Synthesis and properties of hyperbranched polybenzimidazoles via A ₂ + B₃ approach

Hyperbranched polybenzimidazoles (HBPBIs) were successfully synthesized by condensation polymerization of 1,3,5-benzenetricarboxylic acid (BTA) and 3,3′-diaminobenzidine (DAB) in polyphosphoric acid (PPA) at 190°C. Different monomer addition manners and molar ratios resulted in different polymers, that is, simultaneous addition of BTA and DAB with the molar ratio of 1:1 (manner 1) gave carboxyl-terminated HBPBI (HBPBI-1), whereas the addition of BTA portion-wise to DAB solution in PPA with the molar ratio of DAB:BTA = 2:1 (manner 2) yielded amine-terminated HBPBI (HBPBI-2). The free carboxyl and amino groups of HBPBI-1 and HBPBI-2 could further react with o-diaminobenzene and benzoic acid, respectively, to form the chemically modified polymers. Except HBPBI-2, all the HBPBIs showed good solubility in some organic solvents (e.g., dimethyl sulfoxide and N,N-dimethylacetamide). Thermogravimetric analysis measurement revealed that HBPBIs except HBPBI-1 had high thermal stability (>450°C). HBPBI membranes with good mechanical properties were obtained by crosslinking treatment of partially chemically modified HBPBIs with terephthaldehyde (TPA) during the film cast process. The HBPBI membranes had high phosphoric acid uptake and the phosphoric acid-doped HBPBI-6 (40% o-diamino groups were reacted with benzoic acid) membrane showed higher tensile strength than the acid-doped commercial PBI despite the higher doping level of the former.