The impact of microstructured morphology on macroscale properties of anion exchange membranes

Abstract

This research seeks to explore the relationship between polymer morphology and ion transport to contribute to the development of inexpensive and high-performance ion exchange membranes. The goal of the projects described herein, and from future work, is to work towards a structure-property understanding that will enable the design of ionomers that self-assemble efficient hydrated ion-conducting channels. The materials must also have good mechanical integrity: having properties that prevent the excess swelling / water uptake that is known to lead to the failure of membrane materials. This will enable electrochemical energy devices, such as fuel cells and electrolyzers, that employ ion-transporting membranes to become more economically viable solutions to the cleaner and more sustainable production of energy. In this thesis, novel polymer backbones were designed, synthesized by anionic polymerization, functionalized by the post-polymerization addition of alkyl side chains with terminal ionic groups, and fully characterized as anion exchange membranes. Characterization techniques included differential scanning calorimetry (DSC), x-ray scattering, laser scanning confocal microscopy, ion conductivity, water uptake, and tensile stress-strain measurements. In the first project, the backbone employed was the semi-crystalline triblock copolymer poly(ethylene-b-styrene-b-ethylene) (ESE). The ESE triblock copolymer was functionalized via an acid-catalyzed Friedel-Crafts reaction to attach alkyl side chains containing bromofunctional groups for subsequent quaternization, but the high-temperature condition increased the polydispersity of the functionalized ESE copolymers and we found that the resulting ionomer membranes formed different microstructures (either long-range ordered lamellar or disordered) depending on the polydispersity. We found that the long-range ordered lamellar microstructures caused adverse morphological effects that negatively impacted the ion transport and mechanical properties of the ionomer membranes, leading to the conclusion that long-range ordered structures in block copolymers are not necessarily beneficial to the performance of block copolymer membranes. In the second project, the two backbones investigated were higher glass transition temperature polystyrene-analogs: poly(1,1-diphenylethylene-alt-styrene) (DPE/S) and poly(1,1-diphenylethylene-alt-4-tert-butylstyrene) (DPE/tBS), with glass transition temperatures of 173 °C and 194 °C, respectively. The DPE monomer cannot self-polymerize, which lends it to the synthesis of precisely tailored alternating copolymer backbones. This work demonstrated the benefits of employing a polymer backbone with a glass transition temperature > 120 °C on the overall mechanical stability and ion conductivity of AEMs operating in hydrated conditions. In addition, evidence for the benefits of having a precisely-tailored backbone where the ion functional groups are regularly spaced along the polymer backbone chain with a minimum distance of four methylene groups is presented. Finally, the feasibility of depolymerizing DPE/S and DPE/tBS styrene-analog polymers is demonstrated at room temperature using triflic acid as the catalyst, unlocking potentials for these polymers to be recycled unlike polystyrene.