The effect of coagulants on the microstructure and mechanical properties of lyotropic fiber-forming polymers

The effect of coagulant on the mechanical properties and microstructure of three lyotropic fiber-forming polymers, poly(p-phenylene benzobisthiazole) (PBZT), poly(p-phenylene terephthalamide) (PPTA) and poly(p-phenylene benzobisoxazole) (PBO) was studied. Previous research found that the imbalance between tensile and compressive/shear properties in these high-modulus fibers is due to a low degree of lateral interaction between microfibrillar elements. In this work, coagulants were chosen which could have strong specific molecular interactions with some portion of the polymer chain; the goal of this research was to increase compressive and shear properties by creating lateral physical crosslinks among the polymer chains.; Results from these coagulation studies show that macroscopic properties such as compressive strength and torsion modulus are dependent on the coagulant; these interactions are strong enough to significantly affect mechanical and structural properties of the fiber. For oxygen-containing lyotropic polymers, the cations present in the coagulant may be important in determining the occurrence of specific interactions.; As-spun PBZT fiber coagulated in iodine/ethanol solution with a spin/draw of 3 had a shear modulus of 1.14 GPa and tensile strength of 2.2 GPa; fiber coagulated in ethanol had respective values of 0.63 GPa and 1.8 GPa. Wide-angle X-ray scattering (WAXS) studies showed that I{dollar}sb3sp-{dollar} and I{dollar}sb5sp-{dollar} anions were present within the fiber, and that there was some disruption of the standard PBZT unit cell. PPTA fiber was coagulated into water, ethanol, iodine/ethanol and aqueous solutions of alkali salts. Coagulation in water produced PPTA fiber with the highest tensile, shear and compressive properties. The shear and/or compressive properties of PPTA fibers could be decreased without a corresponding change in crystal structure; these properties seem to be based on an element of microstructure above that of the unit cell. PBO fibers were coagulated in many of the same coagulants; mechanical properties were unaffected. Coagulation in aqueous potassium iodide produced fiber containing oriented potassium iodide crystals within the fiber.