Poly(amide-imide) bearing imidazole groups/sulfonated polyimide blends for low humidity and medium temperature proton exchange membranes

A poly(amide-imide) (PAI) bearing imidazole groups on the polymer chain was synthesized via direct polycondensation of a synthesized diacid-diimide and 4,4′-(1,4-Phenylenediisopropylidene) bisaniline (PDBA). Diacide-diimide was synthesized by the condensation of an amino acid compound, (S)-(+)-Histidine hydrochloride monohydrate and 3,3′,4,4′-Benzophenone tetracarboxylic dianhydride (BTDA). On the other hand, a sulfonated polyimide (SPI) was also synthesized by the solution imidization of sulfonated, (4,4-diaminostilbene-2,2-disulfonic acid) (DSDSA) and non-sulfonated, 4,4′-(1,4-Phenylenediisopropylidene) bisaniline (PDBA) diamines in reaction with a six-membered naphthalene base dianhydride, 1,4,5,8-Naphthalenetetracarboxylic dianhydride (NTDA). A strong and flexible SPI membrane with good uniformity and proper thermal and mechanical properties was achieved. The SPI was then blended with different amounts of PAI and doped with phosphoric acid (PA), in order to investigate the blending influence of PAI in PA-doped blend membranes compared to the pure SPI membrane. It was found that a proper amount of PAI could effectively improve the water uptake, IEC and proton conductivity of the PA doped SPI/PAI membranes. Nevertheless the excess PAI negatively affected the membrane properties. The pure SPI with an IEC of 1.76 meq.g⁻¹ showed a proton conductivity of 29.4 mS cm⁻¹ at 120 °C, while PA doped SPI/PAI-10 % (w/w) as the most optimal PAI containing sample, with an IEC of 2.23, showed a proton conductivity of 69.7 mS cm⁻¹ at 140 °C. The proton conductivity measurements were performed at 40 % relative humidity.