Facile synthesis of a symmetrical diamine containing bis-benzimidazole ring and its thermally stable polyimides

A new symmetrical diamine that contains benzimidazole ring, 6,6’-bis[2-(4-aminophenyl)benzimidazole], was synthesised directly from p-aminobenzoic acid and 3,3’-diaminobenzidine via benzimidazolisation condensation. The diamine monomer was polymerised with various commercial dianhydrides by a conventional two-step polymerisation reaction to obtain the corresponding polyimides. The obtained polyimides were amorphous and could be used to produce flexible, yet tough, transparent films exhibiting excellent thermal stability with glass transition temperatures of 334 °C to 412 °C and 5 % weight loss temperatures above 540 °C in nitrogen. These polymers also demonstrated outstanding mechanical properties with a tensile strength of 177 MPa to 224 MPa and a tensile modulus of 3.5 GPa to 5.7 GPa. The excellent thermal and mechanical properties of these polyimides are most likely attributable to the rigid-rod structure and the presence of hydrogen bond between the macromolecular chains.