Enhanced hydrolytic stability of sulfonated polyimide ionomers using bis(naphthalic anhydrides) with low electron affinity

Abstract

In the pursuit of hydrolytically stable sulfonated polyimide (SPI) membranes as promising candidates for proton exchange membranes, usable at elevated temperature, a series of novel SPI ionomers based on the low electron affinity bis(naphthalic anhydrides), 4,4′-sulfide-bis(naphthalic anhydride) (SBNA) and benzophenone-4,4′-bis(4-thio-1,8-naphthalic anhydride) (BPBTNA), were prepared. Tough, flexible, and transparent membranes were obtained from these polymers, although their inherent viscosities ranged from 0.41 to 0.59 dL g⁻¹. The SPI membranes were thermally stable with the decomposition of sulfonic acid groups over 300 °C, and exhibited good mechanical properties with 65 MPa of tensile strength at 25 °C and 50% RH. The proton conductivities of the SPI membranes increases with increasing temperature and ion exchange capability (IEC), and the S–O(80) with 2.23 mequiv. g⁻¹ of IEC showed a higher proton conductivity than Nafion® 212 at 100% RH. For the high IEC membranes, microscopic analyses revealed the hydrophilic clusters were well-dispersed and connected to each other. The accelerated water stability tests demonstrated that the SPI ionomers based on SBNA and BPBTNA maintained a high mechanical strength after being aged in water for 24 h at 140 °C, which was much more stable than the SPI membranes based on 1,4,5,8-naphthalene tetracarboxylic dianhydride (NTDA). The improved hydrolytic stability of polymers could be well correlated with the results of the electron affinity (E_a) of the dianhydride calculated by the theoretical calculation. This investigation illustrated that this strategy will benefit the further development of hydrolytically stable SPIs applied to high temperature PEFCs.