Theoretical Description of the Structural Characteristics of the Quaternized SEBS Anion-Exchange Membrane Using DFT

For four conforming structures of the quaternized polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene membrane (QSEBS), (a) tetramethylammonium hydroxide (TMA(+)OH(-)), (b) benzyltrimetylammonium hydroxide, and (c and d) QSEBS segments with 1 and 2 side chains (DBQSEBS), spatial distribution, bond distances, and charge-density profiles were obtained with density functional theory (DFT) and compared with structural simulations of DBQSEBS for two different hydration levels. Results for the TMA(+)OH(-) showed that its constituent ions stay metastable in the vicinity of each other and are joined by donor-acceptor interactions. Also, simulations of the other conforming structures show that, in the absence of water, spatial distribution as well as charge-density profiles of trimethylammonium hydroxide do not change with respect to isolated TMA(+)OH(-), demonstrating that the QSEBS chain is a thermodynamically stable backbone to support the functional group, which is in agreement with the literature. When hydrated, simulations of DBQSEBS for water uptake of 4 show that there is a partial dissociation of hydroxide ions due to donor acceptor interactions acting competitively on them. For water uptake of 6, this dissociation is completed, and hydroxide ions conform to hypercoordinated structures similar to the square-planar arrangement described for pure water medium, but with some structural differences associated with location, type, and interactions among the molecules involved.