| This dissertation examines the conversion of poly p-phenylenebenzobisoxazole (PBO) to carbon fiber. The aromatic, rigid-rod backbone of the polymer enables the carbonized fiber to develop a three-dimensionally ordered structure without stabilization. This represents a significant processing advantage over traditional carbon fiber precursors such as pitch and PAN (polyacrylonitrile).; This dissertation presents the first comprehensive analysis of the high temperature behavior of this unique polymeric material. Additionally, the relationship between crystalline misalignments in the polymeric and carbonized fibers are analyzed. Also, the effects of processing conditions on the fiber are examined and explained.; The carbonized PBO fiber was shown to develop lower electrical resistivities and higher tensile moduli than most commercial PAN-based fibers. However, the carbonized PBO fibers achieved tensile strengths of only 0.7 GPa. By contrast, commercial PAN-based fibers often possess tensile strengths in excess of 3 GPa. Thus, understanding the reasons for this reduced tensile strength became essential.; The removal of nitrogen at elevated temperatures was shown to disrupt the developing crystalline structure of the fiber, resulting in a decrease in tensile strength. The rapid heating rates associated with continuous carbonization were found to substantially reduce the damage caused by this "puffing" phenomenon and improve the tensile strength of the carbonized fiber.; A Weibull analysis of single filament tensile data indicated that the same flaw distribution present in the as-received fibers resulted in the tensile failures in the carbonized fibers. This implied that the flaws in the precursor fiber persist throughout the carbonization process. Thus, the nitrogen puffing essentially exacerbates the severity of the previously existing crystallite misalignments.; Perhaps the most intriguing aspect of carbonized PBO fibers is their development of exceptional lattice dependent properties. Thus, the carbonization of the polymer was examined in detail. Thermogravimetric analysis and mass spectrometry studies showed that the carbonization process could be represented by a free-radical depolymerization model. An activation energy for the free-radical initiation reaction was determined, and the kinetic model was applied to predict the mass losses experienced by the fiber during carbonization.; Finally, the graphitization kinetics of the fiber were examined. The growth of graphene crystallites was shown to have a temperature dependence described by the Arrhenius relationship, and an activation energy of 120 kcal/mol was determined for the process. |
