Micro-phase separation of diblock copolymer in a nanosphere: Dissipative particle dynamics approach

Micro-phase separation and morphologies of diblock copolymer in a nanosphere with different size and interfacial energy were studied by dissipative particle dynamics approach. The surface of a nanosphere is modeled by a layer of frozen soft particles. For symmetric diblock copolymer A5B5, if the surface is neutral, i.e., with the same interactions toward different blocks, the morphology consists of bended distorted lamellae connected by channels. They might be mushroom-like, helical or bi-continuous depending on the sphere size. When the surface is non-neutral, i.e., with the different interactions toward different blocks, the meso-structure is onion like. The outmost layer is the block with weaker repulsion toward the surface. The number of the alternative inner layers is determined by the size. For asymmetric diblock, the morphology is strong associated with the composition. For A3B7, owing to the strong restriction of the nanosphere, the hexagonal phase originally in the bulk does not occur. When the surface is neutral, the minority block forms bended sticks touched the surface. With the increase of the sphere size, more and more cylindrical or star like joints link these sticks. When the surface is non-neutral and less repulsive toward the minority block, more bended sticks near the nanosphere surface than those in the neutral nanosphere. Increasing the sphere size causes spherical and cylindrical domain formed in the center of the sphere. For A2B8, the minority A-block forms spheres or peanut like domains consisting concentric spheres; the number of them is related to the sphere size. However, for A1B9, no micro-phase separation was found. The polymer surfactant in aqueous solution was also studied. The results indicate that the polymer in a nanosphere can be used to mimic the polymer surfactant in some cases.