Self-Assembly in Block Copolymer Thin Films upon Solvent Evaporation: A Simulation Study

The self-assembly in cylinder-forming diblock copolymer thin films upon solvent evaporation is studied by lattice Monte Carlo simulations under the assumption that the solvent evaporation starts from the free surface and gradually propagates toward the substrate. The effects of solvent selectivity, surface preference, and solvent evaporation rate on the morphology evolution during solvent evaporation are systematically investigated. It is found that the perpendicular cylinder morphology tends to form under weak surface preference, whereas under strong surface preference this morphology is promoted by the fast solvent evaporation rate and the strong solvent selectivity. The surface preference window for forming perpendicular cylinders with solvent evaporation is found to be wider than with thermal annealing, and especially much wider when the solvent evaporation starts from random (disordered) initial states. A new mechanism of perpendicular cylinder formation is proposed and elucidated. Hexagonally packed short perpendicular cylinders formed in the earlier stage of the solvent evaporation may remain to the dry film when the solvent selectivity for the majority block is strong or the solvent evaporation rate is fast, which results in the enlargement of the surface preference-window of perpendicular cylinder morphology. Mix-orientated morphology with one or two layers of parallel cylinders at the top of the film and perpendicular cylinders throughout the remaining film is also predicted, and its formation mechanism is discussed.