Polymer, 2018, vol 150pp. 254-266
A series of high-performance polyimide copolymer (co-PI) fibers containing phenylenebenzoxazole moiety with one hydroxyl group are synthesized based on the copolymerization of 3,3′,4,4′-biphenyl tetracarboxylic dianhydride (BPDA) with two diamine monomers, namely, p-phenylenediamine (p-PDA) and 5-anino-2-(2-hydroxy-4-aminobenzene)-benzoxazole (p-mHBOA), through a two-step method. The molar ratio of p-PDA/p-mHBOA varies from 10/0 to 0/10. Influences of the p-mHBOA moieties on the thermal stability, crystal structure, crystal orientation, microvoid morphology, and mechanical properties are systemically investigated. Fourier transform infrared results confirm that intra/inter molecular hydrogen bonds come from the OH group and nitrogen atom of the benzoxazole group and/or OH group and the oxygen atom of the carbonyl group of cyclic imide. The glass transition temperatures (Tgs) of co-PI fibers are in the range of 290 °C–325 °C. The prepared fibers show excellent thermal stabilities, and their T5% is within 528 °C–542 °C in air. Two-dimensional wide-angle X-ray diffraction spectra indicate that homo-PI and co-PI fibers present a regularly arranged polymer chains along the fiber axial direction. Moreover, the ordered molecular packing along the transversal direction of fibers is destroyed by the copolymerization. Small-angle X-ray scattering results show that co-PI fibers with optimal mechanical properties exhibit the shortest average length (L) and the smallest radius (R¯) of microvoids. When the p-mHBOA/p-PDA molar ratio is 5/5, the fracture strength and initial modulus can reach approximately 30.31 cN/dtex (4.40 GPa) and 894.88 cN/dtex (129.8 GPa), respectively. The relationship between structure and mechanical properties is also discussed.