Synthesis and properties of soluble sulfonated polybenzimidazoles

A new sulfonated aromatic diacid, 4,8-disulfonyl-2,6-naphthalenedicarboxylic acid (DSNDA), was prepared from 2,6-naphthalenedicarboxylic acid by direct sulfonation. The chemical structure of the sulfonated monomer was characterized by FTIR, 1H NMR, 13C NMR and MS. A series of sulfonated polybenzimidazoles (sPBI-NF) were successfully synthesized from DSNDA, 2,2-bis(4-carboxyphenyl)hexafluoropropane and 3,3′-diaminobenzidine in poly(phosphoric acid). All the sulfonated samples having high-molecular-weights (Mn, 74,100-109,500) were soluble in some polar aprotic solvents such as DMF, DMSO and DMAc, and they could be easily formed into tough and flexible films via solution casting. The films presented good thermal stabilities (T5% > 453 °C), high storage moduli and glass-transition temperatures (Tg > 220 °C), as well as tensile strengths of about 95 MPa and swelling degrees lower than 12%. The conductivity values of sPBI-NF60H measured at the temperature range from 30 to 90 °C vary from 5.7 × 10−4 to 2.7 × 10−3 S/cm at 100% relative humidity.