聚丙烯腈预氧化纤维组织结构的遗传与演变研究

采用扫描电镜(SEM)和透射电镜(TEM)仪器和方法,对聚丙烯腈(PAN)原丝和不同温度阶段预氧丝的表面形貌、断面形貌和内部组织结构进行了系统的分析。结果表明,PAN原丝的原纤结构在整个预氧化过程中是具有遗传性的,随着预氧化的不断进行,原纤的韧性逐渐降低,原纤之间的结合更为紧密;预氧化反应由纤维外部向芯部逐步进行,形成了组织致密、脆性高的皮层和组织疏松、韧性高的芯部。皮层区域的晶粒尺寸细小,分布均匀,无择优取向,非晶组织较为致密;而芯部区域的组织粗大,呈现出沿纤维轴向排列的层片状结构,越靠近芯部,层片的取向越明显。     

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     

Heredity and difference of multiple-scale microstructures in PAN-based carbon fibers and their precursor fibers

Multiple-scale microstructures, including skin-core structure, fibril structure, lamellar structure, crystal/amorphous structure, were found co-present in the fibers during the whole production process of polyacrylonitrile (PAN)-based carbon fibers. The structural heredity and difference among them were systematically investigated for the first time by scanning electron microscope, optical microscope, transmission electron microscope, and X-ray diffraction. The relations between the four kinds of structures and their formation mechanisms were analyzed. The skin-core structure is contributed to inhomogeneous distribution of composition and chemical structure along the radial direction in oxidative stabilized fibers, which is proved different in PAN precursor fibers and carbon fibers. Fibrils are successively separated from low-temperature oxidative stabilized fibers by ultrasonic etching in dimethylsulfoxide solution. The separation of individual fibril becomes harder and even impossible in those fibers prepared at temperatures higher than 245°C. This suggests a stronger bonding force between fibrils in high-temperature oxidative stabilized fibers and carbon fibers. The lamellar structures within fibrils are observed in all of these fibers but with thicker lamella width with increasing temperature. They are unlikely due to the alternatively alignment of crystal regions and amorphous regions as reported by many previous literature, because the oxidative stabilized fibers are amorphous but have lamellar structures. The (002) diffraction arc gives the evidence that the lamellar structure in carbon fibers is not strictly perpendicular to the fiber axis, but have an angle of about 45° with it. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Study on the chemical structure and skin-core structure of polyacrylonitrile-based fibers during stabilization

The chemical structure and the skin-core structure of the fiber during stabilization were investigated by FT-IR, elemental analysis, optical microscopy (OM), SEM, TEM and Energy Dispersive X-ray Detector (EDX). The experimental results show that the oxygen content in the skin of the stabilized fiber is more than that in the core, resulting in the inhomogeneity structure of the stabilized fiber. The cross-section of the stabilized fiber presents concentric multi-layer ring-like morphology, the outmost thin and dense skin layer, followed by the cortex layer, the endothelium layer and the core layer. The stabilized fiber is consisted of sheet-like structure in the microstructure images, but there is some difference between the skin and the core. Not only the orientation of the skin is better than the core, but also the skin structure is more compact and homogenous than the core.     

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.     

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     

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.     

Permeability measurement on high strength concrete without and with polypropylene fibers at elevated temperatures using a new test setup

A new test setup for permeability measurement at room and high temperature is presented. The experimental results obtained by employing the new setup are reported and validated. The experiments are performed on high performance concrete, without and with addition of polypropylene fibers under temperatures ranging from 20 °C to 300 °C as well as after cooling of previously heated specimens to the room temperature. The results show that plain concrete exhibits steady increase in permeability with increasing temperature, whereas concrete with fibers exhibit a sudden increase of permeability at temperatures between 80 °C and 130 °C. The results confirm the governing role of permeability on explosive spalling and suggest the existence of mechanisms of pressure relief other than just melting of fibers. The microstructure of concrete with fibers is investigated using SEM before and after exposure to high temperature. It is observed that the melted polypropylene flows only into the micro-cracks and does not penetrate into cement paste.     

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     

Evolution of aggregation structure of polyacrylonitrile fibers in the cyclization reaction

The aggregation structure of polyacrylonitrile precursor evolves gradually with progress of cyclization. In this work, the variety of cyclization degrees were determined by Fourier transform infrared spectroscopy and the evolution of aggregation structure of PAN fibers were characterized by wide‐angle X‐ray diffraction. Experimental results showed that the cyclization occurred first in the amorphous parts when the heating temperature was below 200°C. After heated at 200°C for 30 min, molecular chains in the pseudo‐crystalline regions started to pack into crystalline regions due to the increasing stress which was produced by cyclization occurred in the amorphous phase, and the crystallinity and crystallite size increased slowly. When the temperature reached to 220°C, pseudo‐crystalline regions rearranged obviously under stress, while molecular chains in the crystalline region started to participate in cyclization, and the original crystalline structures were destructed. The two competitive processes induced that the crystallinity and crystallite size grew to the maximum values at 30 min. When the temperature up to 240°C, the cyclization occurred in the crystalline region became more intensely, while the crystallinity and crystallite size decreased out of synchronize. A scheme of evolution of aggregation structure in cyclization was modified based on the above results. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012