Microstructural evolution in polyacrylonitrile fibers during oxidative stabilization

Microstructural evolution in polyacrylonitrile fibers during oxidative stabilization was investigated by high‐resolution transmission electron microscopy and wide‐angle X‐ray diffraction. Nanocrystallites and two types of amorphous structures, that is, onion‐like spheres and maze‐like matrix, are observed not only in the precursor fibers but also in the fibers stabilized at 230, 255, and 275 °C. During oxidative stabilization, changes take place earlier in the amorphous onion‐like spheres than in the ordered regions. With increasing temperature, the outer layers of the onion‐like spheres gradually interlace with each other, microscopically demonstrating crosslinking reactions among neighboring macromolecules. The interplanar spacing in crystalline regions increases and the crystallite size decreases as a result of an order–disorder transition. The transition in large crystallites is not completed even in the fibers that are stabilized at 275 °C, suggesting that uniform microstructure and fine crystallites are conducive to homogenous stabilization. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 759–765, 2008     

Effect of the oxygen-induced modification of polyacrylonitrile fibers during thermal-oxidative stabilization on the radial microcrystalline structure of the resulting carbon fibers

The oxygen-induced modification of polyacrylonitrile (PAN) fibers during the final stage of thermal-oxidative stabilization is used to control the degree of chemical reactions and the radial structural homogeneity of fibers. A radial structure model for oxidized PAN fibers (OFs) and carbon fibers (CFs) has been established by Raman spectroscopy and wide angle X-ray diffraction. According to the model, the cross-section of OFs is divided into the internal and external regions; the oxygen-induced modification has a greater effect on the structural evolution of internal regions than that of external regions. When the oxygen volume content for the modification is 22.2%, the OFs possess the highest level value for degree of disorder (DD) in internal regions. This is inherited by the corresponding CFs with the best radial structure homogeneity and optimum mechanical properties; meanwhile, the coefficient of variation for DD is defined to characterize the radial homogeneity of CFs. The mechanism of the oxygen-induced modification demonstrates that the mechanical properties of the resulting CFs depend on the degrees of the intermolecular cyclization and oxidation which are beneficial to the decrease in CF crystallite size.     

Microstructure and properties of polyacrylonitrile based carbon fibers

The microstructure of polyacrylonitrile (PAN)-based carbon fibers with different mechanical properties was investigated. It was found that the tensile strength of the PAN-based carbon fibers generally decrease with the increase in the modulus. The properties of PAN-based carbon fiber are mainly controlled by the microstructure and microvoids. The increase in size and orientation of graphite crystallites follows the decrease in interlayer space of graphite sheets, which accompanies the increase in modulus and decrease in tensile strength of the carbon fibers. Simultaneously, the increase in the modulus of the carbon fibers accompanies the merging of the elliptical microvoids along the fiber axis and the turbostratic graphite in the carbon fibers transforms into 3D ordered graphite lamellar structure. This work provides useful information on tailoring the mechanical properties of carbon fibers by adjusting the microstructure.     

Fibrillar structure development of polyacrylonitrile fibers treated by ultrasonic etching in oxidative stabilization

Polyacrylonitrile fibers were oxidatively stabilized through 10 gradient‐elevated temperature zones in sequence. The ultrasonic etching method was used for fibril separation of fibers heated at different temperatures, and the fibrillar structure development was studied by scanning electron microscopy. The voids among fibrils are the weak combination points. Under ultrasonic etching, the voids are enlarged. Subsequently, the solvent enters and spreads among fibrils, which results in the separation of fibrils. Separated fibrils with diameters of 100–400 nm appear in fibers heated at less than 235°C. Fibrils in fibers heated from 195°C to 235°C tend to adhere to each other, and the observed macrofibrils are composed of several to dozens of fibrils. For fibers heated from 195°C to 245°C, only a few fibril bundles emerge on the skin near the fiber end, and the fibrils manifest themselves as numerous protuberances on the cross section. In the ranges of 255–275°C, fibrils compactly combine with each other, which suggests insolubility and infusibility, and no separated fibrils appear. The fibrils arrange in a systematic way along the fiber axis and grooves parallel to the fiber axis on the fibers' surface. These grooves are the macro behavior of fibrils arranging on the fiber surface. Copyright © 2017 John Wiley & Sons, Ltd.     

Effects of deformation-induced orientation on cyclization and oxidation of polyacrylonitrile fibers during stabilization process

Orientation of copolymer polyacrylonitrile(PAN) chains during their deformation prior to stabilization and the further effect on the stabilization were investigated in detail.Results reveal that the orientation of PAN chains presents a saturation point of 69.51%when the deformation ratio reaches approximately 1.07,meanwhile the cyclization rather than the oxidation has a stronger dependence on the orientation of PAN chains during stabilization.The cyclization is facilitated that the cyclization degree is increasing while the activation energy is decreasing obviously as a consequence of the developing orientation of PAN fibers before the saturation point;however,it is restrained during the further deformation of PAN fibers after the point.The resulting carbon fibers obtained from the PAN fibers prepared at the saturation point possess the highest mechanical properties of 4.07 GPa in tensile strength and 249.0 GPa in tensile modulus.     

Ultrafine porous carbon fibers for SO2 adsorption via electrospinning of polyacrylonitrile solution

This work describes the potential capability of ultrafine porous carbon fibers (UPCF) prepared via electrospinning in the removal of SO2 from a mixture gas stream. A series of conventional PCF (CPCF) and ultrafine PCF (UPCF) were produced under the identical conditions and UPCF was also modified. Compared to the CPCF, experimental results showed that the UPCF had a better adsorption capacity for SO2 due to its higher surface area and microporous volume. After the modification of the UPCF, adsorption capacities of UPCF for SO2 were improved further via increasing the N-containing amount of UPCF and substrate which was followed by few changes in its specific surface area. The optimum concentration of modified reagent is 10%. From the results of the fatigue test, it has been found that both the UPCF and the modified UPCF showed a good durability.     

Properties and structure of polyacrylonitrile fibers

With the use of the methods of dynamic mechanical analysis and thermomechanical analysis, the structural transformations of polyacrylonitrile fibers during heating are studied. It is shown that, regardless of the degree of macromolecule orientation, the fibers not exposed to heating are characterized by an amorphous ordered structure with an ordered mesophase. During heating, the fiber acquires a single-phase structure owing to transition of the amorphous component to the mesophase.     

Characteristic assessment of stabilized polyacrylonitrile nanowebs for the production of activated carbon nano-sorbents

Polyacrylonitrile(PAN) nanofibers with average diameter of 300 nm were produced by electro-spinning. The nanofibers were stabilized at different temperatures in the range of 180-270 ℃ in several duration times and heating rates. Fourier transforms infrared(FTIR) spectroscopy, differential scanning calorimetry(DSC) and X-ray diffraction(XRD) analyzing techniques were employed to measure the extent of stabilization reaction. By all procedures, the ranges of temperature and duration time recommended were about 250-270 ℃ and 1-2 h, respectively. Increasing the activation temperature from 800 ℃ to 1200 ℃ caused porosity and pore volume development up to 60% and 0.532 cm3/g, respectively. Pore width of all samples was calculated to be about 0.7 nm confirming micro-pore structure of the produced PAN based activated carbon nanofibers. Comparing dye adsorption for different adsorbents including chitin and granular activated carbon(GAC) showed the highest efficiency for the produced activated carbon nanofibers(ACNFs).     

A novel methodology for designing thermal processes in order to optimize stabilization of polyacrylonitrile (PAN) fibers

The aim of this work is to describe a novel methodology for optimizing the stabilization of polyacrylonitrile (PAN) fibers, through designing of proper thermal treatment. The methodology is based on a set of design rules and the procedure for implementing them, utilizing the time‐temperature‐transition (TTT) and the maximum permittable stress (max.stress) plots. The proposed approach is implemented in order to optimize the stabilization of commercial PAN fibers, resulting in a series of multistage thermal treatments. The changes of both physical and chemical structures of PAN during the progress of the multistage treatments were investigated and showed that the fibers were progressively converted into completely stabilized material; this gradual transformation permitted improvement of fiber annealing and minimized the effect of the decomposition reactions. The proposed methodology can be universally applied for achieving the global optimum of the stabilization process for any PAN precursor.     

XPS studies on the chemical structure of the stabilized polyacrylonitrile fiber in the carbon fiber production process

In the carbon fiber production process from polyacrylonitrile (PAN), PAN precursor is heated first in air to secure stabilization in the succeeding carbonization process at higher temperature. The mechanism of the stabilization reaction and chemical structure of the stabilized PAN have been examined by x-ray photoelectron spectroscopy and elemental analysis. The stabilized PAN was determined to have a ladderlike structure consisting of 40% acridone ring, 30% naphtyridine ring, 20% hydronaphtyridine ring, and others. This structure well explains the stability of the polymer in the succeeding carbonization process on carbon fiber production with conjugated π-electron systems over the whole polymer chain and intermolecular hydrogen bonds. A comonomer addition to the precursor was found to accelerate the dehydrogenation reaction in the stabilization process.     

Microstructural evolution during oxidative ablation in air for polyacrylonitrile based carbon fibers with different graphite degrees

Polyacrylonitrile‐based carbon fibers with different graphite degrees were oxidative ablated at 500 and 600 °C in air. By Thermal gravimetric (TG), Raman spectroscopy, X‐ray diffraction, and SEM, the mass loss, microstructure, and surface morphology of carbon fibers were investigated. The mass loss of carbon fiber increases linearly with increasing oxidative ablated time under 500 and 600 °C. The carbon fiber with higher graphite degree shows higher oxidative resistance, and the surface roughness increases gradually because of chemical ablation during the whole oxidation. A gloss morphology appears on the surface primarily because of physical denudation for carbon fibers with lower graphite degree and then burn off according to carbon and oxygen reaction. The crystallite size (La) decreases significantly, while interlayer spacing(d₀₀₂) remains nearly unchanged. SEM observation suggests the two kinds of ablation mechanisms for carbon fibers with different graphite degrees indicating that CC band in sp³ hybridization prefers to be attacked by oxygen molecule more than that in sp² hybridization during oxidation ablation in air. Copyright © 2012 John Wiley & Sons, Ltd.     

Mechanism and kinetics of the stabilization reactions of itaconic acid-modified polyacrylonitrile

The structural evolution and thermal behavior of polyacrylonitrile (PAN) homopolymer and copolymer [P(AN-IA)] containing about 1.5 mol% itaconic acid (IA) during stabilization in air were studied by Fourier transform infrared (FTIR) spectroscopy, differential scanning calorimetry (DSC) and thermogravimetry (TG). A new parameter Es=A1595cm⁻¹/A2243cm⁻¹ was defined to evaluate the extent of stabilization. The kinetic parameters, viz. activation energy (E_a) and pre-exponential factor (A) of the stabilization reactions were calculated by Kissinger method and Ozawa method. FTIR analysis indicated that the cyclization of nitrile groups was initiated at a lower temperature by the IA comonomer and the stabilization proceeded at a more moderate pace in P(AN-IA) than in PAN, while an IA additive was found to be decomposed and failed to initiate the cyclization at a lower temperature. The improvement effect of IA comonomer on the stabilization reactions was further confirmed by the dynamic thermal analysis and kinetic study.     

FT-IR study on interactions between solutes and entrainers in supercritical carbon dioxide

Fourier transform infrared (FT-IR) spectroscopy has been used to measure the molarities of hydrogen bonding species between carboxylic acids (acetic acid and palmitic acid) and water in supercritical CO₂. The equilibrium constants of dimerization for the carboxylic acids were determined in supercritical CO₂ with octane. Further, the interactions of propanol-d (1- and 2-propanol-d) or xylenol (2,5-, 2,6- and 3,4-xylenol) isomers with acetone in supercritical CO₂ were studied. Experiments were carried out at 308.2–313.2 K and 7.0–20.0 MPa. The molarities of hydrogen bonding species between the carboxylic acids and water in supercritical CO₂ increase with the increasing molarity of water. The carboxylic acids interact more easily with ethanol than water in supercritical CO₂. For supercritical CO₂+carboxylic acid+octane systems, the equilibrium constants between the carboxylic acid monomer and dimer increase with the increasing molarity of octane. The equilibrium constants of the carboxylic acids seem to approach to those in liquid paraffin according to addition of octane in supercritical CO₂. The amount of the interaction species between 1-propanol-d and acetone is larger than that between 2-propanol-d and acetone. The amount of acetone interacting with OH group for 3,4-xylenol is the largest among those for xylenol isomers. These differences among the isomers may be caused by the screen effects of methyl groups around hydroxyl group for the isomers.     

Substituent effects in the reaction of phenols with polyacrylonitrile radicals

Substituent effects in the reactivities of phenols towards polyacrylonitrile radicals have been studied in terms of Swain and Luptons' field (F_k) and resonance (R_k) components of the substituent parameters and the unique positional weighting factors (f_j and r_j) proposed by Williams and Norrington, with the aid of the following equation:

$\text{P}{}_{\mathrm{i}}\text{= x}{}_{\mathrm{i}}\text{?}{}_{\mathrm{i}}\text{F}{}_{\mathrm{k}}\text{+ ß}{}_{\mathrm{i}}\text{r}{}_{\mathrm{j}}\text{K}{}_{\mathrm{i}}\text{+ e}{}_{\mathrm{i}}\text{+}\text{P}\text{i}\text{o}$

P_i′s are the rate parameters, P_i⁰ being that for a standard reference compound. Two types of rate parameters have been employed—one, suggested by Simonyi, Tüdös and Pospisil (β) and another, suggested by us, (K), which is obtained from a plot of [Monomer]/(rate of polymerisation) vs [Phenol]. The correlations have been found to be quite satisfactory with both β and K. An attempt has been made to ascertain the nature of the transition state from the reaction parameters α_i and β_i; a dipolar transition state is suggested. The anomalous kinetic behaviour of hydroxy phenols has been discussed.     

Theoretical study of the P-Ylide reaction in the carbon nanotube

Recent studies have shown that the inner phase of carbon nanotubes (CNTs) can not only change the properties of molecules inside the tube, but also enhance or restrain the S_N2 reactions. Thus, the CNTs can be considered a form of solid solvent. In this paper, we study the [2+2] cycloaddition reaction between CH₂O and PH₃CH₂ in the gas phase, benzene solution and inner phase of CNT using the density functional theory (DFT). The results indicate that the inner phase of CNT has little effect on the [2+2] cycloaddition reaction. This can be explained as that while taking the linear arrangement for S_N2 reaction, the reactants do not possess the axial symmetry for the studied [2+2] cycloaddition reaction. Therefore, although the CNT has large axial polarizability, it can exert little influence on the [2+2] cycloaddition reaction. Our studies will be helpful for further understanding of the inner phase chemistry of CNTs.     

碳纳米管内P—Ylide反应的理论研究

碳纳米管空腔不仅可以改变填充到其内部分子的性质,而且对其内部的SN2取代反应有促进或抑制作用,因而被视为一种新型的“固体溶剂”．本文通过密度泛函理论研究了CH2O和PH3CH2在气相、苯溶液和碳纳米管空腔内的[2＋2]加成反应．结果表明,与气相相比,碳纳米管空腔对于[2＋2]环加成反应影响不大,这是因为所研究的[2＋2]环加成反应中,反应物并不是轴对称的,不同于SN2取代反应的一维线性排列,所以具有较大轴向极化率的碳纳米管对[2＋2]环加成反应影响较小．     

Detection of anti-tetanus toxoid antibody on modified polyacrylonitrile fibers

Accurate determination of concentration of immunoglobulin (IgG) to tetanus toxoid is important in order to evaluate the immunogenicity of tetanus toxoid vaccines, immune competence in individual patients and to measure the prevalence of immunity in populations. Surface modified polyacrylonitrile (PAN) fibers were evaluated as a matrix to develop highly sensitive method for the detection of anti-tetanus antibody in a sandwich ELISA format. In the proposed method tetanus toxoid immobilized on modified PAN fibers was used to detect anti-tetanus antibody (raised in horse hence represented as horse anti-tetanus toxoid or HAT-Ab) with horse raddish peroxidase enzyme conjugated with Rabbit anti-Horse IgG (RAH-HRP) as the label within 2.5 h. A sigmoidal pattern for the detection of different concentration of antibody ranging from 1.0 to 0.0001 IU mL⁻¹ was validated. The immunoassay recorded a very high sensitivity as concentration as low as 0.0005 IU mL⁻¹ of HAT-Ab was detected. The intra- and inter-assay precision for 3 parallel measurements of 0.01 and for 0.001 IU mL⁻¹ of antibody varied from 5.4% to 11% and 5.7% to 20% respectively. PAN fibers were also used to qualitatively access the presence of different level of anti-tetanus antibody spiked in human blood. Seroepidemiological studies to measure the immunity against tetanus were conducted with twenty-five human beings belonging to various age groups using modified PAN-ELISA. The sensitivity, specificity and the reproducibility of the developed immunoassay indicate the potential application of modified PAN fibers in the field of immunodiagnostics.