Sulfonated naphthalenic polyimides containing ether and ketone linkages as polymer electrolyte membranes

Two series of sulfonated naphthalenic polyimides containing ether and ketone linkages (SPI-KK-X and SPI-K-X) were prepared by a one-pot polymerization reaction of sulfonated diamine monomers, 1,4-bis[3-sodium sulfonate-4-(4-aminophenoxy)benzoyl]benzene or 4,4-bis[3-sodium sulfonate-4-aminophenoxy]benzophenone, with 4,4-diaminodiphenyl ether and 1,4,5,8-naphthalenetetracarboxylic dianhydride. The membranes cast from the resulting polymers had good thermal and oxidative stability, dimensional stability and mechanical properties. At 80 °C, the proton conductivities of several samples, including SPI-KK-1, SPI-K-1 and SPI-K-2 were higher than 0.10 S/cm, which were comparable to that of Nafion. Methanol permeabilities of the obtained polymer electrolyte membranes (PEMs) were in the range of 1.43–2.03 × 10−7 cm2/s, which were several times lower than that of Nafion 117. It is interesting to note that the SPI-KK-X series, having a more rigid phenyl–ketone–phenyl–ketone–phenyl moiety, had a lower dimensional swelling ratio and lower methanol permeability in comparison with the corresponding SPI-K-X series at the same level of ion exchange capacity (IEC).