Effects of metal-polymer complexation on structure and transport properties of metal-substituted polyelectrolyte membranes.

Morphological and transport properties of hydrated metal-substituted Nafion membranes doped with metal ions of different valency and coordination strength are explored using coarse-grained dissipative particle dynamics simulations. To incorporate the effects of metal-polymer complexation, we introduce a novel metal ion complexation model, in which the charged central metal ion is surrounded by dummy sites that coordinate with ligands. The model parameters are determined by matching the metal-ligand running coordination numbers and the diffusion coefficients obtained from atomistic simulations and/or experiments. The increase of valency and coordination strength is found to strongly influence both the morphology and transport characteristics of the membrane at all hydration levels. The membrane segregation into hydrophobic and hydrophilic sub-phases is affected by metal-sulphonate coordination induced crosslinking at the hydrophilic/hydrophobic interface. The simulation results indicate that the interfacial crosslinking influences the interfacial tension and thereby affect the growth and coalescence of water clusters upon the increase of hydration. Multivalent complexation hinders water and ion mobility and causes anomalous sub-diffusion and dramatic decrease of the water permeability and ionic conductivity. Our DPD model is found efficient in elucidating the mechanisms of coordination-induced cross-linking and complexation and predicting on a semi-quantitative level the morphological and transport properties of metal-substituted Nafion membranes depending on the ion valency and coordination strength. The proposed model can be further advanced and adopted for other polyelectrolyte systems, such as sulfonated block-copolymers, polysaccharide solutions and composites, and biopolymer assemblies.