Effect of High Molecular Weight PPTA on Liquid Crystalline Phase and Spinning Process of Aramid Fibers

High molecular weight poly (p-phenylene-terephthalamide) (h-PPTA) was blended with the commercial PPTA in concentrated sulfuric acid to improve the spinnability of the polymer solutions and the mechanical properties of the as-spun fibers. h-PPTA in the solution has an influence on the temperature of the formation of liquid crystalline phenomenon. The temperature range with the existence of the liquid crystalline phase increases upon the contents of h-PPTA in the solution, and the extended temperature window is helpful for the preparation of PPTA fibers by the dry-jet wet-spinning technology. The long-chains of h-PPTA enhance the inter-macromolecular interactions and induce the orientation of short-chains for PPTA along the fiber axis under the shear stress in the spinneret and the stretching stress at the air gap. These effects also increase the maximum drawing ratio in the spinning process and improve the mechanical properties of the obtained fibers. The crystallinity and crystal orientation of the fibers are investigated by X-ray diffraction, and results from sonic velocity test further confirm ordering state of the macromolecular chains. The fibril morphologies of the fibers are also studied by a scanning electric microscope.