Dissipative Particle Dynamics Simulation on Self-Assembly of Comb-Coil Copolymers

Abstract

Using dissipative dynamics simulation, we studied self-assembly behavior of (A-g-B)-b-A comb-coil copolymers in selective solvents. The comb-coil copolymers, having two competitive length scales, are able to self-assemble into aggre- gates of different types, i.e., type I and type II. For the aggregates of type I, the phase separation occurs between the solvo- phobic and solvophilic blocks, which behave as asymmetric graft copolymers. While in the aggregates of type II, the phase separation takes place between coil and comb blocks, acting as diblock copolymers. The self-assembly of the comb-coil block copolymers in solvents selective to either graft arms or backbone was investigated. The effects of the number and length of graft arms on the self-assembly behavior were examined. In the solvents selective to graft arms, the comb-coil copolymers tend to assemble into spherical micelles of type II, where the comb and coil blocks form the shell and core, respectively. This is due to the fact that the crowd of the comb blocks in the shell can be alleviated by forming high-curvature structures. In addition, such a crowd can also be alleviated by decreasing the length of graft arms and therefore, vesicles were observed when the graft arms are short. In addition, the decrease in the interaction strength between backbone and graft arms (and sol- vents) favors the formation of the aggregates of type II. In the solvents selective to backbones, the comb-coil copolymers incline to form low-curvature aggregates of type II, such as disklike micelles and vesicles. By forming low-curvature struc- tures, the rod-like comb blocks can be tightly packed in the cores of the aggregates. When the comb-coil copolymers form the aggregates of type I in both solvents, the morphologies are very sensitive to the length of the graft arms. For example, in sol- vents selective to graft arms, as the length of graft arms increases, a morphological transformation of large-compound micelle → vesicle → cylindrical micelle → spherical micelle was observed. The simulation results were compared with the avail- able experimental findings reported in the literatures, and an agreement was observed. In addition, the simulations predict some behaviors that have not been observed yet. The present work is helpful for further understanding the competitive self-assembly behavior of the comb-coil copolymers. Keywords comb-coil copolymer; self-assembly; dissipative particle dynamics simulation; micelle; phase separation