Synthesis and characterization of novel sulfonated polyimides from 4,6-bis(4-aminophenoxy)-naphthalene-2-sulfonic acid

A novel sulfonated diamine monomer, 4,6-bis(4-aminophenoxy)-naphthalene-2-sulfonic acid(BAPNS), was synthesized. A series of sulfonated polyimide copolymers was prepared from BAPNS, 1,4,5,8-naphthalenetetra-carboxylic dianhydride(NTDA) and nonsulfonated diamine 4,4′-diaminodiphenyl ether(ODA). Flexible, transparent, and mechanically strong membranes were obtained. The novel sulfonated polyimide(SPI) membranes show higher conductivity, for example, SPI-100 shows a conductivity of 0.0698 S/cm at 80 °C(SPI-X: X refers to molar fraction of BAPNS). The membranes exhibit the permeability of methanol from 2.18×10−7 cm2/s to 2.57×10−7 cm2/s, which is much lower than that of Nafion(2.00×10−6 cm2/s). The copolymers were thermally stable up to 330 °C. The sulfonated polyimide copolymers also show reasonable mechanical strength; for example, the maximum tensile strength at break of the sulfonated polyimide copolymer with 100%(molar fraction) BAPNS is 1.35 GPa under high moisture conditions. The optimum concentration of BAPNS was found to be 100%(molar fraction) from the view point of proton conductivity, methanol permeability, and membrane stability.