A HIGH-PERFORMANCE AROMATIC POLYIMIDE FIBER .1. STRUCTURE, PROPERTIES AND MECHANICAL-HISTORY DEPENDENCE

A new segmented rigid-rod polyimide has been synthesized from 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA) and 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl (PFMB). This polyimide is soluble in hot m-cresol, allowing fibres to be spun from an isotropic solution using a dry-jet wet spinning method. The as-spun fibres have low tenacities and low moduli, but they can be drawn at high temperatures (> 380-degrees-C) under tension to large draw ratios (up to 10 times), which produces a remarkable increase in strength and modulus. Drawn fibres display a tensile strength of about 25 g den-1 (3.2 GPa) and an initial modulus higher than 1000 g den-1 (130 GPa). BPDA-PFMB fibres show excellent thermal stability and retain relatively high strength and modulus at elevated temperatures. Annealed BPDA-PFMB fibres display distinct wide-angle X-ray patterns, from which a monoclinic unit cell has been determined. Furthermore, changes in crystallographic c-axis, apparent crystal sizes, degree of crystallinity, crystal orientation and thermomechanical properties have been observed with different draw ratios.