Phosphonium-based poly(Ionic liquid) membranes: The effect of cation alkyl chain length on light gas separation properties and Ionic conductivity

Phosphonium poly(ionic liquid)s (PILs) have been studied as alternatives to more common ammonium and imidazolium PILs for potential transport and separation applications. This work characterizes the CO2, H-2, N-2, O-2, CH4, and C2H4 single-gas permeability, diffusivity, solubility, and selectivity of freestanding films of poly([(tri-n-alkyl)vinylbenzylphosphonium][bis(trifluoromethylsulfonyl)imidel) PILs (i.e., poly([P-nnnVB][Tf2N) where n=4, 6, 8). The gas permeability was found to increase approximately linearly with increasing alkyl chain length on the phosphonium group. To our knowledge, the CO2 permeability of 186 barrers observed for poly([P888VB][Tf2N]) is the highest reported for neat PIL materials. In contrast, gas selectivity was observed to decrease with an increase in phosphonium alkyl chain length from n = 4 to n = 6, then remain approximately constant between n = 6 and n=8. Additionally, the ionic conductivity of these materials was observed to increase from ca. 10(-8) to ca. 10(-5) S cm(-1) as the measurement temperature was increased from 25 to 105 degrees C. At 25 degrees C, the PIL with the shortest cation alkyl chain (n=4) was observed to have the lowest ionic conductivity. However at ca. 90 degrees C, the expected trend of increasing ionic conductivity in the order n=4 > n =6 > n=8 was observed. (C) 2015 Elsevier B.V. All rights reserved.