Asymmetric Vesicle Constructed by AB/CB Diblock Copolymer Mixture and Its Behavior: A Monte Carlo Study

Asymmetric vesicles constructed from AB/CB diblock copolymer mixture in a selective solvent for A and C blocks are studied using Monte Carlo simulation. The effects of the mixed ratio of the two diblock copolymers, the solution pH, and the hydrophilic chain length on the distributions of hydrophilic blocks on the surfaces of asymmetric vesicles are studied systematically. The simulation results show that asymmetric vesicle, in which the inner and outer surfaces are constructed from different hydrophilic blocks, can be obtained from AB/CB diblock copolymer mixture. The formation of ABC or CBA three-layer asymmetric vesicle depends on the composition of the mixture, the chain length of hydrophilic block, and the solution pH. The hydrophilic block with the same charge (induced by the solution pH), or longer chain length, or lower content in the mixture is more likely to distribute on the outer surface of the vesicle. Moreover, the transition from ABC to CBA three-layer asymmetric vesicle in which blocks C are charged can occur by adjusting the composition of the mixture. On the other hand, the investigations of the interfacial energy density of asymmetric vesicles elucidate the distribution regularity of hydrophilic blocks. When the hydrophilic chain lengths are equal, the difference between the outer and inner interfacial energies is the main factor that determines the asymmetric vesicle structures; that is, the distributions of different hydrophilic blocks on asymmetric vesicles always tend to gain the largest difference between the outer and inner interfacial energies. However, when the hydrophilic chain lengths are different, the chain conformational entropy becomes the main driving force for determining the distribution of hydrophilic blocks on asymmetric vesicles.