The water stability of sulfonated copolyimides (SPIs) derived from 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTDA), sulfonated diamines of 4,4‘-bis(4-aminophenoxy)biphenyl-3,3‘-disulfonic acid (pBAPBDS), and 2,2‘- or 3,3‘-bis(3-sulfopropoxy)benzidine (2,2‘- or 3,3‘-BSPB) and nonsulfonated diamines was investigated in detail from viewpoints of viscosity, mechanical strength, proton conductivity, weight loss, and hydrolysis products eluted into the soaking water. With the aging in water or 100% relative humidity vapor at 130 °C, the polymer chain scission took place mainly in the early stage but thereafter slightly, and as a result the SPI membranes kept the reasonably high mechanical properties even after 192 h. The branched/cross-linked SPI membranes prepared by incorporating a flexible triamine kept better mechanical properties. The weight loss and sulfur loss of <10% were observed for the pBAPBDS-based SPIs. This was due mainly to the elution of hydrolysis product, the oligomer of NTDA, and sulfonated diamine, which did not contain the nonsulfonated diamine moieties. With the aging at 130 °C, the proton conductivity did not change for the pBAPBDS-based SPIs, but for the BSPB-based SPIs it decreased 20% in water and much more at the lower relative humidities because of the cleavage of the sulfopropoxy group. The accelerated water stability tests reveal that the water stability of the present SPI membranes is not sufficiently high at 130 °C but is high enough for PEFC and DMFC applications at least at 80 °C.

A series of sulfonated copolyimides (co‐SPIs) bearing pendant sulfonic acid groups were synthesized from 1,4,5,8‐naphthalenetetracarboxylic dianhydride (NTDA), bis(3‐sulfopropoxy) benzidines (BSPBs), and common nonsulfonated diamines via statistical or sequenced polycondensation reactions. Membranes were prepared by casting their m‐cresol solutions. The co‐SPI membrane had a microphase‐separated structure composed of hydrophilic and hydrophobic domains, but the connecting behavior of hydrophilic domains was different from that of the homo‐SPIs. The co‐SPI membranes displayed clear anisotropic membrane swelling in water with negligibly small dimensional changes in the plane direction of the membrane. With water uptake values of 39–94 wt %, they showed dimensional changes in membrane thickness of about 0.11–0.58, which were much lower than those of homo‐SPIs. The proton conductivity σ values of co‐SPI membranes with ion exchange capacity values ranging from 1.95–2.32 meq/g increased sigmoidally with increasing relative humidity. They displayed σ values of 0.05–0.16 S/cm at 50 °C in liquid water. Increasing temperature up to 120 °C resulted in further increase in proton conductivity. The co‐SPI membranes showed relatively good conductivity stability during the aging treatment in water at 100 °C for 300 h. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 1545–1553, 2005

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