On the Way to Transport Controlled Membranes: Solvent Effects on Structured Ionic Polymers

Complex ion containing polymers are in the core of numerous current and potential applications including clean energy, water purification membranes, piezoelectric materials and sensors. The ability to facilitate ion and electron transport is a key to their function and is controlled by their structure. Moving from a laboratory concept to devices requires processing means in which well controlled structures are formed on a large scale. As these polymers are often processed from solvents, one effective path to gain structural control is tuning their conformation by solvent interactions. These polymers often consist of transport facilitating blocks tethered to mechanical stabilizing ones. Here we present results of molecular dynamics simulations of the effects of solvents on the conformation of one complex macromolecule of the shape of ABCBA symmetric ionic pentablock copolymer. The center block C is a randomly sulfonated polystyrene that can facilitate transport, tethered to a flexible block, B, and t-butyl polystyrene A, that provide mechanical stability. The effects of two solvents, a cyclohexane and n-heptane mixture, a commercially viable solvent, which is a good solvent for nonpolar blocks, and propanol which is a preferential solvent for the center block will be presented. We find that while in cyclohexane:n-heptane the ionic block is fully collapsed, the propanol tunes the conformation of the ionic block. Understanding solvent effects to enable prediction of the conformation of the polymers offers a design tool for structured polymeric membranes.