Polyacrylonitrile-based Fibers Research Articles

Stabilization of polyacrylonitrile-based fiber with a quasi-traveling microwave applicator

This study introduces a novel microwave applicator and optimized processing conditions to enhance the stability of Polyacrylonitrile (PAN)-based precursor fiber (PF). The innovative microwave applicator facilitates the propagation of the electromagnetic (EM) field akin to a quasi-traveling wave, thus circumventing standing wave nodes. This ensures a uniform thermal distribution and broadens the heating zone. Utilizing this applicator, the PF undergoes thermal stabilization in a streamlined two-step process, completing in just 13 min, a significant improvement over the conventional 90-min process. This not only saves manufacturing time, promoting energy efficient manufacturing but also aligns with the global trend towards green energy and lightweight carbon fiber-reinforced polymer matrix composites, potentially catalyzing rapid economic growth. Fiber characterization through Raman spectroscopy (RS), X-ray diffraction (XRD), differential scanning calorimetry (DSC), and complex permittivity measurements reveals that the microwave-processed fiber meets the standard of commercial stabilization fiber (SF).

Microstructure Evolution of Polyacrylonitrile-Based Fibers during Thermal Pre-Oxidation

In this work, pre-oxidized polyacrylonitrile fibers are treated with ultrasonic etching and solution etching to produce ultra-thin sections. The evolution of the fibers’ microstructure in the pre-oxidation process is observed, and the transformation model of the microstructure of the pre-oxidized fibers is proposed. Scanning electron microscopy and high-resolution transmission electron microscopy were used to observe the microstructure changes of the fibers. Fourier transform infrared spectroscopy and X-ray diffraction were used to observe the chemical structure transformation and crystallization degree of the fibers in the pre-oxidation process. The results revealed that pre-oxidized fibers exhibited a smooth surface, while their interior consisted of fibrils. The longitudinal microfibrils were connected by the transverse microfibrils and amorphous regions. The fracture morphology of the fibers shifted from ductile to brittle, and the cross-section gradually became smoother. The linear molecular chain of PAN transformed into a ring structure as pre-oxidation progressed, subsequently leading to the cross-linking of this ring structure into an orderly trapezoidal configuration. The connection between the fibrils was enhanced, and the fiber structure became more compact and stable.

Preparation of hierarchical porous polyacrylonitrile-based fiber sponges and electrochemical performance study

Preparation of hierarchical porous polyacrylonitrile-based fiber sponges and electrochemical performance study

Study on the relationship between ring formation and properties of pre-oxidized polyacrylonitrile-based fibres

The relationship between the degree of ring formation and the overall properties of the pre-oxidized fibre was studied by treating PAN precursor with KClO3/H2SO4 solution and different pre-oxidation techniques. PFs are polar materials and the dielectric constant increases after solution treatment. The bulk density of Cl-MSFs is always higher than that of MSFs and CSFs in the same temperature zone. The bulk density of Cl-MSFs5 is 1.401 g/cm3, which is 1.6% higher than that of MSFs5. The EOR value of Cl-MSFs5 is 0.767, which is 11.97% higher than the value of MSFs5. The modified polyacrylonitrile precursor showed a high degree of cyclization during microwave heat treatment. The temperature can be reduced by about 40 °C by modifying and microwave heat treatment to achieve the same level of cyclization. Cl-MSFs5 has a higher O content than MSFs5. The microscopic morphology shows that the number of "grooves" and fibre diameter of Cl-MSFs5 is significantly smaller than that of MSFs5, indicating that it has a higher degree of stability. There is no Cl element on the surface of Cl-MSFs5, which indicates that the structure of the pre-oxidized fibre is not affected after the pre-oxidation treatment. In addition, the structural transformation of KClO3-modified PFs during microwave pre-oxidation is proposed. The cyclization reaction occurs first, followed by dehydrogenation and oxidation reactions. In the carbon fibre production process, the pre-oxidation stage takes a long time and consumes a lot of energy. The new composite treatment technology used in this study can effectively reduce the treatment temperature and shorten the treatment time in the pre-oxidation stage, which provides a reference for the development of low-cost and efficient pre-oxidation process.

Effect of the fabrication process on the microstructural evolution of carbon fibers and flexural property on C/SiC composites by the NITE method

Nano-infiltration and transient eutectic phase (NITE) SiC matrix composites are designed for application in aerospace propulsion systems, particularly in fasteners and thrusters. A variety of carbon fibers with different properties have been selected as reinforcements for SiC matrix composites. Carbon fibers are known to be stable at high temperatures; however, the effects of high applied pressure at high temperatures on the fiber microstructure evolution and mechanical properties are not well-known. As a scoping study for fabricating NITE C/SiC composites, the behaviors of various carbon fibers in SiC composites. Pitch-based fibers, namely, GRANOX XN-05 and YS-90A, and a polyacrylonitrile-based fiber, namely, TORAYCA T-300B, were selected for matrix reinforcement. The 3-point bending test results indicated pseudo-ductile behaviors in the cases of YS-90A and T-300B fiber reinforcements. Fracture resistance evaluation based on the single-notch bending test indicated that the YS-90A fiber reinforced composite afforded the highest fracture resistance among the three C/SiC composites. The microstructure evolution on YS-90A and T-300B fibers was limited to near the fiber surface. Therefore, YS-90A and T-300B carbon fibers are potential candidates for reinforcement in NITE C/SiC composites.

Polyamine-modified polyacrylonitrile fibers for efficient removal of U(VI) from real fluorine-contained low-level radioactive wastewater

It is of great significance to develop an adsorbent with high adsorption capacity and excellent resistance to anion and cation interference toward the removal of U(VI). Herein, a novel polyamine-modified polyacrylonitrile-based fiber (PANPA) has been synthesized through hydrothermal method, which can validly remove U(VI) from solution. Combined with mesoscopic, spectral characterization and simulation method, the removal behavior and mechanism of U(VI) from high fluorine uranium-containing wastewater by PANPA are systematically investigated. The results show that, based on the strong coordination principle of polyamine group and UO22+, PANPA can selectively remove U(VI) from wastewater. In addition, the qmax of 459.27 mg g−1 was more than that of many other adsorbent materials. More importantly, PANPA is not affected by high concentration of F−, and exhibits higher distribution coefficient (559,900 mL g−1) and removal efficiency (99.5%) to U(VI) than other coexisting ions in real wastewater. Furthermore, the column experiment was also implemented to remove U(VI). The results indicate that PANPA is a promising material to effectively remove U(VI) from real wastewater produced during the fabrication of nuclear fuel elements.

Flexible nickel disulfide nanoparticles-anchored carbon nanofiber hybrid mat as a flexible binder-free cathode for solid-state asymmetric supercapacitors

Nickel disulfide nanoparticles (NiS2NPs)-anchored carbon nanofibers (NiS2NPs@CNF) hybrid mats were fabricated via the sequential process of stabilization and carbonization of electrospun polyacrylonitrile-based fibers followed by hydrothermal growth of NiS2NPs on the porous surface of CNFs. The vertical growth of NiS2NPs on entire surfaces of porous CNFs appeared in the SEM images of hybrid mat. The hierarchical NiS2NPs@CNF core–shell hybrid nanofibers with 3D interconnected network architecture can endow continuous channels for easy and rapid ionic diffusion to access the electroactive NiS2NPs. The conductive and interconnected CNF core could facilitate electron transfer to the NiS2 shell. Moreover, the porous CNF as a buffering matrix can resist volumetric deformation during the long-term charge–discharge process. The NiS2NPs@CNF electrode can yield high specific capacitance (916.3 F g−1 at 0.5 A g−1) and reveal excellent cycling performances. The solid-state asymmetric supercapacitor (ASC) was fabricated with NiS2NPs@CNF mat as a binder-free positive electrode and activated carbon cloth as a negative electrode. As-assembled ASC not only produce high specific capacitance (364.8 F g−1 at 0.5 A g−1) but also exhibit excellent cycling stability (∼92.8% after 5000 cycles). The ASC delivered a remarkably high energy density of 129.7 Wh kg−1 at a power density of 610 W kg−1. These encouraging results could make this NiS2NPs@CNF hybrid mat a good choice of cathode material for the fabrication of flexible solid-state ASC for various flexible/wearable electronics.

Comparative study of conventional and microwave heating of polyacrylonitrile-based fibres

Abstract The effects of the conventional heating method and the microwave heating method on polyacrylonitrile-based fibres in the temperature range of 180–280 °C were investigated. Fourier transform infrared spectroscopy, X-ray wide-angle scattering, Raman spectroscopy, energy-dispersive spectrometer, scanning electron microscopy and bulk density were used to characterise the properties of the samples. Results show that the microwave heating method can shorten the pre-oxidation time, reduce pre-oxidation temperature and reduce the number of surface defects. The pre-oxidised fibres obtained by the microwave heating method exhibit not only good crystallite size but also a smooth surface. Atomic morphology and molecular arrangement are orderly inside the fibre. The FT-IR spectrum shows that the oxidation reaction occurs at 220 °C, and the CI value of PAN fibers stabilised by microwave heating is the larger than the fibers stabilised by conventional heating. XRD analysis shows that fibers stabilised by microwave heating have low stack domains. The SEM and Raman spectra indicate that hydrogen peroxide can improve the surface finish of the fibers and reduce defects. Microwave heating can reduce the pre-oxidation temperature by about 20 °C and shorten the heating time. The economic benefits of using this method are significantly improved.

Preparation of polyacrylonitrile-based fibres with chelated Ag ions for antibacterial applications.

The need for an excellent antibacterial material that is sufficiently powerful to never develop bacterial resistance is urgent. In this study, a series of novel polyacrylonitrile-based fibres with chelated Ag ions (referred to as Ag-SH-PANF) were prepared by a two-step chemical modification process: grafting and chelating. The properties of the as-prepared Ag-SH-PANF were characterized by Fourier transform infrared (FT-IR) spectroscopy, thermogravimetric analysis (TGA), X-ray diffraction (XRD), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The antibacterial activities of Ag-SH-PANF were examined against pathogenic bacteria, and an antibacterial mechanism was explicated based on the release of Ag ions from the fibres' surfaces. The results showed that, although chelation occurred between the Ag ions and the grafted amino, sulfhydryl and disulfide groups, Ag-SH-PANF retained its fine microstructure and thermal stability. Moreover, Ag-SH-PANF displayed excellent antibacterial ability against pathogenic bacteria as well as good washing durability. In terms of the antibacterial mechanism, Ag ions are the main bactericidal agents in the role of catalysts and are not consumed in the antibacterial process. Nonetheless, a relatively higher concentration of Ag ions can accelerate the bactericidal process.

Ultrathin Li7La3Zr2O12@PAN composite polymer electrolyte with high conductivity for all-solid-state lithium-ion battery

Composite polymer electrolyte is promising in future lithium-ion battery due to its high safety, easy-processing feature and electrochemical stability. However, it is still a great challenge to obtain the electrolyte with both low thickness and high conductivity. In this work, we synthesize a ~25 μm thin electrolyte membrane with the Li7La3Zr2O12-embedded polyacrylonitrile-based fiber as the 3D backbone and the in-situ polymerized gel electrolyte as the fillers. The electrolyte with the multi-dimensional lithium ion conducting channels exhibits a high conductivity of 1.60 × 10−3 S·cm−1 at 25 °C. The wide electrochemical window of 4.5 V vs. Li/Li+ enables the electrolyte to match with high voltage cathode materials. The Li||Li symmetric cell with the electrolyte shows a high cycling stability with more than 350 h at a current density of 0.1 mA·cm−2. In addition, the solid-state LiFePO4||Li full cell has a stable charge-discharge cycle at 25 °C. This thin electrolyte can be used in lithium-ion batteries to improve energy density. The method of preparing thin composite polymer electrolyte membrane can also be extended to other polymer-filler systems.

Effects of chemical pre-treatment on structure and property of polyacrylonitrile based pre-oxidized fibers

Polyacrylonitrile-based pre-oxidized fibers with improved thermal stability, flame retardant, and mechanical properties were made from the pristine polyacrylonitrile fibers through chemical pretreatment followed by pre-oxidation in air. The morphological structure of the polyacrylonitrile-based pre-oxidized fibers was investigated by Fourier transfer infrared spectra, X-ray diffraction, scanning electron microscopy, and X-ray energy dispersive spectrometer. The changes of characteristic functional groups and chemical compositions confirmed the successful modification of the polyacrylonitrile fibers during pre-treatment. The grooves and cracks on the surface of polyacrylonitrile-based pre-oxidized fibers were remarkably decreased in comparison with that of pristine polyacrylonitrile fibers. The evolution of crystalline structure of the polyacrylonitrile fibers proved the occurrence of cyclization reactions during pre-oxidation. Meanwhile, thermal stability, flame retardant, and mechanical properties of polyacrylonitrile-based pre-oxidized fibers were also investigated by thermogravimetric analyzer, oxygen index meter, micro combustion calorimeter, and single fiber tensile tester, respectively. The results demonstrated that the polyacrylonitrile-based pre-oxidized fibers initially pre-treated by hydroxylamine hydrochloride, followed by monoethanolamine, had a high limiting oxygen index of 40.1 and breaking strength of 2.03 cN/dtex. The peak of heat release rate and total heat release of polyacrylonitrile-based pre-oxidized fibers decreased significantly while its charred residues increased, contributing to the improved flame retardant property.

Determination of the optimum porosity for 2-CEES adsorption by activated carbon fiber from various precursors

In this study, we prepared ACFs with a high specific surface area from various precursors (rayon, pitch, and oxidized polyacrylonitrile-based fibers) by a steam-activation technique and investigated the effects of the micropore and mesopore fraction on 2-CEES adsorption behaviors. The activation time was precisely controlled so that the activation yield was in the range of 35–40% to ensure the mechanical properties of the ACFs. The N2 adsorption isotherm characteristics at 77K were confirmed by Brunauer–Emmett–Teller, Barrett–Joyner–Halenda and non-local density functional theory equations. The adsorption capacities of the ACF were measured by breakthrough experiments in the gas phase (750 μg/mL of 2-CEES in N2 flow). The removal efficiency of the ACFs was evaluated and compared with that of AC. From the results, specific surface areas and total pore volume of the ACF were determined to be 1380–1670 m2/g and 0.61–0.82 cm3/g, respectively. It was also observed that various pore characteristics of ACF were found to be dependent on crystallite structure of each precursor. The break through time (C/C0 = 0.10) was in the order of Oxi-Pan-H-9-2 < Saratoga AC < Rayon-H-9-3 < Pitch-H-9-4. This indicates that 2-CEES adsorption capacity could be a function not only of specific surface area or total pore volume, but also of sub-mesopore volume fraction in the range of 1.5–2.5 nm of adsorbents.

Highly electroconductive mesoporous activated carbon fibers and their performance in the ionic liquid-based electrical double-layer capacitors

The development of supercapacitors with high energy density calls for multi-functional electrode materials without obvious drawbacks in capacitance performance and in device processing. The highly electroconductive mesoporous activated carbon fibers (MACFs) for 4 V supercapacitors in ionic liquids are proposed. Preparing by the controlled carbonization and activation of polyacrylonitrile-based fibers by CO2 at high temperature, MACFs exhibit high special surface area (2404 m2/g), large mesopore volume (2.3 cm3/g), large packing density (0.25 g/cm3), high electrical conductivity of 57–195 S/cm, good chemical stability at high voltage and low liquid intake ability. As tested in EMIMBF4 electrolyte at 4 V, MACFs exhibit high capacitance (204 F/g at 0.5 A/g), high energy density (113 Wh/kg) and excellent capability of capacitance retention. Such excellent capacitance performance is also due to the one-dimensional structure of MACFs, with the long carbon in-plane length for electron transfer in axial direction and the short radial diffusion distance for ions of ionic liquids. To the best of our knowledge, the obtained MACFs are the first material combining all advantages of conventional electrode material (activated carbon) and new generation electrode materials (mainly carbon nanotubes and graphene) together, as well as minimizing their major drawbacks.

Hydrogen peroxide modified polyacrylonitrile-based fibers and oxidative stabilization under microwave and conventional heating – The 1st comparative study

Abstract Polyacrylonitrile (PAN) precursor was modified with hydrogen peroxide, and oxidative stabilisation studies were carried out using conventional and microwave heating. Wetting property, bulk density, Fourier transform infrared spectroscopy (FT-IR), extent of reaction (EOR), X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy and mechanical properties were also investigated. Results show that PAN fibers have excellent wettability with hydrogen peroxide. Hydrogen peroxide modification can shorten the oxidation stabilisation time of PAN fibers. The FT-IR spectrum shows that the stabilised fibers modified by hydrogen peroxide have a conjugated structure, and the EOR value of hydrogen peroxide-modified PAN fibers stabilised by microwave heating is the largest. XRD analysis shows that hydrogen peroxide-modified fibers stabilised by microwave heating have low stack domains and height of the interlayer spacing. The SEM and Raman spectra indicate that hydrogen peroxide can improve the surface finish of the fibers and reduce defects. In addition, hydrogen peroxide-modified fibers stabilised by microwave heating exhibit excellent mechanical properties, with fineness of 0.78 dtex, strength of 1.60 dtex, and elongation at break of 3.49%.

Carbonization of polyacrylonitrile-based fibers under defined tensile load: Influence on shrinkage behavior, microstructure, and mechanical properties

The influence of temperature and tensile mechanical load during carbonization of stabilized polyacrylonitrile fibers on the shrinkage behavior, the crystalline parameters and the mechanical properties of the resulting fibers is investigated. To this end, a newly developed experimental setup enables a defined heat treatment of fiber bundles up to a temperature of 1200 °C under control of tensile load. The attachment devices for the fiber bundles are located in the hot zone of the furnace. Additionally, carbonization treatments up to 1600 °C without application of load are performed.Three characteristic temperature regions of the carbonization process are identified.Between 250 °C and 450 °C the still ductile fibers contain rests of the polyacrylonitrile phase experiencing stabilization reactions. Stacked, heterocyclic planar structures grow due to molecular crosslinking. Between 450 °C and 1000 °C graphite-like turbostratic carbon is formed. Crystallites grow laterally, crystallinity increases, and the material becomes brittle. Above 1000 °C a pronounced increase in stacking height, lateral crystallite size and crystallinity occurs, while the interlayer distance decreases. Mechanical properties reach the values typical for commercial carbon fibers.Application of a tensile stress during carbonization influences fiber dimensions at temperatures between 250 °C and 450 °C, where the material shows ductile behavior. Shrinkage in fiber direction is hindered and radial shrinkage is favored. At temperatures above 1000 °C, crystalline parameters and mechanical properties of the resulting carbon fibers depend on the applied tensile stress. Carbonization without tensile stress results in significantly lower mechanical properties.The impact of heat treatment temperature and applied stress on carbon fiber properties demonstrate the possibility of a defined tailoring of fiber characteristics.

Dialysis membranes: A 2018 update.

Dialysis membranes are the basic element of a hemodialyzer. Synthetic and natural materials characterized by various fiber arrangements are used in their production. The most up-to-date ones are made of synthetic polymers such as polyamide, phosphatidylserine (PS), polyacrylonitrile-based fiber (PAN), polyarylethersulfone, polyethersulfone, or polymethylmethacrylate. Dialysis membranes are characterized by the ability to remove uremic molecules, which can be divided into small water-soluble compounds, protein-bound compounds and larger "middle molecules". Newer membranes such as medium cut off membranes (MCO) allow the removal of a wider spectrum of uremic molecules, which reduces the risk of late complications of dialysis. Dialysis membranes are used in therapy methods such as low flux, high flux or HDx therapy. An important aim in dialysis membrane development is to increase their biocompatibility. Insufficient biocompatibility can result in complement activation or platelet activation, which can lead to an increased risk of cardiovascular complications. The aim of the study is to discuss the latest reports on dialysis membranes.

High-resolution imaging of the nanostructured surface of polyacrylonitrile-based fibers

In our study, we present atomic force microscopy (AFM) investigations of the surface of Polyacrylonitrile-based carbon fibers utilizing two different AFM probes, a standard tip as used in literature up to now and a recently made available super sharp tip. Using the super sharp tip, we identified so far not reported pore-like nanostructures distributed homogeneously over the surface of the fibers. We show that such nanopores are already present on the surface of the corresponding precursor fiber, indicating that these structures are characteristic for the fiber along the production process. To investigate a possible correlation between the surface structures and the mechanical properties of carbon fibers, we further analyzed the surface of various carbon fibers showing different tensile strengths. All investigated fibers show characteristic nanoporous surface structures and a correlation was found between the Nanopore size and shape and the mechanical properties. The effective nanopore area and the aspect ratio of the nanopores decrease with increasing tensile strength of the fibers. In addition, the nanoroughness of the fiber surface is correlated to the nanopore size and also decreases with increasing tensile strength.

Polyacrylonitrile-based fiber modified with thiosemicarbazide by microwave irradiation and its adsorption behavior for Cd(II) and Pb(II)

A novel thiosemicarbazide modified adsorbent (PANMW–TSC) based on polyacrylonitrile fiber was successfully synthesized under microwave irradiation, which was applied for the uptake of Cd(II) and Pb(II) from aqueous solution subsequently. Microwave irradiation method is a new approach to achieve the modification and it turns out that just a 30min process is enough for the anchoring of functional groups in the fiber matrix. The surface characterization was performed by fourier transform infrared spectroscopy (FT–IR), scanning electron microscopy (SEM) elemental analysis (EA) and thermogravimetric analysis (TGA), indicating that the modification was successfully accomplished. Batch adsorption experiments including equilibrium isotherms, kinetics and the effects of pH and temperature on the adsorption of Cd(II) and Pb(II) were systematically studied. Among three kinetic models, the pseudo-second-order kinetic model provides the best correlation for the process. The nonlinear resolution of the Langmuir isotherm equation has been found to show the closest fit to the equilibrium date. Thermodynamic parameters, involving △G, △H and △S were also calculated from graphical interpretation of the experimental data, which suggest that metal ions adsorption onto PANMW–TSC fibers is spontaneous and exothermic. Regeneration of PANMW–TSC fibers loaded with metal ions was efficiently done with 0.5M HNO3, by which the investigated adsorbent could be used reproductively for five times with a small decrease in sorption capacity. The feasible preparation of PANMW–TSC fibers with high adsorption capacities opens a new perspective in the potential application for wastewater treatment.

The effect of nanostructure on the tensile modulus of carbon fibers

Using the Eshelby equivalent inclusion theory and the Mori–Tanaka method, a new micromechanical model is proposed to predict the tensile modulus of carbon fibers by considering crystallites, amorphous components, and microvoids of the fiber structure. Factors that affect the tensile modulus included the aspect ratio of crystallites, the aspect ratio of microvoids, the volume fraction of crystallites, the volume fraction of microvoids, and the orientation degree of crystallites. To follow the dependence of the tensile modulus of the fibers on microstructure, thirty different types of polyacrylonitrile-based fibers were prepared. The aspect ratios and orientation degrees of crystallites were calculated directly by X-ray diffraction. The aspect ratios and volume fractions of microvoids were obtained by small-angle X-ray scattering. The average tensile modulus of amorphous was estimated by dealing with thirty types of PAN-based fibers. The volume fractions of crystallites were obtained by the micromechanical model. Some relationships are concluded: (1) the tensile modulus increased with increasing volume fractions of crystallites, aspect ratios of crystallites and microvoids, and orientation degree of crystallites; (2) the tensile modulus increased with decreasing volume fractions of microvoids.

Iodine Treatment of Lignin–Cellulose Acetate Electrospun Fibers: Enhancement of Green Fiber Carbonization

Pure biopolymer-based electrospun precursor carbon fibers are fabricated using an abundant and inexpensive biopolymer lignin blended with renewable resource-based cellulose acetate (CA). Iodine treatment on the fabricated green fiber was successfully performed in order to enhance the carbonization process as well as the retention of fiber morphology. The absorption mechanism of iodine by lignin and cellulose acetate and their derived electrospun green fibers has been investigated by means of thermal behavior and morphological retention. It was found that iodine treatment plays a vital role in altering the graphitization behavior as well as morphology retention during the carbonization process. With the help of iodine treatment, the green precursor fibers were successfully converted into thin carbon fibers, and scanning electron microscopy analysis confirmed the retention of fibrous structures with diameters around 250 nm. Raman spectroscopy revealed that although the overall level of graphitization was lower compared to polyacrylonitrile-based fibers, the graphitic crystallite size was larger in the produced carbon fibers. The produced pure biopolymer fibers and iodine treatments show promise for the production of green and cost-reduced carbon fibers.