ABSTRACT
Abstract-Three kinds of novel sulfonated diamine monomers, 4,4’-diaminodiphenyl ether-2,2’- disulfonic acid (ODADS), 9,9-bis(4-aminophenyl)fluorene-2,7-disulfonic acid (BAPFDS), and 4,4’- bis(4-aminophenoxy)biphenyl-3,3,-disulfonic acid (BAPBDS), were successfully synthesized by di rect sulfonation of the corresponding parent diamines, 4,4’-diaminodiphenyl ether (conventional name: 4,4’-oxydianiline (ODA)), 9,9-bis(4-aminophenyl)fluorene (BAPF), and 4,4’-bis(4aminophenoxy)biphenyl (BAPB), respectively, using fuming sulfuric acid as the sulfonating re agent. Various sulfonated polyimides were prepared from 1,4,5,8-naphthalenetetracarboxylic dian hydride (NTDA), ODADS, BAPFDS, BAPBDS, and common non-sulfonated diamines. Proton conductivities of the resulting sulfonated polyimide membranes were measured as functions of rela tive humidity and temperature. With increasing relative humidity (at a given temperature) or tem perature (in water) the proton conductivities increased. The proton conductivity was mainly deter mined by the ion exchange capacity (IEC), i.e., the higher the IEC, the larger the conductivity. All the sulfonated polyimide membranes displayed much better stability toward water than those de rived from the widely used sulfonated diamine 2,2’-benzidinedisulfonic acid (BDSA) with similar IEC, and among them NTDA-BAPBDS exhibited the best water stability. This is because of the flexible structure and/or the high basicity of the sulfonated diamine moieties of these polyimides in comparison with that of BDSA-based ones. Fenton’s reagent test revealed that these polyimide membranes also had fairly good stability towards oxidation. Polyimide membranes with good water stability as well as high proton conductivity were developed. NTDA-BAPBDS polyimide mem brane, for example, did not lose mechanical properties after being soaked in water at 80°C for more than 1000 h, while its proton conductivity was still at high level (comparable to that of Nafion 117).
Keywords: Polyimide; sulfonated diamine; synthesis; membrane; proton conductivity; water stability.
Aromatic polyimides, known for their excellent thermal stability, high mechanical strength, good film forming ability, and superior chemical resistance, have found
