Abstract: Poly(p-phenyleneterephthalamide) fibers having a very high tensile strength is described. The fibers consist essentially of poly(p-phenyleneterephthalamide) having an inherent viscosity of at least 5 dl/g as determined at 25.degree. C. in sulfuric acid having a concentration of 98 wt. % at a polymer concentration of 0.5 g/dl. The fibers are composed of single filaments of 0.5 to 3 denier, wherein the single filament tensile strength is at least 35 g/denier. The macrovoid number, hereinbefore defined, of the fibers is not more than 10 per 100 mm. The asymmetry, hereinbefore defined, of the fibers is not more than 0.2.

Abstract: There is provided a shaping composite dope composition comprising a cellulose derivative, an acrylonitrile polymer and a solvent capable of substantially dissolving these polymeric materials simultaneously therein, and exhibiting flow birefringence or optical anisotropy. A composite fiber made from the dope has a strucute such that the cellulose derivative is present in the form of a dispersed phase of fine fibers in a matrix of the acrylonitrile polymer, the crystalline orientation angle of the acrylonitrile polymer phase is 18.degree. to 63.degree. as measured by wide angle X-ray diffractiometry, and the ratio of the crystalline orientation angle of the cellulose derivative phase to the crystalline orientation angle of the acrylonitrile polymer phase satisfies a specified requirement. The composite fiber is excellent in the thermal dimensional stability and mechanical properties, such as tensile strength and tensile elongation and loop strength and loop elongation.