| Summary: | The mechanical properties of high modulus/high strength fibers of poly-(p-phenylene benzobisthiazole) (PBT) have been investigated in relation to fiber microstructure and fibrillar morphology. Heat treatment processing of dry-jet wet spun PBT fibers was undertaken to provide fiber samples for a systematic comparison of structure-property relationships. Heat treated PBT fibers possessing a tensile modulus as high as 300 GPa with tensile strength of 3 GPa have been produced by employing heat treatment conditions bordering on polymer degradation; temperatures of 630(DEGREES)C - 680(DEGREES)C in a nitrogen atmosphere with applied tensions approaching fiber breakage. Quantitative measures of mechanical anisotropy were obtained for PBT fibers in torsional and bending experiments. Torsional shear modulus values of 1 - 1.5 GPa were measured in free torsional oscillation experiments and shear strengths of 60 - 140 MPa were obtained from twisting experiments. Bending studies provided a measure of compressive strength, with fibers exhibiting buckling for compressive stresses of approximately 700 MPa. The state of stress in model cylindically orthotropic fiber structures was analyzed using anisotropic elasticity theory. Tension.compression, torsion and bending were examined in order to compare the predicted level of transverse stress with the measured (relatively low) lateral strength. Optical microscopy, transmission and scanning electron microscopy and x-ray diffraction were used to examine the structure of PBT fibers. Crystallite size perpendicular to the fiber axis increases from approximately 2 nm in as-spun fibers to 10 - 12 nm in fibers heat treated at temperatures above 600(DEGREES)C. Fiber tensile strength was found to increase with increase in the extent of the lateral molecular order. However, tensile modulus and tensile strength did not depend directly on heat treatment parameters of temperature, time but rather indirectly through the affect of applied tension during heat treatment on the overall axial orientation. The highest values of fiber tensile modulus and tensile strength were exhibited by the more highly oriented fibers.
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