Polyacrylonitrile based carbon fibers: Spinning technology dependent precursor fiber structure and its successive transformation

In this study, the effects of different spinning methods including traditional wet and dry-jet wet spinning, and newly developed dry-jet gel spinning, on the structures and performances of polyacrylonitrile fibers, as well as the structural evolution during stabilization and carbonization, are compared in detail. The structural differences along radial direction, surface roughness, and chain orientation of carbon fibers are inherited from their precursor fibers, and these factors are determined by spinning technologies and processing conditions. Among all spinning methods, dry-jet gel spinning could prepare fibers with the best chain orientation, the highest tensile properties, and the lowest surface roughness, which would be favorable for achieving higher mechanical performance. Additionally, for the resultant carbon fibers, the surface modification of dry-jet gel spun carbon fibers is easier than dry-jet wet spun carbon fibers, and comparable to wet spun carbon fibers. Overall, dry-jet gel spinning is promising to make carbon fibers with both excellent tensile properties and good interfacial adhesion with epoxy matrix.     

Effect of the solvent type on the formation and physical properties of polyacrylonitrile fibers via a solvent‐free coagulation bath

Polyacrylonitrile (PAN) fibers were fabricated via a dry‐jet wet‐spinning technique, and a solvent‐free coagulation bath system was adopted. The effects of different types of dope solvent on the formation and physical properties of the PAN fibers were investigated. Dimethylformamide and dimethyl sulfoxide (DMSO) were selected as the solvents and were added to a spinning solution consisting of 18 wt % PAN. The PAN fibers were examined with field emission scanning electron microscopy, differential scanning calorimetry, and thermogravimetric analysis. The field emission scanning electron micrographs revealed that the PAN fibers with the DMSO solvent exhibited a more circular shape and a smoother skin. The PAN fibers with the DMSO solvent had their glass‐transition temperature (T_g) at 121°C. This study indicated that the different types of dope solvent used in the dope preparation did not affect T_g of the PAN fibers because of the solvent‐free coagulation bath system; however, they significantly affected the physical formation of the PAN fibers. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Wet-spinning and carbonization of graphene/PAN-based fibers: Toward improving the properties of carbon fibers

Graphene/polyacrylonitrile (PAN)-based composite fibers, as monofilaments, multifilaments, and yarns, were prepared through a facile solution mixing and wet-spinning method. The PAN-based (PANb) precursor was synthesized via reversible addition–fragmentation chain transfer polymerization with N-isopropylacrylamide as a comonomer. Following wet-spinning, the PANb yarns were carbonized at 900 °C. Scanning electron microscopy images confirmed the presence of a homogenous dispersion of graphene nanosheets inside the polymer matrix. It has been shown that the addition of graphene not only enhanced the thermal and mechanical properties of the PANb fibers, but also improved their graphitic structure after heat treatment. Tensile strength and Young's modulus of the PANb yarns were increased by 28 and 20%, respectively, on addition of 0.5 wt % graphene. Raman spectra demonstrated improvement in the graphitic structure of the carbonized yarns even at low graphene content. These graphene/ PANb fibers show potential as a suitable precursor for the development of next generation carbon fibers. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47932.     

Polyimide fibers prepared by a dry‐spinning process: Enhanced mechanical properties of fibers containing biphenyl units

Polyimide (PI) fibers with enhanced mechanical properties and high thermal and dimensional stability were prepared via a two‐step dry‐spinning process through the introduction of 3,3′,4,4′‐biphenyl tetracarboxylic dianhydride (BPDA) containing biphenyl units into rigid homopolyimide of pyromellitic dianhydride (PMDA) and 4,4′‐oxydianiline. The attenuated total reflectance–Fourier transform infrared spectra results imply that the incorporated BPDA moieties accelerate the imidization process and increase the imidization degree (ID) of the precursor fibers; this was attributed to the increased molecular mobility of the polymer chains. Two‐dimensional wide‐angle X‐ray diffraction spectra indicated that the prepared PI fibers possessed a well‐defined crystal structure and polymer chains in the crystalline region were highly oriented along the fiber axis. The PI fiber, with the molar ratio of PMDA/BPDA being 7/3, showed optimum tensile strength and modulus values of 8.55 and 73.21 cN/dtex, respectively; these were attributed to the high IDs and molecular weights. Meanwhile, the PI fibers showed better dimensional stability than the commercial P84 fiber, and this is beneficial for its security applications. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43727.     

Lignin‐based carbon fibers: Accelerated stabilization of lignin fibers in the presence of hydrogen chloride

Organosolv switchgrass lignin (SGL) and hardwood lignin (HWL) polymers are used as precursors to prepare low cost carbon fibers (CFs). Lignin powder and fiber samples are stabilized and carbonized at different conditions to investigate the effect of HCl on thermal‐oxidative stabilization time, mass yield, fiber diameter, and mechanical properties. The results show that HCl can accelerate stabilization and reduce the stabilization time from many hours to 75 min for the SGL fibers, and to 35 min for the HWL fiber. The rate of rapid stabilization in HCl/air is at least four times faster than conventional stabilization in air. The CFs prepared with two different stabilization methods have almost the same strength and modulus, but higher carbon yield is obtained with the rapid stabilization due to a short time of oxidation. Pores and defects observed on the surface and the cross‐section of the CFs across all stabilization conditions contribute to low fiber strength. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45507.     

Steady shear and viscoelastic properties of a micro‐crosslinked acrylamide‐based terpolymer

A water‐soluble micro‐crosslinked associating polymer (PASA‐PL): poly (acrylamide/butyl styrene/sodium 2‐acrylamido‐2‐methylpropane sulphonate) (PASA)–phenolic aldehyde (PL) was prepared to reduce the critical associated concentration and enhance the thickening properties for the linear PASA polymer in aqueous and brine solutions. The consecutive steady shear and viscoelastic properties were investigated to explore the correlations between the rheologic performance and supramolecular structures for the PASA‐PL brine solutions. Upon consecutive steady shear, the intermolecular hydrophobic association is greatly reinforced because of the expansion of the coiled PASA‐PL chains at the suitable shear rate, and the brine solution exhibits obvious shear thickening behavior. The steady shear results show that the intermolecular hydrophobic association is reversible, and that the polymer chains do not degrade upon shearing. The PASA‐PL brine solutions with 50 g L⁻¹ NaCl have predominantly elastic character over the angular frequency range at the polymer concentration higher than 1.0 g L⁻¹, which is remarkably strengthened with a slight increase in polymer concentration. The PASA‐PL brine solutions display a salt‐thickening effect and predominantly exhibit elastic character over the angular frequency range at 10–50 g L⁻¹ NaCl. These results demonstrate that the viscoelastic behavior of the PASA‐PL solutions mainly depends on the formation of hydrophobically associated structures via the intermolecular association strengthened by the micro‐crosslink of PASA with PL. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Polycarbonate polyurethane elastomers synthesized via a solvent‐free and nonisocyanate melt transesterification process

Soluble and thermally stable polycarbonate polyurethane elastomers were synthesized through a solvent‐free and nonisocyanate melt transesterification process. The conditions of this process were studied, and the optimum conditions were as follows: the catalyst dibutyltin oxide dosage was 0.125 wt %, the raw material molar (n) ratio of dimethyl 1,6‐hexamethylene dicarbamate (HDC) to poly(carbonate macrodiol) (PCDL) was n(HDC)/n(PCDL) = 1:0.99, the reactants were prepolymerized under 100°C for 1 h and then under 185°C and a high vacuum for 4 h. Three different PCDLs were selected to participate in the reaction under the conditions mentioned previously, and their structures were characterized by Fourier transform infrared spectroscopy and X‐ray diffraction. The products obtained from this process were still stable under 280°C. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41377.     

The effects of carbonization temperature on the properties and structure of PAN‐based activated carbon hollow fiber

Polyacrylonitrile (PAN) hollow fibers were pretreated with ammonium dibasic phosphate and then further oxidized in air, carbonized in nitrogen, and activated with carbon dioxide. The effects of carbonization temperature of PAN hollow fiber precursor on the microstructure, specific surface, pore‐size distribution, and adsorption properties of PAN‐based carbon hollow fiber (PAN‐CHF) and PAN‐based activated carbon hollow fibers (PAN‐ACHF) were studied in this work. After the activation process, the surface area of the PAN‐ACHF increased very remarkably, reaching 900 m² g^?1 when carbonization is 1000°C, and the adsorption ratios to creatinine and VB₁₂ of ACHF were much higher than those of CHF, especially to VB₁₂. The different adsorption ratios to two adsorbates including creatinine and VB₁₂ reflect the number of micropores and mesopores in PAN‐ACHF. The dominant pore sizes of mesopores in PAN‐ACHF are from 2 to 5 nm. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 2155–2160, 2005     

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     

Thermal,fire, and mechanical properties of solvent‐free processed BN/boehmite‐filled prepregs

Within the scope of this research, platelet‐shaped hexagonal Boron Nitride (h‐BN) with a size of 2 and 12 μm, and oval‐shaped Boehmite (BT) with a size of 2 μm were incorporated in a glass fiber‐reinforced epoxy novolac matrix cured with a diamine‐based hardener. The effects of the platelet size (BN 2 and 12 μm) and filler nature (BT vs. BN) were correlated with the final thermal and fire‐related properties. The incorporation of the fillers shows that not only the thermal conductivity (σ) was increased from approximately 0.2 up to 1.04 W/mK (through‐plane) but also the flame retardancy was improved by using a hybrid combination. The time to ignition (t_ig) was increased by 48 s and the FIGRA value was decreased from 6.5 to 3.3 indicating a much lower fire hazard for the material. scanning electron microscopic micrographs of the laminate cross sections show that the fillers are distributed and oriented randomly in the fiber‐reinforced matrix, and also highlight the fiber wetting. Furthermore, the results show that the resulting 3D filler network and infiltration of the intratow regions is strongly dependent on lateral filler size and filler combination. With increasing the filler aspect ratio, the effect on thermal properties and filtration is more evident. Moreover, the hybrid combination of BN and Boehmite fillers has a strong effect on the network formation during processing, resulting in enhanced thermal and mechanical properties. A synergy was observed when using BN 12 μm in combination with Boehmite 2 μm as the larger platelets tend to assemble themselves in the intertow region (resin‐rich region) and the smaller particles infiltrate into the intratow regions. This leads to a formation of a thermal pathway throughout the glass fabric barrier. Considering the cost factor, the through‐plane (z‐direction) heat dissipation and the flame retardancy can be tailored by optimizing the size, aspect ratio/geometry, and nature of the fillers. POLYM. ENG. SCI., 59:1840–1852, 2019. © 2019 The Authors. Polymer Engineering & Science published by Wiley Periodicals, Inc. on behalf of Society of Plastics Engineers.     

pH and organic solvent‐sensitive properties of crosslinked polymeric spheres formed by a soap‐free emulsion polymerization

Narrowly distributed poly (styrene‐co‐(4‐vinylpyridine)) microspheres are facilely prepared by a soap‐free emulsion polymerization, and their structures and properties are investigated by TEM, FTIR spectra, DSC, and DLS, respectively. The sizes and glass transition temperatures of the polymeric spheres increase with an increase of 4‐vinylpyridine in the reactive system. In addition, these polymeric spheres show good stability in water and a series of organic solvents due to their crosslinked structures. When poly(styrene‐co‐(4‐vinylpyridine)) microspheres are obtained in the reactive system where the weight ratio of 4‐vinyl pyridine to styrene is less than 4/6, they can be well dispersed in water as well as in organic solvents such as ethanol, toluene and DMF, and show obvious pH sensitive and organic solvent‐sensitive characteristics. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Synthesis and characterization of a calcium‐ and sodium‐containing acrylamide‐based polymer and its effect on soil strength

Soil degradation is a significant problem throughout the world. One strategy that can be used to improve soil physical properties is the use of soil conditioners, particularly anionic polyacrylamides. Synthetic water‐soluble anionic acrylamide‐based polymers have wide applications in agriculture, such as mulch and agrochemicals, and can also be used as soil modifiers for erosion control, nondesertification, and soil stabilization. In this study, anionic polyacrylamides containing cationic metal ions were prepared by free‐radical polymerization. Anionic polymer with negative charges was produced by the reaction of mono‐ and divalent inorganic salts with acrylamide monomer via solution polymerization. Hydrolysis of the polymer was carried out by using calcium chloride and sodium carbonate in their soluble forms, and the negative charges on the polymer were regulated by variation of the molar ratios of inorganic salts with respect to the acrylamide monomer concentration. The molecular weight and charge density of the anionic charged polymers were improved and manipulated by using these methods. The molecular structure of the polymers was characterized and confirmed by common techniques. The effect of the polymers on soil strength was evaluated, and the results showed that the addition of anionic polymer having a high molecular weight improved the behavior of soil components. J. VINYL ADDIT. TECHNOL., 19:140–146, 2013. © 2013 Society of Plastics Engineers     

Solvent‐free acetylation of lignocellulosic fibers at room temperature: Effect on fiber structure and surface properties

Acetylation is one of the most interesting chemical treatments to improve the affinity of lignocellulosic fibers with polymeric matrices for the elaboration of several types of composites. In this paper, the acetylation of flax and wood pulp (bleached softwood Kraft pulp and thermomechanical pulp) fibers was carried out at room temperature in a solvent‐free system with acetic anhydride in the presence of sulfuric acid as catalyst. The effect of acetylation on the fine structure of fibers was investigated by spectroscopic methods, while the extent of acetylation was quantified by weight percent gain. The effect of reaction time on fiber morphology was studied at macro‐ and microscale using scanning electron microscopy, optical microscopy, and fiber quality analysis. The evolution of the hydrophobic/hydrophilic character of fibers was determined by contact angle measurements. The wettability of fibers by liquid epoxy resin was also evaluated to confirm the improvement of the affinity of acetylated fibers with the epoxy matrix. It was found that the hydrophilic character of fibers decrease with increasing reaction time, whereas the trend was less pronounced beyond specific reaction times. Acetylated fibers can therefore be potential candidates for replacing nonbiodegradable reinforcing materials in composite applications. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42247.     

Performance and structure changes of the aromatic co‐polysulfonamide fibers during thermal‐oxidative aging process

The changes in performance during thermal‐oxidative aging process of the aromatic co‐polysulfonamide (co‐PSA) fibers over a broad temperature range from 250 °C to 320 °C have been investigated. In addition, the mechanism of thermal‐oxidative aging process has been studied by using structural information obtained from the fibers at varying length scales. The results showed that a significant reduction in tensile strength was observed compared with that of initial modulus during aging process. Macroscopically, thermal‐oxidative aging mainly causes color changes of fibers and thermally induced macro defects begin to appear only at 320 °C for 100 h. On a micro level, the crystal structure of fibers remained stable and did not show significant changes expect that aging at 320 °C. In addition, thermo‐degradation as well as crosslinking has been observed primarily in amorphous region. With the increase of temperature and time duration, the crosslinking became more dominant and crosslinking density increases. Correspondingly, the fibril length decreases due to degradation and then increases due to the formation of crosslinked structures within the fibers. The results suggest that molecular degradation is the main cause of strength loss and the formation of crosslinking structure within the fibers contributes to the retention of modulus and improvement of creep resistance. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 44078.     

Comparison about the structure and properties of PAN‐based activated carbon hollow fibers pretreated with different compounds containing phosphorus

Polyacrylonitrile (PAN) hollow fibers were pretreated with five different compounds containing phosphorus, including ammonium dibasic phosphate, ammonium dihydrogen phosphate, triammonium phosphate, phosphoric acid, and metaphosphoric acid, and then further oxidized in air, carbonized in nitrogen, and activated with carbon dioxide. The effects of different compounds containing phosphorus as pretreating agents on the properties and structure of the resultant oxidized hollow fibers, carbon hollow fibers, and activated carbon hollow fibers are discussed. Comparing the Brunaner‐Emmett‐Teller (BET) surface area of PAN‐activated carbon hollow fibers (ACHF) pretreated with five different compounds, ammonium dibasic phosphate > triammonium phosphate > ammonium dihydrogen phosphate > phosphoric acid > metaphosphoric acid, and the surface area of mesopores in PAN‐ACHF pretreated with ammonium dibasic phosphate reaches maximum, 174 m² g^?1. The adsorption ratio to mesomolecule adsorbate, VB₁₂, of PAN‐ACHF pretreated with ammonium dibasic phosphate also reaches maximum, 97.7 wt %. Moreover, the dominant pore sizes of PAN‐ACHF range from 2 to 5 nm in diameter. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 294–300, 2005     

Effects of ammonium dibasic phosphate pretreatment time on the structure and properties of PAN‐based activated carbon hollow fibers

Polyacrylonitrile (PAN) hollow fibers were pretreated with ammonium dibasic phosphate aqueous solution, then further oxidized in air, carbonized in nitrogen, and activated with carbon dioxide. The effects of pretreatment time of PAN hollow fibers in ammonium dibasic phosphate aqueous solution on the microstructure, specific surface, pore‐size distribution, and adsorption properties of PAN‐based activated carbon hollow fibers (PAN‐ACHF) were studied in this work. After the activation process, the Brunaner–Emmett–Teller (BET) surface area of the PAN‐ACHF and surface area of mesopores in the PAN‐ACHF increases and reaches 513 m² g^?1 and 66 m² g^?1 respectively, when the dipping time of PAN hollow fibers in ammonium dibasic phosphate aqueous solution is 30 min. The adsorptions to creatinine and VB₁₂ of PAN‐ACHF are much high, reach 95% and 86% respectively, when dipping time is 30 min. The dominant pore sizes of mesopores in PAN‐ACHF range from 2 nm to 5 nm. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 2448–2453, 2006     

Effect of DMSO/H2O coagulation bath on the structure and property of polyacrylonitrile fibers during wet‐spinning

The effect of coagulation bath condition on the structure and property of the nascent fibers and polyacrylonitrile fibers during wet‐spinning is studied. The best coagulation condition to produce polyacrylonitrile fibers has been found by examination of EA, XRD, SEM, and so on. The results indicated that when the coagulation bath was DMSO/H₂O system, the temperature was 60°C, the concentration was 65%, the minus stretch ratio was ?10%, fine crystallites and high degree of crystallization in the nascent fibers and polyacrylonitrile fibers could be achieved, and less solvents remained in the nascent fibers with circular cross section morphology. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Preparation and properties of emulsifier‐/solvent‐free polyurethane‐acrylic hybrid emulsions for binder materials: Effect of the glycidyl methacrylate/acrylonitrile content

To obtain binder materials, emulsions of emulsifier‐/solvent‐free waterborne polyurethane‐acrylic hybrids with a fixed acrylic monomer content (30 wt %) were prepared in this study. This study focused on the effect of glycidyl methacrylate (GMA)/acrylonitrile (AN) wt % on the shelf stability, mean particle size and viscosity of hybrid emulsion samples, water swelling %/dynamic mechanical thermal properties/mechanical properties of hybrid film samples, and the failure mode and adhesive strength of binder materials prepared in this study. Characterization of the chemical structures of prepolymers, hybrid materials (binder materials), and atmospheric pressure plasma‐treated polyethylene (PE) has been performed by means of Fourier transformed infrared spectroscopy to determine the presence/disappearance/peak intensity change of functional groups. Various properties such as mean particle size, viscosity, T_g, water swelling %, hardness and mechanical properties, and failure mode and adhesive strength for leather/leather, control PE/leather, and plasma‐treated PE/leather were found to be significantly dependent on the weight ratio of GMA/AN. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44497.     

Lipase‐catalyzed ethanolysis of soybean oil in a solvent‐free system using central composite design and response surface methodology

BACKGROUND: In this work we describe the synthesis of ethyl esters, commonly known as biodiesel, using refined soybean oil and ethanol in a solvent‐free system catalyzed by lipase from Thermomyces lanuginosus. Central composite design and response surface methodology (RSM) were employed to optimize the biodiesel synthesis parameters, which were: reaction time, temperature, substrate molar ratio, enzyme content, and added water, measured as percentage of yield conversion. RESULTS: The optimal conditions obtained were: temperature, 31.5 °C; reaction time, 7 h; substrate molar ratio, 7.5:1 ethanol:soybean oil; enzyme content, 15% (g enzyme g⁻¹ oil); added water, 4% (g water g⁻¹ oil). The experimental yield conversion obtained under these conditions was 96%, which is very close to the maximum predicted value of 94.4%. The reaction time‐course at the optimal values indicated that 5 h was necessary to obtain high yield conversions. CONCLUSION: A high yield conversion was obtained under the optimized conditions, with relative low enzyme content and short time. Comparison of predicted and experimental values showed good correspondence, implying that the empirical model derived from RSM can be used to adequately describe the relationship between the reaction parameters and the response (yield conversion) in lipase‐catalyzed biodiesel synthesis. Copyright © 2008 Society of Chemical Industry     

Preparation and properties of carbon nanotube/binary‐polymer composites with a double‐segregated structure

A carbon nanotube (CNT)/poly(methyl methacrylate) (PMMA)/ultrahigh molecular weight polyethylene (UHMWPE) composite containing a double‐segregated structure was formalized by means of a facile mechanical mixing technology. In the composite, the CNTs were decorated on the surfaces of PMMA granules, and the CNTs decorated granules formed the continuous segregated conducting layers at the interfaces between UHMWPE particles. Morphology observations confirmed the formation of a specific double‐segregated CNT conductive network, resulting in an ultralow percolation threshold of ～0.2 wt %. The double‐segregated composite containing only 0.8 wt % CNT loading exhibited a high electrical conductivity of ～0.2 S m^?1 and efficient electromagnetic shielding effectiveness of ～19.6 dB, respectively. The thermal conductivity, temperature‐resistivity behaviors, and mechanical properties of the double‐segregated composites were also studied. This work provided a novel conductive network structure to attain a high‐performance conducting polymer composite at low filler loadings. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39789.