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 degrees 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 degrees with it. (C) 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012