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.     

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.     

A procedure for precise determination of thermal stabilization reactions in carbon fiber precursors

Thermal stabilization of polyacrylonitrile (PAN) fibers is an important step in production of carbon fibers. Understanding the onset and temperature range of the stabilization reactions is a key for adjusting processing parameters such as tension, stretching, etc. However, stabilization reactions are very complex and overlap. In order to separate the stabilization reactions, we combined the results of FTIR (Fourier Transform Infrared Spectroscopy), DSC (Differential Scanning Calorimetry), TGA (Thermogravimetry analysis), TMA (Thermomechanical analysis), and densitometry. It was shown that combination of DSC and TGA allowed separation of reactions regardless of the composition of initial PAN fibers. FTIR, TMA and densitometry results verified the validity of proposed method. Accordingly, three special and commercial grade PAN fibers with different chemical composition were studied. FTIR results indicated that during thermal stabilization of PAN fibers chemical reactions including cyclization, oxidation and dehydrogenation occurred in the fibers and a ladder polymer was formed. According to DSC and TGA curves, initiation temperature, temperature range and order of occurrence of these reactions were a function of chemical composition of initial fibers. In fibers containing itaconic acid plus methyl acrylate comonomers, oxidation reactions already started at 175 °C. Cyclization started above 210 °C, and reactions occurring above 250 °C were mainly dehydrogenation. In fibers containing only itaconic acid cyclization initiated above 210 °C, dehydrogenation started after 242 °C and oxidation occurred only after 284 °C. In fibers containing vinyl acetate comonomers, the initial reactions above 240 °C were attributed to cyclization. Oxidation occurred below 290 °C and dehydrogenation started above 290 °C.     

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).     

Unusual deactivation in the asymmetric hydrogenation of itaconic acid

The formation of unusual Rh(III) substrate complexes from [Rh(DIPAMP)(MeOH)₂]BF₄ and itaconic acid has been detected which leads to the deactivation of the catalyst. The influence of different parameters on the formation of such complexes, namely substrate concentration, reaction time, temperature, acidic and basic additives, was investigated with different NMR methods. Two different Rh(III) substrate complexes are formed whose ratio is strongly dependent on substrate concentration and reaction time. The pH value of the solution shows a strong influence on the chemical shifts of the ³¹P NMR signals of such complexes. A catalyst-mediated esterification of itaconic acid in methanol was detected. Extended investigations provide detailed ¹H, ¹³C and ³¹P NMR data for the Rh(III) complexes and information about their stability in solution.     

Microwave-Assisted Synthesis and Characterization of Poly(itaconic acid) Grafted Gellan Gum

Poly(itaconic acid) was grafted on GG in aqueous medium under microwave irradiation using a catalytic amount of BPO. Grafted copolymers (GG-g-PIA) were characterized by FT-IR, TGA, and SEM techniques. The influence of microwave power, exposure time, and composition of the reaction mixture on extent of grafting was studied. Conditions for obtaining the highest extent of grafting were optimized. The rate of grafting was determined from weight measurements. The overall activation energy for grafting is found to be to be 28.3 kJ/mol, indicating the possibility of occurrence of the grafting process with absorption of low thermal energy.     

Studies on isothermal kinetics of some reactions of aluminum oxides and hydroxides

Thermal decomposition and hydrothermal reactions of aluminum oxides and hydroxides were studied. The starting material was gibbsite pretreated by grinding in different ways. Sixteen kinetic models were fitted and the rate constants were estimated at different temperatures. Mechanical activation of the starting gibbsite had an effect on the reactivity of samples in several subsequent reactions.A new quantity, the relative residual variance is defined for comparison of different linearized models, and it is proposed as a criterion of fit. This was applied in making statistics in order to find the well-fitting models.

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Zusammenfassung Es wurden die thermische Zersetzung und hydrothermische Reaktionen von Aluminiumoxiden und -hydroxiden untersucht. Das Ausgangsmaterial war Gibbsit, welches durch Zerkleinern auf verschiedene Weise vorbereitet wurde. Es wurden sechzehn kinetische Modelle angewendet und bei verschiedenen Temperaturen die Gesch-windigkeitskonstanten geschätzt. Die mechanische Aktivierung des Ausgangsmateriales Gibbsit beeinflu\t die Reaktivität der Proben in einigen Folgereaktionen.Für den Vergleich verschiedener Linearisationsmodelle wurde eine neue Grö\e, die relative Restvarianz definiert, die als Kriterium für die Anpassungsgüte vorgeschlagen wird. Dies wurde bei statistischen Untersuchungen angewendet, um die bestangepa\ten Modelle zu finden.

     

Measurement of heterogeneously catalyzed gas reactions by DSC

Gas reactions, catalyzed by solid catalysts, can be measured by DSC. In the experimental set-up an open sample pan with catalyst (powder or pellet) is placed on the sample side of the DSC sensor. The reactive gas mixture flows through the cell and reacts on the catalyst surface. The heat effect, caused by this reaction, results into a DSC signal.The calibration procedure is described for quantitative evaluation of the DSC measurements. For illustration four different reaction systems are discussed.     

Investigation into the gelation and crystallization of polyacrylonitrile

Polyacrylonitrile (PAN) is soluble in dimethyl sulfoxide (DMSO) and the resulting solution can gel by various mechanisms. The effects of temperature and water on the thermoreversible gelation of PAN have attracted much attention because of their importance in the fiber formation and film casting. Rheological tests were employed in this study to examine the gelation behavior and determine the gel point temperature of PAN-DMSO solution. Calorimetric studies of PAN-DMSO gels were conducted through Differential Scanning Calorimetry and no crystallization was discovered in these gels prepared at low temperature. X-ray diffraction of different PAN gels indicated that in the absence of water, gels resulted from PAN-DMSO solution by decreasing temperature were not crystallizable. In contrast, water-induced gelation led to crystallization of PAN gel. The water content in the formed gel is responsible for its crystallinity and average crystallite size.     

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.     

衣康酸-天冬氨酸-苯乙烯磺酸钠三元共聚物合成及阻垢性能研究

以衣康酸(IA)、天冬氨酸(ASP)及苯乙烯磺酸钠(SSS)为单体,过硫酸铵为引发剂,采用水溶液自由基聚合法合成了IA-ASP-SSS三元共聚物,并对产物结构进行红外表征。探讨单体配比、引发剂用量、聚合温度、反应时间对共聚物阻垢性能的影响,通过正交实验和单因素实验确定最佳合成条件。采用静态阻垢法考察阻垢剂用量、钙离子浓度对阻垢性能的影响。结果表明：共聚单体的物质的量比(nASP:nIA:nSSS)为1:1:0.1、引发剂用量为单体总质量的9%、反应温度85℃且反应时间3h时,合成的IA-ASP-SSS共聚物对碳酸钙、磷酸钙及硫酸钙的阻垢率皆在95%以上,且此共聚物阻垢剂适用于高硬度水环境。     

Spontaneous proton-transfer copolymerization of 2-methyl-2-oxazoline with itaconic acid

Copolymerization of two monomers, 2-methyl-2-oxazoline (1) and itaconic acid (2) was carried out. The copolymerization proceeded spontaneously in 1:1 monomer feed ratio to give 2:1 copolymer 3. The mechanism for the production of such a 2:1 copolymer was explained by the in situ formation of monomer 5 from 1 and 2. Then, the propagation proceeded between 5 and 1 via zwitterionic intermediates. When the ratio of 1 in the feed was higher, the homopropagation of 1 took place.     

To what extent does the substituent conformation influence the kinetics of addition reactions on 5X-bicyclo[4.4.0]decan-2-ones?

Stereochemistry and relative rates k_ax and k_eq of addition reactions on the title compounds have been measured under four different reaction conditions (CH₃MgI in Et₂O and C₆H₆, and CH₃Li in Et₂O at 20°C and −78°C). In strict accordance with previous findings we show that the axial substituents are far less electronegative than their equatorial counterparts in equatorial attack reactions. Axial attack, however, is independent of the substituent conformation.     

Use of itaconic acid-based polymers for solid-phase extraction of deoxynivalenol and application to pasta analysis

Molecular modelling and computational design were used to identify itaconic acid (IA) as a functional monomer with high affinity towards deoxynivalenol (DON), a Fusarium-toxin frequently occurring in cereals. IA-based polymers were photochemically synthesised in dimethyl formamide (porogen) using ethylenglycol dimethacrylate as cross-linker and 1,1′-azo-bis(cyclohexane carbonitrile) as initiator, and the relevant binding interactions with DON in solvents with different polarity were investigated. The performances of the non-imprinted IA-based polymer (blank polymer, BP) and the corresponding molecularly imprinted polymer (MIP) were compared using DON as a template. Both BP and MIP were able to bind about 90% DON either in toluene, water or water containing 5% polyethylene glycol. Non-imprinted polymers with different molar ratios of IA to cross-linker were evaluated as adsorbents for solid-phase extraction (SPE) clean-up and pre-concentration of DON from wheat and pasta samples prior to HPLC analysis. Samples were extracted with PBS/0.1 M EDTA solution and cleaned up through a cartridge containing blank IA-based polymer. The column was washed with PBS (pH 9.2) and the toxin was eluted with methanol and quantified by reversed-phase HPLC with UV detector (λ = 220 nm), using methanol:water:acetic acid (15:85:0.1, v/v/v) as the mobile phase. Effective removal of matrix interferences was observed only for pasta with DON recoveries higher than 70% (RSD < 7%, n = 3) at levels close to or higher than EU regulatory limit.     

Modelling of diffusion-controlled solid-solid reactions

A model describing the initial step of diffusion-controlled solid-solid reactions is presented. The kinetic equation obtained was used to evaluate the activation energy of the reaction LiCl(s)+NaF(s)=LiF(s)+NaCl(s) from DTA data.

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Zusammenfassung Es wird ein Modell zur Beschreibung des einleitenden Schrittes von diffusionskontrollierten Feststoffreaktionen beschrieben. Die erhaltene kinetische Gleichung wurde zur Bestimmung der Aktivierungsenergie der Reaktion LiCl(s) + NaF(s)=LiF(s) + NaCl(s) aus DTA-Daten angewendet.

     

Optimization of amine-terminated polyacrylonitrile synthesis and characterization

Amine-terminated PANs were prepared in two steps. The first step includes free radical polymerization of acrylonitrile (AN) using initiator pair of ammonium persulfate and sodium thiosulfate as redox system. In the second step, the amino groups were introduced through the reaction of polyacrylonitrile with excess of different diamines (10-fold) including ethylenediamine (EDA), hexamethylenediamine (HMDA) and octamethylenediamine (OMDA), to yield PAN–EDA, PAN–HMDA and PAN–OMDA, respectively. Optimization of the amine-terminated PANs synthesis was carried out at different temperatures (30–90 °C) and different time intervals (4–24 h). In addition, the introduction of the amino group was followed by the piperidine test and recording of the FT-IR spectra. All polymers were characterized by, ¹H NMR spectra, thermogravimetric analysis (TGA), and FT-IR spectra.     

Mechanism of formation of Cu(II) carboxylates in the solid state 1. Kinetics of reactions between benzoic acid, salicylic acid and CuCO3 Cu(OH)2

The kinetic data for reactions between aromatic acids, i.e. benzoic acid, salicylic acid and basic copper carbonate in the solid state have been recorded. The measurements have been made at various temperatures for fixed sizes of the particles at constant compression. The products are characterised by infrared and reflectance spectra. The reactions are diffusion-controlled with activation energy of 71.8 and 115.0±5 kJ mol⁻¹ for the formation of Cu(II) benzoate and Cu(II) salicylate respectively.     

Properties of the forpolymer of N-vinylpyrrolidone with itaconic acid, acrylamide and 2-acrylamido-2-methyl-1-propane sulfonic acid as a fluid-loss reducer for drilling fluid at high temperatures

The forpolymer of N-vinylpyrrolidone (NVP), itaconic acid (IA), acrylamide (AM) and 2-acrylamido-2-methyl-1-propane sulfonic acid (AMPS) was synthesized through free-radical polymerization and was characterized using Fourier transform IR spectroscopy. The colloidal properties of the drilling fluid were investigated in the form of fresh-water or salt-water mud. It was found that the NVP–IA–AM–AMPS forpolymer had strong effects on the properties of the mud. The filtrate volume decreased with the increase of the forpolymer concentration before or after the aging test at 220 °C, and the filtrate volume after the aging test was larger than that before the aging test, but was still very small compared with the corresponding base mud. In addition, the rheological properties of both fresh-water mud and salt-water mud were modified by the forpolymer. The particle size data demonstrated that the average size of the clay particle after the aging test was larger than that before the aging test and that the particle size of the fresh-water mud was narrower compared with that of the salt-water mud before and after the aging test, respectively. The drilling fluid containing the forpolymer had an excellent tolerance to salt and high temperature. Received: 13 September 2000/Accepted: 9 January 2001     

Glass transition kinetics of some Se-Te-Ag chalcogenide glasses

Summary The present paper reports the Differential Scanning Calorimetric (DSC) study of some Ag doped Se-Te chalcogenide glasses. DSC runs were taken at different heating rates. Well-defined endothermic and exothermic peaks were obtained at glass transition and crystallization temperatures. The variation of glass transition temperature T_gwith Ag concentration has been studied. It has been found that T_gdecreases with increase in Ag concentration. The heating rate dependence of T_gis used to evaluate the activation energy of glass transition (DE_t). The value of<span style='font-size:10.0pt; font-family:"SymbolProp BT";mso-bidi-font-family:"SymbolProp BT"'>DE_thas been found to increase with increase in Ag concentration followed by nearly constant value at higher concentrations of Ag.     

A study of the thermodynamic properties and dehydration reaction kinetics of some salt-hydrates

Correlations were determined between heat capacity and temperature and phase change enthalpy of Ba(OH)₂·8H₂O. The phase diagram and DSC curve of the binary system Na₂CO₃·10H₂O?Na₂HPO₄·12H₂O were determined The kinetics of the dehydrating reaction of Ba(OH)₂·8H₂O, Na₂CO₃·10H₂O and Na₂HPO₄·12H₂O were measured and theoretically analyzed by TG.