Synthesis and characterization of sulfonated polyimides containing aliphatic linkages in the main chain

A series of six-membered sulfonated polyimides with aliphatic linkages (SPIAs) was successfully synthesized using 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTDA), 4,4′-diaminobiphenyl 2,2′-disulfonic acid (BDSA) as the sulfonated diamine, and aliphatic diamines H₂ N(CH₂)_nNH₂ where n = 6, 8, 10, 12. These SPIAs were evaluated for thermal stability, ion exchange capacity (IEC), water uptake, proton conductivity, and hydrolytic stability. Proton conductivity and hydrolytic stability of the SPIAs were compared with the fully aromatic polyimide (MDA-SPI) prepared from 4,4′-methylenedianiline (MDA), BDSA, and NTDA. All the SPIAs exhibited high thermal stability. As the chain length of the aliphatic diamine decreased, the IEC and water uptake of the SPIAs increased. The SPIAs showed higher proton conductivity than commercially available membranes such as Nafion 117 at high temperatures and higher proton conductivity than MDA-SPI at all temperatures. All SPIAs exhibited a hydrolytic stability more than twice as high as that of MDA-SPI.