Water vapor sorption and diffusion properties of sulfonated polyimide membranes

Abstract

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 a w were very similar among SPIs and Nafion 117. The enthalpy of sorption Δ H s was determined from the van’t Hoff plots of solubility coefficient S. With an increase in a w, the |Δ H s| for SPIs decreased from 52 kJ/mol at a w=0.02 (or λ=1) to 45 kJ/mol at a w=0.8 ( λ=7). The steady-state permeation of water vapor was measured for SPIs at 323 K and the concentration averaged diffusion coefficient D ̄ was determined as a ratio of permeability coefficient P over S. The D ̄ and P values for the SPIs and Nafion 117 increased with an increase in a w. The SPIs displayed very large D ̄ and P values of 10 −7 cm 2/s and 10 5 Barrer (1 Barrer=10 −10 cm 3 (STP) cm/(cm 3 s cmHg)), respectively, which were still much smaller than those of Nafion 117.