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The diamine monomer, 5-amino-2-(p-aminophenyl)benzoxazole(ABO), was successfully synthesized and a series of new naphthalenic sulfonated polyimides (SPIs) were prepared from 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTDA), 2,2’-benzidinedisulfonic acid (BDSA), ABO and 1,10- diaminedecane (DADE) using one-step high temperature polymerization method. Their structures were characterized by FTIR,  1 HNMR and TG-DSC. The sulfonated degree of the SPI copolymers was controlled through varying the molar ratio of BDSA to the non-sulfonated diamines. Tough and transparent membranes with the typical polyimide brown colour were prepared by casting from the polymer solution. They showed clear anisotropic membrane swelling in water with larger dimensional changes in the thickness direction of membrane. The sulfonated polyimide membrane containing 20mol% benzoxazole moieties (SPI-20) showed desirable thermal stability with the decomposition temperature of 272°C and mechanical stability with the maximum stress of 42 MPa. The proton conductivity (6.2×10 -3 S/cm) of SPI-20 membrane was comparable to that of Nafion ® 117 membrane measured under the same condition (9.8×10 -3  S/cm). The effects of temperature on the proton conductivity, and highly anisotropic proton conductivity in the thickness and plane direction were also investigated.

Synthesis and properties of new sulfonated naphthalenic polyimides as proton exchange membrane for fuel cell applications