Copolymerization of 4-(3,4-diamino-phenoxy)-benzoic acid and 3,4-diaminobenzoic acid towards H3PO4-doped PBI membranes for proton conductor with better processability

A series of copolymers, which can also be regarded as ABPBIs containing phenoxyl in the main chain, were synthesized by copolymerization of two kinds of self-polycondensation monomers: 3,4-diaminobenzoic acid (DABA) and 4-(3,4-diamino-phenoxy)-benzoic acid (DPBA). The phenoxyl-containing ABPBIs exhibit better processability and acid stability than original ABPBI, despite their affinities towards acid decrease with the addition of phenoxyl moieties. By adjusting the phenoxyl content of the polymer, we made a tradeoff between acid doping levels and proton conductivity, with mechanical properties. The optimal polymer therein, PO-AB-50 (50 M percentages of DPBA in total monomers), exhibits considerable proton conductivity (53.47   mS/cm at 160   °C) and tensile strength (1.9   MPa) after doped with phosphoric acid. In addition, the oxidation stability, water uptake and swell ratio properties of the polymers were also improved with the introduction of phenoxyl moieties.