在氮气中处理聚丙烯腈纤维的不熔化工艺

聚丙烯腈纤维不熔化过程的前期(220℃)在氮气中热处理,后期(220℃)在空气中热处理,与常规全程空气中热处理进行对比,希望能够降低不熔化处理的成本。借助元素分析(EA)、热重分析仪(TGA)、环境扫描电子显微镜(ESEM)、力学性能分析等表征手段,研究了不同热处理方法对PAN不熔化纤维的氧化程度和皮芯结构影响以及碳化收率和碳纤维的力学性能。结果发现,与全程空气处理相比,经过氮气气氛处理后的不熔化纤维在后续处理过程中最终不熔化温度可以降低20℃,900℃碳化后的收率增加了3.8%,碳纤维的强度相当,初步估算节能8.7%。     

Preparation of polyacrylonitrile/graphene oxide by in situ polymerization

Highly oriented molecular structure is essential for high‐performance carbon fibers. The addition of a small amount of graphene sheets may enhance the degree of molecular orientation of precursor fibers during spinning and stabilization by limiting the disorientation of the chain segments. Graphene sheets merge into the carbon fiber structure during carbonization. The structure and properties of polyacrylonitrile containing graphene oxide (GO) prepared by in situ polymerization were investigated. With increasing GO loading, the molecular weight of the polymer decreased gradually from 69 000 g mol^?1 for the sample without GO to 60 600 g mol^?1 for the sample with 2.5 wt% loading of GO. Scanning electron microscopy and X‐ray diffraction results indicated that GO was dispersed in single layers in the polymer matrix. The degree of crystallization of the polymer with 0.5 wt% GO was increased by 8%. Moreover, differential scanning calorimetry and thermogravimetric analysis showed that an appropriate amount of GO, e.g. 0.5 wt%, made the carbon yield of the polymer increase by 5.0 wt%, because the GO in the composite improved the intermolecular crosslinking reaction. Copyright © 2012 Society of Chemical Industry     

张力对聚丙烯腈纤维不熔化过程中氧化反应的影响

通过连续的不熔化实验,在不同温度段对聚丙烯腈纤维施加不同的张力,得到一系列不熔化纤维。借助差示扫描量热法(DSC)、元素分析(EA)、场发射扫描电镜(FESEM)等表征手段,研究张力在不熔化过程中对氧化反应的影响。结果表明,在175~218℃,随着张力的增大,纤维内氧含量减少,氧含量梯度(氧梯度)变化不大;在226~232℃,氧含量随张力增大先增多后减少,张力较大使得更多氧参与反应,纤维皮部与芯部氧含量差异增大,氧梯度较大;在238~270℃,随张力增大纤维内氧含量增多氧梯度增大。     

Influence of oxygen on the stabilization reaction of polyacrylonitrile fibers

Stabilized polyacrylonitrile (PAN) fibers pretreated under N₂ and air atmospheres were prepared and their thermal behaviors were compared by differential scanning calorimetry and thermogravimetry methods. The results indicated that the subsequent stabilization reaction of PAN pretreated in air was more obvious than that in N₂. In addition, the thermal stability of PAN pretreated in air is better than that in N₂. The structural analysis by Fourier transform infrared spectroscopy and solid state ¹³C nuclear magnetic resonance implied that oxygen promoted dehydrogenation and a compact conjugated structure was formed in PAN. In addition, the C?O structures were generated in air and increased gradually with temperature. The contents of oxygen in PAN fibers studied by elemental analysis corresponded with the structural evolution. Further investigation indicated that the C?O structures helped dehydration and also promoted formation of the cross‐linked structures. A mechanism for structural evolution in PAN during stabilization in air was proposed. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

聚丙烯腈纤维预氧化工艺条件对孔隙结构的影响

借助场发射扫描电镜和小角X-射线散射对聚丙烯腈纤维的孔隙结构进行了研究。结果表明,聚丙烯腈纤维中的孔洞多为长条形且具有取向性的微孔。在预氧化温度区域,孔隙结构参数(平均孔径和孔隙率)变化不明显;但随着拉伸比的增大,其均呈下降趋势。因此,对聚丙烯腈纤维孔隙结构在预氧化过程中演变情况的研究,为进一步探讨孔隙结构对预氧化过程的影响奠定了基础。     

Effect of oxygen uptake and aromatization on the skin–core morphology during the oxidative stabilization of polyacrylonitrile fibers

The isothermal oxidative stabilization of polyacrylonitrile fibers has been carried out at 210, 230, and 250°C. The stabilized fibers, treated for different times, have been characterized with elemental analysis, wide‐angle X‐ray diffraction, optical microscopy, and field emission scanning electron microscopy. A parabola relationship has been established between the oxygen uptake and stabilization time, whereas the aromatization index shows a trend of moderate ascension, retention, and acceleration. With increasing temperature and time, the skin–core morphology of the stabilized fibers becomes more and more distinct, but the skin thickness is almost unchanged for 60 and 120 min at 250°C. The fracture mechanism is ductile fracture in the core but is brittle fracture in the skin. The results indicate that the initial rapid oxygen uptake at a high temperature and the subsequent intense aromatization are responsible for the formation of the skin–core morphology. On the basis of the isothermal stabilization, an onion‐like model is proposed for the structure of stabilized fibers that are treated by stepwise increasing temperatures in industrial production. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Evolution of the crystalline structure and cyclization with changing tension during the stabilization of polyacrylonitrile fibers

The effects of tension on the crystalline structure and cyclization of polyacrylonitrile fibers during stabilization were investigated. The degree of cyclization was measured by Fourier transform infrared spectroscopy and differential scanning calorimetry. The crystalline structure was characterized by wide‐angle X‐ray diffraction. When the fibers were heat‐treated at temperatures between 175 and 218°C, the tension mainly affected the cyclization in the amorphous regions through changes in the spatial distance of the chain segments; this led to a relatively higher cyclization degree under moderate tension. When the temperatures ranged from 226 to 232°C, the reactions extended to the crystalline regions. The chemical bonds became greater in the cross section of the fibers, and this was caused by cyclization structures formed in the former stage. Therefore, the optimum tension was higher than in the prior temperature range. At higher temperatures ranging from 238 to 270°C, a crosslinked structure formed, so the optimum tension increased continually. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42182.     

Mass DSC/TG and IR ascertained structure and color change of polyacrylonitrile fibers in air/nitrogen during thermal stabilization

The structure evolution was studied by mass spectrum (MS), differential scanning calorimetry (DSC) and thermogravimetry (TG), Fourier transform infrared (FTIR) spectroscopy. The results indicated that the CN and CC groups appeared gradually with the increase of the temperature in air and nitrogen. The CO groups appeared because of oxidative reaction in air. The CN, CC and CO groups were all chromophores. The effect of conjugated CN and CC on the absorption of the visible light was shifted to longer wavelengths and indicated π‐π* transition. There was a strong bathochromic effect as the number of CC bonds were increased. The effect of CO and –NH₂ on the absorption of the visible light was shifted to longer wavelengths and indicated n‐π* transition. Oxygen could facilitate chemical reactions in air. Hence, the color of PAN in air was deeper than in nitrogen at the same temperature. The structural change of PAN in air was faster and more complex than in nitrogen. PAN fibers treated in air turned black after 230°C. However, PAN fibers turned black at 350°C in nitrogen. The MS and FTIR indicated that cyclization occurred before dehydrogenation during stabilization in air and nitrogen. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

The role of structural evolution of polyacrylonitrile fibers during thermal oxidative stabilization on mechanical properties

The effects of chemical and physical structural evolution of polyacrylonitrile (PAN)-based carbon fibers precursor during thermal oxidative stabilization (TOS) on the mechanical properties of stabilized fibers were systematically studied. The results of Fourier transform infrared spectroscopy, wide-angle X-ray diffraction, and density gradient column showed that the PAN fibers treated with high temperature and for long time have higher extent of cyclization, oxygen content, and crosslinking content. The crystallinity and crystallite size decreased with the increase of TOS time and temperature, whereas the bulk density of the stabilized fibers increased. The mechanical property results indicated that the decrease in tensile strength was inseparable from the formation of the cyclic structure and the amorphization transition of the crystal structure. The fibers have better structural stability when the extent of cyclization was 80–83%, the crystallinity was 34–45%, and the bulk density of stabilized fibers was 1.33–1.35 g/cm³, but exceeding these ranges, a serious skin-core structure appeared.     

Physicochemical evaluation of nanocomposite hydrogels with covalently incorporated poly(vinyl alcohol) functionalized graphene oxide

To avoid the negative effect of graphene oxide (GO) nanosheets aggregation in aqueous solutions on physicochemical properties of GO incorporated nanocomposite hydrogels, poly(vinyl alcohol)-functionalized GO (GO-es-PVA) are synthesized and are used for preparation of nanocomposite hydrogels. By graft copolymerization of GO-es-PVA with poly(AA-co-AAm) chains, the nanocomposite hydrogel samples with covalently incorporated GO-es-PVA are achieved. FTIR spectroscopy, XRD analysis, and SEM and EDAX techniques confirm successful synthesis process. It is clear that GO-es-PVA content has significant effect on physicochemical properties of nanocomposite hydrogels, such as improvement of the water uptake properties, porosity, and gel strength. The hydrogel sample with 1:80 mass ratio of GO-es-PVA/AAm has the best physicochemical properties due to the optimum amount of GO-es-PVA, which gives the hydrogel proper viscoelasticity as well as fine porosity and water uptake rate. Interpenetration of PVA chains into the polymeric networks makes the movement of the polymer chains easier, which leads to softer polymeric networks. This phenomenon is called plasticizing effect. The plasticizing nature of PVA and its high hydrophilicity are the main reasons for the fine physicochemical properties. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 48025.     

Effect of base‐deposited graphene oxide on the thermal stabilization of poly(vinyl chloride)

Graphene oxide was deposited in a base solution to form base‐deposited graphene oxide (bd‐GO) particles. The structure and properties of the bd‐GO particles were evaluated using transmission electron microscopy, powder X‐ray diffraction and X‐ray photoelectron, Fourier transform infrared, UV‐visible and fluorescence spectroscopies. The effect of the bd‐GO particles on the thermal stabilization of poly(vinyl chloride) (PVC) was investigated using the Congo red test and thermogravimetric analysis. The results showed that the thermal stability of PVC was greatly improved by the bd‐GO particles. Furthermore, this stabilization mechanism was investigated using UV‐visible spectroscopy and nitrogen adsorption–desorption isotherm analysis. It was found that the improvement of thermal stability was mainly related to the deactivation of thermally labile structural defects in the PVC chains by the carboxylate and alkoxide moieties of the basic groups in the bd‐GO particles, and the highly efficient adsorption of the bd‐GO particles with hydrogen chloride produced during PVC degradation. © 2015 Society of Chemical Industry     

Investigation on promotion effect of oxidative stabilization for pitch-based fiber by co-carbonization of coal tar pitch and atmospheric residual

As a pivotal step in the preparation of carbon fiber, oxidative stabilization not only plays a crucial role in maintaining fibrous morphology but also contributes significantly to enhance mechanical properties of resultant carbon fiber. Due to high activation energy of pitch molecules reaction with oxygen and the sluggish diffusion of oxygen within the fiber, the improvement of oxidative stabilization efficiency faces significant challenges. Atmospheric residual (AR) has a high and easily oxidized aliphatic structure. Spinnable pitch is synthesized by co-carbonization of coal tar pitch (CTP) and AR at a ratio of 3:1 in this work. Its methylene bridge bond ratio is 4.45% and have an appropriate amount aliphatic structure, which makes pitch molecular more linear and naphthenic. Excessive addition of AR is detrimental to spinning performance. The most optimal oxidative stabilization temperature of as-spun fiber was 280°C, which is lower than that of fiber produced by CTP alone (300°C), displaying a higher oxidative stabilization efficiency. The obtained pitch-based carbon fiber shows excellent mechanical properties with tensile strength of 999.0 ± 80.1 MPa and Young's modulus of 57.7 ± 3.5 GPa. The co-carbonization by two different substances has been applied in manufacturing carbon fiber, providing a facile approach to accelerate the oxidative stabilization of pitch fiber.     

The synthesis and characteristics of sodium alginate/graphene oxide composite films crosslinked with multivalent cations

Association of a method of the incorporation of graphene oxide (GO) into sodium alginate (Na‐alg) polymer matrix with a method of the use multivalent cations crosslinker was put forward to synthesize novel Na‐alg/GO nanocomposite films. The structures, morphologies, and the properties of Na‐alg/GO films were characterized by Fourier transform infrared (FTIR) spectroscopy, X‐ray diffraction (XRD), field‐emission scanning electron microscopy (FE‐SEM), thermogravimetric analysis (TGA), and tensile tests. The results revealed that the interlayer distance of GO sheets increased from 0.83 nm to 1.08 nm after assembling with Na‐alg, and Na‐alg inserted into GO layers crosslinking with multivalent cations increased the interlayer distance further. Ionic crosslinking significantly enhanced thermal and mechanical properties of Na‐alg/GO nanocomposite films. In particular, Fe³⁺ led to Na‐alg/GO nanocomposite films of significantly higher tensile strength and modulus than Ca²⁺ and Ba²⁺. The excellent thermal and mechanical properties of these novel Na‐alg/GO nanocomposite films may open up applications for Na‐alg films. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43616.     

Preparation of polyimide/siloxane‐functionalized graphene oxide composite films with high mechanical properties and thermal stability via in situ polymerization

An effective approach to prepare polyimide/siloxane‐functionalized graphene oxide composite films is reported. The siloxane‐functionalized graphene oxide was obtained by treating graphene oxide (GO) with 1,3‐bis(3‐aminopropyl)‐1,1,3,3‐tetra‐methyldisiloxane (DSX) to obtain DSX‐GO nanosheets, which provided a starting platform for in situ fabrication of the composites by grafting polyimide (PI) chains at the reactive sites of functional DSX‐GO nanosheets. DSX‐GO bonded with the PI matrix through amide linkage to form PI‐DSX‐GO films, in which DSX‐GO exhibited excellent dispersibility and compatibility. It is demonstrated that the obvious reinforcing effect of GO to PI in mechanical properties and thermal stability for PI‐DSX‐GO is obtained. The tensile strength of a composite film containing 1.0 wt% DSX‐GO was 2.8 times greater than that of neat PI films, and Young's modulus was 6.3 times than that of neat PI films. Furthermore, the decomposition temperature of the composite for 5% weight loss was approximately 30 °C higher than that of neat PI films. © 2015 Society of Chemical Industry     

In situ observation of plaster microstructure evolution during thermal loading

Gypsum microstructural evolution at elevated temperatures has been characterized in situ by environmental scanning electron microscopy and high‐temperature X‐ray tomography. As temperature increases, the observed changes in gypsum crystal morphology are related to the consequences of water loss that leads to crystal reorganization in order to minimize the specific surface. The influence of environmental scanning electron microscopy experimental conditions on the mechanisms observed is also discussed. High‐temperature X‐ray tomography gives quantitative information on porosity evolution with thermal loading. The results obtained are successfully correlated with macroscopic shrinkage measurements performed during dilatometry tests. Copyright © 2016 John Wiley & Sons, Ltd.     

Phase evolution and thermochromism of vanadium oxide thin films grown at low substrate temperatures during magnetron sputtering

Vanadium oxides (VO_x) have been studied extensively for applications in thermochromic materials, electrochomics, and infrared detectors due to their unique phase transition characteristics. However, various vanadium oxide phases usually occur under different deposition conditions due to their particularly complex vanadium-oxygen system. In this research, V₃O₇, VO₂(B), VO₂(M), and V₂O₅ thin films were obtained as pure or mixed phases by controlling the substrate temperatures between 250 °C and 400 °C during magnetron sputtering. The microstructure and phase composition of vanadium oxide thin films were characterized and analyzed using X-ray diffraction and Raman spectroscopy. The phase evolution was dependent on the substrate temperature and could be clarified. Metastable V₃O₇ and VO₂(B) phases were obtained at substrate temperatures of 250–300 °C, while stable VO₂ and V₂O₅ phases were obtained at 350–400 °C. The surface morphology and optical properties of vanadium oxide thin films with different substrate temperatures were investigated in detail. Our results provide methods for transforming vanadium oxide phases under well controlled substrate temperatures.     

Intercalation structure and toughening mechanism of graphene/urea‐formaldehyde nanocomposites prepared via in situ polymerization

Based on the industrialized graphene (GN) product, a series of graphene/urea‐formaldehyde nanocomposites were synthesized via in situ polymerization by incorporation of silicon coupling agent with terminal amino groups (SA) as the compatibilizer. The results showed that addition of SA coupling agent led to much more efficient grafting of UF molecules on the GN surface with high layer thickness by formation of hydrogen bonding, and thus complete exfoliation and uniform dispersion of GN were achieved for the composites. Compared with neat UF, the addition of 1.0 wt% GN resulted in a roughly 25% increase in tensile strength and 12% increase in impact strength; meanwhile the impact fracture surfaces of the composite showed obvious ductile fracture characteristics, indicating the reinforcing and toughening effect of GN on the UF matrix. With increasing GN content, the storage modulus, glass transition temperature and crosslinking density of UF increased, while the tan δ_max decreased, suggesting that a double crosslinking network structure with GN centered crosslinking point and chemical crosslinking point of UF molecular chains formed, leading to improvement in the stiffness of the composites. The present work showed promising potential for developing high performance UF resin on an industrial scale. © 2017 Society of Chemical Industry     

High-temperature in situ FTIR spectroscopy study of LaOF and BaF2/LaOF catalysts for methane oxidative coupling

In situ FTIR spectroscopy was used to characterize the oxygen adspecies and its reactivity with CH₄ over LaOF and 15 mol% BaF₂/LaOF catalysts at OCM temperature (750-800°C). It was found that gas-phase oxygen was activated on the surface of LaOF and 15 mol% BaF₂/LaOF, which had been pretreated under vacuum at 750 or 800°C, forming O ₂ ^- species at high temperature (750-800°C). At 750°C, the adsorbed O ₂ ^- species can react with pure CH₄ accompanied by formation of gas-phase C₂H₄ and CO₂, and there is a good correlation between the rate of disappearance of surface O₂and the rate of formation of gas-phase C₂H₄. The O ₂ ^- species was also observed over the catalysts under working condition, and it reacted with CH₄ in a manner that was consistent with its role in a catalytic cycle. These results suggest that O ₂ ^- may be the active oxygen species for OCM reaction over these catalysts. This revised version was published online in July 2006 with corrections to the Cover Date.     

Structure and mechanism of functional isotactic polypropylene via in situ chlorination graft copolymerization

This paper discusses the structure and mechanism of maleic anhydride (MAH) grafted onto isotactic polypropylene (iPP) via in situ chlorination graft copolymerization (ISCGC). The molecular structure of the grafted iPP was characterized using ¹H NMR and ¹³C NMR spectroscopy, viscosity‐average molecular weight and gel content. The structure of un‐grafted MAH present in the reaction system was investigated using Fourier transform infrared spectroscopy in order to explore the grafting of MAH on iPP. The main side‐reactions, including iPP chain scission and crosslinking, during the grafting reaction were explored. From the experimental results obtained, the reason for controlled macromolecular chain degradation and crosslinking of grafted iPP in ISCGC is proposed. Based on the structural characterization of the grafted polymer, the mechanism of grafting onto iPP obtained via ISCGC was deduced. Mechanical properties, both static and dynamic, of grafted iPP were also investigated and the results showed that the properties of the material changed due to grafted MAH. Copyright © 2011 Society of Chemical Industry     

Structure evolution of melt‐spun poly(vinyl alcohol) fibers during hot‐drawing

The drawability of melt‐spun poly(vinyl alcohol) (PVA) fibers and its structure evolution during hot‐drawing process were studied by differential scanning calorimetry (DSC), two dimensional X‐ray diffraction (2‐D WAXD) and dynamic mechanical analysis (DMA). The results showed that the water content of PVA fibers should be controlled before hot‐drawing and the proper drying condition was drying at 200°C for 3 min. PVA fibers with excellent mechanical properties could be obtained by drawing at 200°C and 100 mm/min. The melt point and crystallinity of PVA fibers increased with the draw ratio increasing. The 2‐D WAXD patterns of PVA fibers changed from circular scattering pattern to sharp diffraction point, confirming the change of PVA fibers from random orientation to high degree orientation. Accordingly, the tensile strength of PVA fibers enhanced by hot‐drawing, reaching 1.85 GPa when the draw ratio was 16. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012