Water vapor sorption and diffusion properties of sulfonated polyimide membranes

Sulfonated polyimides (SPIs) were prepared from 1,4,5,8-naphthalene tetracarboxylic dianhydride (NTDA), sulfonated diamines of 5,5′-dimethylbenzidine-2,2′-disulfonic acid (DMBDSA) and 4,4′-diaminodiphenylether-2,2′-disulfonic acid (ODADS) and common non-sulfonated diamines, and their water vapor sorption, diffusion and permeation properties were investigated in comparison with Nafion 117. Transient sorption and desorption of water vapor in SPI membranes displayed non-Fickian diffusion behavior, indicating the molecular relaxation of polymer chains took place during the experimental time scale. Water vapor sorption isotherms, which were measured by the successive differential sorption experiments at 298–343 K, were sigmoidal and similar to type II BET adsorption. The water uptake significantly depended on the ion exchange capacity, but the sorption isotherms in form of the number of sorbed water molecules per sulfonic acid group λ versus water vapor activity aw were very similar among SPIs and Nafion 117. The enthalpy of sorption ΔHs was determined from the van’t Hoff plots of solubility coefficient S. With an increase in aw, the |ΔHs| for SPIs decreased from 52 kJ/mol at aw=0.02 (or λ=1) to 45 kJ/mol at aw=0.8 (λ=7). The steady-state permeation of water vapor was measured for SPIs at 323 K and the concentration averaged diffusion coefficient D̄ was determined as a ratio of permeability coefficient P over S. The D̄ and P values for the SPIs and Nafion 117 increased with an increase in aw. The SPIs displayed very large D̄ and P values of 10−7 cm2/s and 105 Barrer (1 Barrer=10−10 cm3 (STP) cm/(cm3 s cmHg)), respectively, which were still much smaller than those of Nafion 117.