Sulfonated Pentablock Copolymer Membrane Morphological Anisotropy and Its Impact on Dimensional Swelling, Proton Conductivity, and the Transport of Protons and Water
Wenjian Zheng, Chung‐Hao Liu, Mu‐Ping Nieh, Chris J. Cornelius
Abstract
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.