Gel spun PAN and PAN/CNT based carbon fibers: From viscoelastic solution to elastic fiber
Newcomb, Bradley Allen
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This study focuses on the processing, structure, and properties of gel spun polyacrylonitrile (PAN) and polyacrylonitrile/carbon nanotube (PAN/CNT) carbon fibers. Gel spun PAN based carbon fibers are manufactured beginning with a study of PAN dissolution in an organic solvent (dimethylformamide, DMF). Homogeneity of the PAN/DMF solution is determined through dynamic shear rheology, and the slope of the Han Plot (log G’ vs log G’’). Solutions were then extruded into gel spun fibers using a 100 filament fiber spinning apparatus in a class 1000 cleanroom. Fibers were then subjected to fiber drawing, stabilization, and carbonization, to convert the PAN precursor fiber into carbon fiber. Carbon fiber tensile strength was shown to scale with the homogeneity of the PAN/DMF solution, as determined by the slope of the log G’ vs log G’’ plot. After the development of the understanding between the homogeneity of the PAN/DMF solutions on the gel spun PAN based carbon fiber tensile properties, the effect of altering the fiber spinning processing conditions on the gel spun PAN based carbon fiber structure and properties is pursued. Cross-sectional shape of the gel spun PAN precursor fiber, characterized with a stereomicroscope, was found to become more circular in cross-section as the gelation bath temperature was increased, the amount of solvent in the gelation bath was increased, and when the solvent was switched from DMF to dimethylacetamide (DMAc). Gel spun fibers were then subjected to fiber drawing, stabilization, and carbonization to manufacture the carbon fiber. Carbon fibers were characterized to determine single filament tensile properties and fiber structure using wide-angle x-ray diffraction (WAXD) and high resolution transmission electron microscopy (HRTEM). It was found that the carbon fiber tensile properties decreased as the carbon fiber circularity increased, as a result of the differences in microstructure of the carbon fiber that result from differences in fiber spinning conditions. In the second half of this study, the addition of CNT into the PAN precursor and carbon fiber is investigated. CNT addition occurs during the solution processing phase, prior to gel spinning. As a first study, Raman spectroscopy is employed to investigate the bundling behavior of the CNT after gel spinning and drawing of the PAN/CNT fibers. By monitoring the peak intensity of the (12,1) chirality in the as-received CNT powder, and in differently processed PAN/CNT fibers, the quality of CNT dispersion can be quickly monitored. PAN/CNT fibers were then subject to single filament straining, with Raman spectra collected as a function of PAN/CNT filament strain. As a result of the PAN/CNT strain, stress induced G’ Raman band shifts were observed in the CNT, indicating successful stress transfer from the surrounding PAN matrix to the dispersed CNT. Utilization of the shear lag theory allows for the calculation of the interfacial shear strength between the PAN and incorporated CNT, which is found to increase as the quality of CNT (higher aspect ratio, increased graphitic perfection, and reduced impurity content), quality of CNT dispersion, and fiber drawing increase. PAN/CNT fibers were then subjected to stabilization and carbonization for the manufacture of gel spun PAN/CNT based carbon fibers. These fibers were then characterized to investigate the effect of CNT incorporation on the structure and properties of the carbonized fibers. The gel spun PAN/CNT based carbon fibers were compared to commercially produced T300 (Toray) and IM7 (Hexcel) carbon fibers, and gel spun PAN based carbon fiber. Fiber structure was determined from WAXD and HRTEM. Carbon fibers properties investigated include tensile properties, and electrical and thermal conductivity. PAN/CNT based carbon fibers exhibited a 103% increase in room temperature thermal conductivity as compared to commercially available IM7, and a 24% increase in electrical conductivity as compared to IM7. These studies provide a further understanding of the processing, structure, property relationships in PAN and PAN/CNT based carbon fibers, beginning at the solution processing phase. Through the manufacture of more homogeneous PAN/DMF solutions and investigations of the fiber spinning process, gel spun PAN based carbon fibers with a tensile strength and modulus of 5.8 GPa and 375 GPa, respectively, were successfully manufactured in a continuous carbonization facility. Gel spun PAN/CNT based carbon fibers exhibit room temperature electrical and thermal conductivities as high as 74.2 kS/m and 33.5 W/m-K.