Two novel sulfonated diamine isomers, 2, 2′-bis (3-sulfopropoxy) benzidine (2, 2′-BSPB) and 3, 3′-bis (3-sulfopropoxy) benzidine (3, 3′-BSPB), were successfully synthesized and the highly sulfonated polyimides (SPIs) with sulfonic acid groups in the side chains were prepared from 1, 4, 5, 8-naphthalenetetracarboxylic dianhydride (NTDA) and BSPB monomers. Transmission electron microscopy (TEM) analysis revealed that these side- ...