Effect of solvent annealing on the nano- and micro-structure of block copolymer thin film

The development of structure of symmetric polystyrene-b-poly(2 vinyl pyridine) (PS-PVP) thin film annealed under a neutral solvent vapor was investigated by atomic force microscopy and optical microscopy. Initially, spin cast film showed a micellar structure. With subsequent neutral solvent vapor treatment, a lamellar structure gradually developed. It was found that the structural evolution of the thin film was a strong function of the quantity of solvent diffused into the film. In addition, with an increase in swelling, the orientation of the lamellae changed from parallel to perpendicular. During solvent annealing, the solvent also induced dewetting via primary and secondary heterogeneous nucleation processes, depending on the quantity of solvent in the film. Secondary nucleation was independent of primary nucleation for lower solvent concentration. With higher amount of solvent, the solvent expedited secondary nucleation, and it occurred within the dewetted area caused by primary nucleation.