Synthesis and characterization of sulfonated naphthalenic polyimides based on 4,4′-diaminodiphenylether-2,2′-disulfonic acid and bis[4-(4-aminophenoxy)phenylhexafluoropropane] for fuel cell applications

The paper describes the synthesis and characterization of sulfonated polyimides based on 1,4,5,8-naphthalene tetracarboxylic dianhydride (NTDA), 4,4′-diaminodiphenylether-2,2′-disulfonic acid (ODADS) and (bis[4-(4-aminophenoxy)phenylhexafluoropropane] (BDAF)). Several copolymer samples were prepared by varying the molar ratio of ODADS: BDAF (0.5:1.50, 0.75:1.25, 1:1 and 1.50: 0.5) in the initial monomer feed. Structural characterization of the copolymers was done using FT-IR and 1H NMR. 1H NMR was also used to calculate the copolymer composition. Thermal characterization was done using thermogravimertry and dynamic mechanical analysis. Polymer films were prepared by solution casting using m-cresol as solvent. The membranes thus prepared were characterized for water uptake, water stability, methanol permeability and proton conductivity. The obtained sulfonated polyimides (SPI’s) had proton conductivities in the range of 0.137–3.94 mS/cm. SPI’s with 50% degree of sulfonation had proton conductivity comparable to that of Nafion with methanol permeability lower than that of Nafion. It was found that the degree of sulfonation of polyimide had a large effect on the thermal stability, water uptake, ion-exchange capacity and proton conductivity.