Characterization of Aluminum-Neutralized Sulfonated Styrenic Pentablock Copolymer Films

Abstract

Suspension-phase neutralization of an acid-form sulfonated styrenic pentablock copolymer was used to prepare aluminum cross-linked polymer films. Aluminum neutralization was confirmed by 27Al solid state NMR, and the aluminum-neutralized polymer resisted ion exchange to the sodium counterion form when the polymer was soaked in 1 mol/L sodium chloride for 30 days. The effects of aluminum neutralization on polymer morphology, mechanical properties, water and salt transport properties, and free volume were explored. The primary spacing of the morphology, as measured by small-angle X-ray scattering (SAXS), decreased upon neutralization of the sulfonated polymer. The hydrated aluminum-neutralized polymer was mechanically stronger than the non-neutralized polymer, but the dry aluminum-neutralized polymer was more brittle than the non-neutralized polymer. Neutralization of the polymer’s sulfonic acid functionality resulted in a 5.5-fold decrease in water uptake. This water uptake decrease upon aluminum neutralization resulted in a decrease, relative to the non-neutralized polymer, in water and sodium chloride permeability and water vapor transport rate. These decreases in transport rates and water uptake were consistent with one another, based on an analysis of changes in free volume of the material upon aluminum neutralization. Sodium chloride permeability of the aluminum-neutralized polymer was less dependent on salt concentration than that of the non-neutralized polymer, which is consistent with neutralization of the polymer’s charged sulfonate groups. Aluminum neutralization increased the polymer’s water/sodium chloride permeability selectivity because of a 1.8-fold increase in the material’s diffusion selectivity. Free volume element size, characterized by ortho-positronium lifetime, decreased upon aluminum neutralization in both dry and hydrated film samples.