Wet‐spun,photoinitiator‐modified polyacrylonitrile precursor fibers: UV‐assisted stabilization

The role of dilute concentrations (～1 wt %) of a photoinitiator, 4,4 ′ ‐bis(diethylamino)benzophenone, on the processability and properties of the resulting wet‐spun polyacrylonitrile (PAN) fibers are reported. Rheology measurements show no adverse effect on the viscosities of solutions by the addition of the photoinitiator. Fibers containing photoinitiator were successfully wet‐spun from PAN – DMSO solution. FTIR results prove that 4,4 ′ ‐bis(diethylamino)benzophenone was retained in the fibers after coagulation and post‐stretching. SEM micrographs show no deterioration of the post‐stretched fiber microstructure due to the presence of photoinitiator. Tensile testing results show a small reduction in the strain‐at‐break of post‐stretched fibers containing photoinitiator when compared with pure (control) PAN fibers. After UV treatment, fibers with 4,4 ′ ‐bis(diethylamino)benzophenone display a higher tensile modulus compared with the other sets. Wide‐angle X‐ray diffraction results show no significant decrease in interplanar spacing and size of the crystals within the fibers containing photoinitiator, but such fibers retain a higher extent of molecular orientation after being UV treated. Conversion indices were measured from the WAXD spectra and compared with conventional thermal stabilized fibers. This correlation confirms that the addition of 1 wt % photoinitiator to PAN followed by 5 min of UV treatment leads to a conversion index that is observed in control fibers after more than an hour, which could reduce the conventional thermo‐oxidative stabilization time significantly. These results indicate the potential of the dual stabilization route in generating precursor fibers with higher molecular orientation, and possibly reducing the thermo‐oxidation time during carbon fiber processing. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 2494–2503, 2013     

The influence of continuous stabilization on the properties of stabilized fibers and the final activated carbon fibers. Part I

This study found that, during continuous stabilization and carbonization, the shrinkage behavior of polyacrylonitrile (PAN) fibers affects the morphology and properties of the stabilized fibers, the carbon fibers, and the final activated carbon fibers. In the stabilized fibers, a higher shrinkage of the PAN fibers during the stabilization process increased the oxygen content and the core proportion and decreased the formation of ladder polymers. The effect of the shrinkage behavior of the PAN fibers on the fracture surfaces of the stabilized fibers is discussed. A microstructure model of stabilized fibers is presented, depicting fine radial structure at the fiber center. When stabilized fibers were carbonized during a continuous carbonization process, a hole structure was found in the fiber center at the temperature of 800°C, and a hollow core was found at the temperature of 1300°C. The shrinkage behaviors during the stabilization stage and the formation of the hole and the hollow core in the fiber's center during the carbonization stage are discussed. The carbon fibers developed from shrunk stabilized fibers have a lower density and lower preferred orientation than fibers developed from unshrunk stabilized fibers. But the fibers developed in this new process have greater nitrogen and oxygen content, and have a greater porosity than the traditionally-produced fibers. The mechanical properties df the new and the traditional fibers are comparable. These characteristics are very valuable in the production of activated carbon fibers, which will be described in our next paper.     

Mass production of carbon nanotube‐reinforced polyacrylonitrile fine composite fibers

A facile and large‐scale production method of polyacrylonitrile (PAN) fibers and carboxyl functionalized carbon nanotube reinforced PAN composite fibers was demonstrated by the use of Forcespinning® technology. The developed polymeric fibers and carbon nanotube‐reinforced composite fibers were subsequently carbonized to obtain carbon fiber systems. Analysis of the fiber diameter, homogeneity, alignment of carbon nanotube and bead formation was conducted with scanning electron microscopy. Thermogravimetric analysis, electrical, and mechanical characterization were also conducted. Raman and FTIR analyses of the developed fiber systems indicate interactions between carbon nanotubes and the carbonized PAN fibers through π–π stacking. The carbonized carbon nanotube‐reinforced PAN composite fibers possess promising applications in energy storage applications. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40302.     

UV‐induced crosslinking and cyclization of solution‐cast polyacrylonitrile copolymer

An alternative, rapid stabilization route for polyacrylonitrile (PAN) precursors is reported based on UV‐induced crosslinking and cyclization reactions. Two mechanisms of photoinitiation were investigated: homolytic cleavage and hydrogen abstraction. Solution‐cast PAN copolymer samples were irradiated for different durations (100, 300, and 600 s) and temperatures (～65 and 100°C, below and above glass transition temperature respectively). FTIR spectra show the formation of carbon–oxygen, carbon–nitrogen, and carbon–carbon double bonds (1450–1700 cm^?1 region) attributed to the development of cyclized structure. Conversion indices estimated from the FTIR spectra indicate samples containing hydrogen abstraction photoinitiator show higher extents of cyclization among the three main set of samples. This observation was also confirmed by higher gel percentages measured on the same set of samples. FTIR conversion indices of samples UV‐treated above glass transition temperature were higher compared with that for the same specimens UV‐treated below glass transition temperature. DSC results show that samples containing hydrogen abstraction photoinitiator enable a higher extent of post‐UV thermal cyclization. FTIR spectra of the UV treated samples were compared with conventional thermal stabilized specimens. This comparison confirms that the addition of 1 wt % photoinitiator to PAN followed by 5 min of UV treatment increases the rate of the cyclization reaction and reduces the thermal oxidation time by over an hour, which could significantly reduce the conventional stabilization time by half. These results indicate the potential for an energy‐efficient, cost‐effective route for producing carbon fibers. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

An investigation on structure characterization of thermally stabilized polyacrylonitrile precursor fibers pretreated with guanidine carbonate prior to carbonization

Thermal stabilization of polyacrylonitrile (PAN) precursor fiber was performed with a pretreatment of an aqueous guanidine carbonate solution and its structure was thoroughly characterized using a combination of infrared spectroscopy (IR), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), density, elemental analysis, and X‐ray diffraction measurements. The use of guanidine carbonate pretreatment of polyacrylonitrile precursor fiber was found to be very useful for the acceleration of thermal stabilization of polyacrylonitrile precursor fiber prior to the carbonization stage. The results obtained from density, thermal analysis (TGA and DSC), infrared‐spectroscopy and X‐ray diffraction methods suggested an accelerated thermally stable aromatic ladder structure formation resulting in much reduced thermal stabilization time. X‐ray observations showed the transformation of the original structure from a highly ordered phase to a totally disordered amorphous phase which seemed to be a direct consequence of the crosslinked and cyclized structure present in the stabilized fibers. The results obtained from the infrared spectra of thermally stabilized samples showed a rapid and simultaneous cyclization and dehydrogenation reactions aided by the oxygen uptake in the form of oxygen containing functional groups. Guanidine carbonate pretreated and thermally stabilized PAN precursor fibers showed a carbon yield of 52.5% at 1100°C obtained from TGA measurements. The use of guanidine carbonate pretreatment is expected to significantly increase the productivity of carbon fiber manufacturing at a substantially reduced cost by significantly reducing the time necessary for thermal stabilization of polyacryonitrile fiber. POLYM. ENG. SCI., 2012. © 2011 Society of Plastics Engineers     

Processing,structure, and properties of gel spun PAN and PAN/CNT fibers and gel spun PAN based carbon fibers

Polyacrylonitrile (PAN) and PAN/carbon nanotube (PAN/CNT) fibers were manufactured through dry‐jet wet spinning and gel spinning. Fiber coagulation occurred in a solvent‐free or solvent/nonsolvent coagulation bath mixture with temperatures ranging from ?50 to 25°C. The effect of fiber processing conditions was studied to understand their effect on the as‐spun fiber cross‐sectional shape, as well as the as‐spun fiber morphology. Increased coagulation bath temperature and a higher concentration of solvent in the coagulation bath medium resulted in more circular fibers and smoother fiber surface. as‐spun fibers were then drawn to investigate the relationship between as‐spun fiber processing conditions and the drawn precursor fiber structure and mechanical properties. PAN precursor fiber tows were then stabilized and carbonized in a continuous process for the manufacture of PAN based carbon fibers. Carbon fibers with tensile strengths as high as 5.8 GPa and tensile modulus as high as 375 GPa were produced. The highest strength PAN based carbon fibers were manufactured from as‐spun fibers with an irregular cross‐sectional shape produced using a ?50°C methanol coagulation bath, and exhibited a 61% increase in carbon fiber tensile strength as compared to the carbon fibers manufactured with a circular cross‐section. POLYM. ENG. SCI., 55:2603–2614, 2015. © 2015 Society of Plastics Engineers     

Effects of oxidation time on the structure and properties of polyacrylonitrile‐based activated carbon hollow fiber

Polyacrylonitrile (PAN) hollow fibers were pretreated with ammonium dibasic phosphate, then further oxidized in air, carbonized in nitrogen, and activated with carbon dioxide. The effects of oxidation time of PAN hollow fiber precursor on the microstructure, specific surface, pore size distribution, and adsorption properties of PAN‐based activated carbon hollow fiber (PAN‐ACHF) were studied in this work. Both of specific surface area and adsorption ratio to VB₁₂ reach maximums when PAN hollow fibers are oxidized for 5 h in air. The adsorption ratios for creatinine are all higher than 90% over all oxidation time. After 5 h of oxidation, the number of pores on the surface obviously increases, and the pore size is uniform. After 7 h of oxidation, the number of macropores in PAN‐ACHF increases. The dominant pore sizes of mesopores in PAN‐ACHF range from 2 to 5 nm. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

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.     

Effects of the oxidation temperature on the structure and properties of polyacrylonitrile‐based activated carbon hollow fiber

Polyacrylonitrile (PAN) hollow fibers were pretreated with ammonium dibasic phosphate, oxidized in air, carbonized in nitrogen, and activated with carbon dioxide. The effects of the oxidation temperature of the PAN hollow fiber precursor on the microstructure, specific surface, pore size distribution, and adsorption properties of PAN‐based activated carbon hollow fiber (PAN‐ACHF) were studied. When PAN hollow fibers were oxidized at 270°C, because of drastic oxidation, chain scission occurred, and the number of pores within and on the surface of the resultant PAN‐ACHF increased, but the pores were just in the thinner region of the skin of PAN‐ACHF. The surface area of PAN‐ACHF reached a maximum when the oxidation temperature was 270°C. The adsorption ratios to creatinine were all higher than 90% at all oxidation temperatures, and the adsorption ratio to VB₁₂ reached a maximum (97%) at 230°C. The dominant pore sizes of the mesopores in PAN‐ACHF ranged from 2 to 5 nm. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 203–207, 2005     

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     

Preparation and characterization of carbon fibers from polyacrylonitrile precursors

The present work deals with the preparation of carbon fibers from polyacrylonitrile (PAN) fibers. The chemical composition and physical properties of the starting fibers were determined. The PAN fibers were stabilized in air at the temperatures (230, 270, and 300°C) with the heating time from 40 to 420 min. The effects of both final stabilization temperature and heating rate on the chemical and physical properties of the prepared stabilized fibers were studied. The chosen stabilized fibers samples were carbonized in argon atmosphere at the temperatures (1000, 1200, and 1400°C) with different heating rates 5, 10, 15, and 20°C min^?1. The effects of both carbonizing temperature and heating rate on the weight loss, density, elemental composition, and IR absorption spectra of carbonized fibers were also studied. The fiber sample, which was carbonized at 1400°C, contains 97.55% carbon, 1.75% nitrogen, and 1.4% hydrogen. This means that carbonizing the stabilized fibers at 1400°C in argon atmosphere is suitable to get oxygen‐free carbon fibers. Therefore, the used carbonizing temperature in the present work (1400°C) is suitable to produce moderate heat‐treated carbon fibers with the heating rate of 15°C min^?1. The modulus of the prepared carbon fibers was compared to that of industrially produced fibers using the results of X‐ray analysis. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Stabilization and carbonization of gel spun polyacrylonitrile/single wall carbon nanotube composite fibers

Gel spun polyacrylonitrile (PAN) and PAN/single wall carbon nanotube (SWNT) composite fibers have been stabilized in air and subsequently carbonized in argon at 1100 °C. Differential scanning calorimetry (DSC) and infrared spectroscopy suggests that the presence of single wall carbon nanotube affects PAN stabilization. Carbonized PAN/SWNT fibers exhibited 10-30 nm diameter fibrils embedded in brittle carbon matrix, while the control PAN carbonized under the same conditions exhibited brittle fracture with no fibrils. High resolution transmission electron microscopy and Raman spectroscopy suggest the existence of well developed graphitic regions in carbonized PAN/SWNT and mostly disordered carbon in carbonized PAN. Tensile modulus and strength of the carbonized fibers were as high as 250 N/tex and 1.8 N/tex for the composite fibers and 168 N/tex and 1.1 N/tex for the control PAN based carbon fibers, respectively. The addition of 1 wt% carbon nanotubes enhanced the carbon fiber modulus by 49% and strength by 64%.     

UV stabilization route for melt-processible PAN-based carbon fibers

Ultraviolet radiation-based stabilization routes were explored to produce carbon fibers from melt-processible PAN-based copolymers. An acrylonitrile/methyl acrylate (AN/MA) copolymer was melt-spun into fibers that were crosslinked using UV radiation. The fibers could then be stabilized by oxidative heat treatment, and subsequently carbonized. Physical and mechanical testing was performed to determine the degree of stabilization and the properties of the stabilized and carbonized fibers.     

Structure and properties of partially cyclized polyacrylonitrile‐based carbon fiber—precursor fiber prepared by melt‐spun with ionic liquid as the medium of processing

Carbon fiber has many excellent properties. Currently, the precursor fiber of polyacrylonitrile (PAN)‐based carbon fiber is made from solution by wet or dry spinning process that requires expensive solvents and costly solvent recovery. To solve this problem, we developed a melt‐spun process with ionic liquid as the medium of processing. The melt‐spun precursor fiber exhibited partially cyclized structure. The structure and properties of the melt‐spun PAN precursor fiber were analyzed by combination of scanning electron microscope, Fourier transform infrared spectroscopy, differential scanning calorimetry, X‐ray diffraction, thermogravimetry, ultraviolet spectroscopy, flotation technique, sound velocity orientation test, linear density, and tensile strength tests. The results showed that the tensile strength of melt‐spun PAN precursor fiber was fairly high reached up to 7.0 cN/dtex. The reason was the low imperfect morphology and a cyclized structure formed by in situ chemical reaction during melt‐spun process. Due to the existence of partially cyclized structure in the melt‐spun PAN precursor fiber, exothermic process was mitigated and the heat evolved decreased during thermal stabilization stage in comparison with commercial precursor fibers produced by solution‐spun, which could shorten the residence time of thermal stabilization and reduce the cost of final carbon fiber. POLYM. ENG. SCI., 55:2722–2728, 2015. © 2015 Society of Plastics Engineers     

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     

Gel-spun carbon nanotubes/polyacrylonitrile composite fibers. Part I: Effect of carbon nanotubes on stabilization

Addition of carbon nanotubes (CNTs) in polyacrylonitrile (PAN) fibers significantly improves the mechanical properties of the resulting carbon fibers. This study focuses on the effect of different types of CNTs on chemical, mechanical and structural changes during the stabilization of gel-spun CNT/PAN composite fibers. Among the different types of CNTs, it was observed that CNTs containing more walls had lower reinforcement efficiency than CNTs containing fewer walls. Similarly CNTs containing fewer walls exhibited higher orientation of the ladder polymer and greater effect on the formation of β-amino nitrile in the stabilized fibers. Wide angle X-ray diffraction, infrared spectroscopy, and scanning electron microscopy were used to determine the optimum stabilization time. Additionally, it was found that the higher tension applied during stabilization improved the properties of the stabilized fibers, and the addition of CNTs increased the maximum tension that the fiber can bear.     

Effects of activation time on the properties and structure of polyacrylonitrile‐based activated carbon hollow fiber

Polyacrylonitrile (PAN) hollow fibers were pretreated with ammonium dibasic phosphate, then further oxidized in air, carbonized in nitrogen, and activated with carbon dioxide. The effects of activation time of a precursor fiber on the microstructure, specific surface area, pore‐size distribution, and adsorption properties of PAN‐based activated carbon hollow fibers (PAN‐ACHF) were studied in this work. The BET surface area of PAN‐ACHF and surface area of mesopores gradually increase with activation time extending, and reach the maximum values, 780 and 180 m² g^?1, respectively, when fibers are activated at 800°C for 100 min. The adsorption ratio to creatinine changes little with activation time extending and all values over all activation time are above 90%. The adsorption ratio to VB₁₂ gradually increases with activation time extending before 60 min, and then becomes relatively constant from 60 to 100 min. The number of pores on the surface of PAN‐ACHF increases with activation time extending. The amount of mesopores in PAN‐ACHF made of fibers activated for different time increases with activation time extending and the dominant pore sizes of mesopores in PAN‐ACHF range from 2 to 5 nm. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 2565–2569, 2006     

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 heating and stretching polyacrylonitrile precursor fibers in steam on the properties of stabilized fibers and carbon fibers

Polyacrylonitrile (PAN) fibers were heated and stretched in a steam bath mixed with nitrogen in the early stage of stabilization. During this process, the flexible linear chain of PAN molecule was converted to a rigid ladder structure by intramolecular cyclization. The shrinkage tension along the fiber axis decreased during this heat treatment, because water molecules here acted as a plasticizer for stretching. Consequently, the preferred orientation of ladder molecules in the stabilized fibers was enhanced. As a result, the preferred orientation, crystallite dimensions, and mechanical properties of the resultant carbon fibers (CFs) and graphite fibers (GFs) were improved, comparing with the corresponding CFs and GFs without pretreatment in the steam bath. POLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers.     

Influence of continuous stabilization on the physical properties and microstructure of PAN-based carbon fibers

A continuous stabilization and carbonization process was used to prepare polyacrylonitrile (PAN)-based carbon fibers. The stepwise stabilization of PAN fibers was tried at various temperatures. The effect of stepwise stabilization on the physical properties and microstructure of the final carbon fibers is reported in this article. The fixed temperature in stepwise stabilization is kept below the fusion temperature of PAN precursors to avoid overstabilization of the fibers. The optimum stepwise stabilization process not only increases the amount of ladder polymer in stabilized fiber but also improves the physical and mechanical properties of the resultant carbon fibers. The formation of closed pores from open pores in carbon fiber occurs at 1100°C, but the formation of closed pores occurs at 200°C lower for carbon fiber developed from overstabilized fiber. The effect of continuous stepwise stabilization on the properties of resulting stabilized fibers and the variation in physical properties, element composition, and microstructure of carbon fibers during the carbonization process are also reported in this article.