealed at elevated temperature form plate-like lamellar crystals tightly linked by tie molecules. iii

5 Electrospinning can provide ultrafine nanofibers with cross-sections that contain only a few polymer molecules. Ultrafine polymer nanofibers are extremely stable in transmission electron microscope. Electrospun nanofibers suspended on a holey carbon film showed features of individual polymer molecules. Carbon fibers with diameters ranging from 100 nm to several microns were produced from mesophase pitch by a low cost gas jet process. The structure of mesophase pitch-based carbon fibers was investigated as a function of heat treatment temperatures. Submicron-sized graphene oxide flakes were prepared by a combination of oxidative treatment and ultrasonic radiation. Because pitch is a cheap raw material, graphitic fibers appear to be another useful starting material for mass production of graphene sheets. iv

6 DEDICATION To the memory of my mother, Baozhu Ge. v

7 ACKNOWLEDGEMENTS My deepest gratitude is to my advisor, Dr. Darrell H. Reneker. I have been amazingly fortunate to have an advisor who gave me the freedom to do research and showed me directions when my steps faltered. His patience, encourage, and support helped me overcome many hurdles and finish this dissertation. Special thanks are to the committee members, Dr. Gary R. Hamed, Dr. Stephen Z. D. Cheng, Dr. Shi-Qing Wang, and Dr. George G. Chase, who took time to read my dissertation and helped me to improve it with thoughtful comments and suggestions. I am grateful to Dr. Mark D. Foster for his thoughtful comments and suggestions on my research presentation. I would like to acknowledge Dr. Bojie Wang for his training and help on my microscopy work. I am also thankful to Mr. Edward Laughlin, Mr. Steve Robert, and Dr. Daniel Galehouse for their technical support. I want to acknowledge the group members in Dr. Reneker s lab for their useful discussions and technical supports. Finally, I would like to express my love and appreciation to my family. Without the family, I cannot go anywhere. vi

8 TABLE OF CONTENTS Page LIST OF TABLES... xi LIST OF FIGURES... xii CHAPTER I.INTRODUCTION...1 II. LITERATURE REVIEW Polymeric nanofibers Nanofiber production techniques A brief history of the electrospinning process Electrospinning setup and process Morphology and structure of electrospun nanofibers Properties of electrospun nanofibers Applications of electrospun nanofibers Carbon fibers PAN-based carbon fiber Mesophase pitch-based carbon fibers III. EFFECT OF EVAPORATION AND SOLIDIFICATION OF THE CHARGED JET IN ELECTROSPINNING OF POLY(ETHYLENE OXIDE) AQUEOUS vii

9 SOLUTION Introduction Experimental Results and discussion Conclusions IV. MORPHOLOGY AND INTERNAL STRUCTURE OF ELECTROSPUN POLYMER NANOFIBERS Introduction Experimental Results and Discussion Low voltage high resolution scanning electron microscopy Morphology of electrospun PVDF nanofibers Structure formation in electrospun PVDF nanofibers Orientation of polymer chain in polymer fiber Conclusion V. TEM OBSERVATION OF ELECTROSPUN NANOFIBERS WITH SMALL DIAMETERS Introduction Experimental viii

10 5.2.1 Materials Electrospinning Characterization Results and discussion TEM imaging of electrospun nanofiber on different support membranes Radiation damage of polymer nanofibers in transmission electron microscope Electrospinning of polymer nanofiber with small diameters High resolution images of ultrafine PVDF nanofibers Conclusion VI. STRUCTURE OF MESOPHASE PITCH-BASED CARBON FIBERS PRODUCED BY A NGJ PROCESS Introduction Experimental Materials KMnO4 oxidation treatment Characterization Results and Discussion Conclusion VII. SUMMARY ix

11 REFERENCES x

12 LIST OF TABLES Table Page 3.1. Relative humidity, temperature, and average fiber diameter xi

13 LIST OF FIGURES Figure Page 3.1 Experimental setup A sketch of measurements of fiber diameter (r), bead diameter (R), bead length (L), and distance between beads (D) on poly(ethylene oxide) nanofibers (a) and beaded nanofibers (b) Electron micrographs of poly(ethylene oxide) nanofibers electrospun from aqueous solution under: (a) 8.8%, (b) 20.7%, (c) 40.8%, (d) 52.6%, (e) 57.3%, (f) 61.2% and (g) 63.5% relative humidity Average fiber diameters of poly(ethylene oxide) nanofibers electrospun at different relative humidity Average bead diameters, bead lengths and distances between beads at different relative humidity Optical micrographs, between crossed polarized, of poly(ethylene oxide) electrospun from aqueous solution at: (a) 8.8%, (b) 20.7%, (c) 40.8%, and (d) 57.3% relative humidity SEM images of porous PVDF fibers SEM image of cylindrical PVDF fibers (a) SEM image of a branched PVDF fiber electrospun at 10 kv from a 15 wt.% DMSO solution and (b) TEM image of an isolated branched fiber obtained from a 3 wt.% DMSO solution with 3 wt.% trifluoroacetic acid additive PVDF fibers electrospun at 20 kv from a 15 wt.% DMSO solution TEM images of a split PVDF fiber xii

14 4.6 SEM images of PVDF fibers electrospun from (a) a 15 wt.% acetone solution and (b) a 10 wt.% acetone solution SEM image of porous PVDF fibers FT-IR spectra of PVDF fibers electrospun from different solutions. The solid content for all the solutions is 15 wt.% SEM images of PVDF fibers electrospun from (a) a 15 wt.% DMSO solution and (b) a 15 wt.% DMSO solution with 3 wt.% acetic acid FT-IR spectra of PVDF fibers produced from 15 wt.% DMSO solutions with different additives Electron diffraction patterns of (a) an as spun PVDF fiber and (b) a PVDF fiber after annealed at 150 o C for 30 min. The insets are the fibers used to obtain electron diffraction patterns (a) TEM image of as spun P(VDF-co-TrFE) fiber and (b) SEM image of P(VDF-co-TrFE) fibers annealed at 130 o C for 2 hours TEM images of electrospun PVDF fiber on different subtracts: (a) a continuous carbon membrane and (b) a holey carbon membrane TEM image of electrospun PVDF fiber supported on a graphene flake Electron diffraction pattern of a PVDF nanofiber TEM images of electrospun fiber suspended on a holey carbon membrane before and after 90 minutes beam irradiation: beam intensity is about 650,000 e/nm2/s TEM image of a broken PVDF nanofiber damaged by electron beam TEM images of PVDF nanofibers electrospun from DMSO solutions with different PVDF solid contents: (a) 15 wt.%. (b) 5 wt.%, (c) 3 wt.% and (4)3 wt.% with 2 wt.% trifluoroacetic acid additive...76 xiii

15 5.7 Diameter of PVDF nanofibers versus the weight percent concentration of PVDF in DMSO solution TEM images of ultra-thin carbon fibers Schematic diagram of several possible cross-sections of ultra-thin diameter PVDF nanofibers Diameter of PVDF nanofibers versus the weight percent concentration of PVDF in DMSO solution High resolution images of ultrafine PVDF nanofibers One of the possible arrangements of polymer molecules in a PVDF nanofiber with a diameter of 3.5 nm High resolution TEM image of an ultrafine PVDF nanofiber A typical SEM image of carbonized NGJ nanofibers SEM images of individual NGJ carbon fibers heated at different temperatures: (a) 250 o C, (b) 1500 o C, and (c) 3200 o C X-ray diffraction patterns of NGJ carbon fibers before and after oxidative treatment TEM image of a typical carbonized (left) and graphitized (right) NGJ carbon fiber Electron diffraction patterns of (a) carbonized and (b) graphitized NGJ nanofibers TEM images of fragments of graphitized carbon nanofibers ultrosonicated at NMP for 30 min Raman spectra of NGJ carbon fibers xiv

16 6.8 TEM image of a typical dispersion of graphene oxide sheets produced from graphitized NGJ carbon fibers by oxidative treatment A graphene oxide flake containing only a few graphene layers. The inset is the electron diffraction patter from this isolated sheet SEM image of carbon sheets produced by oxidative treatment of carbonized NGJ fibers xv

17 CHAPTER I INTRODUCTION The formation of polymeric fibers through human invention was achieved at the end of 19th century. The first artificial fiber, known as viscose around 1894 and finally rayon in 1924, was made from wood and plants. Nylon, the first synthetic fiber, was invented as a replacement for silk by DuPont in 1930s. The invention of nylon became the model for scientifically based industrial research in the chemical industry. Throughout the remainder of the 20th century, numerous new polymers were synthesized that could be formed into synthetic fibers, covering an enormous range of properties. Synthetic fibers can be tailor-made from a wide range of synthetic materials by different spinning technologies to provide specific properties that natural fiber can not provide. Now synthetic fibers account for about half of all the fiber usage with applications in every field of fiber and textile technology. Clothing, apparels, cosmetics, cigarette filters, air conditioning filters, fishing nets, composites, surgical masks, extracorporeal devices, vascular grafts, heart valves, are a few examples of these applications. Synthetic fibers based on polyamides, polyesters, polyolefins, acrylics, polymer urethanes are some of the 1

18 most widely used. 1, 2 In general, most synthetic and cellulosic fibers are created by forcing fiber forming materials through holes (called spinnerets) into the air, forming a continuous or short filament. For extrusion, the solid fiber-forming materials must be first converted into a fluid state by melting or dissolving in a suitable solvent. The most important fiber spinning processes developed are dry spinning, wet spinning, melt spinning, and gel spinning. 2 Solution wet spinning is the oldest process. It is used for fiber forming substances that have been dissolved in a solvent. The spinnerets are submerged in a chemical bath and as the filaments emerge, they precipitate from solution and solidify. Dry spinning is also used for fiber-forming substances in solution. However, instead of precipitating the polymer by dilution or chemical reaction, solidification is achieved by evaporating the solvent in a stream of air or inert gas. In melt spinning, the fiber-forming substance is melted for extrusion through the spinneret and then directly solidified by cooling. Gel spinning is a special process used to obtain high strength or other special fiber properties. The polymer is not in a true liquid state during extrusion. Not completely separated, as they would be in a true solution, the polymer chains are bound together at various points in liquid crystal form. This produces strong inter-chain forces in the resulting filaments that can significantly increase the tensile strength of the fibers. Fibers produced from the traditional spinning processes are typically in 5 to 50 micrometers diameter range. Since 1990s, researchers around the world are searching for 2

19 new methods to produce polymer fibers with smaller diameters due to the rise of nanotechnology. In last two decades, numerous efforts have been conducted on research and development in the production, characterization, and emerging use of polymer nanofibers from different polymers from a variety of novel fiber spinning technologies. Advanced fiber spinning technologies, such as electrospinning, nanofiber by gas jet, and rotary jet-spinning, have been developed to produce fibers in the nanometer range. 3-5 Fibers produced from those methods have diameters two to three orders smaller than fibers produced by conventional technologies. Both the surface area to weight ratio and surface area to volume ratio of fibers increase significantly with the decease of fiber diameter. Because of the ultra high surface volume ratio of the polymer nanofibers, numerous applications of the electrospun fibers have been explored, such as air filtration, protective clothing, drug delivery systems, polymer nanocomposites, tissue engineering, catalyst support, biomedicine, and nanoelectronics. 6 Compared with the conventional spinning methods which use mechanical forces, either extrusion or air blowing, to convert bulk materials to fibers, electrospinning is a unique fiber making process. Electrospinning uses electrostatic charges to draw polymer solution or melt into fine fibers. 3 Research on electrospinning and structure formation in electrospun nanofibers has both benefited from and contributed to the rapid advances in nanotechnology and nanomaterials over the last two decades. Electrospinning and the study of nanofiber formation are no longer in their infancy, as they were twenty years ago, 3

20 but much remains to be learned. Some aspects of morphology and structure development of polymer nanofibers have been established, but other remains stubbornly obscure due to the inability of characterization methods to provide the level of understanding at nanometer and molecular scale. Little works has been done on the systematic studies of morphology and internal structure of polymer nanofibers on the same polymer system. Nanofibers by gas jet (NGJ) process is another emerging patented technology to produce polymeric or carbon nanofibers from a polymer melt by high speed gas streams. 5 It is a promising low cost method to produce mesophase pitch carbon fibers with desirable diameter, structure and properties. The objectives of this research are to investigate the effect of electrospinning conditions on the diameter, morphology, and structure of electrospun fibers, to provide a experimental method to produce ultrafine nanofiber specimens and use them for studying polymer structure at molecular and atomic scale, to study the electron beam damage mechanism of polymer nanofibers, and to study the structure formation in mesophase-pitch based carbon fibers produced by a NGJ process. Chapter II summarized the literature review and background of this research. Effect of evaporation and solidification of the charged jet in electrospinning of poly(ethylene oxide) aqueous solution was investigated in Chapter III. In Chapter IV, various fiber morphologies, such as split, branched, wrinkled, and porous fibers were obtained by controlling the electrospinning parameters and were observed by low voltage high 4

21 resolution scanning electron microscopy and transmission electron microscopy. The structure formation in electrospun nanofibers was also investigated by FT-IR, DSC, X-ray or electron diffraction, and bright field transmission electron microscopy imaging. In Chapter V, the experimental approach to obtained ultrafine nanofiber with a diameter under 10 nm was developed. The direct imaging of polymer molecules in ultrafine nanofiber was attempted by high resolution transmission electron microscopy. The effect of electron beam irradiation on the changes of morphology and structure of ultrafine nanofiber was also discussed. In Chapter VI, an innovative, low cost carbon fiber manufacturing process was discussed. The structure formation of NGJ carbon fiber at different heat temperatures was studied. Chapter VII summarized the results of this dissertation. 5

22 CHAPTER II LITERATURE REVIEW Nanofibers, including polymeric and carbon nanofibers, are of interest to many areas of research, as their unique properties make them appealing for a wide number of applications. Compared with bulk materials, nanofibers exhibit extremely large specific surface area and ultra high modulus and strength when their diameters are below 500 nm. Various technologies have been studied to produce fibers with desired properties by controlling fiber diameter, morphology, and structure for different applications. 2.1 Polymeric nanofibers Nanofiber production techniques The nanofiber production techniques can generally be categorized into two classes, bottom-up and top-down techniques. Bottom-up techniques includes nanofiber 6

23 production by self assembly, template synthesis, phase separation, chemical vapor deposition, and nanolithography. Bottom-up nanofiber manufacturing techniques allow for precise control over nanofiber diameters and structures. However, these processes are often expensive and label intensive. Collection and alignment of nanofibers produced from these techniques are usually difficult. Several top-down nanofiber production techniques have been studied in recent years. Those techniques include rotary jet-spinning, nanofiber by gas jet, ultradrawn gel spinning, and electrospinning. 3-5, 7 Among them, electrospinning is one of the most investigated techniques to produce nanofibers and thicker fibers. 8 It is a highly versatile method as the process works with a wealth of materials with a simple setup. Electrospinning is currently the only technique that allows the fabrication of continuous fibers with diameters down to a few nanometers A brief history of the electrospinning process Electrospinning may be considered as a variant of the electrospraying process. The first patent that described the operation of electrospinning appeared in Formalas disclosed an apparatus for the production of polymer fibers by using the electrostatic repulsions between surface charges. 9 Until 1993, there were only a few publications dealing with electrospinning as a thin fiber production techniques. Electrospun fibers were first commercialized for filter application as part of the nonwoven industry. 7

24 Electrospinning gained substantial academic attention in the early 1990s, which was partially initiated by the activities of the Reneker group. 10 Electrospinning became one of the most fascinating subjects since 1990s. A lot of experimental and theoretical studies related to electrospinning were performed because of the remarkable simplicity, versatility, and potential uses of this technique. 6, 11, 12 The number of publications research groups in this field has been increasing exponentially in the past two decades Electrospinning setup and process An electrospinning setup generally consists of three major components: a high voltage power supply, a spinneret, and a collector. A polymer solution is introduced to the electrospinning system through a nozzle. A syringe pump may be used to control the solution feeding rate. High voltage, direct current power supplies (positive or negative polarity) are usually used for electrospinning. When a high voltage (usually in the range of several hundred volts to 30 kv) is applied, the pendant drop of polymer solution at the nozzle will become highly electrified. The charges introduced by the electrodes are evenly distributed on the liquid drop surface. As the electrostatic repulsion force overcomes the surface tension of the polymer solution, a liquid jet is initiated from the tip of a Taylor cone and moves towards the counter electrode (collector). 13 On the way to the counter electrode, the jet is elongated through a series of bending instabilities to form a 8

25 continuous polymer fiber with diameters ranging from micrometer to nanometer. The solidified charged fiber is attracted by the counter electrodes beneath the spinneret and is often deposited as a randomly oriented non-woven mat. With the use of this simple and straightforward technique, hundreds of natural and synthetic polymers have been used for the production of polymeric nanofibers for a wide range of applications. Ceramic, carbon, and metal nanofibers were also created from polymeric nanofibers by electrospinning, followed by heat treatment and chemical reactions Morphology and structure of electrospun nanofibers During electrospinning from solution, structure formation with polymer nanofibers is controlled by the simultaneous processes of the evaporation of the solvent and extreme elongation of electrospinning liquid jets. 14 A liquid jet travels from the electrospinning nozzle to the collection substrate or electrode in a time frame of 0.1s. 11 The high ratio of stretching (as much as 10 5 ) during the electrospinning process, which is similar to uniaxial mechanical stretching, aligns the polymer molecules along the fiber axis. For semi-crystalline polymers, solidification is connected with the formation of crystals. The fast solvent evaporation and short solidification time of electrospinning jet lead to small and imperfect crystallites in electrospun fibers. There have been several studies on structure formation during electrospinning of polyamide, polyvinylidene fluoride, and 9

26 polyethylene It was demonstrated by X-ray diffraction, and Raman spectroscopy that nanofibers of nylon 6 form the less-ordered gamma modification. The less-ordered gamma phase could be converted into more ordered alpha phase by annealing the fibers at high temperatures. 15 The chain orientation and the orientation of the crystallites in the electrospun fibers, which is of greater significance with respect to mechanical properties, were investigated using Raman Spectroscopy, and X-ray or electron diffraction. Very high degrees of crystallite orientation were confirmed in nanofibers by electron diffraction. 15 The orientation of polymer molecules and crystallites can be enhanced by annealing the fiber and a post-stretching process. 18 Electrospinning generally produces smooth fibers with a circular cross-section. Different fiber shapes and topology may be produced by controlling the electrospinning conditions, especially by choosing particular solvents or solvent mixtures, by varying relative humidity, or by using polymer blends Branched fibers, flat ribbons, ribbons with other shapes, and fiber that were split longitudinally from larger fibers were observed by Koombhongse et al. 20, 24 Fibers with wrinkled surfaces were produced by fast phase separation into polymer-rich and polymer-poor regions during electrospinning process. 25 Another route to porous fibers surface is through condensation processes during electrospinning in a very humid environment. 21 Porous fibers were produced by selectively removal of one of the two phases in polymer blend fibers. 10

27 2.1.5 Properties of electrospun nanofibers Polymer nanofibers have been advocated for use in filters, composites, fuel cells, personal protection, catalyst supports, drug delivery devices, tissue scaffolds, and other applications. Of fundamental necessity for many of these applications is an understanding of the physical properties of individual electrospun nanofibers and their structures. Various attempts have been made to quantify the mechanical properties of isolated polymer nanofibers with diameters at sub-micron range via direct experimental measurements. These studies have shown that electrospun nanofibers feature very good mechanical properties. Mechanical properties of single electrospun nanofibers composed of polycaprolacton and poly(ε-caprolactone-co-ethylethylene phosphate) were measured under uniaxial tension, indicating an increase in both stiffness and strength as the fiber diameter decreased from 5 μm to 250 nm. 26 Young s moduli of electrospun nylon-6 nanofibers were found to increase from 20 to 80 GPa as the fiber diameter decreased from 120 to 70 nm. 27 Young s moduli of up to 50 GPa were reported for polyacrylonitrile nanofibers with a high degree of orientation. For comparison, bulk samples of polyacrylonitrile shows moduli of only 1.2 GPa. 28 The stiffening and strengthening effects are attributed to the improved crystallinity and chain alignment in smaller polymer fibers. Electrospun fibers with superior mechanical properties are suitable for applications in nanofiber reinforcement. For the structure and physical properties of 11

28 polymer nanofibers with diameters smaller than 50 nm, experimental data are not available because several challenges still exist that limit the precision and accuracy of physical property measurements. One of challenges is the difficulty of preparing, isolating, and manipulating such small fibers with controlled diameters without compromising them Applications of electrospun nanofibers Electrospinning is a simple and powerful technology for producing polymer nanofibers with a wealth of different materials. Advantages over the conventional spinning of fine fibers, such as a conjugated spinning method, include a simple apparatus, a compact spinning station, a wide range of materials that can be used and, best of all, its capability of producing fibers with very small diameters. Electrospun nanofibers are expected to have high axial strength and stiffness, as well as incredible flexibility. Electrospun nanofibers also exhibit very high porosity and specific surface area due to their small diameters. The unique properties of continuous nanofibers make them attractive for a number of applications. The possible applications include filtration, composite reinforcement, smart cloths, catalysis support, electrodes energy, biomedical applications, etc

29 2.2 Carbon fibers Carbon fiber is a material consisting of extremely thin filaments composed mostly of carbon atoms. Carbon fibers are fabricated by the controlled pyrolysis of a carbonaceous material in its filament form. The historical development of carbon fibers goes back to 1879 when produced by Edison for the electric light bulb. Rayon-based carbon fibers were the first to be commercialized and remained dominant up to the early 1970s. Today, the majority of carbon fibers produced are based on pitch or polyacrylonitrile as a precursor. Fibers made from pitch and polymer fibers are in short and continuous forms. Carbon fibers made from pitch are more graphitizable than those made from polymers, so they can attain higher thermal conductivity and low electrical resistivity. The raw material cost is much lower for making fibers from pitch than from polymers. However, the current market is dominated by PAN-based fibers because of their combination of good mechanical properties, such as tensile strength, and reasonable cost. In contrast, highly graphitic mesophase pitch-based carbon fibers are very expensive, though they have high tensile modulus, high thermal and electrical conductivities. Typical high performance carbon fibers, such as ToRay s T1000 PAN based fiber, have a young s modulus of 300GPa and a tensile strength of 7 GPa. In addition to their high strength to weight ratio, carbon fibers have many desirable qualities including low thermal expansion, high electrical and thermal conductivity, high creep resistance and 13

30 corrosion resistance. 33 Originally used in aerospace, carbon fiber has moved into the mainstream and can be found in luxury automobiles, mountain bikes, electronics, and sports equipment because of the development of low-cost high performance carbon fibers. Wider-spread use of carbon fibers for vast applications is limited due to its high cost. Large effort was made to find low cost alternative precursors. Many other polymeric materials have been investigated as potential precursors for carbon fibers. They include phenolics, polyimides, polyamide, polyphenylene, polybenzimidazole, polyetheyene, polypropylene, and polyvinylchloride. 34 Through some of these polymers exhibit high yield, the properties and structure of the ultimate carbon fibers are not suitable for their application as a high performance material. Oak Ridge National Lab has studied a number of recycled and renewable polymers as carbon fiber feedstocks for use in transportation composite application PAN-based carbon fiber Among all possible carbon fiber precursors, PAN turned out to be the most suitable precursor from the point of view of overall carbon content (67%), high carbon yield (~54%), processing, structure, properties, and cost. As a result, PAN-based carbon fibers account for more than 70% of the world s total carbon fiber production and are used in the majority of current advanced composites

31 PAN is a linear polymer containing highly polar nitrile pendant groups. The compositions of the PAN precursor, as well as the spinning solution and other spinning conditions, have a great influence on the final carbon fiber properties. The PAN polymer used for conversion into high strength or high modulus has an average molecular weight of ~3* 10 5 g/mol and contains more than 90% acrylonitrile. The co-monomers are usually methyl acrylate, methacarylic amines, or ethyl methyl propenoate with some itaconic acid or sodium styrene sulfonated. 37 The fundamental fiber structure needed to develop high strength and stiff carbon fibers is created during the initial fiber forming step. Wet, dry, and melt spinning processes have been used to spin PAN precursor fibers. PAN fibers spun from inorganic solvents are found to be superior precursor for the production of carbon fibers. Wet spinning and dry spinning result in circular and dog bone shaped fiber whereas multilobal cross-section fibers can be formed by melt spinning. In wet spun fibers, polymer molecules are organized into fibrils which are generally oriented parallel to the fiber axis and joined together in a 3D network. This fibrillar network appears to be the precursor for the graphene network that develops during final heat treatment. Leading carbon fiber producers now use dry jet spun PAN fibers, which result in carbon fibers with much higher mechanical properties than those from wet spun fibers. Moreover, to produce high quality carbon fibers, the starting PAN precursor fiber should have small diameter, maximum possible orientation of the molecular chains along the fiber axis, maximum 15

32 crystalline content, and low activation energy for cyclization. Improvement in the precursor fiber can be achieved by post-spin stretching of fiber before stabilization. The basic process for the making of carbon fibers involves three steps: preoxidation stabilization, carbonization, and a high temperature heat treatment. The stabilization step converts the precursor from a linear polymer to a highly condensed, thermally stable ladder polymer structure. The oxidation temperature ranges from 200 o C to 300 o C. In the presence of an acid initiator in the copolymer structure, an ionic mechanism predominates and occurs at a lower temperature. The process combines oxygen molecules from the air with the PAN fibers and cross-links the as-spun structure ensuring that both molecular and the fibrillar orientations will not be lost during final heat treatment. The stabilization is considered to the most decisive step in the production of carbon fibers because the stabilized fibers are considered to be the template for graphitic structures, and hence mechanical properties of the ultimate carbon fibers. Oxidation time varies by specific precursor chemistry. Another important parameter in processing PAN-based carbon fibers is restraining the molecular chains so that they do not become disoriented during heat treatment at all stages. In order to obtain high modulus carbon fibers it is essential to restrain the fibers from shrinking or, even to elongate them by applying tension. The preferred orientation needed for the final high performance carbon fiber is achieved by stretching the polymeric precursor fiber when it is still thermoplastic. Carbonization occurs in an inert atmosphere inside a series of specially designed 16

33 furnaces that progressively increase the processing temperature. In the early stage ( o C), a pyridinic structure is formed which on further heat treatment ( o C) collapses into a turbostratic, stacked ring structure. The low temperature carbonization is extremely important since it is during this stage that the maximum mass transfer takes place and the carbon building block is laid. So a low heating rate should be adopted in this temperature range to avoid fast evolution of gaseous products, and hence, defects in the carbon fibers. In the temperature range of o C, H 2, HCN, and nitrogen are evolved due to the intermolecular cross-linking of the polymeric chains. During carbonization, the fibers suffer a weight loss of about 50% and shrink in diameter by 40-50%. Fibers heat treated to 1500 o C are termed high strength carbon fibers and consist of oriented small crystallites. Heat treatment up to o C continues to improve the graphitic structure with an increase in crystallite size, preferred orientation, and hence elastic modulus. The strength of the fiber, however decrease reaching a minimum at about 1800 o C and a slight improvement after heat treatment of 2000 o C. The carbon fibers heated at high temperature and consist of more than 99% carbon are termed high modulus carbon fibers. 17

34 2.2.2 Mesophase pitch-based carbon fibers Mesophase pitch is a liquid crystalline materials consisting of large polynuclear aromatic hydrocarbons. The properties of mesophase, its formation, and mode of growth have been the subject of numerous articles. The first commercial mesophase precursors were produced by Union Carbide using a thermal polymerization process. 38 Coal tar pitch produces a mesophase product with higher aromaticity, whereas petroleum pitch yields a mesophase product with a more open structure and a higher content of aliphatic side chains. Mesophase pitch possesses a highly condensed aromatic structure with relatively good thermal stability. The mesophase pitches used to form carbon fibers soften and flow well below their degradation temperature. Therefore, they can be melt spun into fiber form. Because its viscosity is highly temperature dependent, mesophase pitch fibers draw down and cool very quickly during fiber formation. As a result, they can break easily during spinning process and are extremely difficult to handle before they are carbonized. Although the rheology of mesophase makes control of the melt-spinning more difficult, its liquid crystalline nature gives mesophase pitch advantages compared to polymeric precursors. In contrast to PAN based carbon fiber processing, the pitch has to be first spun into fibers. The type and composition of pitch yields ultimate carbon fibers with desired properties. Unlike polymeric fibers, the molecular orientation within a mesophase 18

35 precursor fiber can be improved by increasing spinning temperature. Stabilization before carbonization is necessary to prevent the as-spun pitch fibers from melting or relaxing during the final heat treatment. This is usually accomplished by heating the pitch fiber to a temperature below its softening point in the presence of oxygen for an extended period. The large plate-like molecules become crosslinked through carbonyl and phenoxy groups formed by oxygen. The heat treatment time and temperature to stabilize as spun pitch fiber have to be optimized from both a processing point of view as well as the fiber properties. Typically, the temperature range is o C and a period of 2-5 hours is used for the stabilization process of pitch fibers. The stabilized pitch fibers are further heat treated to o C in an inert atmosphere during which most of the noncarbon elements escape in the form of hydrogen, carbon monoxide, ethane, etc. Carbonization of pitch fibers is carried out in steps, firstly at ~700 o C, and then at a desired temperature of o C. The graphitic content, as well as the degree of preferred orientation of the crystallites within the fiber, increase with heat treatment temperature. The fundamental structure of pitch-based carbon fibers is very different from that of PAN-based based carbon fibers, and each structure offers certain advantages. The structure of pitch-based fibers is largely developed during fiber formation process. Mesophase pitch carbon fibers have a wide variety of internal structures, such as radial, random, onionskin and their intermediates. The internal structure is determined by the shear conditions in the spinneret and the dimensions of the die. To obtain proper 19

36 structures and high tensile properties, it is most important to control the spinning process since molecular orientation of mesophase pitch is determined almost exclusively in this process. 20

37 CHAPTER III EFFECT OF EVAPORATION AND SOLIDIFICATION OF THE CHARGED JET IN ELECTROSPINNING OF POLY(ETHYLENE OXIDE) AQUEOUS SOLUTION 3.1 Introduction Electrospinning is receiving attention as a straightforward route to nanofibers. Electrospun fibers and electrospinning processes have many potential applications including filtration 39, biomedical application 40, fuel cells 41, 42, solar sails, 42 and composites. 43 Many polymer and ceramic precursor nanofibers have been successfully electrospun with diameters in the range from 1 nm to several microns. The electric force causes the jet to emerge from a Taylor cone. 44 The charged jet of polyethylene oxide solution elongates and moves toward the collector in a straight line for some distance, and then it begins to bend and develop a spiral path. The repulsive force between charges carried by the jet causes the jet to elongate and become thinner. The elongation and thinning of the charged jet continue until solidification take place. 21

38 The formation of beaded fibers is also widely reported. 45, 46 Solution viscosity, net charge density and surface tension affected formation of beaded fibers. Less viscous solution, lower charge density and higher surface tension favored formation of beaded fibers. 45 Shenoy et al. developed a semi-empirical analysis to predict the transition of uniform fiber to beaded fiber based on the entanglement number of the polymer solution. At critical polymer concentration (c c*), beaded fibers were formed. At higher polymer concentration (c > c*), the increased number of entanglements stabilized the charged jet by inhibiting the capillary instability. 46 A mathematical model was proposed to calculate the bead-on-string formation due to capillary instability of polymer jets in the absence of electric field. 47 Many factors affect the fiber diameter and morphology. Development of useful applications requires a thorough knowledge of parameters of the electrospinning process and their effect on fiber diameters, morphology and properties of electrospun fibers. A wide range of parameters, both intrinsic solution properties and processing parameters, have been investigated in a series of papers. 19, 22, 46, The intrinsic solution properties studied include viscosity, concentration, surface tension, relaxation time, and solution entanglement number. Processing parameters, including applied potential, distance from polymer droplet to the collector, size of the orifice, humidity and temperature, are also known to affect the fiber diameter and morphology. Vapor concentration of solvent and temperature affects evaporation and solidification of the jet. 44 Most investigations were 22

39 carried out in an open atmosphere without controlling the vapor concentration of solvent. No systematic study was conducted on the influence of evaporation and solidification of the jet. Yarin et al. proposed a mathematical model to calculate jet path and fiber diameter 14, 44 during the electrospinning process. The mathematical model was based on the balance of forces acting on the charged jet, including the Coulombic force between charges carried with the jet, force from the electric field, surface tension force, and viscoelastic force. Two publications on this mathematical model were reported, the first without evaporation and solidification, 14 and the second which included evaporation and solidification. 44 In the model without evaporation, the jet diameter could, in principle, decrease to the diameter of a single molecule. When the effect of evaporation and solidification was accounted for in the calculation, a quantitative agreement between experiment and calculation was achieved. Evaporation of the solvent changed the viscoelastic properties of the polymer solutions, and stopped the elongation. Another theoretical model predicts a terminal diameter of the fluid jet controlled by the balance between surface tension and self-repulsion of surface charge. 51 Although it was reported that the fiber diameter can be controlled by varying polymer solution concentrations and excess charge densities carried by the jet, the goal of obtaining finer fibers was compromised by the change of the fiber uniformity at lower polymer concentrations. Controlled production of ultra-fine electrospun nanofibers with 23

40 uniform diameter remains a challenge. In this paper, the effect of evaporation and solidification of the charged jet on fiber diameters and morphology was investigated. The evaporation rate of the solvent, in this case water, was controlled by changing the relative humidity in the air surrounding the jet. Decreased evaporation rate of solvent from the jet allowed the charged jet to remain fluid, to continue to elongate, and to become thinner Experimental Poly(ethylene oxide) with molecular weight of 400,000 g/mol obtained from Scientific Polymer was used. The aqueous solution of poly(ethylene oxide), 6% by weight, was prepared. The solution was electrospun in a closed chamber in which the relative humidity during the electrospinning process was controlled. Figure 3.1 shows the arrangement used in this study. An ultrasonic humidifier and dry ice were used to increase or decrease humidity in the chamber. Circulation of the air in the chamber was maintained by a fan. The poly(ethylene oxide) aqueous solution was held in a glass pipette, and connected to a high voltage power supply. A flat metal collector was placed 18 cm below the tip of the glass pipette. The applied potential difference between the tip and the collector was 5 kv. The applied potential and distance between the tip of the pipette and the collector were selected to ensure that by the time the charged jet hit the collector, it was solidified. Current, temperature and humidity of the air in the chamber 24

41 were monitored during the electrospinning process. The humidity and temperature sensors used in this experiment were HTM 1505 (±0.2% RH, ±0.5 C) manufactured by Humerel. Morphological features of the electrospun fibers were observed with a JEOL model JSM-7401F scanning electron microscope (SEM) and with an Olympus model DP70 optical microscope. The average fiber diameter, bead diameter, bead length and distance between beads were obtained by using Digimizer image analysis software version The fiber diameter measurements were done at points separated by 2 μm on 10 segments randomly selected from SEM images, as shown in Figure 3.2a. The measurements of bead diameter, bead length, and distance between beads on beaded fibers were made as shown in Figure 3.2b. The total number of data points for each parameter was about 50 (Table 3.1, Figure 3.4 and Figure 3.5). 25

42 Figure 3.1 Experimental setup. 26

43 Figure 3.2 A sketch of measurements of fiber diameter (r), bead diameter (R), bead length (L), and distance between beads (D) on poly(ethylene oxide) nanofibers (a) and beaded nanofibers (b). 27

44 Table 3.1. Relative humidity, temperature, and average fiber diameter Relative humidity (%) Temperature ( C) Average fiber diameter a (nm) ± ± ± ± ± ± ± ± ± ± ± 16 Humidity inside chamber at each data point was kept at ±0.4% RH. The observed current flowing from the tip was constant at around 400 ± 10 na. a The beaded fibers were formed at 52.6% relative humidity and higher. The average fiber diameters of beaded fibers were calculated from the segments between beads. 28

45 3.3 Results and discussion Figure 3.3a-g show scanning electron micrographs of the poly(ethylene oxide) nanofibers electrospun in air at % relative humidity. Figure 3.4 shows the average fiber diameters at different relative humidity measured from SEM images. The charged jet solidified at the largest diameter when electrospun at low humidity since water in the jet evaporated rapidly, as shown in Figure 3.3 a-d. The average diameter of poly(ethylene oxide) nanofibers gradually decreased from around 253 nm when electrospun at 5.1% relative humidity to around 144 nm when electrospun at 48.7% relative humidity. The uniformity of electrospun fibers, indicated by the error bars in Figure 3.4, was consistent when electrospun at % relative humidity. The evaporation rate from the fluid jet decreased at higher relative humidity, which allowed the charged jet to continue to elongate. During the same time interval, the surface area increased, and the charge per unit area on the surface of the jet decreased. The capillary instability developed, and created thin fiber segments between beads with diameters larger than the fibers. Beaded fibers were first observed at 52.6% relative humidity. More beads with smaller bead lengths, larger bead diameters, and shorter spacing between beads appeared at higher humidity, as shown in Figure 3.3 and Figure 3.5. The diameter of the fiber between beads continued to decrease even more rapidly as the relative humidity increased after the beads began to form. After the beads formed, the volume of the beads and their segments were combined and the average fiber diameters 29

46 were calculated and plotted as open triangles in Figure 3.4. The dashed line in Figure 3.4 indicates the linear extrapolation of the hypothetical fiber diameters at relative humidity higher than 52.6% if no capillary breakup happened. The apparent increase in the calculated average mass per unit length after the bead formed suggested that the growth of beads shortened the length of the fiber segments between the beads, but the random errors in the observation of the shape of the beads did not permit a firm conclusion to be drawn. The occurrence of the capillary instability precluded subsequent observation of a lower limit on the diameter of the liquid jet

47 Figure 3.3 Electron micrographs of poly(ethylene oxide) nanofibers electrospun from aqueous solution under: (a) 8.8%, (b) 20.7%, (c) 40.8%, (d) 52.6%, (e) 57.3%, (f) 61.2% and (g) 63.5% relative humidity. 31

48 Figure 3.4 Average fiber diameters of poly(ethylene oxide) nanofibers electrospun at different relative humidity. 32

49 Figure 3.5 Average bead diameters, bead lengths and distances between beads at different relative humidity. 33

50 The charged jet bent and developed a spiral path in three dimensions. Optical images of the electrospun fibers in Figure 3.6 show the homogeneity of the fibers over a large area. The fibers are birefringent, as seen by the dependence of the brightness of the fibers on their azimuthal orientation with respect to the polarizer, indicating that the polymer molecules were aligned with the axis. The fibers were collected after the first electrical bending instability coils formed. The fiber segments in Figure 3.6 actually have a measurable curvature, corresponding to a coiled diameter of about 100 mm, because of the electrical bending of the jet path. No second or higher order electrical bending coils were seen at low humidity as shown in Figure 3.6a The thinner and more flexible fibers electrospun at higher humidity were collected with an increasing proportion of bends or coils with radii of curvatures ranging downward to 0.05 mm. The fibers made at higher humidities have radii of curvatures that were smaller, as expected for the buckling of thinner jets as they were collected. 52 At above 50% relative humidity, beads developed on most segments of the fibers. 34

51 Figure 3.6 Optical micrographs, between crossed polarized, of poly(ethylene oxide) electrospun from aqueous solution at: (a) 8.8%, (b) 20.7%, (c) 40.8%, and (d) 57.3% relative humidity. 35

52 3.4. Conclusions Evaporation and solidification affected the fiber diameter and morphology. Fiber diameter became smaller when evaporation and solidification happened more slowly because of the higher vapor concentration of solvent. For poly(ethylene oxide) in water, the linear decrease in the diameters of electrospun fibers with increasing humidity provides an effective process control parameter. Beaded fibers were formed when jet diameter was very thin and the charge per unit area was smaller. The size of the beads and the length of fibers between them changed systematically as the relative humidity increased. 36

53 CHAPTER IV MORPHOLOGY AND INTERNAL STRUCTURE OF ELECTROSPUN POLYMER NANOFIBERS 4.1 Introduction Electrospinning is a simple and cost effective way to create polymer fibers at the nanometer scale. Electrospinning depends on the complex interplay of surfaces, shapes, rheology, and electrical charges although the equipment required for electrospinning is simple and inexpensive. The electrospinning process involves many physical instabilities which produce fibers with different forms of morphology and fiber diameters. A number of interesting experimental observations as well as theoretical considerations on those 6, 14, 44 instabilities were reported in a number of papers. Over a dozen process variables may affect the morphology and structure formation in electrospun nanofibers. A fundamental understanding of how to control the formation, shape, texture, and structure of these fibers is essential for the design and precise control 37

54 of polymeric fibers for specific application needs. In most reports on electrospinning and polymeric nanofiber experiments, the diameter and the morphology from dense fiber mats have been analyzed and controlled and average data on the fiber structures were studied and discussed by X-ray diffraction, infrared spectroscopy, and differential scanning calorimetry. There are few reports on the morphology, orientation of polymer chains, and structure formation on individual electrospun fibers. Scanning electron microscopy (SEM) is a routine technology to investigate the morphology and size of nanomaterials. Due to low contrast and poor electron conductance nature of polymeric materials, a thin layer coating of silver or gold nanoparticles with a thickness up to several tens of nanometers is usually needed for improving contrast and reducing charging phenomenon on polymeric materials in SEM. Almost all the fiber diameters and morphology of electrospun nanofibers reported in the literatures were obtained from the sputter-coated specimens. The sputtered metal coating may cover or smear the true morphology of polymer nanofibers and exaggerate the fiber diameters. Transmission electron microscopy (TEM) is a useful tool to study the morphology and fine structure of ultra-thin polymeric materials. Because of the instability of polymeric materials in electron beam irradiation, it is a very delicate task to obtain information on the orientation with nanofibers. The reports on the orientation of polymer molecules and the structure formation within electrospun nanofibers by using TEM are limited. 15 A more detailed understanding of the electrospinning process on the 38

55 fiber formation and structure is needed for improving the mechanical, electrical, and optoelectronic properties. In this study, the morphology and internal structure of individual electrospun nanofibers were investigated by SEM, TEM, X-ray and electron diffraction techniques. Polyvinylidene fluoride was chosen as a model polymer because it has at least five different forms of crystals depending on its processing condition. The morphology of electrospun nanofiber was studied by a low voltage high resolution SEM technique which eliminates the need of a thin layer of metal coating on specimens. The orientation of crystals and polymer molecules in electrospun fibers was studied by electron diffraction technique. The annealing effect on the morphology and structure of as-spun fiber were also investigated. 4.2 Experimental Polyvinylidene fluoride (PVDF) (Kynar 741) was supplied by Arkema Inc (USA) with an average molecular weight of 250,000 g/mol and a density of 1.78 g/cm3. Acetone, DMF, and DMSO were purchased from Aldrich and used without further purification. Polymer was dissolved at 50 o C with 2 hours of stirring. The PVDF solutions were held in a glass pipette, and connected to a high voltage power supply. A flat metal collector was placed 20 cm below the tip of the glass pipette. The applied 39

56 potential difference between the tip and the collector was from 5 to 20 kv. The electrospinning process was carried out at room temperature and at atmosphere pressure. The electrospun PVDF fibers were characterized using optical microscopy (Olympus BX60) and FE-SEM (JEOL JSM-7401F). Fibers were directly spun onto a pre-cleaned silicon wafer and were studied by scanning electron microscope directly. SEM images were analyzed by using Image J Imaging Software. The structure and crystal orientation of electrospun nanofibers were examined using transmission electron microscopes, Jeol 1230 and FEI Tecnai 12. Fiber was collected on a bare TEM grid or a holey carbon membrane supported on TEM grid. WAXD patterns were recorded with a Bruker GADDS X-ray diffractometer equipped with a two-dimensional (2D) area detector using Cu Kα radiation. The random mats were rolled to make tubes of ~1 mm diameter for the X-ray measurement while the aligned samples were measured directly without rolling, as shown in Fig. 1. For all samples, 2D diffraction patterns were collected in the 2θ range of 15 50, and integrated to obtain azimuthal average intensity against 2θ plots using GADDS software package. To examine crystal orientation, the same software was used to obtain the radial average intensity of equatorial reflections versus azimuthal angle (χ) plots. 40

57 The DSC curves of the materials were measured using a TA Instruments Modulated DSC The sample was heated at 10 C/min from 25 to 200 C. All experiments were performed under a nitrogen purge. FTIR spectra were collected using a Perkin Elmer FTIR System Spectrum in the fiber mat form. 4.3 Results and Discussion Low voltage high resolution scanning electron microscopy Significant work using SEM to study synthetic polymers began in the late 1960s. An advantage associated with all SEM is the ease and speed of sample preparation. A thin layer of metal coating is usually needed to suppress charging on the polymer surface and to produce image contract. The evolution of the low voltage high resolution scanning electron microscopy (LVHRSEM) provides polymer scientists with the opportunity to topographically image structures on the order of 5 nm at 1 kv, a level of microstructure usually investigated with TEM or X-ray scattering. 53 The information limit of advanced aberration-corrected SEM has approached 1 nm at 1 KV. Low voltage operation reduces, or eliminates, the need of coating. The smaller electron-sample interaction volume at low operating voltage provides more surface topology information with better contrast. Figure 4.1 shows SEM images of porous electrospun fibers, obtained at different operating 41

58 voltages. The SEM image obtained at an operating voltage of 2 kv shows the porous structure of PVDF fibers. The larger electron-sample interaction volume at higher operating voltage averaged the signals from the fiber surface. The porous fibers appear smoother when a high operating voltage is used. We might get a wrong impression that the surface of fiber is smooth if only the image obtained at 10 kv was presented. In this study, LVHRSEM technique was used for true surface imaging. SEM operating voltage, which depends on the size and conditions of specimens, was varied from kv. 42

59 2kV 4kV 10kV Figure 4.1 SEM images of porous PVDF fibers. 43

60 4.3.2 Morphology of electrospun PVDF nanofibers Figure 4.2 shows two uniform, cylindrical PVDF fibers electrospun from a 15 wt.% DMSO solution at 7 kv. Electrospinning generally produce smooth fibers with a circular cross section if the electrical repulsion force can balance the capillary force continually before the jet is solidified and collected on the subtract. When fibers were electrospun at a high voltage or a solution with high conductivity was used, more charges were carried with the liquid jet. When local surface charge density reached a limit value at which electrical force overbalances the force from surface tension, a secondary jet will initiate from the place which has the highest local charge density. Figure 4.3a shows a branched PVDF fiber electrospun from a 15 wt.% DMSO solution at 15 kv. Figure 4.3b shows an isolated branched fiber with branches at nanometer scale. It was produced from a 3 wt% DMSO solution with 3 wt.% trifluoroacetic acid additive. When fibers were electrospun at higher voltage than 15 kv, more charges were carried with the electrospinning jet. The high local charge density on the surface of an electrospinning jet may induce splitting of the primary jet. Splitting of the charged jet is another way for the jet to reduce its charge per unit surface area by creating more surface area. Figure 4.4 shows PVDF fibers electrospun at 20 kv from a 15 wt.% DMSO solution. The split locations were enlarged and showed in the lower two images. Figure 4.5 shows TEM images of a split PVDF fiber. 44

61 Figure 4.2 SEM image of cylindrical PVDF fibers. 45

62 (a) (b) Figure 4.3 (a) SEM image of a branched PVDF fiber electrospun at 10 kv from a 15 wt.% DMSO solution and (b) TEM image of an isolated branched fiber obtained from a 3 wt.% DMSO solution with 3 wt.% trifluoroacetic acid additive. 46

63 Figure 4.4 PVDF fibers electrospun at 20 kv from a 15 wt.% DMSO solution. 47

64 Figure 4.5 TEM images of a split PVDF fiber. 48

65 PVDF fibers with wrinkled surface were also produced by electrospinning of a PVDF acetone solution. Figure 4.6 shows SEM images of PVDF fibers electrospun from (a) a 15 wt.% acetone solution and (b) a 10 wt.% acetone solution. The rapid evaporation of acetone induces phase separation in the electrospinning jet and forms solvent-rich and polymer-rich regions on the surface. The solvent final evaporates to form pores on the surface. For a variety of applications, such as tissue engineering, filtration, catalysis, and nanofiber reinforcement, it could be advantageous if the pores function as anchoring points for cells in tissue engineering, increase the surface area in filtration or catalysis, and modify the matrix-fiber coupling in fiber strengthening. Porous fibers are obtained if nanofibers of a mixture of two polymers are electrospun from the same solvent and followed by selectively removal of one component. Figure 4.7 shows porous PVDF fiber obtained by this method. A mixture of PVDF and PEO (50:50 by weight) in DMSO was electrospun into PVDF/PEO blend fibers. The porous structure was obtained after PEO was removed by suspending the blend fibers in water for 24 hours. The increased surface area per unit mass of porous fibers may be beneficial for some applications, such as filtration and catalyst applications. 49

66 (a) (b) Figure 4.6 SEM images of PVDF fibers electrospun from (a) a 15 wt.% acetone solution and (b) a 10 wt.% acetone solution. 50

67 Figure 4.7 SEM image of porous PVDF fibers 51

68 4.3.3 Structure formation in electrospun PVDF nanofibers Although PVDF has a simple chemical structure, it is well established that it can exhibit five different polymorphs depending on its processing conditions. When PVDF chains are packed into crystal lattices, their dipoles are either additive, which leads to a net dipole as in β, γ and δ phases, or canceled among themselves, resulting in no net dipole as in α and phases. 54 Among the three polar phases, the β-phase has the largest spontaneous polarization per unit cell and thus exhibits the highest piezo-, pyro- and ferroelectric activities, which endues PVDF with great potentials for various device applications. During electrospinning from solution, structure formation within polymer fibers is controlled by the simultaneous processes of the evaporation of the solvent and the extreme elongation of the solidifying fibers. The liquid jet travels from the electrospinning to the collector in a time flame of 0.1 s. The high ratio of stretching (as much as 10 5 ), as well as fast solvent evaporation and short solidification time in electrospinning process, produces polymer fibers with unique structure and properties. The effect of electrospinning processing conditions on the structure formation of PVDF electrospun nanofiber was investigated in this study. The effect of solvents on the crystal structure of electrospun PVDF fibers was studied by IR spectra. As shown in Figure 4.8, PVDF fibers electrospun from an acetone 52

69 solution had contained both alpha and beta phase crystals. For fibers electrospun from a DMF solution, the content of beta phase increased because of slow evaporation of DMF solvent and longer structure formation time. DMSO is known polar solvent to grown beta phase PVDF single crystal out of solution. Fibers electrospun from a DMSO solution contained almost pure beta phase crystals. The trace amount of alpha phase crystals remaining in electrospun fibers was due to the beads formed in the electrospinning process, as shown in Figure 4.9a. By adding a small amount of polar additive to the solution, the conductivity of DMSO solutions increased. The beaded fiber structure was eliminated and only beta phase crystals were obtained, as shown in Figure Figure 4.9b shows the bead-free PVDF fibers produced from a DMSO solution with 3 wt.% acetic acid additive. 53

70 Acetone β α β α α α Transmission (arb. unit) DMF DMSO Wavenumber (cm -1 ) Figure 4.8. FT-IR spectra of PVDF fibers electrospun from different solutions. The solid content for all the solutions is 15 wt.%. 54

71 (a) (b) Figure 4.9 SEM images of PVDF fibers electrospun from (a) a 15 wt.% DMSO solution and (b) a 15 wt.% DMSO solution with 3 wt.% acetic acid. 55

72 β α β αα α w/ 3% trichloroacetic acid Transmittance (arbi. unit) w/ 3% acetic acid no additive Wavenumber (cm -1 ) Figure 4.10 FT-IR spectra of PVDF fibers produced from 15 wt.% DMSO solutions with different additives. 56

73 4.3.4 Orientation of polymer chain in polymer fiber The chain orientation and the orientation of the crystallites in individual electrospun fibers, which is of greater significance with respect to mechanical properties, were investigated using electron diffraction. PVDF is a type of highly beam sensitive polymer. A low dose diffraction pattern was obtained from an as spun PVDF fiber with a fiber with a diameter of about 100 nm, as shown in Figure 4.11a. The (001) arc reveals the presence of orientation order of polymer chain in the as-spun fibers. Polymer molecules and crystals were aligned along the fiber axis with a certain degree of order. The diffuse arcs in electron diffraction pattern indicate small and imperfect crystals formed in rapidly solidified as spun fibers. The orientation of polymer molecules and crystallites can be enhanced by annealing the fiber and post-stretching process. After PVDF nanofibers were annealed at 150 o C for 30 min, short azimuthal arcs appeared in the electron diffraction patterns of PVDF nanofiber, as shown in Figure 4.11b. The electron diffraction of PVDF fibers obtained from a DMSO solution was indexed to be beta-form of PVDF. The annealing not only changes the internal structure of electrospun polymer fiber, it also alters the morphology of polymer fiber. Figure 4.12 shows the images of P(VDF-co-TrFE) fibers before and after annealing at 130 o C for 2 hours. The as spun fiber is smooth and uniform as demonstrated in Figure 4.12a. As shown in Figure 4.12b, annealed fiber forms plate-like lamellar crystals which are tightly linked by interlamellar tie molecules. 57

74 (a) (001) (200) (201)β (400) (b) Figure 4.11 Electron diffraction patterns of (a) an as spun PVDF fiber and (b) a PVDF fiber after annealed at 150 o C for 30 min. The insets are the fibers used to obtain electron diffraction patterns. 58

75 (a) (b) Figure 4.12 (a) TEM image of as spun P(VDF-co-TrFE) fiber and (b) SEM image of P(VDF-co-TrFE) fibers annealed at 130 o C for 2 hours 59

76 4.4 Conclusion The morphology of electrospun PVDF nanofibers were studied by a LVHRSEM. Control of electrospinning process produced fibers with various morphological forms. Fibers that were coiled, branched or split were obtained when different instabilities dominated in the electrospinning process. Fibers with wrinkled surface and porous structure were obtained and imaged. The high ratio of stretching (as much as 10 5 ) during electrospinning, like uniaxial mechanical stretching, aligns the polymer molecules along the fiber axis. The rapid evaporation of solvent during electrospinning process creates small and imperfect crystallites. These can be perfected by thermal annealing. Fibers annealed at elevated temperature forms plate-like lamellar crystals tightly linked by interlamellar tie molecules. 60

77 CHAPTER V TEM OBSERVATION OF ELECTROSPUN NANOFIBERS WITH SMALL DIAMETERS 5.1 Introduction Observing the individual building blocks of materials is one of the primary goals of microscopy. With the advancement of aberration correction technology, atomic-scale features in an ultra-thin specimen now could finally be readily imaged. Individual atoms in GaN, germanium, gold and others have been resolved by the advanced aberration-corrected transmission electron microscopes. 55, 56 In the high resolution TEM images of graphene, not only the atoms, but also the chemical bonds could be also observed. 57 Because of the very low contrast of light elements, detecting an individual low-atomic number atom, such as carbon and oxygen atoms in organic materials, is still extremely challenging. Meyer et al demonstrated a means to observe the smallest atoms and molecules on a clean graphene membrane. 58 Adsorbates, such as atomic carbon and 61

78 hydrogen, were imaged by conventional TEM if they were suspended in free space. Molecular scale carbon chain adsorbates on a graphene layer were demonstrated sufficiently stable and can be localized for characterization. To study more complex objects, such as organic or polymeric materials by this method, challenges are difficulties of preparing, isolating, and manipulating ultrathin objects for TEM observation. Electrospinning is a simple and straightforward technique for the production of polymer nanofibers. Almost any polymer, which is soluble in a suitable solvent and forms an entangled network in the solution, can be electrospun into fine fibers. Nylon fibers with diameters as small as 1.6 nm, supported on a lacey carbon film, has been obtained and imaged by a transmission electron microscope. 59 The thinner nylon fibers were also demonstrated but not measurable because only low-magnification, faint images were produced. Direct image of polymer nanofibers at high magnification is not available because of radiation damage of ultrafine nanofiber on a lacey carbon film. In this study, electrospun polymer fibers with diameters containing only a few polymer molecules were obtained from different polymer systems. A variety of support films, including continuous carbon film, holey film, and graphene layers were used for direct imaging of ultrafine polymer fibers. Ultrafine fiber suspended on a holey carbon membrane showed features from individual polymer molecules. The thinnest polymer fiber cross-section, measured from a high resolution TEM image, is 0.5 nm. Radiation damage of polymer nanofibers in transmission electron microscope was also discussed. 62

79 5.2 Experimental Materials Polyvinylidene fluoride (Kynar 741) was obtained from Alf Inc. Nylon 11, Poly(Trimethylene Terephthalate) were obtained from Scientific Polymer Inc.. DMF, DMSO, acetone, trifluoroacetic acid, hexafluoroisopropanol (HFIP) were obtained from Aldrich and used as received Electrospinning The polymer solution was held in a glass pipette, and connected to a high voltage power supply. A flat metal collector was below the tip of the glass pipette. The applied potential difference between the tip and the collector was varied from 5-10KV. The applied potential and distance between the tip of the pipette and the collector were selected to ensure that by the time the charged jet hit the collector, it was solidified. The as-spun fiber was dried at 50 o C in a vacuum to remove the residual solvent. 63

80 5.2.3 Characterization Holey carbon membranes with different hole sizes were prepared according to a method described in reference The morphology and microstructure were characterized by a JEOL model JSM-7401F scanning electron microscope (SEM), a JEOL model 1230 transmission electron microscope (TEM), and a FEI model Technai-12 TEM. High resolution images were obtained from a FEI model Tecnai-F20 TEM. For TEM experiments, electrospun fibers were collected onto a continuous or holey carbon membrane coated on 400-mesh copper grid. TEM observation was made at an acceleration voltage of 120 KV. The average fiber diameters were obtained by using a Digimizer image analysis software version The fiber diameter measurements were done at points separated by 2 μm on 10 segments randomly selected from TEM images. The total number of data points for each parameter was about Results and discussion TEM imaging of electrospun nanofiber on different support membranes The typical diameter range of electrospun fibers reported is from 10 nm to several microns. Electronspun fibers with a diameter less than 10 nm may have been produced by 64

81 many researchers but was not detected because of the low contrast of polymeric materials and the resolution limits of some instruments. Scanning electron microscopy is a routine technique for studying nanomaterials with a resolution of about 10 nm. The resolution of the traditional SEM is not high enough to image fibers with a diameter less than 10 nm. The more advanced aberration corrected SEM, such as Magellan system from FEI Company, has achieved a high resolution of 0.9 nm at 1 kv, which might be useful for the imaging of ultrafine polymer nanofibers. 61 Transmission electron microscopy (TEM) is capable of imaging at a higher resolution than the conventional SEM because of the shorter electron wavelength at higher emission energy. For most of nanomaterials, a support film or substrate is needed for high resolution TEM imaging. In high-resolution TEM, any support film provides a background signal that is most significant for the smallest objects under investigation. Figure 5.1a shows electrospun polyvinylidene fluoride (PVDF) nanofibers supported on a conventional continuous carbon membrane with a thickness of about 20 nm. Fibers with a diameter of about 5 nm are faintly seen in the image. Amorphous carbon membrane limits the capabilities of these advanced microscopes because they contribute to overall electron scattering and diminish the contrast of low-atomic number specimens. The diameter of PVDF nanofibers can not be accurately determined from this image because of the strong background noise from thick carbon support film. In this study, holey carbon membranes were used to collect and image nanofibers samples in TEM. Figure 65

82 5.1b shows electrospun PVDF nanofiber across a hole in a holey carbon membrane. The size of hole can be controlled from 10 nm to several microns. The thick carbon membrane provides a robust mechanical substrate for delicate electrospun nanofibers. A thick amorphous carbon membrane is a good electrical and heat conductor which helps dissipating heat and electrons generated by electron illumination. The fiber, suspended across holes in carbon membrane, provides an ideal specimen for studying the morphology and structure of polymeric materials at molecular and atomic level with no background noise contribution from substrates. Electrospun nanofibers are continuous fibers which have one dimension in macro scale. Polymer fiber specimens can be easily spotted and identified under TEM observation. Direct imaging of fibers supported on a lacey carbon membrane was also attempted. The lacey carbon network can only provide a limited and restricted thermal contact with electrospun nanofibers. High temperature generated by electron beam irradiation destroyed the ultra-thin fiber on a lacey carbon membrane before a focus high resolution image was obtained. 66

83 (a) (b) Figure 5.1 TEM images of electrospun PVDF fiber on different subtracts: (a) a continuous carbon membrane and (b) a holey carbon membrane. 67

84 Graphene is a single atomic layer of carbon atoms tightly packed in a two dimensional honey comb lattice. Graphene is electrically and thermally conductive, and is the strongest material ever measured. These remarkable properties make graphene the ideal support film for electron microscopy. Figure 5.2 shows a TEM image of electrospun nanofibers supported on a graphene flake. The clean areas of graphene are near invisible in this image and can be used to directly observe polymer structure at molecular and atomic level with advanced aberration corrected electron microscopes. Figure 5.2 TEM image of electrospun PVDF fiber supported on a graphene flake. 68

85 4.3.2 Radiation damage of polymer nanofibers in transmission electron microscope Polymer molecules are known as very beam sensitive. A high energy electron beam can alter organic and polymeric materials to some extent in the electron microscope. There are several reviews of radiation damage of organic materials in the TEM, but almost all of them are dealing with bulk polymers in the electron microscopy with a thickness of at least 50 nm The electrons in the beam of the TEM interact with electrons in the specimen. They typically transfer tens of electron-volts of energy to an electron at the site of the interaction. The energy transferred may excite outer electrons and cause bond ruptures in organic materials. Most of excited electrons will recombine very rapidly and reform the original local chemical structure by dissipating the absorbed energy as heat. If radiation breaks a bond in a polymer that is part of the main chain, it will undergo degradation to produce polymer with lower molecular weight. If the side groups of the polymer are ruptured by electron irradiation, the free valences may bond to another chain and form a crosslinked network. High energy electrons can also interact with atomic nuclei in the sample by knocking them out of position, which is not an important damage mechanism in polymeric material and can be alleviated by using lower operating voltage. The effect of electron beam irradiation on morphology and structure of polymer nanofibers was investigated by TEM bright field imaging and electron diffraction. Figure 69

86 5.3 shows the crystalline electron diffraction pattern from a single electrospun PVDF nanofiber with a diameter of about 100 nm. The short arc of (001) indicates a preferred alignment of polymer molecules along the fiber axis. The short arc fades, spreads, and finally becomes a ring pattern with the increasing irradiation time. The physical or chemical changes caused by electron irradiation ruins the regularity of the polymer chains and destroy any existing crystallinity in polymeric specimens. The crystals have become completely amorphous because of irradiation damage. (001) (200) (201)β (400) Figure 5.3 Electron diffraction pattern of a PVDF nanofiber. 70

87 Figure 5.4 shows electrospun nanofibers supported on a holey carbon membrane before and after 90 minutes of strong beam irradiation. The image on the right was taken at the initial stage of beam irradiation. The image on the left shows the same area irradiated with electron beam with an intensity of 650,000 electron/nm 2 /s for 90 minutes and compared with the rest of specimen. The dimension of fibers did not change much and remains about 10 nm after 90 min s beam irradiation. On the other hand, thick amorphous carbon membrane was etched and damaged by such high dose electron beam irradiation. Some part of membrane was completely damaged via mass loss. The amount of energy deposited on specimens in TEM will depend on their chemical composition and thickness, on the operating voltage of the incident beam, and on the current density at specimen plane. In carbon materials, each electron with V 0 = 60 kv loses on average about 1 ev per nanometer traversed. 63 For thick samples, the energy transferred will cause the bond rupture in polymeric materials. Low molecular weight fragments will rapidly diffuse to the surface and evaporate. If the sample is very thin (<10nm), a significant fraction of the energy transferred to it may be lost by the escape of secondary electrons to free space. Figure 5.5 shows a fiber damaged by electron irradiation during image processing, indicating bond ruptures on the polymer main chain by electron beam irradiation. 71

88 Figure 5.4 TEM images of electrospun fiber suspended on a holey carbon membrane before and after 90 minutes beam irradiation: beam intensity is about 650,000 e/nm2/s. 72

89 Figure 5.5 TEM image of a broken PVDF nanofiber damaged by electron beam. 73

90 5.3.3 Electrospinning of polymer nanofiber with small diameters Electron beam may alter the morphology and structure of electrospun nanofiber in transmission electron microscope. However, the high resolution images of electron spun nanofiber are useful for measuring the size of the polymer nanofiber with small diameters. Figure 5.6 shows the TEM images of PVDF nanofibers electrospun from DMSO solutions. Figure 5.7 shows the relationship of fiber diameter vs polymer solution concentration. Fibers with diameters ranging from 1 nm to 1 micron were produced from DMSO solution with different concentrations of PVDF. Electrospinning is a complicated process which has over a dozen processing parameters. The concentration of polymer in the solution may be the main factor influencing the fiber diameter. A jet from a more concentrated solution tends to produce a fiber with larger diameter. The average diameter of PVDF fiber produced from a 15 wt.% DMSO solution is ~820 nm (Figure 5.6a). A jet of lower PVDF concentration elongated more to form thinner fiber before it was solidified and collected on a substrate. The thinner fibers produced from a DMSO solution with 3 wt.% of PVDF are beaded fibers because of lower viscosity and therefore higher surface tension in a solution with lower solid content (Figure 5.6c). PVDF solution with a solid content less than 3 wt.% produced only nanoparticles by electrospraying. Many salts or acid which are soluble in organic solvents were used to increase the electrical conductivity of the solution to reduce the bead concentration in polymer fiber and increase the solution concentration processing windows. The salts dissolved in the 74

91 polymer solution increase the electrical conductivity so the electrospinning liquid jets could carry more currents. The electrical repulsion was increased by the higher charges density on the surface of liquid jet. The jets can be elongated more with higher repulsion force on the surface to produce fibers with smaller diameters. The bigger beads were no longer observed when a small percentage of acid was added to the solution. Figure 5.6d shows uniform, ultrafine polymer nanofibers produced from 3 wt.% PVDF solution by adding 3 wt% of trifluoroacetic acid. Figure 5.8 shows the ultra-thin PVDF nanofiber produced from DMSO solution with 1 wt.% of PVDF and 3 wt.% of trifluoroacetic acid. The small cross-section indicated by a black arrow in image 6a is only 0.8 nm, which contains only 2 or 3 polymer molecules. And, the small necking indicated in Figure 5.7b has a cross-section of 0.5 nm. The diameter of a polyethylene was measured to be 0.4 nm. The chemical structure of PVDF is similar with polyethylene and should have a similar molecular diameter. Figure 5.9 shows the possible arrangement of polymer molecules in several ultrafine PVDF nanofibers with small diameters. 59 Ultrafine fibers with diameters less than 10 nm were also produced from other polymer systems, including nylon, PTT, and polyethylene oxide. Thin fibers with a diameter in a couple nanometers range were electrospun by optimizing the concentration of polymer in a good solvent to this polymer. Figure 5.10 shows the diameter of electrospun PTT fiber vs polymer concentration in HFIP solutions. Ultrafine fiber with an average diameter of 3 nm was produced from HFIP solution with 0.5 wt.% PTT. 75

92 (a) (b) (c) (d) Figure 5.6 TEM images of PVDF nanofibers electrospun from DMSO solutions with different PVDF solid contents: (a) 15 wt.%. (b) 5 wt.%, (c) 3 wt.% and (4)3 wt.% with 2 wt.% trifluoroacetic acid additive. 76

93 Diameter of fibers (nm) no additive with 2% trifluoroacetic acid PVDF concentration (%wt) Figure 5.7 Diameter of PVDF nanofibers versus the weight percent concentration of PVDF in DMSO solution. 77

94 0.8 nm (a) 0.5 nm (b) Figure 5.8 TEM images of ultra-thin carbon fibers. 78

95 Figure 5.9 Schematic diagram of several possible cross-sections of ultra-thin diameter PVDF nanofibers

96 Fiber Diameter (nm) PTT concentration in HFIP (%wt.) Figure 5.10 Diameter of PVDF nanofibers versus the weight percent concentration of PVDF in DMSO solution. 80

97 5.3.4 High resolution images of ultrafine PVDF nanofibers High resolution transmission electron microscopy has revealed nearly atomically precise images of single hydrocarbon molecules confined in carbon nanotubes. 65 In addition to individual atoms, the dynamics of a variety of molecular scale adsorbates from the vacuum contamination were also observed by conventional transmission electron microscope. The individual organic molecules were proved to be extremely electron beam stable under controlled beam conditions. Electrospinning provides a simple and unique way to produce nanofiber containing a few polymer molecules from a wide range of materials. Fibers suspended across holes in carbon membranes are ideal specimens for studying polymer materials at the molecular scale. The combination of the high beam stability of ultrathin polymer nanofibers and the absence of an amorphous background signal enables remarkable light-atom sensitivity in TEM. Figure 5.11 shows high resolution TEM images of two electrospun nanofibers suspended on a holey carbon membrane. The enlarged picture shows features from individual polymer molecules. The diameter of the PVDF nanofiber shown in the high resolution image is about 3.5 nm. The line features on the small nanofibers are mostly alignment along fiber axis, which is different from anisotropic morphology in amorphous carbon membrane in the same image. Figure 5.12 show one of the possible arrangements of polymer molecules in a PVDF nanofiber with a diameter of 3.5 nm. About 40% of polymer 81

98 molecules are on the surface, indicating a very high surface area per unit mass ratio. Figure 5.13 shows another example of high resolution TEM images of ultrafine PVDF nanofibers. The smallest part of the ultrafine nanofiber is about 0.5nm which can hold only one polymer molecule. If each line feature resembles one polymer molecule, we can count the total number of polymer molecules in this electrospun nanofibers with small diameters. The conventional TEM has an information limit which can t resolve the atomic structure of organic or polymer molecules. Direct imaging of the atomic configuration of polymer molecules in ultra-fine nanofibers is being attempted using a spherical aberration-corrected transmission electron microscope. 82

99 Figure 5.11 High resolution images of ultrafine PVDF nanofibers. 83

100 e nm Figure 5.12 One of the possible arrangements of polymer molecules in a PVDF nanofiber with a diameter of 3.5 nm. 0.5 nm 84

101 Figure 5.13 High resolution TEM image of an ultrafine PVDF nanofiber. 85

102 5.4 Conclusion A sample preparation method for the preparation of ultrathin polymeric specimens for high resolution TEM imaging was developed. Polymer nanofibers with a diameter ranging from 1 nm to several microns were produced by optimizing the polymer solution concentration and adding organic acid or salt as an additive. Polymer nanofibers are continuous filaments with one dimension in the macro-scale. The polymer fibers specimens can be easily spotted in TEM. The combination of the high beam stability of ultrafine polymer specimens and the absence of amorphous background enables the unprecedented sensitivity of polymer molecules in TEM. The high resolution TEM images show the features of individual polymer molecules in electrospun PVDF nanofibers. The advanced aberration-corrected electron microscope may resolve the atomic configuration of polymer molecules. 86

103 CHAPTER VI STRUCTURE OF MESOPHASE PITCH-BASED CARBON FIBERS PRODUCED BY A NGJ PROCESS 6.1 Introduction Carbon fibers are light weight, strong, and stiff synthetic fibers with long aromatic molecular chains, comprising mainly carbon. Carbon fibers have proved to be an excellent reinforcement for advanced composites for a wide range of applications since the 1960s. Originally used in aerospace, carbon fiber has moved into the mainstream and can be found in luxury automobiles, mountain bikes, electronics, and sports equipments with the development of high performance carbon fibers at lower cost. Commercial carbon fibers are fabricated by using pitch or polyacrylonitrile (PAN) as the precursor. PAN-based carbon fibers account for more than 70% of the world total carbon fiber production and are used in the majority of current advanced composites. 36 Pitch-based carbon fibers represent growing fraction of the total carbon fibers produced in recent 87

104 years due to the lower cost and higher carbon content of pitch compared to PAN. Carbon fibers produced from mesophase pitch possess a high modulus, good electrical and thermal conductivity in an oriented fibrillous microstructure obtained during spinning process. Mesophase pitch-based carbon fibers are still relatively expensive due to the high processing cost. Large efforts have been made to develop fiber production technologies for low cost mesophase pitch based carbon fibers for a variety of 35, 66, 67 commercial applications. Carbon nanofibers belong to the new class of superior engineered materials because of their exceptional mechanical, electrical properties and high specific surface area. Vapor grown carbon nanofibers are produced by catalytic decomposition of a hydrocarbon at o C in the presence of a transition metal or metallo-organics such as ferrocene, (C 5 H 5 ) 2 Fe. 68 They can be produced commercially, in large quantities, at a lower cost than the production of single wall carbon nanotubes. Several varieties of vapor grown carbon nanofiber with diameter under 200 nm are commercially available. Vapor grown carbon nanofibers are characterized by a high tensile strength (12 GPa) and a high Young s modulus (600 GPa) that is approximately 10 times that of steel. 69 The structure of vapor grown carbon fiber is independent of the precursor gas source employed, but is extremely dependent on processing parameters, such as temperature of growth and type of catalyst. Electrospinning is another emerging technique to produce continuous nanoscale carbon fibers with superior mechanical strength. 70 Unlike conventional fiber spinning methods, 88

105 electrospinning utilizes electrical forces to create fibers ranging from a few nanometers to tens of micrometers. 3, 6 Carbon nanofibers with diameters of nm were produced 71, 72 from electrospun PAN copolymer nanofiber precursor. The tensile strength and Young s modulus of carbon nanofibers produced by electrospinning technique were in the ranges of 300 to 600 MPa and 40 to 60 GPa, respectively. 71 The mechanical properties of carbon nanofibers produced by electrospinning were reported to be enhanced by the post-spinning stretching process. 72 The exceptional mechanical and electrical properties of carbon nanofibers are being explored in a variety of ways to impart functionalities. Applications include nanocomposite filler, electrode materials, catalyst support, and scanning probe microscopy tips. A new class of carbon fibers with diameters in the range of 100 nm to several microns was prepared from mesophase pitch by using a nanofiber by gas jet (NGJ) process. The NGJ process is a patented technique, which uses a high velocity hot gas stream to elongate molten mesophase pitch as it passes through a specially designed nozzle. 5 NGJ is a promising technique to produce carbon fibers with valuable structural, thermal, and electrical properties at cost levels as much as an order of magnitude less than current processes. The mesophase pitch-based carbon fibers made by the NGJ process have controlled fiber diameter and high length/diameter aspect ratios. The microstructure and properties of pitch-based carbon fibers have been 33, investigated by a number of researchers. Pitch based carbon fibers primarily 89

106 consist of axially oriented carbon layer planes having different transverse microstructures. The fibers with high strength and elongation to failure were found to be composed of turbostratic carbon structure, which was different from the three dimensional graphite structure in ultra-high modulus carbon fibers. 74 In this study, the surface texture and internal structure of NGJ carbon fibers were investigated by field emission scanning electron microscopy, transmission electron microscopy, electron diffraction, X-ray diffraction, and Raman spectroscopy. The effect of thermal treatments on the microstructure of NGJ carbon fibers was studied as a function of heat treatment temperatures. Ultrasonic exfoliation in a liquid and a chemical oxidation approach were used to reveal the internal structure of NGJ carbon fibers. 6.2 Experimental Materials Sulfuric acid, 1-methyl-2-pyrrolidinone (NMP), tetrahydrofuran, hydrogen peroxide, and potassium permanganate were purchased from Aldrich Inc. Mesophase pitch was supplied by Union Carbide Inc. The mesophase based carbon fibers were produced by a nanofiber by gas jet process. Mesophse pitch was melt-spun at 300 o C using a lab-scale gas jet spinning apparatus. The as-spun pitch fibers were stabilized at 250 o C for 2 hours. 90

107 The stabilized fibers were heated in a furnace in an inert atmosphere to 1500 o C or 3200 o C, respectively, to induce carbonization or graphitization of pitch fibers KMnO4 oxidation treatment A chemical oxidation approach was used to prepare graphene oxides from graphitized NGJ carbon fibers. The NGJ carbon fibers were suspended in concentrated sulfuric acid for 2 hours and then treated with 1000 wt% potassium permanganate. The reaction mixture was stirred for 24 hours and then poured over water containing 2 wt.% hydrogen peroxide. The resulting solution was filtered and washed with acidic water followed by ethanol. The treated fibers were dried in vacuum oven at 50 o C for 24 hours. A uniform dispersion of graphene oxide sheets was obtained by ultrasonicating a 1-methyl-2-pyrrolidinone suspension of the resulting sample for 60 min, followed by centrifugation at 1000 rpm for 30 min Characterization The morphology and microstructure were characterized by a JEOL model JSM-7401F scanning electron microscope (SEM) and a JEOL model 1230 transmission electron microscope (TEM). All samples were observed by a low voltage high resolution 91

108 SEM technique without sputter-coating of specimens. For TEM observation, the specimens were prepared by placing a drop of NMP suspension of NGJ carbon fibers or graphene oxide sheets onto a holey carbon membrane supported on a 400-mesh copper grid. TEM observations were made at an acceleration voltage of 120 KV. The microstructure of the carbon fibers was observed in bright field imagies and selectived area electron diffraction. The structure of the samples was observed using wide angle X-ray diffraction (WAXD). Powder methods were used to analyze the diffraction patterns of the samples. 6.3 Results and Discussion Figure 6.1 shows a typical SEM micrographic image of NGJ carbon fiber. Fibers produced by the NGJ process have a high aspect ratio with diameters ranging from a few hundred nanometers to several microns. The average diameter of fibers shown in Figure 6.1 is ~540 nm, which is an order of magnitude smaller than that of pitch based carbon fiber spun by a conventional melt-spinning apparatus. The lengths of carbon fiber vary from tens of micron to a few tenths of a millimeter. The larger diameter NGJ fibers are typically much longer. The length, diameter, and distribution of the diameters of NGJ carbon fibers can be adjusted by changing process parameters. 92

109 Figure 6.1 A typical SEM image of carbonized NGJ nanofibers. 93

110 Figure 6.2 shows the lateral surfaces and the cross-sectional areas of NGJ fibers heated at different temperatures. Mesophase pitch carbon fibers are based on the liquid crystal characteristics of the mesophase pitch precursor. The molecules that constitute the liquid crystal pitch are readily oriented during the NGJ spinning process and can result in highly oriented internal structure of carbon fibers. As shown in Figure 6.2a, a smooth surface with slight fibril texture was presented on the lateral surface of stabilized pitch fibers. High resolution-sem micrographs of the longitudinal surfaces of the carbonized and graphitized NGJ fibers revealed the fibril units. Heat treatment at 1500 o C caused rough surface to develop. A layered texture developed in the cross-section of a carbonized fiber (Figure 6.2b). A well-developed layered graphitic structure in NGJ fibers heated to 3200 o C was clearly observed in Figure 6.2c. Radical carking along the fiber axis, which is often seen in commercially available carbon fibers, was not observed in NGJ carbon fibers. Mesophase pitch-based carbon fiber are observed to have one of several different cross-section structures such as radial, layered, random structures, and complicated structure of these three. The structure is affected by process conditions and shapes of die. Radial and layered structures are reported to exhibit higher tensile properties than more random structures because of the higher orientation. 33 Carbon fibers produced by the NGJ process are expected to have higher tensile property because of their highly ordered layered structure and a smaller number of crakes in the fibers. 94

111 (a) 95

112 (b) (c) Figure 6.2 SEM images of individual NGJ carbon fibers heated at different temperatures: (a) 250 o C, (b) 1500 o C, and (c) 3200 o C. The crystalline structure of NGJ fibers heat treated to various temperatures was characterized using wide angle X-ray diffraction technique. Figure 6.3 shows the X-ray diffraction patterns of NGJ fibers. The interlayer spacing d 002 was determined using the Bragg equation, while the average crystallite size was determined using the Scherrer equation. 71 The d spacing decreases from a turbostratic spacing of 0.35 nm for carbonized fiber to a graphitic spacing (0.34 nm) at the high heat treatment temperature of 3200 o C. 96

113 The wide interlayer spacing peak in carbonized NGJ fiber is due to the distorted microstructure, including disordered stacking of graphene layers, curved graphene layers, and other structure defects. The crystallite thickness increased with the increasing processing temperature. Graphitized fibers prepared at 3200 o C have larger crystal size of 15.0 nm compared with a crystal size of 3.6 nm for carbonized fibers. carbonized NGJ fiber Intensity(a. u.) after oxidative treatment graphitized NGJ fiber after oxidative treatment θ Figure 6.3 X-ray diffraction patterns of NGJ carbon fibers before and after oxidative treatment. 97

114 Figure 6.4 shows TEM bright field images of NGJ carbon fibers. The image on the left is from a fiber that was heated to 1500 ºC. It shows the early development of imperfect sheet like structures. The fiber on the right was graphitized at 3200 ºC. Torn graphene sheets extend from the broken end of the fiber. The rows of dark spots are Bragg diffraction contours which indicate that the long graphene sheets are slightly wrinkled. Figure 6.5 shows the electron diffraction patterns from individual NGJ carbon fibers. The (002) arc from the carbonized fiber indicates a little scattered preferred orientation; the orientation of the net plane of carbon to the fiber axis has some degree of distortion. Graphitized fiber has a spot-type orientation of (002), which indicates a high degree of preferred orientation of carbon layers arranged in parallel to the fiber axis. Figure 6.6a demonstrates the internal structure of graphitized fibers after liquid-phase exfoliation in NMP by ultrasonication. Dispersion and exfoliation of graphite in organic solvents, such as NMP, is a known method to produce single layer graphenes. 76 Because NMP has a high surface energy, the solvent-graphene interaction is comparable to those existing between the stacked graphenes in graphite. The layered structure of graphenes in graphitized NGJ fibers was revealed after some of graphene layers were peeled off from NGJ graphitized carbon fibers by liquid phase exfoliation, as evidenced in figure 6.6b. Ultrasonic treatment in NMP did not produce observable changes in carbonized NGJ fibers because energy applied by ultrasonication is not large enough to break the bonding in turbostratic carbon layers. 98

115 Figure 6.4 TEM image of a typical carbonized (left) and graphitized (right) NGJ carbon fiber. 99

116 (a) (b) Figure 6.5 Electron diffraction patterns of (a) carbonized and (b) graphitized NGJ nanofibers. 100

117 (a) (b) Figure 6.6 TEM images of fragments of graphitized carbon nanofibers ultrasonicated in NMP for 30 min. 101

118 Raman spectroscopy is another powerful tool to study microstructures. Figure 6.7 shows Raman spectra of carbonized and graphitized NGJ fibers. A typical spectrum of carbon fibers reveals two peaks at 1580 and 1330 cm -1. D-band centered at the wavenumber of 1330 cm -1 that is related to disordered turbostratic structures of carbon material, and G-band at 1580 cm -1 is related to ordered graphitic structures. The intensity ratio (known as R-value) of the D-band to G-band indicates the amount of structurally ordered graphite crystallites in the carbonaceous materials. It was evident that the R-values of NGJ graphitized fibers is much lower than that of carbonized fibers, indicating disordered carbonaceous components were converted into more ordered graphite crystallites when heated at 3200 o C. Along with the transmission electron micrographic bright field image, electron diffraction, and X-ray diffraction data, it may be concluded that NGJ fibers treated at 3200 o C is characterized by a high degree of preferred orientation and a three dimensional graphitic structure, and fiber treated at 1500 o C has a crystal structure of turbostratic carbon layers with a distorted orientation along the fiber axis. 102

119 NGJ graphitized fiber Normalized Intensity (a.u.) NGJ carbonized fiber Wavenumber (cm -1 ) Figure 6.7 Raman spectra of NGJ carbon fibers Graphene has emerged as an exciting two dimensional material showing great potential for the fabrication of nanoscale devices. Several mechanical, chemical, and synthetic procedures have been developed to produce microscopic samples of graphene or graphene oxide form different starting materials, including pristine graphite, multiwalled carbon nanotubes, and vapor grown carbon nanofibers Graphitic NGJ fiber possesses a well-developed layered graphene structure after heat treatment at a temperature of 3200 o C. It could be another useful starting material to produce graphene or graphene oxide microscopic samples. Graphene oxide flakes were produced from graphitized NGJ 103

120 carbon fibers by a solution-based oxidative process introduced by Kosynkin et al. 81 Figure 6.8 shows graphene oxide flakes produced by a combination of oxidative treatment and ultrasonic radiation. Submicron-sized graphene oxide flakes contain graphene oxide sheets with a d spacing of 0.88 nn, as evidenced by XRD spectra in Figure 6.3. The increase of the interlayer spacing in graphene oxide sheets is due to the introduction of carbonyls, carboxyls, and hydroxyls groups at the edges and the surface of graphene layers in graphitized NGJ carbon fiber by oxidative treatment. 81 Figure 6.9 shows a graphene oxide flake prepared by oxidative treatment, which contains only a few layers of graphene oxide sheets. The inset is the electron diffraction from this graphene oxide sheet. The array of diffraction spots from this isolated graphene oxide sheet shows the regular graphene structure of the sheet. Oxidative treatment of carbonized NGJ fiber produces only non-crystallite turbostratic carbon sheets, as shown in Figure No electron diffraction spots or X-ray diffraction peaks were detected from microscale carbon sheets or from bulk power samples prepared by oxidative treatment of NGJ carbonized fibers. 104

121 Figure 6.8 TEM image of a typical dispersion of graphene oxide sheets produced from graphitized NGJ carbon fibers by oxidative treatment. 105

122 Figure 6.9 A graphene oxide flake containing only a few graphene layers. The inset is the electron diffraction patter from this isolated sheet. 106

123 Figure 6.10 SEM image of carbon sheets produced by oxidative treatment of carbonized NGJ fibers. 107