Fabrication of poly(bis-benzimidazole imide) fibers with enhanced mechanical properties and high limit oxygen indexes

A kind of diamine monomer 2,2′-p-phenylenebis (5-aminobenzimidazole) (PBABI) with bis-benzimidazole moiety was synthesized and a facile two-step wet-spinning method was employed to fabricate a series of polyimide fibers containing bis-benzimidazole units (PBIPI) with the 3,3′,4,4′-biphenyltetracarboxylic anhydride (BPDA), p-Phenylenediamine (p-PDA) and self-synthesized PBABI as the monomers. The structures and purity of self-synthesized PBABI was characterized in detail. Various temperatures for thermal-drawn process were studied and the 2D-WAXD showed that the higher the temperature for thermal-drawn process, the more orderly of the polymer chains. Attributed to the stronger intermolecular hydrogen bonding forces combined with the orientation and arrangement of molecular chains, the as-spun PBIPI fiber displayed improved mechanical properties with the tensile strength of 1.97 GPa and the initial modulus 59.5 GPa. Furthermore, the PBIPI fiber also showed improved thermal stability and flame resistance properties due to the existent of benzimidazole units. Correspondingly, the glass transition temperature (Tg) and the temperature at the maximum degradation rate (Td-max) of the PBIPI fiber in nitrogen increased by 76 °C and 17 °C respectively and the limit oxygen index (LOI) enhanced by 13%, making these novel polymeric fibers as good thermo-resistant and flame-retardant fiber in fire-fighting domain.