Morphology and diffusion within model membranes: Application of bond counting method to architectures with bimodal side chain length distributions

Abstract

The consequence of a bimodal side chain length distribution on micro-phase separation and solvent diffusion within amphiphilic grafted polymeric model membranes is studied by dissipative particle dynamics (DPD). Fifteen polymeric architectures are modeled that are of the same ion exchange capacity and molecular weight. The backbones consist of hydrophobic A fragments. At regular intervals along the backbones two side chains are simultaneously branching off. One of the side chains contains p and the other q consecutively connected A fragments, and each of them is end-linked with a hydrophilic C fragment. Water is represented by W beads. The water volume fraction is 0.16. Diffusion through the hydrophilic pore networks is derived from Monte Carlo tracer diffusion calculations through selected snapshots and from the W bead motions. The distance between the water containing pores and water diffusion decreases with difference in side chain length (q−p). The results are explained by introducing a parameter, based on bond counting, which measures the average number of bonds between A fragments toward the nearest C fragment within each architecture. This parameter also explains previous results on diffusion and pore size obtained for ∼50 grafted architectures with various types of side chains (linear or Y-shaped), side chain lengths, side chain distributions, and ion exchange capacity. A consistent picture evolves that might guide pore network design strategies for obtaining low percolation thresholds for solvent and proton diffusion.