Effect of Selective Solvent Addition Rate on the Pathways for Spontaneous Vesicle Formation of ABA Amphiphilic Triblock Copolymers

In this study, the kinetics of vesicle formation of ABA amphiphilic triblock copolymers from an initially homogeneous state was theoretically and experimentally investigated by adding a selective solvent into the system. The pathway of spontaneous vesicle formation depended greatly on the selective solvent addition rate. At a slow addition rate, the pathway followed three stages: (1) the amphiphilic triblock copolymer combined into a large irregular aggregation; (2) the large irregular aggregation broke into big irregular spheres; and (3) some hydrophilic molecules in the big irregular spheres diffused toward the surface, and some hydrophilic molecules diffused toward the center, forming vesicles. However, at a fast addition rate, the pathway was as follows: (1) the amphiphilic triblock copolymer aggregated into many small spheres; (2) the small spheres merged to form rod-like micelles first and then oblate membranes; and (3) the oblate membranes closed up to form vesicles. This pathway difference for vesicle formation can be attributed to the existence of many metastable states in the system. This finding not only provides new insight into the origins of vesicles but also provides further understanding on the self-assembly kinetics of amphiphilic block copolymers in a selective solvent.