Effect of copolymerization and heat treatment on the structure and x-ray diffraction of polyacrylonitrile

Wide-angle x-ray diffraction studies are reported on polyacrylonitrile (PAN) (homopolymer powder and fiber samples) and its random copolymers with methyl methacrylate (MMA) and methacrylonitrile (MAN). A new method of determination of degree of order which involves resolution of the individual maxima in the diffraction pattern is described and compared with the methods used in the literature. Comparison of the results on untreated fibers with those on untreated homopolymer powder samples, and the effect of heat treatment, suggest the molecular rod conformation of the PAN molecules in the amorphous phase. Heat treatment or the orientation induces a parallel alignment of the molecular rods in the amorphous phase, similar to the cyrstalline structure of PAN. Copolymerization with both the comonomers MMA and MAN produces qualitatively similar effect on the structure of PAN, which depends on the comonomer content. Below a critical comonomer content c* (c* = 12 mol % for MMA and 25 mol % for MAN comonomer), the crystalline lattice remains that characteristic of PAN, whereas above the critical comonomer content some modification in the crystalline lattice is apparent.     

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.     

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.     

A review of heat treatment on polyacrylonitrile fiber

Developing carbon fiber from polyacrylonitrile (PAN) based fiber is generally subjected to three processes namely stabilization, carbonization, and graphitization under controlled conditions. The PAN fiber is first stretched and simultaneously oxidized in a temperature range of 200-300 °C. This treatment converts thermoplastic PAN to a non-plastic cyclic or a ladder compound. After oxidation, the fibers are carbonized at about 1000 °C in inert atmosphere which is usually nitrogen. Then, in order to improve the ordering and orientation of the crystallites in the direction of the fiber axis, the fiber must be heated at about 1500-3000 °C until the polymer contains 92-100%. High temperature process generally leads to higher modulus fibers which expel impurities in the chain as volatile by-products. During heating treatment, the fiber shrinks in diameter, builds the structure into a large structure and upgrades the strength by removing the initial nitrogen content of PAN precursor and the timing of nitrogen. With better-controlled condition, the strength of the fiber can achieve up to 400 GPa after this pyrolysis process.     

Effect of copolymerization on the crystalline structure of polyacrylonitrile

X-ray diffraction of polyacrylonitrile (PAN), poly-2-hydroxyethyl methacrylate (poly-HEMA) and their random copolymers containing low proportions of HEMA has been carried out in order to investigate the changes in the PAN structure produced by introducing some HEMA units into the chain. Both crystalline and amorphous phases were distinguishable in PAN and the copolymers. Considerable decrease in crystallinity on copolymerization was found, but not showing any quantitative relationship with the HEMA content. The crystalline lattice in the copolymers remained similar to that of PAN. A two-phase (crystalline and amorphous) structure of PAN was found more suitable to explain the results.     

Studies of structure formation in grafting of polyacrylonitrile to polycaproamide fibers

The relationship between the basic structure parameters of a polycaproamide (PCA) substrate and the structure of grafted polyacrylonitrile (PAN) was studied. The influence of the total degree of crystallinity of the PCA fiber, its crystal modification, and the orientation of crystallites on the PAN structure in the copolymer were analyzed. The structure of PCA-PAN graft copolymers was studied by infrared spectroscopy (dichroism and ATR), X-ray structure analysis, nuclear magnetic resonance, and scanning electron microscopy. The development of an oriented isotactic structure was found in PAN grafted to a PCA substrate having a bigh degree of orientation. The type of crystal modification of PCA does not affect the structure formation process in PAN. The oriented structures in PAN arise directly in the course of synthesis rather than on the subsequent rearrangement of the molecular chains under the action of orientation drawing. In contrast to earlier findings, our studies indicate that synthesis under the conditions specified increases the degree of crystallinity of PCA and crystallite size. The grafted PAN chains are presumed to grow epitaxially under the influence of PCA crystallites.     

Influence of silver nano‐additive amount on the supramolecular structure,porosity, and properties of polyacrylonitrile precursor fibers

This work examines the influence of the amount of silver nanoparticles added to polyacrylonitrile spinning solutions on their rheological properties as well as the structure and properties of the fibers produced. The influence of the amount of silver nanoparticles on the supramolecular structure of nanocomposite polyacrylonitrile precursor fibers, their porosity, as well as thermal and tensile strength properties was determined. The distribution of the nano‐ additive in fiber cross‐sections and on the surface was estimated. It was found that the addition of silver nanoparticles to polyacrylonitrile precursor fibers in an amount of up to 1.5% does not cause a decrease in the susceptibility of the fiber matter to deformation at the drawing stage. The produced fibers were characterized by an increased total volume of pores of 0.35 cm³/g and tenacity of more than 34 cN/tex. Copyright © 2009 John Wiley & Sons, Ltd.     

Separation and kinetic analysis of the thermo-oxidative reactions of polyacrylonitrile upon heat treatment

A polyacrylonitrile (PAN) homopolymer and a copolymer with methyl acrylate were analyzed by differential scanning calorimetry (DSC) in order to establish a method for the kinetic analysis of the stabilization reaction steps during heat treatment. The application of a program rarely used for the study of the chemical transformation of PAN and its copolymers resulted in five separate steps, which can be assigned with large probability to various cyclization and oxidation reactions. A more exact identification of the reactions needs further measurements and considerations. Overlapping peaks appearing in the DSC traces were deconvoluted and then quantitatively analyzed to obtain kinetic parameters. The kinetics of the processes were described by the Kissinger model. The model could be fitted well to four of the five processes that may indicate that these belong to individual reactions of the first order. The parameters obtained agree well with the few values of similar investigations published in the literature up to now. The results have large practical relevance, since the kinetic parameters obtained can be used in the preparation of intermediates and for the optimization of the stabilization process.     

碱性热处理对豌豆蛋白结构及溶解性的影响

本文研究了碱性热处理(pH 12+60℃)对豌豆蛋白结构及溶解性的影响.单独碱处理(pH 12)和碱性热处理均不能改变豌豆蛋白的亚级结构,但圆二色谱和内源荧光结果显示两种处理方法能够引起豌豆蛋白的二级和三级结构发生变化.相比单独碱处理,碱性热处理能够更加显著地使豌豆蛋白的折叠结构展开,使原来埋于内部的疏水基团暴露出来,...     

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.     

NMR and x-ray studies on the morphology of thermoreversible polyacrylonitrile gels

The morphology of thermoreversible polyacrylonitrile–propylene carbonate (PAN-PC) gels was examined using solid-state carbon-13 nuclear magnetic resonance (NMR) spectroscopy and x-ray diffraction. Following complete dissolution of the polymer at elevated temperature and cooling of the concentrated PAN-PC solutions, a gel was formed. The PAN-PC gels consisted of regions of mobile polymer chains, rich in PC, “cross-linked” by regions of rigid polymer. The mobile regions of the gels showed solution-type NMR spectra with resolution of tacticity effects. The rigid component detected by NMR would correspond to the crysttallites detected previously by x-ray diffraction. Wide-angle x-ray diffractograms of the gels showed different peaks when compared with the dry polymer powder. After solvent extraction and drying of the gel, the diffractogram reverted to that of the original dry powder. This new result is the strongest evidence to support the view advanced earlier that the new peaks found in the diffraction pattern of the wet gels arises from solvated polymer crystallites rather than from ordinary polymer crystallites. © 1993 John Wiley & Sons, Inc.     

Adsorption and desorption of humic acid on aminated polyacrylonitrile fibers

Aminated polyacrylonitrile fibers (APANFs) were prepared by surface modification and were used as an adsorbent to remove humic acid from aqueous solutions. The APANFs were found to be very effective in removing humic acid at the pH range from 2 to 10. The adsorption isotherm obeyed both the Langmuir and Freundlich models, and the adsorption kinetics followed an initial diffusion-controlled and then an attachment-controlled adsorption pattern. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectroscopy revealed that chemical bonds were formed between the nitrogen atoms in the amine groups on the fibers and humic acid molecules adsorbed, suggesting that, besides electrostatic interaction, surface complexation also played an important role in humic acid adsorption on the APANFs. The humic acid adsorbed on the APANFs can be effectively desorbed in a 0.1 M NaOH solution, and the regenerated APANFs can be reused in the subsequent adsorption cycles without significant loss of the adsorption capacities.     

Behaviors and mechanisms of copper adsorption on hydrolyzed polyacrylonitrile fibers

Polyacrylonitrile fiber (PANF) was hydrolyzed in a solution of sodium hydroxide and the hydrolyzed polyacrylonitrile fiber (HPANF) was used as an adsorbent to remove copper ions from aqueous solution. Scanning electron microscopy (SEM) showed that the hydrolysis process made the surface of HPANF rougher than that of PANF. Fourier transform infrared (FTIR) spectroscopy revealed that the HPANF contained conjugated imine (-Cz=Nz-) sequences. Batch adsorption results indicated that the HPANF was very effective in adsorbing copper, and the adsorption equilibrium could be reached within 10-20 min. Atomic force microscopy (AFM) showed that some aggregates formed on the surface of the HPANF after copper ion adsorption and the average surface roughness (R(a)) value of the HPANF changed from 0.363 to 3.763 nm due to copper adsorption. FTIR analysis indicated that copper adsorption caused a decrease of the light adsorption intensity of the imine (-Cz=Nz-) groups at 1573 and 1406 cm(-1) wavenumbers, and X-ray photoelectron spectroscopy (XPS) showed that the binding energy (BE) of some of the nitrogen atoms in the HPANF increased to a greater value due to copper adsorption. The FTIR and XPS results suggest that the adsorption of copper ions to the HPANF is attributed to the imine groups on the surface of the HPANF.     

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.     

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.     

Effect of macro-RAFT agent on the morphology of polymer dispersed liquid crystals

In this study, macro-(RAFT) reversible additional fragmental chain transfer agent prepared by reversible additional fragmental chain transfer polymerisation has been incorporated into polymer dispersed liquid crystals (PDLCs). The effects of concentration, molecular weight and glass transition temperature of macro-RAFT agent were studied in terms of morphology, polymerisation kinetics, molecular weight of polymer matrix and electro-optical properties of the films. It was found that the key factor influencing morphology was the mobility of macro-RAFT agent chain rather than polymerisation rate and molecular weight of polymer matrix. Furthermore, the decrease in the mobility of macro-RAFT agent chain caused less liquid crystal nematic fraction, smaller liquid crystal domain size and greater driving voltage.     

热处理对HZSM-5结构、酸性及催化乙醇脱水反应性能的影响

对商用HZSM-5分子筛进行高温热处理,利用X射线衍射(XRD)、核磁共振波谱法(²⁷Al-MAS NMR)低温氮吸附、电感耦和等离子体放射光谱(ICP-AES)、氨-程序升温脱附(NH₃-TPD)和吡啶吸附傅里叶变换红外(FT-IR)光谱等技术对高温热处理改性前后样品进行了表征,并考察了高温热处理对分子筛的结构、酸性及催化乙醇脱水制乙烯反应性能的影响。结果表明,热处理改性后的HZSM-5分子筛发生骨架脱铝,B酸减少,L酸增多,孔容增大,出现新的介孔;适中的酸性位和复合型的介-微双孔道结构使其减少了反应中的副反应,乙烯的选择性得到提高,催化剂寿命显著提高。     

Radiation-induced crosslinking of polyacrylonitrile fibers and the subsequent regulative effect on the preoxidation process

To investigate the radiation effect on polyacrylonitrile (PAN) fibers as well as on the preoxidation process, PAN fibers were irradiated by γ-rays at room temperature at 50–500 kGy in vacuum and then were thermally oxidized in air. Gel fraction determination indicated that γ irradiation led to the predominant crosslinking of PAN fibers, with G values (the number of event per 100 eV absorbed) of G(X)=0.28 and G(S)=0.16 for chain crosslinking and scission, respectively. It was found that irradiation caused a slight change in the crystal structure and tensile strength at low dose. Radiation led to a reduction of the onset temperature of cyclization reaction and moderated the exothermic behavior. The density of the PAN fibers after thermal oxidation was used to evaluate the preoxidation extent. It was proven that radiation could significantly accelerate the preoxidation process and consequently shortened the preoxidation time. Radiation crosslinking may have potential application in the production of PAN-based carbon fibers.     

Fast microscopic method for large scale determination of structure, morphology, and quality of thin colloidal crystals

We present a novel fast microscopic method to analyze the crystal structures of air-dried or suspended colloidal multilayer systems. Once typical lattice spacings of such films are in the range of visible light, characteristic Bragg scattering patterns are observed. If in microscopic observations these are excluded from image construction, a unique color coding for regions of different structures, morphologies, and layer numbers results. Incoherently scattering defect structures, however, may not be excluded from image construction and thus remain visible with high resolution.