Complex micelles from the self-assembly of coil-rod-coil amphiphilic triblock copolymers in selective solvents

We report an extensive simulation study on the spontaneous formation of complex micelles from coil-rod-coil amphiphilic triblock copolymers in dilute solution resulting from solvent selectivity. The amphiphilic molecule is built from one hydrophilic block on each side and a hydrophobic block in the middle. The rigidity of the rod block is introduced by adding a bond-bending potential of the angle among three subsequent particles in the hydrophobic block. The incorporation of rigid-rod block into the amphiphilic block copolymer results in the self-assembled microstructures and their corresponding properties that differ from those built from fully flexible amphiphilic molecules with the same conditions. By changing the selectivity of solvents, defined in terms of the repulsive interactions between the solvent and the hydrophilic/hydrophobic particles, we find that the aggregation morphology changes from bundle-like micelles to spherical and cylindrical micelles to elongated micelles and then to ring-like toroidal micelles, revealing that the selectivity of solvents is a key factor that determines aggregation morphology. In addition, we observe that the formation of toroidal micelles from coil-rod-coil amphiphilic triblock copolymers proceeds via the growth pathway, which is quite distinct from the conventional toroidal micelle coalescence pathway observed in the self-assembling process of fully flexible amphiphilic triblock copolymer systems. Chain packing in toroidal micelles formed from amphiphilic triblock copolymers with fully flexible hydrophobic and rigid-rod hydrophobic blocks is likewise investigated. The simulation results show that the rigid-rod middle blocks adopt only extended conformations while flexible-coil middle blocks can adopt both folded and extended conformations in toroidal micelles. These findings demonstrate that the bond-bending potential in amphiphilic molecules is an effective and relatively simple method to model the behavior of coil-rod-coil amphiphilic block copolymers.