Improving proton conduction pathways in di- and triblock copolymer membranes: Branched versus linear side chains

Phase separation within a series of polymer membranes in the presence of water is studied by dissipative particle dynamics. Each polymer contains hydrophobic A beads and hydrophilic C beads. Three parent architectures are constructed from a backbone composed of connected hydrophobic A beads to which short ([C]), long ([A₃C]), or symmetrically branched A₅[AC][AC] side chains spring off. Three di-block copolymer derivatives are constructed by covalently bonding an A₃₀ block to each parent architecture. Also three tri-blocks with A₁₅ blocks attached to both ends of each parent architecture are modeled. Monte Carlo tracer diffusion calculations through the water containing pores for 1226 morphologies reveal that water diffusion for parent architectures is slowest and diffusion through the di-blocks is fastest. Furthermore, diffusion increases with side chain length and is highest for branched side chains. This is explained by the increase of water pore size with 〈N_bond〉, which is the average number of bonds that A beads are separated from a nearest C bead. Optimization of 〈N_bond〉 within the amphiphilic parent architecture is expected to be essential in improving proton conduction in polymer electrolyte membranes.