Effects of neutralization with Et3Al on structure and properties in sulfonated styrenic pentablock copolymers

The effect of neutralization with Et3Al on morphologies in a sulfonated pentablock copolymer solution and cast membrane was investigated using small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). The sulfonated pentablock copolymer in a non-polar solvent mixture (cyclohexane/heptane) persists as spherical micelles after neutralization where the micelle core of sulfonated polystyrene (SS) incorporates Al (3+) ions and the corona is solvated hydrogenated isoprene (HI) and t-butylstyrene (tBS). Membranes cast from this micellar solution exhibit a bicontinuous microphase separated morphology with interconnected SS microdomains. The aluminum-neutralized membrane with a continuous transport channel shows excellent water vapor transport rate (WVTR) (22 kg/(m2 day)), which is comparable to the WVTR of membranes in the sulfonic acid form. The strong interaction between Al(3+) ions and the sulfonic acid group in the aluminum-neutralized membrane results in significantly lower water uptake (∼30 wt%) that provides much-improved mechanical stability in the wet state.