(434e) Characterization of Aluminum-Neutralized Sulfonated Styrenic Pentablock Copolymer Films

Sulfonated polymers have been widely considered for membrane applications such as desalination, electrodialysis (ED), reverse electrodialysis (RED), and membrane capacitive deionization (CDI).  Further optimization of these materials, however, is needed in order to extend and improve these technologies.  Sulfonate groups are reactive sites that can be used as a platform for chemical modification of the polymer.  In this work, an acid-form sulfonated styrenic pentablock copolymer was suspension-phase neutralized with triethylaluminum in order to prepare an aluminum crosslinked polymer, and a structure/property study was conducted.  Aluminum neutralization was confirmed by ²⁷Al NMR.  The morphology and free volume of the polymers were probed by small angle x-ray scattering (SAXS) and positron annihilation lifetime spectroscopy (PALS), respectively, and the stability of the aluminum-sulfonate group bond against ion exchange to the sodium form was probed by soaking the films in 1 mol/L NaCl for 30 days.  The neutralized polymer sorbed considerably less water than the acid-form precursor due, primarily, to neutralization of the sulfonic acid groups.  As a result, the hydrated neutralized polymer was mechanically stronger than the non-neutralized polymer, but the dry neutralized polymer was more brittle than the non-neutralized polymer.  The material’s permeability to water and salt decreased as a result of the neutralization process in a manner that is consistent with free volume theory.  The material’s water/salt sorption selectivity decreased due to neutralization of the polymer’s fixed charge, but overall, the polymer’s water/salt permeability selectivity increased upon neutralization due to an increase in the material’s diffusivity selectivity, which stems predominantly from the decrease in water sorption and its effect on the polymer’s free volume.