Highly Proton Conducting Electrolyte Membranes Based on Poly(arylene sulfone)s with Tetrasulfonated Segments

A series of fully aromatic polymers having only sulfone bridges linking the aromatic rings have been synthesized via polycondensations and studied as proton exchange membranes. Mixtures of tetrasulfonated 4,4'-bis[(4-chlorophenyl)sulfonyl]-1,1'-biphenyl (BCPSBP), non-sulfonated BCPSBP, and 4,4'-thiobisbenzenethiol were copolymerized by nucleophilic aromatic substitution reactions to obtain sulfonated poly(arylene thioether sulfone)s (SPATSs) with ion exchange capacities (IECs) between 2.0 and 4.0 mequiv g(-1). The thioether bridges of the SPATSs were quantitatively oxidized to sulfone bridges to obtain the corresponding sulfonated poly(arylene sulfone)s (SPASs). Small angle X-ray scattering of dry SPATS and SPAS membranes showed that the tetrasulfonatecl segments promoted a distinct phase separation of the ionic groups already at quite low ionic contents The SPAS polymers degraded between 300 and 340 degrees C in air, which was significantly above the degradation temperature of the corresponding SPATSs polymers. Moreover, SPAS membranes showed a significantly lower water uptake than the corresponding SPATS membranes. SPATS and SPAS membranes with IEC values of 2.4 and 2.2 mequiv g(-1), respectively, maintained high proton conductivity at low relative humidity (RH). At 30% RH and 80 degrees C, these membranes reached 8 and 10 mS cm(-1), respectively. The latter value coincided with that recorded for the state-of-the-art perfluorinated NRE212 membrane under the same conditions. Thus, the SPAS materials combine a straightforward synthetic pathway with a very robust polymer structure giving high proton conductivity at reduced RH.