Biomimetic membrane control of block copolymer vesicles with tunable wall thickness

Abstract

Our recent simulation study on mechanisms of vesicle formation in ABA triblock copolymer system has revealed the factors or conditions that lead to the transition from mechanism I (a bilayer membrane closing pathway) to mechanism II (a nucleation and growth pathway). The spontaneous vesicle formation involves many metastable states and the two mechanisms do not each lead to a unique intermediate structure, but have a transition period. In this paper, we use dissipative particle dynamics (DPD) simulations to further investigate the effect of vesicle size and hydrophobicity of the blocks on membrane characteristics, with the aim of understanding and controlling the structures of block copolymer vesicle and its formation in different scales, and to try to connect our simulation results to previous experimental studies. We have evaluated the thickness of the hydrophobic layer and observed two types of dependence on the vesicle size. Our results indicate that as the degree of hydrophobicity of the blocks increases, from a totally strong-behavior to a totally weak-behavior relationship, the transformation is observed in large sized vesicles first and then in small sized vesicles. Two characteristics, the chain compaction of the vesicles and the area densities of the inner corona, are thought to be important in controlling the membrane thickness, which are proposed to explain the size-dependent behaviors of bilayer thickness. Finally, the fraction of bridges Φ_b is also calculated. In our model, Φ_b increases with an increase in vesicle size for a given system and fixed interaction and decreases with an increase in the degree of hydrophobicity of the blocks.