Synthesis and properties of sulfonated copoly(phthalazinone ether imides) as electrolyte membranes in fuel cells

A new series of six-member sulfonated copolyimides (SPIs) were prepared by one-step solution copolycondensation from 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTDA), 1,2-dihydro-2-(4-amino-2-sulfophenyl)-4-[4-(4-amino-2-sulfonphenoxy)-phenyl] (2H)phthalazin-1-one (S-DHPZDA), 4,4′-bis(4-aminophenoxy) biphenyl (BAPB) and 1,2-dihydro-2-(4-aminophenyl)-4-[4-(4-(aminophenoxyl)phenyl)](2H)phthalazin-1-one (DHPZDA). The sulfonation degree (DS) of the SPIs was controlled by the mol ratio of the sulfonated diamine and non-sulfonated diamine. The obtained SPI membranes had excellent thermal stability, high mechanical property and proton conductivity as well as low methanol permeability. The tensile strength of the SPI membranes was ranging from 54.7 to 98.1 MPa, which was much higher than that of Nafion®. The SPI membranes exhibited high proton conductivity (σ) and low methanol permeability ranged from 10−3 to 10−2 S/cm and 10−8 to 10−7 cm2/s depending on the DS of the polymers, respectively.