Polybenzazole fiber having high tensile modulus and process of manufacture thereof

A polybenzazole fiber obtained through heat treatment, which has a high tensile modulus of not less than 300 GPa and a tensile strength of not less than 5.0 GPa, said fiber being characterized by an X-ray analysis by a fine structure thereof of at least one of the following (1) and (2): (1) a crystal orientation parameter sin2 of not more than 0.009 as determined by a wide-angle X-ray diffraction method, (2) absence of an equatorial streak, a two-point pattern or a four-point pattern in a small-angle X-ray scattering; and a process of manufacturing a polybenzazole fiber which comprises extruding a dope comprising a polybenzazole polymer and a nonoxidative acid capable of dissolving said polymer, from a spinneret into a non-coagulative gas to give spun filaments, introducing said filaments into a coagulation bath to extract the acid contained in said filaments, neutralizing the filaments, washing the filaments, adjusting a water content of the filaments to not more than 100 %, applying a pretension to the filaments at an optional stage after introduction into a coagulation bath and before heat treatment, and heat treating the filaments at a temperature of not less than 500 DEG C under a certain tension to give the fiber. The polybenzazole fiber of the present invention has a specific fine structure of fiber, as mentioned above, and also has a high tensile strength and a high tensile modulus heretofore unavailable. The inventive polybenzazole fiber can be manufactured at an industrial scale with ease. Thus, the inventive fiber is tremendously effective in expanding the field of possible utilization as an industrial material with high practical advantages.