Sulfonated Pentablock Copolymer Membrane Morphological Anisotropy and Its Impact on Dimensional Swelling, Proton Conductivity, and the Transport of Protons and Water

Poly[t-butylstyrene-b-(ethylene-alt-propylene)-b-(styrene-co-styrene sulfonate)-b-(ethylene-alt-propylene)-b-t-butylstyrene], a pentablock copolymer (PBC), was solution-cast into membranes as a function of the ion-exchange capacity (IEC) using cyclohexane and heptane (PBC[C/H]) or tetrahydrofuran (PBC[THF]) to study its anisotropic morphology and transport. Orientation-dependent transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS) were used to characterize its morphology in the xy-plane (surface) and the xz-plane (thickness). PBC[C/H] had a random morphology of dispersed spherical-like sulfonated domains regardless of the IEC. An orientation-dependent lamellar morphology was formed in PBC[THF]. At IEC = 2.0, PBC2.0[C/H] and PBC2.0[THF] had orientation-dependent proton conductivity through the xz-plane and xy-plane. The membrane proton conductivities of PBC2.0[C/H] were 114.4 (xz-plane) and 103.6 mS/cm (xy-plane), which became 124.4 (xz-plane) and 24.4 mS/cm (xy-plane) for PBC2.0[THF]. Membrane water vapor flux and diffusivity through the xy-plane were 1.26 × 10-3 mol/(m2 s) and 1.52 ×10-10 m2/s for PBC2.0[C/H], respectively. The lamellar morphology within PBC2.0[THF] reduced its water transport properties to 3.84 × 10-4 mol/(m2s) and 3.86 × 10-11 m2/s. A disordered morphology is less anisotropic than an ordered one.