Thermal treatment of pyrolytic lignin and polyethylene terephthalate toward carbon fiber production

In this work, pyrolytic lignin (PL) was thermally co-treated with polyethylene terephthalate (PET) to produce carbon fiber precursor. The produced PL-PET precursors were thoroughly characterized and analyzed, and then being processed into carbon fiber. It was found that a novel precursor, rather than their physical blending, was formed by the thermal co-treatment, indicating there were strong interactions between PL and PET. The novel PL-PET precursors had enhanced thermal properties and rheological characteristics, therefore are more suitable for processing into better carbon fibers based on melt-spinning method. In this study, the precursor fibers derived from the co-treatment of PL and 5% PET were also stretched under tension during stabilization step to reduce the fiber diameter and improve molecular orientation. The resulting carbon fibers with an average diameter of 12.6 μm had the tensile strength of up to 1220 MPa. This work demonstrated that PET could be used to improve the processability and quality of lignin-based carbon fiber when it is chemically bonded with lignin-based precursor. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48843.     

Producing high‐quality precursor polymer and fibers to achieve theoretical strength in carbon fibers: A review

The precursor fiber quality has a large impact on carbon fiber processing in terms of its performance, production yield, and cost. Polyacrylonitrile precursor fibers have been used commercially to produce strong carbon fibers with average tensile strength of 6.6 GPa. There is a scope to improve the average tensile strength of carbon fibers, since only 10% of their theoretical strength has been achieved thus far. Most attempts to increase the tensile strength of carbon fibers have been made during the conversion of precursor fiber to carbon fiber. This review highlights the potential opportunities to enhance the quality of the polyacrylonitrile‐based precursor fiber during polymer synthesis, spinning, and postspinning. These high‐quality precursor fibers can lead to new generation carbon fibers with improved tensile strength for high‐performance applications. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43963.     

A facile method to prepare ZrC nanofibers by electrospinning and pyrolysis of polymeric precursors

Zirconium carbide (ZrC) is one of the most attractive ultra-high temperature ceramics due to its excellent properties. ZrC nanofibers were fabricated via electrospinning and pyrolysis of a novel polymeric precursor, Polyzirconosaal (PZSA), with the addition of polyvinylpyrrolidone (PVP) as the spinning aid. The polymer PZSA was prepared from the chemical reaction between Polyzirconoxane (PZO) and Salicyl alcohol. The as-spun PZSA/PVP fibers were converted to ZrC nanofibers with a diameter ~200 nm after carbothermal reduction at 1300 °C in argon. The obtained ZrC nanofibers maintained its excellent fibrous morphology. The microstructures exhibited that nanoscale ZrC particles dispersed in the fibers containing free carbon. The average crystallite size of ZrC particles using Scherrer method was 42 nm. The obtained ZrC nanofibers were characterized by XRD, SEM and TEM. The current material would be particularly useful for applications such as catalyst support, filters, gas storage, supercapacitors, and phase change material support in thermal management systems.     

Electrospun plasma‐modified chitosan/poly(ethylene terephthalate)/ferrocenyl‐substituted N‐acetyl‐2‐pyrazoline fibers for phosphate anion sensing

Two ferrocenyl‐substituted N‐acetyl‐2‐pyrazolines, N‐acetyl‐3‐(2‐furyl)‐5‐ferrocenyl‐2‐pyrazoline (Fc‐1) and N‐acetyl‐3‐(2‐thienyl)‐5‐ferrocenyl‐2‐pyrazoline (Fc‐2) electrospun fibers, were produced in the presence of plasma‐modified chitosan (PMCh)/poly(ethylene terephthalate) (PET) supporting polymers with an electrospinning method. The morphological and chemical characterizations of the PMCh/PET/Fc‐1 and PMCh/PET/Fc‐2 electrospun fibers were determined by scanning electron microscopy coupled with energy‐dispersive X‐ray spectroscopy analysis. Thermogravimetric analysis results indicated the presence of ferrocene within the PMCh/PET nanofibers. The electrochemical behavior of the PMCh/PET/Fc‐1 and PMCh/PET/Fc‐2 electrospun fibers were investigated by cyclic voltammetry measurements based on the ferrocene/ferrocenium redox couple. The new PMCh/PET/Fc‐1 and PMCh/PET/Fc‐2 electrospun fibers aggregated on the indium tin oxide were used for phosphate anion sensing. The highest oxidation peak currents were observed for the PMCh/PET/Fc‐1 electrospun fibers at about 0.56 V in 0.1M phosphate buffer. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43344.     

Development of lignin carbon fibers: Evaluation of the carbonization process

The use of lignin as a renewable resource for the production of less‐expensive carbon fibers has in recent years attracted great interest. In order to develop the strength properties, the stabilization and carbonization processes have to be optimized. For this reason, the process parameters during carbonization have here been studied on stabilized lignin fibers in the temperature interval from 300 to 1300 °C. The effects of temperature, heating rate, and straining of fibers during carbonization on the strength properties of carbon fibers were investigated. The heating rate, in the range from 1 to 40 °C/min, was shown to have no effect on the property development of the fibers. During carbonization with no load applied to the fibers, a shrinkage of 20% was noted. Counteracting the shrinkage by imposing a load on the fibers during the carbonization resulted in fibers with a greater stiffness. The tensile strength was not, however, affected by this loading. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43965.     

A new softening agent for melt spinning of softwood kraft lignin

Kraft lignin obtained from the pulping of wood is an interesting new precursor material for carbon fiber production because of its high carbon content and ready availability. However, continuous spinning of softwood kraft lignin (SKL) has been impossible because of its insufficient softening characteristics and neat hardwood kraft lignin (HKL) has required extensive pretreatments to enable fiber formation. Softwood kraft lignin permeate (SKLP) and hardwood kraft lignin permeate (HKLP), fractionated by membrane filtration, were continuously melt spun into fibers. To improve the spinnability of SKL and HKL, HKLP was added as a softening agent. SKL‐ and HKL‐based fibers were obtained by adding 3–98 wt % HKLP. A suitable temperature range for spinning was 20–85°C above the T_g of the lignin samples, and this range gave a flawless appearance according to the SEM analysis. Smooth, homogeneous fibers of SKLP, HKLP, and SKL with HKLP were successfully processed into solid carbon fibers. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Potential of producing carbon fiber from biorefinery corn stover lignin with high ash content

The possibility of producing carbon fiber from an industrial corn stover lignin was investigated in the present study. As‐received, high‐ash containing lignin was subjected to methanol fractionation, acetylation, and thermal treatment prior to melt spinning and the changes in physiochemical and thermal properties were evaluated. Methanol fractionation removed most of the impurities in the raw lignin and also selectively removed the molecules with high melting points. However, neither methanol fractionation nor thermal treatment rendered melt‐spinnable precursors. The precursors were highly viscous and decomposed easily at low temperatures, attributed to the presence of H, G phenolic units, and abundant hydroxycinnamate groups in herbaceous lignin. A two‐step acetylation of methanol fractionated lignin greatly improved the mobility of lignin, while enhancing the thermal stability of the precursor during melt‐spinning. Fourier Transform Infrared and 2D‐NMR analysis showed that the contents of phenolic and aliphatic hydroxyls, as well as the hydroxycinnamates, decreased in the acetylated precursors. The optimum precursor was a partially acetylated lignin with a glass transition temperature of 85 °C. Upon oxidative stabilization and carbonization, the carbon fibers with an average tensile strength of 454 MPa and modulus of 62 GPa were obtained. The Raman spectroscopy showed the I_D/I_G ratio of the carbon fiber was 2.53. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45736.     

Impact of lignin extraction methods on microstructure and mechanical properties of lignin‐based carbon fibers

In order to optimize the use of residues of enzymatic hydrolysis of corn stalk (REHCS) and explore the low‐cost and sustainable raw material substitute for carbon fibers, three types of lignin samples were extracted from REHCS by various extraction methods, and then they were converted into carbon fibers (CFs) by electrospinning, thermostabilization, and carbonization under the same process conditions. The microstructure and mechanical properties of the three types of carbonized fibers were different. The CFs from the ethanol organosolv lignin were actually smooth and brittle carbon films. The CFs from the formic acid/acetic acid organosolv lignin had microscopic pores, causing poor mechanical properties. Comparatively, the CFs from the alkaline lignin demonstrated preferable microstructure and mechanical properties. The reasons for the differences were analyzed by characterizing the lignin samples, precursor fibers, and resultant CFs. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45580.     

Poly(ethylene terephthalate) nanosheets prepared by CO2-laser supersonic multi-drawing

Poly(ethylene terephthalate) (PET) nanosheets were fabricated by winding nanofibers onto a spool. The nanofibers were prepared by irradiating PET fibers with radiation from a carbon dioxide laser while drawing them at supersonic velocities. A supersonic jet was generated by blowing air into a vacuum chamber through the fiber injection orifice. A new vacuum chamber was developed to produce nanosheets; it has seven fiber injection orifices and a spool to collect the nanofibers. A rectangular nanosheet that was 17 cm wide, 18 cm long, and 30 μm thick was obtained by collecting nanofibers for 10 min. The nanosheet is composed of nanofibers with an average fiber diameter of 350 nm. This technique is a novel method for producing nanosheets.     

Studying the Potential Application of Electrospun Polyethylene Terephthalate/Graphene Oxide Nanofibers as Electroconductive Cardiac Patch

Nowadays, engineering‐based cardiac patches aim to accelerate cardiac regeneration in myocardial infarcted tissues. Considering the fundamental role of cardiac electrophysiology in myocardial function, this study aims to investigate graphene oxide (GO) incorporation in the polyethylene terephthalate (PET) nanofibrous scaffold, as a conductive cardiac patch. The PET/GO nanocomposites are prepared using the uniaxial nozzle and coaxial nozzle electrospinning processes and comprehensively evaluated. The morphological observation indicates a uniform beaded free morphology with an average diameter of 147 ± 38 and 253 ± 67 nm for solid and core–shell nanocomposite fibers, respectively. Addition of GO to the PET nanofibers in a concentration of 0.05 wt% remarkably increases the Young modulus of mats from 30 ± 0.03 to 60 ± 0.02 and 69 ± 0.08 MPa for solid and core–shell nanofibers, respectively. Also, the electroconductivity is improved from 0.7 × 10^?6 to 1.175 × 10^?6 and 1.3 × 10^?6 S cm^?1 for solid and core–shell nanofibers, which are in the range of cardiac electroactivity values. PET/GO substrate interestingly supports human umbilical vein endothelial cells’ spreading morphology and cardiomyocyte elongated morphology, mainly where the GO nanosheets are distributed near the surface of nanofibers. In conclusion, the core–shell electrospun PET/GO nanocomposite fibers are suggested as a potential electroactive cardiac patch to improve cardiac cell attachment and proliferation.     

Characterization of polyacrylonitrile,poly(acrylonitrile‐co‐vinyl acetate), and poly(acrylonitrile‐co‐itaconic acid) based activated carbon nanofibers

In this study, three different acrylonitrile (AN)‐based polymers, including polyacrylonitrile (PAN), poly(acrylonitrile‐co‐vinyl acetate) [P(AN‐co‐VAc)], and poly(acrylonitrile‐co‐itaconic acid) [P(AN‐co‐IA)], were used as precursors to synthesize activated carbon nanofibers (ACNFs). An electrospinning method was used to produce nanofibers. Oxidative stabilization, carbonization, and finally, activation through a specific heating regimen were applied to the electrospun fibers to produce ACNFs. Stabilization, carbonization, and activation were carried out at 230, 600, and 750 °C, respectively. Scanning electron microscopy, thermogravimetric analysis (TGA), and porosimetry were used to characterize the fibers in each step. According to the fiber diameter variation measurements, the pore extension procedure overcame the shrinkage of the fibers with copolymer precursors. However, the shrinkage process dominated the scene for the PAN homopolymer, and this led to an increase in the fiber diameter. The 328 m²/g Brunauer–Emmett–Teller surface area for ACNFs with PAN precursor were augmented to 614 and 564 m²/g for P(AN‐co‐VAc) and P(AN‐co‐IA), respectively. The TGA results show that the P(AN‐co‐IA)‐based ACNFs exhibited a higher thermal durability in comparison to the fibers of PAN and P(AN‐co‐VAc). The application of these copolymers instead of AN homopolymer enhanced the thermal stability and increased the surface area of the ACNFs even in low‐temperature carbonization and activation processes. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44381.     

A novel approach for extracting cellulose nanofibers from lignocellulosic biomass by ball milling combined with chemical treatment

Cellulose nanofibers (CNFs) were isolated from kenaf fibers and wheat straw by formic acid (FA)/acetic acid (AA), peroxyformic acid (PFA)/peroxyacetic acid (PAA), hydrogen peroxide (H₂O₂) treatment; and subsequently through ball milling treatment. Characterization of extracted cellulose and cellulose nanofibers was carried out through Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X‐ray diffraction (XRD), and thermogravimetric analysis (TGA). TEM images showed that extracted cellulose nanofibers had diameter in the range of 8–100 nm. FTIR and XRD results implied that hemicellulose and lignin were mostly removed from lignocellulosic biomass with an increase in crystallinity, and isolation of cellulose nanofibers was successful. The TGA results showed that decomposition temperature of cellulose nanofibers increased by about 27°C when compared with that of untreated lignocellulosic biomass. No significant change was observed in the decomposition temperature of bleached celluloses after ball milling. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 42990.     

Syndiotactic polypropylene nanofibers obtained from solution electrospinning process at ambient temperature

Fabrication of semicrystalline syndiotactic polypropylene (sPP) nanofibers by solution electrospinning was studied. It was found that sPP nanofibers with an average diameter of 230 nm were successfully fabricated via solution electrospinning with methylcyclohexane as solvent at room temperature (25°C). The obtained diameter was significantly thinner than the minimum diameter of 350 nm of PP fibers that were reported previously. It was also found that increasing viscosity of the sPP solution, which was customarily a useful way of fabricating noncrystalline thin fibers, was found ineffectual in producing thinner semicrystalline sPP fibers in our experiments. In fact, a careful selection of solvent by considering the evaporation rate and the specific viscosity could effectively lead to the fabrication of thinner sPP fibers by imposing proper elongation and preventing the sPP solution from gelation. The results could be applied to other semicrystalline polyolefins with similar gelation characteristics analogous to sPP to produce thinner nanofibers. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43238.     

Carbon Fibers Prepared from Melt Spun Peracylated Softwood Lignin: an Integrated Approach

Different softwood lignin O‐acyl derivatives, i.e., methacrylated, hexanoylated, benzoylated, methoxybenzoylated, and cinnamoylated lignin are synthesized and subjected to melt spinning. In the presence of spinning aids such as vanillin and ethylene glycol dimethacrylate, multifilament melt spinning is accomplished with spinning speeds up to 500 m min⁻¹, which allowed for realizing uniform precursor fibers 17 μm in diameter. Out of all acyl‐derivatives of softwood lignin investigated, cinnamoylated softwood lignin (CL) turned out to be superior in terms of processability. CL‐derived precursor fibers are oxidatively thermostabilized and then carbonized applying carbonization temperatures up to 2200 °C. Carbon fiber structure formation is followed in detail by wide‐angle X‐ray scattering and Raman spectroscopy. An orientation ≤53% and a d ₀₀₂ spacing of 0.353 nm is achieved. According to small angle X‐ray scattering, carbon fibers have a porosity of ≈38%. CL‐derived carbon fibers are also characterized in terms of mechanical properties. Tensile strengths up to 0.93 GPa (average 0.75 GPa) are obtained and follow Weibull statistics. Elastic moduli are ≤66.5 GPa (average 41.1 GPa).

     

Characterization on oxidative stabilization of polyacrylonitrile nanofibers prepared by electrospinning

Ultrafine polyacrylonitrile (PAN) fibers, as a precursor of carbon nanofibers, with diameters in the range of 220–760 nm were obtained by electrospinning of PAN solution using N,N-dimethyl formamide (DMF) as solvent. Morphology of the nanofibers for varying concentration and applied voltage was investigated by field emission scanning electron microscopy (FESEM). The thermal properties and structural changes during the oxidative stabilization process were primarily investigated by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and Fourier transform infrared (FT-IR) and Raman spectroscopy. The nanofiber diameters increase as the applied voltage is increased and they also increase with an increase in the concentration of the polymer solution. It was also concluded that the electrospun fibers displayed a very sharp exothermic peak at 297.34 °C. A transition temperature observed by FT-IR and Raman was approximately 300 °C, which was closely consistent with the results of DSC and TGA studies. It was also found that oxidative stabilization in air was accompanied by a change in color of nanofibers webs.     

Fatigue response and constitutive behavior modeling of poly(ethylene terephthalate) unreinforced and nanocomposite fibers using genetic neural networks

The constitutive behavior of poly(ethylene terephthalate) (PET) unreinforced (control) and PET fibers reinforced with 5 wt% vapor‐grown carbon nanofibers (VGCNFs) under uniaxial tension and subsequent to fatigue loading has been evaluated utilizing various analytical models. Two types of fatigue tests were performed: (1) Long cycle fatigue at 50 Hz (glassy fatigue) to evaluate fatigue resistance and (2) fatigue at 5 Hz (rubbery fatigue) to evaluate residual strength performance. The long cycle fatigue results at 50 Hz indicated that the PET‐VGCNF sample exhibited an increased fatigue resistance of almost two orders of magnitude when compared to the PET unreinforced filament. The results of the fatigue tests at 5 Hz indicated that the constitutive response of both the PET control and PET‐VGCNF samples changed subsequent to fatigue loading. The large deformation uniaxial constitutive response of the PET and PET‐VGCNF fibers was modeled utilizing genetic‐algorithm (GA) based training neural networks. The results showed that the large deformation uniaxial tension constitutive behavior of both PET unreinforced and PET‐VGCNF samples with and without prior fatigue can be represented with good accuracy utilizing neural networks trained via genetic‐based backpropagation algorithms, once the appropriate post‐fatigue constitutive behavior is utilized. Experimental data of uniaxial tensile tests and experimental postfatigue constitutive data have been implemented into the networks for adequate training. The fatigue tests were conducted under tension‐tension fatigue conditions with variations in the stress ratio (R), maximum stress (σ_max), number of cycles (N), and the residual creep strain (ε_R). POLYM. COMPOS., 2012. © 2012 Society of Plastics Engineers     

Structure and mechanical properties of nanofibrous ZrO2 derived from alternating field electrospun precursors

Nanofibrous zirconia (ZrO₂) meshes were prepared from precursor fibers which were synthesized using the method of free-surface, high-yield alternating field electrospinning (AFES). The weight ratio of zirconyl chloride salt to polyvinylpyrrolidone (PVP) polymer in liquid precursors was investigated for its effect on the spinnability and formation of precursor fibers as well as on the resulting fibrous ZrO₂. The precursor fiber generation measured at a rate up to 5.6 g/h was achieved with a single flat 25-mm diameter alternating current (AC) electrode, which corresponded to production of up to 1.5 g/h of fibrous ZrO₂. The calcination process involved annealing the fibers at temperatures which ranged from 600 °C to 1000 °C and produced 0.1–0.2 mm thick fibrous ZrO₂ meshes. Individual nanofibers were found to have diameters between 50 and 350 nm and either a tetragonal (t-ZrO₂) or monoclinic (t-ZrO₂) structure depending on the calcination temperature. The annealed meshes with total porosity between 98.0 ± 0.2% and 94.6 ± 0.2% showed little deformation or cracking. Tensile strength and modulus of fibrous t-ZrO₂ meshes strongly depended on porosity and varied from 0.07 ± 0.03 MPa to 1.05 ± 0.3 MPa and from 90 ± 40 MPa to 388 ± 20 MPa, respectively. The m-ZrO₂ meshes resulted similar moduli, but much lower strengths due to their brittleness. A power-law relationship between the elastic modulus and porosity of AFES-derived nanofibrous t-ZrO₂ meshes, in comparison with other porous zirconia materials, was also investigated. The results of this study have demonstrated the feasibility of free-surface AFES in sizeable production of zirconia nanofibers and highly porous nanofibrous ceramic structures.     

Preparation and characterization of electrospun poly(ε‐caprolactone)–pluronic–poly(ε‐caprolactone)‐based polyurethane nanofibers

In this study, amphiphilic poly(ε‐caprolactone)–pluronic–poly(ε‐caprolactone) (PCL–pluronic–PCL, PCFC) copolymers were synthesized by ring‐opening copolymerization and then reacted with isophorone diisocyanate to form polyurethane (PU) copolymers. The molecular weight of the PU copolymers was measured by gel permeation chromatography, and the chemical structure was analyzed by ¹H‐nuclear magnetic resonance and Fourier transform infrared spectra. Then, the PU copolymers were processed into fibrous scaffolds by the electrospinning technology. The morphology, surface wettability, mechanical strength, and cytotoxicity of the obtained PU fibrous mats were investigated by scanning electron microscopy, water contact angle analysis, tensile test, and MTT analysis. The results show that the molecular weights of PCFC and PU copolymers significantly affected the physicochemical properties of electrospun PU nanofibers. Moreover, their good in vitro biocompatibility showed that the as‐prepared PU nanofibers have great potential for applications in tissue engineering. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43643.     

Carbon fibers derived from wet‐spinning of equi‐component lignin/polyacrylonitrile blends

Equi‐component blends of polyacrylonitrile (PAN) and lignin, i.e., with a lignin content as large as 50 wt %, were successfully used as precursors to produce carbon fibers. Rheological measurements demonstrated that increasing lignin content in spinning solution reduced shear viscosity and normal stress, indicating a decrease of viscoelastic behavior. This was confirmed by Fourier transform infrared results that show no discernable chemical reaction or crosslinking between PAN and lignin in the solution. However, the resulting carbon fibers display a large I_D/I_G ratio (by Raman spectroscopy) indicating a larger disordered as compared to that from pure PAN. The macro‐voids in the lignin/PAN blend fibers typically generated during wet‐spinning were eliminated by adding lignin in the coagulant bath to counter‐balance the out‐diffusion of lignin. Carbon fibers resulting from lignin/PAN blends with 50 wt % lignin content displayed a tensile strength and modulus of 1.2 ± 0.1 and 130 ± 3 GPa, respectively, establishing that the equi‐component wet‐spun L/P‐based carbon fibers possessed tensile strength and modulus higher than 1 and 100 GPa. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45903.     

Synthesis and Characterization of Poly(vinyl alcohol)/Poly(vinyl pyrrolidone)-Iodine Nanofibers with Poloxamer 188 and Chitosan

This study focuses on the fabrication of poly(vinyl alcohol)/ poly(vinyl pyrrolidone)-Iodine nanofibers via electrospinning. Electrospun fibers were characterized by FT-IR, DSC and SEM techniques. DSC results indicated that the thermal stability of nanofibers were improved after the addition of chitosan and poloxamer 188. SEM images showed that the spongiform structure is much more compact and fibrous in the case of added chitosan, with an average fiber diameter of 374 nm, whereas the addition of poloxamer 188 resulted in a more porous and beaded composition, with average fiber diameter of 489 nm.