Low water uptake anion exchange membranes utilizing poly(1,1-diphenylethylene-alt-butadiene) backbones

Abstract

This research seeks to explore the relationship between polymer backbone composition, water uptake, and conductivity to contribute to the development of high-performance, yet inexpensive, ion exchange membranes. The goal of the projects described, including future work, is to work towards a structure-property understanding that enables the design of ionomers which self-assemble hydrated ion-conducting channels. Any such materials must also have good mechanical integrity, as well as properties that prevent excessive swelling from water uptake, which is known to lead to the failure of membrane materials. This will enable electrochemical energy devices, such as fuel cells and electrolyzers, that utilize ion-transporting membranes to become more economically viable routes for the production of cleaner and more sustainable energy.In this work, novel polymer backbones were designed, synthesized by anionic polymerization, functionalized by the post-polymerization addition of alkyl side chains with terminal ionic groups, and characterized as anion exchange membranes. Characterization techniques included differential scanning calorimetry, ion conductivity, and water uptake. In the first project, the backbone investigated was poly(1,1-diphenylethylene-alt-butadiene) (DPE/B). The bulky DPE monomer cannot self-polymerize, which lends itself to the precisely tailored synthesis of alternating copolymer backbones. The DPE/B alternating copolymer was selectively hydrogenated to saturate the polymer backbone (DPE/E), then functionalized via an acid-catalyzed Friedel-Crafts reaction to attach alkyl side chains containing bromofunctional groups for subsequent quaternization. The anion exchange membrane properties, including ion conductivity and water uptake, of the quaternized polymer was then compared to quaternized polystyrene controls of similar ion content for a baseline comparison to a well-known and used polymer for anion exchange membranes. Additionally, the DPE/B polymer was cyclized via an acid-catalyzed Friedel-Crafts reaction to increase the glass transition of the precursor polymer backbone. The cyclized DPE/B (DPE/B-C) had a glass transition temperature of 205°C, up from 103°C for DPE/E. Benefits of the increased glass transition temperature of the polymer backbone are explored.