Controlling the self-assembly pathways of amphiphilic block copolymers into vesicles

Vesicles and membrane properties play critical roles in reproducing the natural environment of living cells such as nutrient transport and DNA protection. We report how to control the morphology evolutionary stages of the self-assembly of amphiphilic block copolymers composed of hydrophilic-hydrophobic-hydrophilic structure using dissipative particle dynamics method. Two unique intermediate states are obtained by controlling the hydrophobic/hydrophilic block ratio, polymer-solvent interaction, and polymer concentration: (1) bilayer-type membrane such as rod-like, disk-like, or bowl-like micelle (mechanism I), (2) semivesicle originating from the rearranged hydrophilic blocks movement into the center or the trapped hydrophilic blocks during merging (mechanism II). Additionally, during the transition period between these two pathways, vesicles are formed through an in-between pathway. Specifically, instead of the typical mechanism I or mechanism II, hydrophilic blocks gradually diffuse toward the center of some irregular spherical micelles, and then become full vesicles. Most importantly, we show that two factors, the degree of hydrophobicity of the blocks and the probability of the adhesive amphiphile collisions are thought to be of key importance to control the vesicle-formation mechanisms. As a consequence, a crucial balance between the segregation of inner-hydrophobic beads and the attraction of outer-hydrophilic beads drastically affects the self-assembly pathways of amphiphilic block copolymer into vesicles from one mechanism over the other. Furthermore, we demonstrate that when the hydrophilic blocks move toward the center to form a cavity, they can move in randomly and maintain a balanced quantity.