High-Strength and High-Modulus Polyimide Fibers with Excellent UV and Ozone Resistance

A series of high-strength and high-modulus polyimide (PI) fibers containing benzimidazole and biphenyl units were synthesized from 2-(4-aminophenyl)-5-aminobenzimidazole (BIA), p-phenylenediamine (PDA), and 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA). The dry-spinning and hot-drawing processes were used to prepare PI fibers with various PDA/BIA molar ratios, wherein the chemical structures were identified by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR). Additionally, the relationship between the mechanical properties and aggregation structure of obtained PI fibers, varying with diamine ratios and hot-drawing ratios, was demonstrated by two-dimensional wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS), showing that the variation trend of mechanical property has a strong consistency with the nanoscale orientation factor calculated by the SAXS. The optimum tensile strength and modulus of PI fibers are 4.3 and 145 GPa, respectively, at a PDA/BIA molar ratio of 8/2. Moreover, after 168 h of UV irradiation and 175 h of ozone aging, this sample shows an excellent UV irradiation resistance and ozone aging resistance with retaining 94% and 95% of tensile strength of the pristine fiber. Meanwhile, the prepared PI fibers exhibit prominent thermal stabilities with the 10% weight loss temperature over 590 °C in a nitrogen atmosphere.