DEVELOPMENT OF CARBON NANOFIBERS FROM ELECTROSPINNING
Lifeng Zhang, Spero Gbewonyo, Alex Aboagye, Ajit D. Kelkar Joint School of Nanoscience and Nanoengineering, North Carolina A&T State University, Greensboro, NC, United States
CHAPTER OUTLINE 1 Carbon Fiber Production ..................................................................................................................867 2 PAN-Based Carbon Nanofibers From Electrospinning ......................................................................... 868 2.1 Electrospinning of PAN .................................................................................................. 868 2.2 Carbonization of Electrospun PAN Nanofibers .................................................................. 870 2.3 Current Advances on Carbon Nanofibers From Electrospun PAN ........................................ 870 3 Exploration of High Strength Carbon Nanofiber Yarns From Electrospinning PAN .................................871 4 Conclusions ....................................................................................................................................874 Acknowledgments ...............................................................................................................................875 References ......................................................................................................................................... 875
1 CARBON FIBER PRODUCTION Carbon fibers are of great technological and industrial importance because of their comprehensive physical properties such as high strength-to-weight ratio, excellent chemical resistance, and superior electric and thermal conductivity [1, 2]. They have been especially used for high-performance fiberreinforced polymer composite material in automotive, aerospace, and sport industries. Activated carbon fibers (ACFs) fall into another category of carbon fibers that bear high specific surface area, which are found mostly in applications like gas adsorption/storage and water treatment [3, 4]. Generally, there are two approaches to produce carbon fibers: vapor growth and spinning techniques. Synthesis of carbon fibers from vapor growth was explored in the 1970s and 1980s [5, 6]. These carbon fibers were acquired from catalytic decomposition of certain hydrocarbons in the presence of catalyst such as metal particles. However, the development of carbon fibers from this route encountered big difficulties in mass production. Spinning thus became the most commonly used route for carbon fiber production, which comprises steps of spinning of polymeric precursor fibers and following thermal treatment. Although any material with carbon atom on its macromolecular backbone can be used as Nanotube Superfiber Materials. https://doi.org/10.1016/B978-0-12-812667-7.00033-1 Copyright # 2019 Elsevier Inc. All rights reserved.
CHAPTER 33 DEVELOPMENT OF CARBON NANOFIBERS
precursor of carbon fibers in theory, carbon fibers are normally produced from three polymeric precursors: polyacrylonitrile (PAN), cellulose, and pitch. Among these three precursors, PAN has received major attention due to its high carbon yield and superior mechanical properties of the resultant carbon fibers. Actually, PAN is the precursor for about 90% of carbon fibers that are manufactured today . It is noteworthy that carbon fibers with the highest mechanical strength have been produced exclusively from PAN copolymer precursors containing 0.5–8 wt% comonomers such as acids (e.g., itaconic acid) and vinyl esters (e.g., methyl methacrylate) . The inclusion of comonomers partially disrupts nitrilenitrile interactions in PAN macromolecules, enables higher solubility of PAN in spinning solvent, allows better macromolecular chain orientation in as-spun fibers, and makes final carbon fibers more structurally homogeneous through stabilization and carbonization. Spinning of PAN is conventionally done by wet spinning and dry-jet wet spinning, while dry spinning and even melt spinning are also available . In wet spinning, a spinneret that contains orifices with diameters in the range of 40–80 μm is immersed in a coagulation bath, and a spin dope, that is, PAN solution, is extruded directly into the coagulation bath to form jets or filaments. In dry-jet wet spinning, a spinneret is positioned a few millimeters above a coagulation bath, and jets or filaments are extruded vertically into the bath. The dry-jet wet spun fibers exhibit finer linear density and higher strength and therefore become more popular. To make carbon fibers, PAN precursor fibers are firstly stabilized under tension through a controlled heating program normally in air between 200°C and 300°C in order to convert PAN to a ladderlike molecular structure, which ensures these precursor fibers to go through further processing at higher temperature. Stabilized PAN fibers are subsequently converted to carbon fibers in process of carbonization that involves heat treatment in an inert atmosphere up to 1500°C. In the carbonization process, almost all elements other than carbon are eliminated in the form of a variety of by-products, and a graphite-like molecular structure is formed. The carbon fibers that are produced from the wet or dry-jet wet spinning typically have diameters ranging from a few to a couple tens of micrometers. With widespread interest in nanomaterials in recent years, interests of making carbon fibers with diameters falling into submicrometer and nanometer range, that is, carbon nanofibers, have been growing. The production of carbon nanofibers falls into the same two categories as their conventional counterparts: vapor growth and spinning. The approach of vapor-phase growth, that is, catalytic synthesis, has been investigated [8–10], and graphitic carbon nanofibers are obtained from carbon-containing gases by using metallic catalysts. Nonetheless, these carbon nanofibers are relatively short and difficult to be aligned, assembled, and processed into applications. In the meantime, low product yield, expensive manufacturing equipment, and significant amount of catalyst residue are problematic. Thus, spinning method, particularly electrospinning, has attracted much more attention these days for carbon nanofiber production.
2 PAN-BASED CARBON NANOFIBERS FROM ELECTROSPINNING 2.1 ELECTROSPINNING OF PAN The rapidly developing technique of “electrospinning” provides a straightforward way to make continuous carbon fibers at submicrometer and nanometer scale (typically 100–1000 nm), which is approximately two to three orders of magnitude smaller than conventional carbon fibers [11–14]. Like conventional carbon fiber production, PAN is the most often used precursor polymer for carbon
2 PAN-BASED CARBON NANOFIBERS FROM ELECTROSPINNING
nanofiber fabrication from electrospinning. Unlike conventional fiber spinning techniques, electrospinning is driven by electric force instead of mechanical force and follows a different thinning mechanism. Extensive research has been conducted on electrospinning of PAN during the last decade [15–18]. In the process of electrospinning, a droplet of PAN solution at the tip of spinneret deforms from the spherical shape caused by surface tension alone to a cone-like shape (termed as Taylor cone) when it is exposed to an electric field . As the applied electric force on the Taylor cone reaches a critical value and overcomes surface tension and viscoelastic force of the PAN droplet, a jet of PAN solution ejects from the tip of Taylor cone, and electrospinning begins. The PAN solution jet follows a bending, winding, and spiraling path in three dimensions and becomes thinner and thinner with the increase of loop circumference (Fig. 1). This phenomenon is referred to as “bending (or whipping) instability” [20–23], which is the dominant thinning mechanism in electrospinning. Typically, the bending instability in electrospinning causes length of a PAN solution jet to elongate by more than 10,000 times in 50 ms or less with concurrent thinning. Thus, elongation or drawing rate of the PAN solution jet during the bending instability is extremely large (up to 1,000,000 s 1 ). Such enormous drawing rate, which is not accessible from other methods, can effectively stretch PAN macromolecular chains in the solution jet and closely align them along axis of the resultant nanofiber . Due to the increase of surface area from the thinning of the PAN solution jet, over 99% solvent can be removed from the PAN solution jet during or shortly after bending instability. In this case, macromolecular orientation of PAN in the electrospun nanofibers is likely to retain. Nonetheless, the chaotic trajectory of the PAN solution jet makes electrospun PAN nanofibers very difficult to form ordered and/or aligned assemblies and intrinsically results in a nonwoven mat that is composed of randomly deposited/stacked PAN nanofibers. Due to ultrahigh specific surface area, electrospun PAN nanofibrous mats (or other equivalent names such as felts or membranes) have seen extensive applications in the fields of adsorption/filtration/separation [25, 26] and catalysis [27, 28].
FIG. 1 Schematic diagram of electrospinning PAN including basic electrospinning setup, Taylor cone, and bending instability.
CHAPTER 33 DEVELOPMENT OF CARBON NANOFIBERS
2.2 CARBONIZATION OF ELECTROSPUN PAN NANOFIBERS Similar to conventional carbon fibers, carbon nanofibers have been successfully prepared by electrospinning PAN followed by a two-step heat treatment: stabilization and carbonization. A variety of stabilization and carbonization conditions have been reported for electrospun PAN nanofibers. The stabilization was carried out in air at temperatures between 200°C and 300°C, while the carbonization was further performed in inert atmosphere up to 2800°C [29–34]. In order to reduce mass loss and dimension shrinkage in the final carbon nanofibers, progressive and multistage heating of electrospun PAN nanofibers has been adopted (Fig. 2). The progressive stabilization and carbonization with successive heating from 30 to 230°C at 5°C/min, from 230 to 270°C at 1°C/min, and then from 270 to 800°C at 5°C/min led to little change in fiber packing, much less planar dimensional shrinkage, and significant increase of carbon yield  compared to the reported procedure in which the stabilization was carried out at 200°C for 30 min, followed by the carbonization at 750°C for 1 h .
2.3 CURRENT ADVANCES IN CARBON NANOFIBERS FROM ELECTROSPUN PAN In spite of their potential to become high-strength fibers due to their smaller sizes and the drawing feature in electrospinning process, individual carbon nanofibers from electrospinning generally exhibit very weak strength. This is because PAN macromolecular chains in electrospun nanofibers, especially in the presence of some trace amount of solvent, may relax to some extent after depositing on collector and lose their formerly drawing-lead orientation. It is well known that many structural imperfections such as diametric bulges, cavities, cracks, and disordered structures in carbon precursor fibers are likely to retain in the resulting carbon fibers, and the amount, size, and distribution of structural imperfections in carbon fibers determine their mechanical strength . The loss of macromolecular orientation in final electrospun PAN nanofibers may be the main reason that causes inferior mechanical properties of the resulting carbon nanofibers. As for nonwoven mat of carbon nanofibers, only a small amount of carbon nanofibers share tensile stress inside the nonwoven mat due to their random depositing and stacking feature, and these nanofibers are easily separated from each other at junction points under stress when an external force is applied to the nonwoven mat. Therefore, the mechanical strength
FIG. 2 Representative SEM images of electrospun PAN nanofibers from 8 wt% PAN solution in N,N-dimethylformamide (DMF) (A); carbon nanofibers derived from (A) with a two-step heating: 200°C for 30 min and 750°C for 1 h (B); carbon nanofibers derived from (A) with a multistep progressive heating: 5°C/min from 30 to 230°C, 1°C/min from 230 to 270°C, and then 5°C/min from 270 to 800°C (C) .
3 EXPLORATION OF HIGH STRENGTH CARBON NANOFIBER YARNS
of either individual carbon nanofibers or their nonwoven mats is inevitably weak . Up-to-date carbon nanofibers from electrospun PAN are restricted to those applications that do not rely on mechanical properties but rely on their superior physical properties such as high specific surface area, great electric conductivity, and good biocompatibility.
3 EXPLORATION OF HIGH STRENGTH CARBON NANOFIBER YARNS FROM ELECTROSPINNING PAN The research in developing strong carbon nanofibers from electrospun PAN, however, does not fade out. Instead, it has become a point of interest due to the potential to overcome current technological obstacles to further improve mechanical strength of carbon fibers . In current fiber industry, there are consecutive steps that are needed for the preparation of conventional textile fibers including but not limited to spinning, drawing, and annealing. Drawing and annealing operations are very important to get strong industrial fibers. The drawing aligns polymer molecules in the direction of the long axis of fiber, and the annealing increases polymer crystallinity in fiber. Polymer fibers are significantly reinforced with well-developed molecular orientation and crystallization. Therefore, postspinning stretching may be required for electrospun PAN nanofibers to improve their macromolecular alignment and reach higher mechanical strength for consequent high-strength carbon nanofibers especially when consider the loss of macromolecular orientation of PAN in final PAN nanofiber product from electrospinning. It is easy to handle postspinning processes (such as stretching) of polymer fibers whose diameters are generally in the range of 20–200 μm from conventional spinning techniques. Single PAN nanofiber from electrospinning, however, has a diameter from 100 to 1000 nm, which is too thin and too fragile for postspinning handling. Aligned PAN nanofiber bundle or yarn that is composed of hundreds or thousands of nanofibers may outperform either single nanofiber or nonwoven nanofibrous mat from the point of view of enough mechanical property to sustain postspinning handling. A number of attempts have been taken to improve order and/or alignment of electrospun nanofibers and to prepare nanofiber bundles or yarns [37, 38]. Among these attempts, there are generally two strategies to collect nanofiber bundles or yarns (Fig. 3). The first one is to use dynamic collector setup such as rotating drum, wire drum, or thin disk [39–44]. When the collector rotates at high speed, electrospun nanofibers start to align along the rotating axis of the collector. The other one is to manipulate electric field profile between the tip of spinneret and collector by using modified electrodes such as parallel electrode, array of counter electrode, ring electrodes, or two oppositely charged spinnerets [45–53] to control the trajectory of electrospinning jet and make aligned nanofibers or nanofiber yarns. These two strategies can also be combined to achieve greater order in fiber assembly . Although aligned electrospun nanofiber bundles and yarns have been acquired from these setups, the alignment of nanofibers is gradually reduced and eventually becomes random after some time when a large amount of nanofibers deposit onto the collector, which is a result from repulsive force caused by accumulation of residual charges on the deposited fibers. It is noteworthy that the acquired nanofiber bundles or yarns from these methods lack length and productivity. It is more noteworthy that these aligned nanofiber bundles or yarns only showed limited improvement in their mechanical properties. In the process of electrospinning, a single continuous nanofiber without breaking can be produced if electrospinning conditions are judiciously selected and adjusted. Under the condition that multiple
CHAPTER 33 DEVELOPMENT OF CARBON NANOFIBERS
FIG. 3 Basic methods for controlling the alignment of electrospun nanofibers: (A) high-speed rotating drum collector, (B) tapered wheel, and (C) stationary gap collector [37, 38].
FIG. 4 Schematic diagram of the flowing water bath setup to collect PAN nanofiber yarns.
spinnerets are placed above a fast-moving collector, a continuous fiber bundle that is comprised of loosely oriented PAN nanofibers and possesses desired morphological and structural properties could be obtained. After PAN nanofibers in the bundle are straightened and stretched, a highly aligned PAN nanofiber bundle or yarn can be readyfor subsequent stabilization and carbonization. Nanofibrous yarns have been collected using liquid [55–57], solid substrate , or even in the air . Our research group explored a new way to collect PAN nanofiber yarns in which multiple electrospinning spinnerets were placed above a continuous flowing water bath and PAN nanofiber yarns were taken up from the water during the electrospinning process (Fig. 4). The use of flowing water bath as collecting medium has several advantages: (1) Water will not attract electrospun PAN nanofibers as much as solid substrates, and PAN nanofiber bundles or yarns can be easily lifted off and then drawn to a rotating mandrel for take-up. (2) When flowing water is used as collector, not only the high surface tension of water assists in consolidating and tightening a PAN yarn, but also the water flow can help to straighten and stretch PAN nanofiber yarns.
3 EXPLORATION OF HIGH STRENGTH CARBON NANOFIBER YARNS
(3) Controlling water flow is relatively easy, and PAN nanofiber yarns’ morphology can be adjusted according to the water flow. (4) Water is environmental friendly, and the whole collecting process can be carried out without much safety and environmental concern. (5) Using water as collector will not generate any problem for the subsequent drawing step in water. In this exploration, PAN nanofiber yarns from multineedle electrospinning setup were taken up from the flowing water bath and collected on a rotating wood spindle. The take-up of yarns from the flowing water bath was stable and continuous for hours without breaking. Drawing of the PAN nanofiber yarns was performed through multiple steps afterward. The initial drawing of PAN nanofiber yarns was done in 50–60°C water to up to 50% elongation followed by drawing in 90–100°C water to up to 200% elongation. Then, the stretched PAN nanofiber yarns were dried in 100°C air and saved for further heat treatment. Stabilization and carbonization processing conditions of the stretched PAN nanofiber yarns were adopted from that of carbon nanofibers as derived from previous research . Specifically, the stretched PAN nanofiber yarns were stabilized with tension in a box furnace with a constant flow of air according to the following heating program: the yarns were heated at a rate of 1°C/min from 30 to 280°C and held there for 6 h. Next, the stabilized PAN nanofiber yarns were carbonized in a constant flow of nitrogen at a rate of 5°C/min to 1000°C and maintained at this temperature for 1 h. Compared to regular electrospun PAN nanofibrous mat, as-collected PAN nanofiber yarn from the flowing water bath showed quite good fiber alignment (Fig. 5). Over 60% PAN nanofibers aligned with respect to the yarn axis. After stretching process, over 90% PAN nanofibers became aligned and, in the meantime, elongated and densely packed in the yarn. The electrospun PAN nanofiber yarns retained their morphology through the process of stabilization and carbonization. The individual carbonized nanofibers as well as carbonized yarns shrunk significantly due to extraction of elements other than carbon in the process of carbonization. Tensile test of as-collected, stretched, stabilized, and carbonized electrospun PAN nanofiber yarns was performed on an Instron materials testing machine with a 10 N load cell (accuracy 0.00001 N) according to ASTM D3822. As shown in Fig. 6, the as-collected single PAN nanofiber yarn showed a maximum load of 0.035 0.005 N. After stretching, the maximum load of single PAN nanofiber yarn increased to 0.059 0.018 N, a 68.6% increase. Stabilization reduced the maximum load of single PAN nanofiber yarn to 0.042 0.012 N, a 28.8% decrease from that of the stretched PAN nanofiber yarn. Carbonization further reduced the maximum load of single PAN nanofiber yarn to 0.033 0.007 N. This is another 21.4% decrease compared to that of the stabilized PAN nanofiber yarn, but the maximum load value is still close to that of the as-collected PAN nanofiber yarn. The results of mechanical test did show significant improvement in mechanical strength of PAN nanofiber yarn after stretching. However, the carbonized PAN nanofiber yarn exhibited lower load although it is still comparable with the as-collected PAN nanofiber yarn. This may be due to a few reasons. Firstly, the carbonized PAN nanofibers shrunk significantly in size and led to void in between the space of nanofibers in the yarn; secondly, the PAN we used was pure PAN, which is not a widely adopted carbon precursor for high-strength carbon fibers. A copolymer of PAN with itaconic acid, methyl methacrylate, and others is normally employed for high-strength purpose. In future research, PAN precursor and interfiber void will be investigated to improve the strength of final carbon nanofiber yarn from electrospinning. The research results so far has demonstrated the proof of concept.
CHAPTER 33 DEVELOPMENT OF CARBON NANOFIBERS
FIG. 5 SEM images of PAN nanofiber yarns: (A) as-collected, (B) stretched, (C) stabilized, and (D) carbonized.
4 CONCLUSIONS When scientists and engineers first discovered how to produce carbon fibers over five decades ago, the tensile strength of carbon fibers (T300) quickly reached 3 GPa. Although the theoretical tensile strength of carbon fiber exceeds 180 GPa, after many years of research, the strongest carbon fiber that the industry can produce today (T1000) has the tensile strength of 7 GPa. Decreasing the diameter of PAN precursor fibers can reduce structural imperfections and thus result in stronger carbon fibers. Nonetheless, the conventional spinning methods make it difficult, if not impossible, to prepare precursor fibers with diameters that are orders of magnitude smaller than 10 μm. Therefore, to further improve the structural perfection of precursor fibers and to produce carbon fibers with superior mechanical properties (especially strength), new methods must be investigated. The current attempt of collecting PAN nanofiber yarns by placing multiple electrospinning spinneretts above a flowing water bath followed by postspinning stretching and further carbonization revealed a hope to acquire stronger carbon fibrous material than those from conventional carbon fiber
FIG. 6 Maximum tensile load of electrospun PAN nanofiber yarns.
industry. PAN nanofiber yarns from electrospinning would pave the road for the development of continuous carbon fibers with superior strength. The manufacture of high-strength carbon nanofiber yarns may inject new vitality to spinning and textile industry and uncover new opportunities for developing novel carbon nanofiber yarn-reinforced composite materials. Future research should focus on the fundamental correlations among the processing conditions, the structures, and the final mechanical properties of the carbon nanofiber yarns.
ACKNOWLEDGMENTS This work was performed at the Joint School of Nanoscience and Nanoengineering of North Carolina A&T State University, a member of Southeastern Nanotechnology Infrastructure Corridor (SENIC) and National Nanotechnology Coordinated Infrastructure (NNCI), which is supported by the National Science Foundation (ECCS1542174). The authors would also thank NASA SBIR Program, AxNano, LLC, and the State of North Carolina for financial support.
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