Synthesis, characterization and properties of aromatic copolyimides containing Bi-benzimidazole moiety

A new aromatic heterocyclic diamine monomer, 2,2-bis (4’-aminophenyl)- 5,5-bi-1H-benzimidazole (BAPBBI), was synthesized via a one-step reaction of 3,3’,4,4’-biphenyltetramine and p-aminobenzoic acid in polyphosphoric acids with the high yield and purity. FTIR, ¹H NMR, ¹³C NMR, mass spectra and elemental analysis were employed to confirm the chemical structures. A series of copolyimide films containing the bi-benzimidazole moieties were prepared by the reaction of pyromellitic dianhydride (PMDA), 4,4’-oxydianiline (ODA) and BAPBBI with various molar ratios. The combination of polybenzimidazole and polyimide by introducing BAPBBI into the main chains brings the highly rigid structure and the enhanced macromolecular interactions, resulting in the enhancement in the thermal stability and the mechanical properties. For instance, these polyimides are endowed with a higher glass transition temperature of 408–438 °C and an initial thermal decomposition of 550–580 °C. They exhibit a strong tensile strength of 123–233 MPa, and a tensile moduli of 2.66–12.84 GPa without post stretching. Meanwhile the stretched films at 450 °C exhibited the enhanced mechanical properties with the tensile strength of 195–554 MPa and the tensile modulus of 4.69–38.57 GPa.