A sulfonated diamine monomer, 4,4‘-diaminodiphenyl ether-2,2‘-disulfonic acid (ODADS), was successfully synthesized by direct sulfonation of a commercially available diamine, 4,4‘-diaminodiphenyl ether (ODA), using fuming sulfuric acid as the sulfonating reagent. A series of sulfonated polyimides were prepared from 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTDA), ODADS, and common nonsulfonated diamines. The resulting sulfonated polyimides displayed much better stability toward water than those derived from the widely used sulfonated diamine 2,2‘-benzidinedisulfonic acid (BDSA). This is because ODADS-based polyimide membranes have a more flexible structure than the corresponding BDSA-based ones. Fenton's reagent test revealed that ODADS-based polyimide membranes also had fair good stability to oxidation. Polyimide membranes with good water stability as well as high proton conductivity were developed. NTDA−ODADS/BAPB(1/1) copolyimide membrane (BAPB refers to 4,4‘-bis(4-aminophenoxy)biphenyl)), for example, did not lose mechanical properties after being soaked in water at 80 °C for 200 h, while its proton conductivity was still at a high level (comparable to that of Nafion 117).

A series of wholly aromatic hyperbranched polyimides were successfully prepared by condensation polymerization of a triamine monomer, tris(4-aminophenyl)amine (TAPA), and a series of commercially available dianhydride monomers such as 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA), 3,3′,4,4′-diphenylsulfonetetracarboxylic dianhydride (DSDA), and pyromellitic anhydride (PMDA). The synthesis involves the formation of hyperbranched polyamic acid precursors in the first step and the thermal or chemical imidization in the subsequent step, which is similar to the conventional two-step method for the synthesis of linear polyimides. Different monomer addition orders resulted in different hyperbranched polymers even if the same kinds of monomers but different molar ratios were used. When a dianhydride solution was added to the TAPA solution (manner 1), an amineterminated hyperbranched polyimide was obtained, whereas the addition of TAPA solution to a dianhydride solution (manner 2) yielded an anhydride-terminated hyperbranched polyimide. The appropriate molar ratios of TAPA to a dianhydride are 1:1 for manner 1 and 1:2 for manner 2. From 1 H NMR spectra the branching degrees of the amine-terminated hyperbranched polyimides were estimated to be 0.64, 0.72, and 0.68 for 6FDA-TAPA, DSDA-TAPA, and PMDA-TAPA, respectively. All the anhydride-terminated polyimides were found wholly branched. Gel permeation chromatography (GPC) measurement revealed that both the amine-terminated and anhydride-terminated 6FDA-TAPA polyimides had moderate number-averaged molecular weights, but the latter had very broad molecular weight distribution.

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A new sulfonated diamine monomer, 9,9-bis(4-aminophenyl)fluorene-2,7-disulfonic acid (BAPFDS), was synthesized by direct sulfonation of the parent diamine, 9,9-bis(4-aminophenyl)fluorene (BAPF), using fuming sulfuric acid as the sulfonating reagent. A series of sulfonated polyimides with different sulfonation degrees were prepared from 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTDA), BAPFDS, and common nonsulfonated diamines. The resulting sulfonated polyimides generally showed good solubility in m-cresol and DMSO. Proton conductivities of these polyimide membranes were measured as the functions of relative humidity and temperature. The resulting homopolyimide, NTDA-BAPFDS, displayed proton conductivities quite similar to those of Nafion 117 in the whole humidity range (RH < 100%). At 100% relative humidity, all the BAPFDS-based polyimide membranes showed proton conductivities similar to or higher than those of Nafion 117. In addition, BAPFDS-based polyimide membranes exhibited much better water stability than those derived from a widely used sulfonated diamine, 2,2′-benzidinedisulfonic acid (BDSA), with similar IEC. This is probably because of the higher basicity of BAPFDS, which is favorable for maintaining the stability of imido rings.