Mesoscale Simulations of Quaternary Ammonium-Tethered Triblock Copolymers: Effects of the Degree of Functionalization and Styrene Content

The hydrated morphology of anion-exchange membranes (used in alkaline membrane fuel cells) is a key aspect in determining their ion conductivity. Herein, we examine the changes in the morphology of hydrated polystyrene-b-poly(ethylene-co-butylene)-b-polystyrene functionalized with quaternary ammonium groups in the polystyrene phase. The effects of the degree of functionalization and styrene content were studied using dissipative particle dynamics simulations. The morphology was controlled by the following three factors: The hydration level, the degree of functionalization of styrene blocks, and the percentage of styrene in the block copolymers. Perfect lamellae, imperfect lamellae, disordered bicontinuous structures, gyroid-like structures, and the coexistence of layered structures and micelles were all observed under particular conditions. Among the three types, the imperfect lamella was made up of slices or subdomains of perfect lamella. These lamellae formed in different directions and were interconnected by the transitional regions. Pure water domains were formed at a high water content (λ nH2O/TMA+ ≥ 16), and the lowest degree of functionalization (25%) may require the highest hydration. An analysis of the clustering of water molecules revealed that a full percolating aqueous domain occurs when the hydration level reaches approximately λ = 12. Lowering of the functionalization and/or the styrene percentage was observed to postpone the formation of large connected water domains, but increasing the styrene percentage enables the percolation to occur at a lower water content.