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 tetrasulfonated segments promoted a distinct phase separation of the ionic groups already at quite low ionic contents. The SPAS polymers degraded between 300 and 340 °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 °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