Enhanced mechanical and hydrophobic properties of polyimide fibers containing benzimidazole and benzoxazole units

A series of rigid-rod co-polyimides (co-PI) containing benzimidazole and benzoxazole moieties were synthesized by the reaction of 2-(4-aminophenyl)-5-aminobenzimidazole (BIA) and 5-amino-2-(4-aminobenzene)benzoxazole (BOA) with 3,3',4,4'-biphenyltetracar-boxylic dianhydride (BPDA). The corresponding co-PI fibers with various diamine ratios were prepared by the two-step wet-spinning process. The small chemical structure difference between benzimidazole and benzoxazole moieties leads to unexpected great changes on the aggregation structures and properties of the obtained fibers. The two-dimensional wide-angle X-ray diffraction (WAXD) spectra imply that the co-PI fibers containing more than 70% molar ratio of BIA show better-defined ordered crystal structures, while for those samples with more BOA contents, a new ordered lateral packing occurs. Meanwhile, the equatorial scattering streaks in the small-angle X-ray scattering (SAXS) patterns for the co-PI fibers suggest the presence of microvoids, whose sizes were evaluated by means of Guinier and Ruland's method. The optimum tensile strength and modulus of co-PI fibers are as high as 1.74 and 74.4 GPa, respectively, at the BIA/BOA molar ratio of 7/3. Besides, the co-PI fibers exhibit excellent thermal and thermo-oxidative stabilities due to the rigid molecular chains. The introduction of BOA units in the polymer backbones have apparently improved the hydrophobic properties of PI fibers. (C) 2015 Elsevier Ltd. All rights reserved.