Pore morphologies and diffusion within hydrated polyelectrolyte membranes: Homogeneous vs heterogeneous and random side chain attachment

Abstract

Using dissipative particle dynamics pore morphologies within model ionomer membranes are simulated. The ionomers are composed of hydrophobic backbones and side chains that are end-linked with a hydrophilic acid containing site. The separation distance between successive branching points is bi-modal, being alternating short (distance x) and long (distance y). The dependence of morphology on ion exchange capacity and separation distance is investigated. Phase separated morphologies were calculated at a water content of 16 vol. %. An increase of side chain density results in a decreasing size of the water containing pores, distance between them and decreasing Bragg spacing. For fixed side chain density, an increase in difference between the longer and shorter separation distance (y - x) results in a larger Bragg spacing. Monte Carlo calculations demonstrate that a large majority of the water is contained within a percolating network that allows for long-range diffusion. Diffusion constants vary drastically with architecture: Diffusion is fastest for architectures for which the side chains are highly non-uniformly distributed (y ≫ x). For architectures with the same side chain density, the tracer diffusion constants increase linearly with increase of the asymmetry ratio y∕x (y > x). This is caused by the cooperative action of those terminal acidic sites that are topologically close together, allowing them to arrange pair wise along the pore walls and make the pores larger. We verified that for polymer architectures that mimick Nafion1200 similar trends are obtained, resulting in increased H(2)O, O(2), and H(2) permeation for statistical side chain distribution as compared to a uniform distribution of side chains. This trend is most pronounced for H(2)O and less pronounced for H(2).