Novel aromatic polyimide fiber with biphenyl side-groups: Dope synthesis and filament internal morphology control

A new organic-soluble aromatic polyimide with biphenyl side-groups has been synthesized from 4,4'-oxydiphthalic anhydride and 3,5-diamino-benzonic-4'-biphenyl ester (DABBE) via a one-step polymerization in m-cresol. A higher molecular weight polyimide has been obtained by the addition of chlorotrimethylsilane (TMSCI) in the solution of DABBE to form, in situ, silylated diamine. The optimum mole amount of TMSCI relative to the number of amino groups is 100%. This polyimide is soluble in m-cresol, allowing fibers to be spun from isotropic solution using a dry-jet wet spinning method. Based on a ternary phase diagram of m-cresol, ethanol, and water, controlling of the internal morphology of as-spun fibers has been achieved by varying the rate of polyimide coagulation through adjustment of nonsolvent/solvent miscibility in the coagulation bath. Scanning electron microscopic pictures show that filament internal morphologies ranged from porous-like to fully solid. The solid as-spun fibers can be drawn at high temperatures (> 330 degrees C) under tension to high drawn ratios (up to 6x), which produces a remarkable increase in tensile strength to about 1.0 GPa and an initial modulus higher than 60 GPa.