Fabrication and properties of polyimide copolymer fibers containing pyrimidine and amide units

Polyimide (PI) fibers containing pyrimidine and amide moieties were fabricated by the copolymerization of 2,5-bis(4-aminophenyl)pyrimidine (PRM) and 4,4′-diaminobenzanilide (DABA) with 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA) via a two-step method. The effects of chemical structure of PI fibers, i.e., various molar ratios of PRM/DABA, on their thermal property, crystallinity, orientation factor, microvoid defects, and mechanical property, were investigated in detail. The glass transition temperatures (T_g) of all PI fibers ranged from 282 to 310 °C, and the temperatures at 5% weight loss (T_5%) under N₂ and air atmosphere were respectively within 548–584 °C and 482–540 °C, demonstrating the excellent thermal stability and thermal oxidation stability of all fibers. The wide-angle X-ray diffraction data illustrated that the co-PI fibers had lower crystallinity than the homo-PI fibers due to a marked decrease in molecular order packing by copolymerization. However, the co-PI fibers possessed higher degrees of orientation relative to the homo-PI fibers. In particular, the co-PI fiber with PRM/DABA = 5/5 showed the highest orientation factor, f_c = 0.872. Moreover, the small-angle X-ray scattering results, i.e., decrease in microvoid size, revealed that DABA incorporation helps to reduce the defects in PI fibers. When the PRM/DABA molar ratio was 5/5, the co-PI fiber possessed an optimal tensile strength and initial modulus of 3.43 and 136.92 GPa, respectively.