Water Sorption, Proton Conduction, and Methanol Permeation Properties of Sulfonated Polyimide Membranes Cross-Linked with N,N -Bis(2-hydroxyethyl)-2-aminoethanesulfonic Acid (BES)

Sulfonated polyimide (SPI) membranes consisting of 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTDA), 3,5-diaminobenzoic acid (DBA), and 4,4‘-diaminodiphenyl ether 2,2‘-disulfonic acid (SODA) were synthesized over a wide range of DBA/SODA molar compositions. A sulfonic acid-containing cross-linking agent, N,N-bis(2-hydroxyethyl)-2-aminoethanesulfonic acid (BES), was subsequently used to induce cross-linking between sulfonated polyimide chains via a thermally activated reaction. Efforts to prevent excessive water swelling due to the high degree of sulfonation and to avoid undesirable molecular transport by cross-linking typically resulted in a reduction of ion conductivity due to the blockage of a hydrophilic channel or loss of the water pocket after cross-linking. To solve this problem, a sulfonic acid-containing compound was used as both a cross-linking agent and a proton carrier. One of the main goals of this study was to compensate for the probable loss of ion conductivity due to cross-linking by using a cross-linker that included a fixed charge group. As expected, the BES-cross-linked SPI membranes showed excellent proton conductivity (i.e., σ = 0.1 S cm-1 at 90 °C and relative humidity (RH) of 90%) and low methanol permeability (i.e., P = 1.6 × 10-7 cm2 s-1). Proton conductivity increased and methanol permeability decreased with BES content, which is completely contrary to typical behavior as both properties are normally reduced by further cross-linking. In addition, the effect of cross-linking without a fixed charged group (-SO3H) on the transport and permeation properties was compared with that of BES in cross-linked SPI membranes.