Novel proton exchange membranes based on water resistant sulfonated poly[bis(benzimidazobenzisoquinolinones)]

Novel water resistant sulfonated poly[bis(benzimidazobenzisoquinolinones)] (SPBIBIs) were synthesized from 6,6′-disulfonic-4,4′-binaphthyl-1,1′,8,8′-tetracarboxylic dianhydride (SBTDA) and various aromatic ether tetraamines. The resulting polymers with IEC in the range of 2.17–2.87 mequiv g−1 have a combination of desired properties such as high solubility in common organic solvents, film-forming ability, and excellent thermal and mechanical properties. Flexible and tough membranes, obtained by casting from m-cresol solution, had tensile strength, elongation at break, and tensile modulus values in the range of 87.6–98.4 MPa, 35.8–52.8%, and 0.94–1.07 GPa. SPBIBI membranes with a high degree of sulfonation displayed high proton conductivity and a good resistance to water swelling as well. SPBIBI-b with IEC of 2.80 mequiv g−1 displayed the conductivity of 1.74 × 10−1 S cm−1 at 100 °C, which was comparable to that of Nafion® 117 (1.78 × 10−1 S cm−1, at 100 °C). However, the water swelling ratio of SPBIBI-b membranes was merely 8% at 100 °C while the Nafion® 117 was 21.5%. The low swelling ratio was attributed to the strong intermolecular interaction including the electrostatic force and hydrogen bond. Moreover, they also exhibited much better hydrolytic stability than other sulfonated aromatic polymers such as polyimides. Consequently, these materials proved to be promising as proton exchange membranes.