Hydrophobic collapse and ion channel connectivity in polymeric model membranes with gradient side chain length distributions

Abstract

The phase separation of four hydrated polymeric model membranes generated by coarse grained simulations is studied in detail. We consider polymeric architectures with hydrophobic backbones, composed of 25 A beads, along which nine side chains split-off equidistantly. For polymer I, the side chains are all the same (i.e., [A4C]), where each C bead represents a functional hydrophilic pendent site. For polymer II, the side chain lengths increase gradually along the backbone (i.e., [C], [AC], [A2C], …, [A8C]). For polymer III, the side chains are longest at the backbone terminals and shortest near the backbone center (i.e., [A8C], [A6C], [A44C], [A2C], [C], [AC], [A3C], [A5C], [A7C]). For polymer IV, the side chains are shortest near the backbone terminals and increase in length gradually toward the center of the backbone (i.e., [C], [A2C], [A4C], [A6C], [A8C], [A7C], [A5C], [A3C], [AC]). At 16 vol. % water content, the C and water beads are nano-phase separated from the polymer A matrix and form a connected single hydrophilic phase. The hydrophilic phase connectivity is inspected by MC tracer diffusion calculations. A salient feature is that the local hydration around the C sites depends on the side chain length and placement along the backbone. The hydrophilic phases can serve as pathways for proton or hydronium diffusion in ion exchange membranes. Since their connectivity depends on polymeric architecture, control of the side chain length and their placement along the backbone are interesting parameters to optimize the membrane conductivity.