Development of carbon nanofibers from aligned electrospun polyacrylonitrile nanofiber bundles and characterization of their microstructural, electrical, and mechanical properties

Carbon nanofibers with diameters of 200-300 nm were developed through stabilization and carbonization of aligned electrospun polyacrylonitrile (PAN) nanofiber bundles. Prior to the oxidative stabilization in air, the electrospun PAN nanofiber bundle was tightly wrapped onto a glass rod, so that tension existed during the stabilization. We also investigated several carbonization procedures by varying final carbonization temperatures in the range from 1000 to 2200 °C. The study revealed that: (1) with increase of the final carbonization temperature, the carbon nanofibers became more graphitic and structurally ordered; (2) the carbon nanofiber bundles possessed anisotropic electrical conductivities, and the differences between the parallel and perpendicular directions to the bundle axes were over 20 times; and (3) the tensile strengths and Young's moduli of the prepared carbon nanofiber bundles were in the ranges of 300-600 MPa and 40-60 GPa, respectively.     

Gel-spun carbon nanotubes/polyacrylonitrile composite fibers. Part III: Effect of stabilization conditions on carbon fiber properties

The oxidative stabilization process of gel-spun carbon nanotube (CNT)/polyacrylonitrile (PAN) composite fibers have been studied and optimized. Optimum stabilization time depends on both the applied tension and temperature. Various characterization methods including thermal shrinkage, dynamic mechanical analysis, infrared spectroscopy, and wide angle X-ray diffraction are used to monitor the chemical and structural evolution during stabilization and carbonization. The relationship between the stabilization conditions of CNT/PAN composite fiber and the tensile properties of the resulting carbon fibers were investigated. By optimizing stabilization conditions, CNT/PAN based carbon fibers with a tensile strength of 4 GPa and a tensile modulus of 286 GPa were obtained using batch carbonization processing at 1100 °C.     

Mechanical properties of vapor grown carbon nanofibers

Individual as-fabricated, high temperature heat-treated and graphitized/surface oxidized vapor grown carbon nanofibers (VGCNFs), with average diameter of 150 nm were tested for their elastic modulus and their tensile strength by a MEMS-based mechanical testing platform. The elastic modulus increased from 180 GPa for as-fabricated, to 245 GPa for high temperature heat-treated nanofibers. The nominal fiber strengths followed Weibull distributions with characteristic strengths between 2.74 and 3.34 GPa, which correlated well with the expected effects of heat treatment and oxidative post-processing. As-fabricated VGCNFs had small Weibull modulus indicating a broad flaw population, which was condensed upon heat treatment. For all VGCNF grades, the nanofiber fracture surface included the stacked truncated cup structure of the oblique graphene layers comprising its backbone and cleavage of the outer turbostratic or thermally graphitized layer.     

In situ transmission electron microscope tensile testing reveals structure–property relationships in carbon nanofibers

Tensile tests were performed on carbon nanofibers in situ a transmission electron microscope (TEM) using a microelectromechanical system (MEMS) tensile testing device. The carbon nanofibers tested in this study were produced via the electrospinning of polyacrylonitrile (PAN) into fibers, which are subsequently stabilized in an oxygen environment at 270 °C and carbonized in nitrogen at 800 °C. To investigate the relationship between the fiber molecular structure, diameter, and mechanical properties, nanofibers with diameters ranging from ∼100 to 300 nm were mounted onto a MEMS device using nanomanipulation inside the chamber of a Scanning Electron Microscope, and subsequently tested in tension in situ a TEM. The results show the dependence of strength and modulus on diameter, with a maximum modulus of 262 GPa and strength of 7.3 GPa measured for a 108 nm diameter fiber. In particular, through TEM evaluation of the structure of each individual nanofiber immediately prior to testing, we elucidate a dependence of mechanical properties on the molecular orientation of the graphitic structure: the strength and stiffness of the fibers increases with a higher degree of orientation of the 0 0 2 graphitic planes along the fiber axis, which coincides with decreasing fiber diameter.     

Modification of polyacrylonitrile (PAN) carbon fiber precursor via post-spinning plasticization and stretching in dimethyl formamide (DMF)

This study investigates the possibility of using a post-spinning plasticization and stretching process to eliminate suspected property-limiting factors in polyacrylonitrile-based carbon fibers. This process was performed with the intention of removing surface defects (to improve tensile strength), attenuating fiber diameter (to promote more uniform heat treatment), and reducing molecular dipole interactions (to facilitate further molecular orientation). Among the various organic and inorganic solutions tested, treatment using aqueous dimethyl formamide (DMF) offered far and away the best properties and was therefore selected for further testing. Tested individually (as single filaments), fibers exposed to 80% DMF for 10 s gave the highest precursor values of elastic modulus (9.07 GPa) and tensile strength (675 MPa). While fibers treated in 80% DMF gave a 73% improvement in elastic modulus and a 53% improvement in tensile strength over as-received PAN, limitations in sample preparation and carbonization necessitated a reduction in DMF concentration (to 30%) to allow extraction of individual carbon fibers for tensile testing. Despite this compromise, results for fibers carbonized at 1000°C ultimately showed a 32% improvement in carbon fiber elastic modulus and a 14% improvement in carbon fiber tensile strength over regularly prepared carbon fibers. These results show that, to a certain extent, improvements in PAN precursor properties can translate to corresponding improvements in subsequently produced carbon fibers. Additional characterization using wide angle X-ray scattering (WAXS) and scanning electron microscopy (SEM) suggests that these improvements are due in part to improved lateral order as well as the successful elimination of surface defects and prevention of skin-core formation.     

High strength and high modulus carbon fibers

Carbon fibers have been processed from gel spun polyacrylonitrile copolymer on a continuous carbonization line at Georgia Tech (GT) with a tensile strength in the range of 5.5–5.8 GPa, and tensile modulus in the range of 354–375 GPa. This combination of strength and modulus is the highest for any continuous fiber reported to date, and the gel spinning route provides a pathway for further improvements in strength and modulus for mass production of carbon fibers. At short gauge length, fiber tensile strength was as high as 12.1 GPa, which is the highest value ever reported for a PAN based carbon fiber. Structure analysis shows random flaws of about 2 nm size, which results in limiting tensile strength of higher than 20 GPa. Inter-planar turbostratic graphite shear modulus in high strength carbon fibers is 30 GPa, while in graphite the corresponding value is only 4 GPa.     

Mechanical and structural characterizations of simultaneously aligned and heat treated PAN nanofibers

A simple and nonconventional electrospinning technique was employed for producing aligned polyacrylonitrile (PAN) nanofibers. A thermal zone was placed between syringe needles and collector in the electrospinning set up to obtain aligned and heat treated nanofibers. Suitable temperatures for heat treat process of PAN nanofibers was determined using differential scanning spectroscopy (DSC) technique. The influence of treatment temperature was investigated on morphology, internal structure and mechanical properties of collected PAN nanofibers. The average fiber diameter measured from SEM images exhibited decreasing trend at higher temperatures. FTIR spectra indicated no considerable difference between chemical structure of untreated and treated PAN nanofibers. Crystallization degree of PAN nanofibers calculated from WAXD patterns showed relatively low change with treatment temperature. Tenacity values of nanofiber bundles increased with increasing temperature while the extension values had an inverse trend. However, the modulus did not show a regular manner, but treated nanofibers had more modulus than untreated ones. The stress and modulus of PAN nanofibers increased to 112.9 MPa and 7.25 GPa at 270°C, respectively. Nanofibers treated at the highest temperature had the largest amount of crystallinity and strength. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Study of carbon films from PAN/VGCF composites by gelation/crystallization from solution

A carbon film with a cross-sectional area much larger than that of a commercial carbon fiber (>6000 times) and a thickness of about 0.3 mm was obtained using a new method. In this method, composite materials of polyacrylonitrile (PAN) and vapor-grown carbon fiber (VGCF) prepared by gelation/crystallization from dilute solutions were used as starting materials The gelation/crystallization method was adopted to ensure high orientation of PAN chains. The composite materials were heat-treated at 200-300°C in an oxidizing atmosphere for thermal stabilization and then heat-treated to 1500°C in argon gas to promote carbonization. The tensile modulus and electric conductivity for the carbon materials with cross-sectional areas of about 0.6 mm² (thickness 0.3 mm and width 2 mm) reached 18 GPa and 10 Ω⁻¹ cm⁻¹, respectively. The mechanical and electrical properties of the final carbonized materials were sensitive to the PAN/VGCF composition and the draw ratio. These phenomena were analyzed using Fourier transform IR and X-ray diffraction.     

The influence of cobaltous chloride modification on physical properties and microstructure of modified PAN fiber during carbonization

Modification of polyacrylonitrile (PAN) fibers with cobaltous chloride has increased crystal size, crystallinity, and density, and also improved tensile strength and modulus of the resulting carbon fibers. In this study, the effect of cobaltous chloride modification on the physical properties, microstructure, and elemental composition of PAN fibers during the carbonization process was examined. The resultant carbon fibers developed from modified PAN fibers had a lower formation temperature of carbon basal planes than those fibers that developed from the original one. The modification process not only improved the tensile strength but also increased the tensile modulus by about 15% of the resulting carbon fibers at carbonization temperature of 1300°C. A higher stacking size (L_c), or a greater carbon basal plane in crystalline, is one of the reasons to improve the modulus and conductivity of the final carbon fibers. The modification process also increased the electrical conductivity by about 15% at 1300°C and by about 150% at 2500°C. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 70: 2409–2415, 1998     

The tensile behavior of carbon fibers at high temperatures up to 2400 °C

The tensile behavior of four different brands of carbon fibers (a rayon-based, a PAN-based, and 2 pitch-based fibers) has been investigated at various temperatures up to 2400 °C. The tests were carried out using an original fiber testing apparatus. Various mechanical properties including strength and Young's modulus, as well as Weibull statistical parameters were extracted from test data. Typical tensile behaviors were evidenced such as an essentially linear elastic behavior at room temperature and intermediate temperatures up to 1400-1800 °C, then a nonlinear elastic delayed response at higher temperatures and ultimately an inelastic response with permanent deformations at very high temperatures. Such unusual nonlinear responses for homogeneous materials were related to structure and texture features at the nanometer scale, that were described through an X-ray diffraction technique.     

Development of oriented morphology and tensile properties upon superdawing of solution-spun fibers of ultra-high molecular weight poly(acrylonitrile)

The uniaxial drawing of UHMW-PAN fibers spun from a dilute solution into methanol coagulation baths at different temperatures and the resultant structure and tensile properties of the drawn products were studied. Although the initial morphology of the fibers and the deformation mode in a lower draw ratio (DR_t) range were significantly dependent on the temperatures of the coagulation bath, the tensile properties at a given DR_t, as well as the maximum achieved ones, were comparable. Both the tensile modulus and strength increased steadily with the DR_t and reached 35 and 1.8 GPa, respectively, at the highest DR_t of ∼80. These tensile properties are among the highest ever reported for PAN fibers. The achievement of such high tensile properties for extremely drawn fibers is ascribed to the conformational changes of crystalline chains from the 3/1 helix to the planar-zigzag with increasing DR_t, the improvement in the uniformity of the fiber diameter along the fiber axis, and the decrease in fiber diameter. Indeed, the tensile strength of fibers prepared from a dilute solution and having comparable moduli increased with a decrease in the fiber diameters. The reciprocal of the strength was proportional to the square root of the diameter as suggested by the Griffith theory. Extrapolation to a zero diameter yielded an ultimate tensile strength of 2.4±0.1 GPa for a fiber having a maximum achieved tensile modulus of 35±1 GPa.     

Preparation of carbon fibers from linear low density polyethylene

Carbon fibers were produced from linear low density polyethylene (LLDPE) instead of commonly used precursors, such as viscose rayon, mesophase pitch and polyacrylonitrile (PAN). Cross-linked fibers were produced at various temperatures, times and stress conditions during a sulfuric acid treatment using LLDPE fibers obtained from dry-wet spinning. The effects of cross-linking were analyzed using a range of characterization techniques, such as differential scanning calorimetry, color change, fourier transform infrared spectroscopy, elemental analysis, density, scanning electron microscopy, and single filament mechanical properties. The carbonization process of cross-linked fibers was carried out at 950 °C for 5 min in a nitrogen atmosphere. The carbon fibers with the best mechanical properties were obtained from the cross-linked fiber with the highest tensile modulus. In particular, the carbon fibers with the best mechanical properties (tensile strength and tensile modulus of 1.65 GPa and 110 GPa, respectively), similar to commercial-grade carbon fiber, were obtained from the cross-linked fiber that had undergone a carbonization process with a stress of 0.25 MPa after an acid treatment for 150 min at 140 °C and a stress of 0.26 MPa.     

High‐temperature DSC study of polyacrylonitrile precursors during their conversion to carbon fibers

The evolution of structure, the changes of properties during the preoxidation, precarbonization, and carbonization of different polyacrylonitrile (PAN) precursors were firstly studied in detail by means of using high‐temperature differential scanning calorimetry (DSC) to characterize with continuous heating from 25 to 1400°C. It was essential to study the thermal behavior of PAN precursors so that proper temperature was determined. Three precursors with different composition, P1: acrylonitrile/itaconic acid (AN/IA) = 98/2 (wt/wt); P2: acrylonitrile/acrylamide (AN/AM) = 98/2 (wt/wt); P3: acrylonitrile/ammonium salt of itaconic acid (AN/AIA) = 98/2 (wt/wt) were, respectively, selected in this study. Comparative results of the DSC curves showed that the heating history influenced greatly the heat quantities released and exothermic position at low temperature of DSC curve, but influenced slightly at higher temperature. Aromatic index, carbon yield (W_c) and weight loss (η) can be calculated. After adjusting the temperature in preoxidation and precarbonization and carbonization technology depending on high‐temperature DSC, the high performance carbon fibers were obtained that tensile strength is 3.56–4.16 GPa, modulus is 235–243 GPa. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Polyacrylonitrile solution homogeneity study by dynamic shear rheology and the effect on the carbon fiber tensile strength

Poly(acrylonitrile‐co‐methacrylic acid) (PAN‐co‐MAA)/N,N‐dimethylformamide (DMF) solutions were prepared and dynamic shear rheology of these solutions were investigated. With increasing stirring time up to 72 h at 70°C, the polymer solution became less elastic (more liquid‐like) with a ～60% reduction in the zero‐shear viscosity. Relaxation spectra of the PAN‐co‐MAA/DMF solutions yield a decrease in relaxation time (disentanglement time, τ_d), corresponding to an about 8% decrease in viscosity average molecular weight. The log‐log plot of G′ (storage modulus) versus G″ (loss modulus) exhibited an increase in slope as a function of stirring time, suggesting that the molecular level solution homogeneity increased. In order to study the effect of solution homogeneity on the resulting carbon fiber tensile strength, multiple PAN‐co‐MAA/DMF solutions were prepared, and the precursor fibers were processed using gel‐spinning, followed by continuous stabilization and carbonization. The rheological properties of each solution were also measured and correlated with the tensile strength values of the carbon fibers. It was observed that with increasing the slope of the G′ versus G″ log‐log plot from 1.471 to 1.552, and reducing interfilament fiber friction during precursor fiber drawing through the addition of a fiber washing step prior to fiber drawing, the carbon fiber strength was improved (from 3.7 to 5.8 GPa). This suggests that along with precursor fiber manufacturing and carbonization, the solution homogeneity is also very important to obtain high strength carbon fiber. POLYM. ENG. SCI., 56:361–370, 2016. © 2016 Society of Plastics Engineers     

Stretching-induced orientation of polyacrylonitrile nanofibers by an electrically rotating viscoelastic jet for improving the mechanical properties

An additional centrifugal field applied to an electrostatic field in a novel electrospinning technique was proposed in this study. An additional centrifugal field can not only remove bending instability of electrically charged liquid jets during the electrospinning process but can also fabricate aligned and molecularly oriented nanofibers. The results indicated that combining a strong stretching force from an additional centrifugal field and an electrostatic field can be used to align polymer chains parallel to the nanofiber axis, producing polyacrylonitrile (PAN) nanofibers with superior molecular orientation and mechanical properties. The optimal stretching force of an electrically rotating viscoelastic jet was obtained from high-speed videography and dimensionless groups (Re, We, and Oh numbers) analysis. The dichroic ratio (D) was 0.78, and the chain orientation factor (f), measured via Polarized FT-IR was 0.21. These measurements indicated an increase in the molecular orientation for the fabricated PAN nanofibers via the optimal stretching force. The elastic modulus of PAN nanofibers with f = 0.21 was 6.29 GPa and 4.55 GPa when measured by atomic force microscopy (AFM) and nanoindenter experiments, respectively. These results demonstrated that superior mechanical properties of PAN nanofibers could be improved by conducting the proposed electrospinning technique. Furthermore, carbon nanofibers produced from the optimal PAN nanofibers through the proposed method could potentially be applied for the reinforcement of composites.     

Evaluation of carbonized medium-density fiberboard for electrical applications

The conversion of wood-based fiberboard materials into crack-free, monolithic, porous hard carbons is of significant interest due to their ability to perform in a multifunctional capacity. Three varieties of carbonized medium-density fiberboard (c-MDF) were studied for electrical, mechanical, and structural properties. X-ray diffraction data suggested that the volume fraction of large turbostratic crystallites increased with carbonization temperature (T_carb). The volume fraction of large turbostratic crystallites had a positive correlation with elastic modulus and electrical conductivity. The c-MDF materials were approximately isotropic with respect to elastic modulus and exhibited increasing stiffness with increasing T_carb (up to 4.5 GPa). Between 600 and 1400 °C, the electrical resistivity of c-MDF varied by seven orders of magnitude. The electrical resistivity of the hard carbon material in c-MDF 1400 °C was found to be within about an order of magnitude of polycrystalline graphite.     

Magnetic‐field‐assisted electrospinning highly aligned composite nanofibers containing well‐aligned multiwalled carbon nanotubes

Composite nanofiber meshes of well‐aligned polyacrylonitrile (PAN)/polyvinylpyrrolidone (PVP) nanofibers containing multiwalled carbon nanotubes (MWCNTs) were successfully fabricated by a magnetic‐field‐assisted electrospinning (MFAES) technology, which was confirmed to be a favorable method for preparation of aligned composite nanofibers in this article. The MFAES experiments showed that the diameters of composite nanofibers decreased first and then increased with the increase of voltage and MWCNTs content. With the increase of voltage, the degree of alignment of the composite nanofibers decreased, whereas it increased with increasing MWCNTs concentration. Transmission electron microscopy observation showed that MWCNTs were parallel and oriented along the axes of the nanofibers under the low concentration. A maximum enhancement of 178% in tensile strength was manifested by adding 2 wt % MWCNTs in well‐aligned composite nanofibers. In addition, the storage modulus of PAN/PVP/MWCNTs composite nanofibers was significantly higher than that of the PAN/PVP nanofibers. Besides, due to the highly ordered alignment structure, the composite nanofiber meshes showed large anisotropic surface resistance, that is, the surface resistance of the composite nanofiber films along the fiber axis was about 10 times smaller than that perpendicular to the axis direction. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41995.     

Swelling and related mechanical and physical properties of carbon nanofiber filled mesophase pitch for use as a bipolar plate material

Mesophase pitch was investigated as a melt processable precursor to a compression or injection moldable all carbon bipolar plate. After shaping, carbonization to 1000 °C or greater is required to achieve the desired electrical and mechanical properties, but gases evolved during this step lead to swelling. Carbon nanofiber was added to suppress swelling during carbonization and bypass the typical oxidation steps used when processing mesophase pitch. The addition of carbon nanofiber decreased swelling by increasing the viscosity of the melt. Carbonized materials with carbon nanofibers can show strengths (30-50 MPa) and conductivities (20-80 S cm⁻¹) consistent with composite bipolar plate materials. The materials show conductivities below Department of Energy target values at the current carbonization temperatures, which were limited to 1000 °C. The use of glass fibers as a secondary filler led to reduced gas permeability in porous samples.     

Structure and properties of polyacrylonitrile/single wall carbon nanotube composite films

Polyacrylonitrile (PAN)/single wall carbon nanotube (SWNT) composite films have been processed with unique combination of tensile strength (103 MPa), modulus (10.9 GPa), electrical conductivity (1.5×10⁴ S/m), dimensional stability (coefficient of thermal expansion 1.7×10⁻⁶/°C), low density (1.08 g/cm³), solvent resistance, and thermal stability. PAN molecular motion above the glass transition temperature (T_g) in the composite film is significantly suppressed, resulting in high PAN/SWNT storage modulus above T_g (40 times the PAN storage modulus). Rope diameter in the SWNT powder was 26 nm, while in 60/40 PAN/SWNT film, the rope diameter was 40 nm. PAN crystallite size from (110) plane in PAN and PAN/SWNT films was 5.3 and 2.9 nm, respectively. This study suggests good interaction between PAN and SWNT.     

取向PAN/MWCNTs与热塑性聚烯烃复合材料的制备及表征

利用静电纺丝并借助高速旋转的滚筒和热牵引作用制备不同取向度的聚丙烯腈/碳纳米管（PAN/MWCNTs）纳米纤维膜,通过高速滚筒和热牵引提高PAN的结晶度从而提高材料的拉伸强度和弹性模量,但会降低断裂伸长率;MWCNTs含量为0.5%（质量分数,下同）时PAN/MWCNTs力学性能最佳。利用浸渍法将各种取向度的PAN/MWCNTs纳米纤维膜与热塑性弹性体（POE）制备成一系列POE/PAN/MWCNTs复合材料（POE/PM）。结果表明,高取向度POE/h-P2M复合材料的拉伸强度比不取向POE/u-PME复合材料高71%,拉伸强度显著提高,断裂伸长率则减小,PAN/MWCNTs纳米纤维膜含量为6.7%时,复合材料的力学性能最佳。