Polyimide fibers: Structure and morphology

Polyimide fibers were prepared by wet spinning of poly(p,p′ -diaminodiphenylmethanepyro-mellitamic acid). Density measurements and x-ray diffraction studies were carried out to study the structure of the resultant polyimide fibers. Polyamic acid as well as undrawn polyimide fibers were essentially amorphous with two amorphous haloes. Hot drawing of the fibers at 300°C resulted in increase in crystallinity, and a simultaneous decrease in density also took place. X-ray data revealed that meridional reflections correspond to the repeat unit length in the fiber. Scanning electron micrography studies indicated that polyamic acid fibers prepared by a wet-spinning technique developed voids during spinning which increased on cyclodehydration to the polyimide state. Hot drawing of fibers resulted in enlargement of these voids. However, a highly fibrillated structure was developed during drawing which could account for the strength of the fibers.