Synthesis and properties of sulfonated multiblock copolynaphthalimides

Sulfonated multiblock copolynaphthalimides (co-SPIs) with block length of 5–20 were prepared by a two-pot polymerization method from 1,4,5,8-naphthalenetetracarboxylic dianhydride, sulfonated diamines of 2,2′-bis(4-sulfophenoxy)benzidine (BSPOB) and 4,4′-bis(4-aminophenoxy)biphenyl-3,3′-disulfonic acid (BAPBDS) and nonsulfonated diamines. The scanning transmission electron microscopy showed that the BSPOB-based multiblock co-SPI membranes had well-defined microphase-separated structure where the hydrophilic and hydrophobic layer-like domains were oriented in the plane direction of membrane. On the other hand, the BAPBDS-based multiblock co-SPIs and all the random co-SPIs showed the homogeneous morphology. The water uptake, anisotropic membrane swelling, anisotropic proton conductivity and polymer electrolyte fuel cell (PEFC) performance were investigated for the multiblock and random co-SPIs. The results demonstrated strong effects of the membrane morphology on these properties. The BSPOB-based multiblock co-SPI membranes exhibited the larger through-plane swelling and the lower through-plane conductivity than the random ones, and as a result exhibited the lower PEFC performances. This study gives good understanding of the morphology–property relationship in novel block architecturesʹ design.