DPD simulations of anion exchange membrane: The effect of an alkyl spacer on the hydrated morphology

Abstract

An ion conducting polymeric membrane is a key component in an anion exchange membrane (AEM) fuel cell serving as the electrolyte. To enhance both the membrane stability and ion conductivity, phase segregate copolymers have been proposed for use as AEMs. Phase separation results in the formation of ion conducting domains, and hence the morphology of the material is important. Among the candidates, polystyrene-b-poly(ethylene-co-butylene)-b-polystyrene (SEBS), functionalized by quaternary ammonium (QA) head groups and variants with an alkyl spacer, has been synthesized and offers some promising characteristics. Dissipative particle dynamics (DPD), a coarse-grained scheme, was utilized to investigate the hydrated morphology of these model systems. Specifically, this work seeks to understand the effect of an alkyl (C4H8) ‘spacer’ when attached to the functional group on the hydrated morphology of the ionomer. This spacer was grafted between the SEBS and the QA group as a ‘linker’, to the end of the QA group as a ‘tail’, and in both positions as a coexisting ‘linker and tail’. The simulated morphologies were all compared back to the SEBS-QA without any spacer. An analysis of the clustering was also performed to quantify the size, connectivity, and percolation of the water and ion containing domains as a function of the degree of hydration.