Vertical Domain Orientation in Cylinder-Forming Diblock Copolymer Films upon Solvent Vapor Annealing

Dissipative particle dynamics was applied to simulate solvent vapor annealing in films of cylinder-forming diblock copolymers. Our simulations reveal a wide range of conditions, under which the vertical domain orientation is promoted by the fast solvent evaporation. First, the order-to-disorder transition must be induced upon swelling of the film. This condition ensures the independence of the structure formation during drying on the history, i.e., the preparation conditions. Second, the weak segregation regime has to be achievable during the evaporation process. This condition provides more "pathways" for structural transformations which are not accompanied by a strong penalty in the interfacial energy. The third condition for vertical orientation of the cylindrical nanodomains is a weak selectivity of the solvent. In the swollen state, a stronger swelling of the majority domains results in the formation of spherical micelles (rather than cylindrical). Upon drying of the film, these spherical micelles join to form cylinders, which align in parallel to the (vertical) solvent flow. Possible mechanisms of the alignment are discussed. In addition, the effects of varying film thickness, the degree of swelling, segregation regime, and the selectivity of the free surface of the film are studied. Computer simulations of the solvent vapor annealing and thermal annealing of equivalent dry films reveal considerable differences in the final film structures.