Preparation of high-performance polyimide fibers containing benzimidazole and benzoxazole units

A series of aromatic copolyimide (co-PI) fibers containing benzimidazole and benzoxazole were prepared by introducing 5-amino-2-(4-aminobenzene)benzoxazole (BOA) and 2-(4-aminophenyl)-5-aminobenzimidazole (BIA) into the rigid 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA)/p-phenylenediamine (p-PDA) backbone via a typical two-step wet-spinning method. The incorporated BOA and BIA moieties in the polymer chains resulted in significant changes on the comprehensive performances of the resulting PI fibers. The optimum tensile strength was 2.26 GPa with the BIA/BOA molar ratio of 3/1, whereas the optimum initial modulus was 145.0 GPa with the BIA/BOA molar ratio of 1/3. The drastically improved mechanical properties were originated from the hydrogen-bonding intermolecular interactions by BIA moieties and high molecular orientation by BOA moieties. FT-IR results confirmed the presence of hydrogen-bonding interaction in the fibers with high imidization degree. Two-dimensional wide-angle X-ray diffraction (2D WAXD) indicated the existence of highly oriented structures along the fiber axial direction, while this feature varied with different diamine ratios. SEM observations showed that the fibers were obtained with homogeneous and uniform structures at various diamine ratios. Meanwhile, the co-PI fibers exhibited excellent thermal properties with 5 % weight loss temperature ranging from 563 to 570 °C in nitrogen and glass transition temperature ranging from 308 to 321 °C.