High resolution transmission electron microscopy study on polyacrylonitrile/carbon nanotube based carbon fibers and the effect of structure development on the thermal and electrical conductivities

Polyacrylonitrile (PAN) and PAN/carbon nanotube (CNT) based carbon fibers at various CNT content have been processed and their structural development was investigated using high resolution transmission electron microscope (HR-TEM). In CNT containing carbon fibers, the CNTs act as templating agents for the graphitic carbon structure development in their vicinity at the carbonization temperature of 1450 °C, which is far below the graphitization temperature of PAN based carbon fiber (>2200 °C). The addition of 1 wt% CNT in the gel spun precursor fiber results in carbon fibers with a 68% higher thermal conductivity when compared to the control gel spun PAN based carbon fiber, and a 103% and 146% increase over commercially available IM7 and T300 carbon fibers, respectively. The electrical conductivity of the gel spun PAN/CNT based carbon fibers also showed improvement over the investigated commercially available carbon fibers. Increases in thermal and electrical conductivities are attributed to the formation of the highly ordered graphitic structure observed in the HR-TEM images. Direct observation of the graphitic structure, along with improved transport properties in the PAN/CNT based carbon fiber suggest new applications for these materials.     

High-strength superparamagnetic composite fibers

The polymer composites of magnetic nanoparticles can be possibly used in a bulk form by preserving all the novel characteristics of magnetic nanoparticles such as superparamagnetic behavior. By introducing magnetic properties of Fe₃O₄ nanoparticles into polymer fibers, novel magnetic properties combine with the advantages of composite fibers such as light-weight and ease-of-use. Using dry-jet-wet fiber spinning technology, we have successfully fabricated iron oxide/polyacrylonitrile (Fe₃O₄/PAN) composite fibers with 10 wt% nanoparticle in the polymer matrix. Composite fiber with a diameter as small as 15 μm can achieve tensile strength and tensile modulus values as high as 630 MPa and 16 GPa, respectively. Superparamagnetic properties of Fe₃O₄ nanoparticles were preserved in the composite fibers with saturation magnetization at 80 emu/g and coercivity of 165 G.     

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.     

Aging phenomenon of 6FDA-polyimide/polyacrylonitrile composite hollow fibers

We have observed time-dependent drifts in permeability and selectivity for two types of composite hollow fibers used for air separation. One was PVP [poly(4-vinyl pyridine)]/6FDA-durene/polyacrylonitrile (PAN) composite hollow fiber, and the other was 6FDA-3,5-diaminobenzonitrile/6FDA-durene/PAN composite hollow fiber. Their permeabilities dropped 50 to 70% after 3 to 5 months, while selectivities for O₂N₂ deteriorated slightly with time. A systematic study was carried out to investigate the causes of this creep behavior. Various composite fibers, such as polyimide/Celgard and polyimide siloxane/PAN, were fabricated to simulate the aging process. We conclude that the aging phenomenon observed for these two 6FDA-durene/PAN composite fibers was not due to the structure change of the PAN substrate, but mainly to the densification effect of the 6FDA-durene gutter layer on composite fibers. © 1996 John Wiley & Sons, Inc.     

Oxidative stabilization of PAN/SWNT composite fiber

PAN/SWNT composite fibers have been spun with 0, 5, and 10 wt% single wall carbon nanotubes (SWNTs). Tensile fracture surfaces of polyacrylonitrile (PAN) fibers exhibited extensive fibrillation, while for PAN/SWNT composite fibers, tendency to fibrillate decreased with increasing SWNT content. The reinforcing effect of SWNTs on the oxidized polyacrylonitrile (PAN) fiber has been studied. At 10 wt% SWNTs, breaking strength, modulus, and strain to failure of the oxidized composite fiber increased by 100%, 160%, and 115%, respectively. Tensile fracture surfaces of thermally stabilized PAN and the PAN/SWNT fibers exhibited brittle behavior and well distributed SWNT ropes covered with the oxidized matrix can be observed in the tensile fracture surfaces of the fibers. No de-bonding has been observed between unoxidized or the oxidized PAN matrix and the nanotube ropes. Higher strain to failure of the oxidized composite fiber as compared to that of the oxidized control PAN fiber also suggests good adhesion/interaction between SWNT and the oxidized matrix. Thermal stresses generated on the composite fiber during the oxidation process were lower than those for the control fiber. The potential of PAN/SWNT composite fiber as the precursor material for the carbon fiber has been discussed.     

Plasma Modified Polyaramid Fiber Surface and Fiber/Epoxy Interface

Kevlar-49^® was surface-modified through cold air plasma treatment and plasma grafting of poly-acrylic-acid (PAA), polyethylacrylate (PEA) and their copolymer (PAA/EA). To evaluate the effect of modification, the single fiber pull out (SPFO) test was developed; at least three important physical quantities, G _i , F _r L and τ_o were obtained. For grafting with PAA/EA, G _i increased to 55 J/m² in comparision with 36 J/m² for Kevlar fibers with the sizing removed. The surface topography of fibers pulled from epoxy resin was studied by scanning electron microscopy (SEM).     

A comparison of reinforcement efficiency of various types of carbon nanotubes in polyacrylonitrile fiber

Polyacrylonitrile (PAN)/carbon nanotubes (CNTs) composite fibers were spun from solutions in dimethyl acetamide (DMAc), using single wall (SWNTs), double wall (DWNTs), multi wall (MWNTs) carbon nanotubes, and vapor grown carbon nanofibers (VGCNFs). In each case, CNT content was 5 wt% with respect to the polymer. Structure, morphology, and properties of the composite fibers have been characterized using X-ray diffraction, Raman spectroscopy, scanning and transmission electron microscopy, tensile tests, dynamic mechanical tests, as well as thermal shrinkage. While all nanotubes contributed to property improvements, maximum increase in modulus (75%) and reduction in thermal shrinkage (up to 50%) was observed in the SWNT containing composites, and the maximum improvement in tensile strength (70%), strain to failure (110%), and work of rupture (230%) was observed in the MWNTs containing composites. PAN orientation is higher in the composite fiber (orientation factor up to 0.62) than in the control PAN fiber (orientation factor 0.52), and the PAN crystallite size in the composite fiber is up to 35% larger than in the control PAN (3.7 nm), while the overall PAN crystallinity diminished slightly. Nanotube orientation in the composite fibers is significantly higher (0.98 for SWNTs, 0.88 for DWNTs, and 0.91 for MWNTs and VGCNFs) than the PAN orientation (0.52-0.62). Improvement in low strain properties (modulus and shrinkage) was attributed to PAN interaction with the nanotube, while the improvement in high strain properties (tensile strength, elongation to break, and work of rupture) at least in part is attributed to the nanotube length. Property improvements have been analyzed in terms of nanotube surface area and orientation.     

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     

Impact energy absorption of unidirectional glass fiber-epoxy composites

The influence of resin and fiber properties on the impact behavior of composites can be assessed in a three-point drop-weight impact test by varying the length-to-thickness ratio of the specimen. The fracture initiation energy per unit deformed volume, w_i, can be described by the expression: where τ₁₁ is the tensile stress, τ₁₂ is shear stress; E₁₁ is tensile modulus; and G₁₂ is shear modulus. A unidirectional glass fiberepoxy composite was tested at impact velocities of 2.2 m/s (5 mph) and 4.5 m/s (10 mph). The energy to initiate fracture, w_i, was in the range of 2 to 3.5 MJ/m³, apparently independent of impact velocity. The total energy absorbed by the impacted composite was also found to be independent of impact rate but very sensitive to the length to thickness ratio: about 13 and 3.5 MJ/m3 at the corresponding ratios of 4.6 and 23. It was generally observed that high fracture energy is associated with extensive specimen delamination, i.e. failure in shear.     

Oriented and exfoliated single wall carbon nanotubes in polyacrylonitrile

Polyacrylonitrile (PAN)/single wall carbon nanotubes (SWNT) fibers were gel spun at 0, 0.5, and 1 wt% SWNT content to a draw ratio of 51. Structure, morphology, and mechanical and dynamic mechanical properties of these fibers have been studied. PAN/SWNT composite exhibited much higher electron beam radiation resistance than PAN. As a result, PAN lattice images could be easily observed in the composite fiber by high resolution transmission electron microscopy. The PAN/SWNT composite fiber also exhibited higher solvent resistance than the control PAN fiber. UV-vis spectroscopy of highly drawn fiber exhibited van Hove transitions, suggesting SWNT exfoliation upon drawing. SWNT exfoliation was also confirmed by high resolution transmission electron microscopy (HRTEM). At 1 wt% SWNT loading, fiber storage modulus (at 1 Hz) increased by 13.9, 6.6, and 0.2 GPa at −75, 25, and 150 °C, respectively. This suggests that the load transfer ability and hence interfacial strength is increasing with decreasing temperature, even below the polymer's γ transition temperature.     

Electrical conductivity and Joule heating of polyacrylonitrile/carbon nanotube composite fibers

Carbon nanotube (CNT) can exhibit electrical conductivity and introduce electric current into polymer. Using dry-jet-wet spin technology, polyacrylonitrile (PAN)/CNT composite fibers with 15 wt% and 20 wt% of CNT content were fabricated. The electrical conductivity of PAN/CNT fibers was enhanced by the annealing process at different temperatures and changed with time. These fibers could also respond to stretching, and the electrical conductivity decreased by 50% when the elongation reached 3%. In addition, electrical current can induce Joule heating effect and thermally transform PAN/CNT composite fibers. With the application of various electrical currents up to 7 mA at a fixed length, conductivity was enhanced from around 25 S/m to higher than 800 S/m, and composite fibers were stabilized in air. The temperature of composite fibers can increase from room temperature to several hundreds of degree Celsius measured by an infra-red (IR) microscope. Joule heating effect can also be estimated according to one-dimensional steady-state heat transfer equation, which reveals the temperature can be high enough to stabilize or carbonize fibers. As a result, this research provides a new idea of heating fabrics for thermal regulation, and a new approach for stabilizing and carbonizing PAN-based carbon fibers.     

Orientation of multiwalled carbon nanotubes in composites with polycarbonate by melt spinning

A conductive polycarbonate (PC) composite containing 2 wt% multiwalled carbon nanotubes (MWNT) and pure PC were melt spun using a piston type spinning device. Different take-up velocities up to 800 m/min and throughputs leading to draw down ratios up to 250 were used. The composite material of PC with MWNT was prepared by diluting a PC based masterbatch consisting of 15 wt% MWNT by melt mixing in an extruder. The alignment of the nanotubes within melt spun fibers with draw down ratios up to 126 was investigated by TEM and Raman spectroscopy. The nanotubes align in their length axis along the fiber axis increasingly with the draw down ratio, however, the curved shape of the nanotubes still exist in the melt spun fibers. At higher draw down ratios, the MWNT started to align by reducing their curvature. Polarized Raman spectroscopy indicated that the D/D and G/G ratios parallel/perpendicular to the fiber axis increase for both MWNT bands in a similar manner with the draw down ratio. Interestingly, with increasing alignment electrical conductivity of the fibers is lost. Mechanical investigations revealed that at low spinning speeds elongation at break and tensile strength of the composite are lower than those of the pure PC. However, at the highest take-up velocity of 800 m/min the elongation at break is higher and true stress at break of the composite fiber is comparable to the pure PC fiber.     

Stress relaxation in carbon nanotube-based fibers for load-bearing applications

Carbon nanotube (CNT) based continuous fiber, a CNT assembly that could retain the superb properties of individual CNTs on a macroscopic scale, has emerged as a promising candidate for reinforcement in multifunctional composites. While existing research has extensively examined their short-term mechanical properties based upon quasi-static measurements, the long-term durability of CNT fibers has been largely neglected. Here we report time-dependent behavior of CNT fibers, with a particular focus on tensile stress relaxation. Both the pure CNT fiber and the CNT/epoxy composite fiber exhibited significant stress decay during the relaxation process, and this time-dependent behavior became more significant at a higher initial strain level, a lower strain rate and a greater gauge length. The present approach signifies a fundamental difference in the load-bearing characteristics between CNT fibers and traditional advanced fibers, which has major implications for the long-term durability of CNT fibers in load-bearing multifunctional applications.     

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.     

Ultradrawing properties of ultrahigh‐ molecular‐weight polyethylene/carbon nanotube fibers prepared at various formation temperatures

The influence of formation temperature on the ultradrawing properties of ultrahigh‐molecular‐weight polyethylene/carbon nanotube (UHMWPE/CNT) fiber specimens is investigated. Gel solutions of UHMWPE/CNT with various CNT contents were gel‐spun at the optimum concentration and temperature but were cooled at varying formation temperatures in order to improve the ultradrawing and tensile properties of the UHMWPE/CNT composite fibers. The achievable draw ratio (D_ra) values of UHMWPE/CNT as‐prepared fibers reach a maximum when they are prepared with the optimum CNT content and formation temperature. The D_ra value of UHMWPE/CNT as‐prepared fibers produced using the optimum CNT content and formation temperature is about 33% higher than that of UHMWPE as‐prepared fibers produced using the optimum concentration and formation temperature. The percentage crystallinity (W_c) and melting temperature (T_m) of UHMWPE/CNT as‐prepared fiber specimens increase significantly as the formation temperature increases. In contrast, W_c increases but T_m decreases significantly as the CNT content increases. Dynamic mechanical analysis of UHMWPE and UHMWPE/CNT fiber specimens exhibits particularly high α‐transition and low β‐transition, wherein the peak temperatures of α‐transition and β‐transition increase dramatically as the formation temperature increases and/or CNT content decreases. In order to understand these interesting drawing, thermal and dynamic mechanical properties of the UHMWPE and UHMWPE/CNT as‐prepared fiber specimens, birefringence, morphological and tensile studies of as‐prepared and drawn fibers were carried out. Possible mechanisms accounting for these interesting properties are proposed. Copyright © 2010 Society of Chemical Industry     

The interfacial strength and fracture characteristics of ethanol and polymer modified carbon nanotube fibers in their epoxy composites

Carbon nanotube (CNT) fibers spun from CNT arrays were used as the reinforcement for epoxy composites, and the interfacial shear strength (IFSS) and fracture behavior were investigated by a single fiber fragmentation test. The IFSS between the CNT fiber and matrix strongly depended on the types of liquid introduced within the fiber. The IFSS of ethanol infiltrated CNT fiber/epoxy varied from 8.32 to 26.64 MPa among different spinning conditions. When long-molecule chain or cross-linked polymers were introduced, besides the increased fiber strength, the adhesion between the polymer modified fiber and the epoxy matrix was also significantly improved. Above all, the IFSS can be up to 120.32 MPa for a polyimide modified CNT fiber, one order of magnitude higher than that of ethanol infiltrated CNT fiber composites, and higher than those of typical carbon fiber/epoxy composites (e.g. 60–90 MPa). Moreover, the composite IFSS is proportional to the tensile strength and modulus of the CNT fiber, and decreases with increasing fiber diameter. The results demonstrate that the interfacial strength of the CNT fiber/epoxy can be significantly tuned by controlling the fiber structure and introducing polymer to optimize the tube–tube interactions within the fiber.     

Hexadecyl-functionalized lamellar mesostructured silicates and aluminosilicates designed for polymer-clay nanocomposites. Part II: Dispersion in organic solvents and in polystyrene

Layered mesostructured silicates and aluminosilicates with covalently attached hexadecyl groups (denoted as C₁₆-LMS, C₁₆-LMAS, and a sample with layers whose thickness was increased by additional silicate, C₁₆-SiO₂-LMAS) were investigated as synthetic clays for dispersion and exfoliation in polymer melts. The dispersion of these clays in 13 organic solvents and their performance in polystyrene (PS) nanocomposites were examined. The three synthetic clays dispersed and formed gels in aromatic solvents and in a branched alkyl solvent (2,6,10,14-tetramethylpentadecane, TMPD) based on visual observations and rheology. The elastic moduli (G′) of the toluene/clay dispersions for all three clays were similar when compared at equal inorganic content. The synthetic clays were blended with PS samples of various molecular weights. Melt rheology of the PS/clay nanocomposites showed a dramatic increase in elastic modulus compared with neat PS and formation of a G′ plateau at low frequencies. The plateau occurred at higher G′ values for C₁₆-LMAS than for C₁₆-SiO₂-LMAS or C₁₆-LMS, indicating that C₁₆-LMAS has higher strength and/or higher aspect ratio and can thus withstand the stresses of melt mixing. Increasing the molecular weight of PS increased G′ of the PS/C₁₆-LMAS nanocomposites. By small angle X-ray (SAXS) and transmission electron microscopy C₁₆-LMAS showed better dispersion and a higher aspect ratio in the PS-nanocomposite than C₁₆-SiO₂-LMAS.     

Polymerization of acrylonitrile in supercritical carbon dioxide

A series of fluorinated diblock copolymers, consisting of styrene (St)-acrylonitrile (AN) copolymer [poly(St-co-AN)] and poly-2-[(perfluorononenyl)oxy]ethyl methacrylate, with various compositions as well as with different molecular weights were synthesized by atom transfer radical polymerization and characterized. Dispersion polymerization of acrylonitrile in supercritical carbon dioxide (scCO₂) at 30 MPa and at 65 °C with this kind of amphiphilic block copolymer as a stabilizer and 2,2′-azobisisobutyronitrile as an initiator was investigated. The experimental results indicated that, in the presence of a small amount of poly(St-co-AN) (5 wt% to AN), spherical particles of polyacrylonitrile (PAN) were prepared with small diameter and narrow polydispersity (d_n = 153 nm, d_w/d_n = 1.12), resulting from the high stabilizing efficiency of this fluorinated block copolymer. Furthermore, the polymerization of AN in scCO₂ under different initial pressures especially under low pressure (<14 MPa) was studied. When the polymerization was carried out around the critical pressure of CO₂ (7.7-7.8 MPa), the PANs with high molecular weight (M_ν ≈ 130,000-194,000) were synthesized at high monomer conversion (>90%) no matter whether the stabilizer was added, compared to those synthesized by dispersion polymerization at 30 MPa. It was also found that the crystallinity of PAN synthesized at 7.7-7.8 MPa was somewhat higher than that synthesized at 30 MPa, while its crystallite size did not change.     

Electrospun magnetic carbon composite fibers: Synthesis and electromagnetic wave absorption characteristics

Electrospun polyacrylonitrile (PAN)‐based carbon composite fibers embedded with magnetic nanoparticles have been developed as materials for electromagnetic wave absorption. The nanocomposite fibers were prepared by electrospinning from a dispersion of magnetite (Fe₃O₄) nanoparticles stabilized by L ‐glutamic acid in a solution of PAN and N, N‐dimethyl formamide. The Fe₃O₄‐embedded PAN nanofibers were stabilized at 270°C in air and carbonized at 800°C in nitrogen. The Fe₃O₄ nanoparticles were crystalline with a particle size of about 7 nm, most of which was reduced to Fe₃C with agglomerates of up to 50 nm diameter in the carbon fibers. The carbon morphology was mostly disordered, but exhibited a layered graphitic structure in the vicinity of the nanoparticles. The carbon composite fiber exhibited ferromagnetic behavior, and the induced magnetic saturation per unit mass of fibers increased with increasing Fe₃O₄ content in the precursor. The complex relative dielectric permittivity was tuned by adjusting the amount of Fe₃O₄ in the carbon fiber precursor. With increasing Fe₃O₄ content, good electromagnetic wave absorption characteristics were observed below 6 GHz, even for samples with fiber loadings as low as 5 wt %. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Continuum analysis of carbon nanotube array buckling enabled by anisotropic elastic measurements and modeling

For the first time, carbon nanotube (CNT) forests are fully characterized as transversely isotropic continuum material. Each of the five independent elastic constants is experimentally obtained using a combination of nanoindenter-based uniaxial compression and shear testing, in situ SEM compression, and digital image correlation (DIC) of vertically and laterally oriented CNT microstructure columns. Material properties are highly anisotropic, with an axial modulus (165–275 MPa) that is nearly two orders of magnitude greater than the transverse modulus (2.5–2.7 MPa) and the out of plane shear modulus (0.8–1.6 MPa). The Poisson’s ratios along three mutually orthogonal axes, measured directly by simultaneous in situ DIC evaluation of axial and transverse strain, are found to be similarly anisotropic (ν₁₂ = 0.35, ν₂₃ = 0.1, ν₂₁ = 0.005). A Timoshenko beam model is then developed to accurately predict the critical buckling stress of the vertically oriented columns using a subset of these anisotropic properties and considering inelastic column buckling. These results show that the critical bucking stress of CNT microstructures vary predictably with geometry and that continuum models with appropriate material constants may be applied to analyze CNT microstructures and evaluate their stability for many applications.