Effect of Solvent Removal Rate on the Morphology of Solvent Vapor Annealed Block Copolymer Thin Films

The influence of solvent removal rate on the morphology of DMF annealed cylinder-forming poly( styrene-block-4-vinylpyridine) (PS-b-P4VP) thin films with different thickness is investigated by atomic force microscopy (AFM) and transmission electron microscopy (TEM). It is found that the final film morphology can be effectively modulated by controlling the solvent removal rate of the swollen film. When the film thickness was 35 nm, under fast solvent removal rate, long range ordered inverted parallel PS cylinders were obtained, as solvent removal rate decreased, the ring-shaped PS nanostructure formed in the whole film, and finally slow solvent removal rate resulted in terrace lamella morphology. Such morphological transformation is mainly due to kinetic effect and preferential affinity of annealing solvent for the minority block, which brings remarkable solvation effect. The unique ring-shaped morphology is caused by the shrinkage of the continuous P4VP phase and bending of the PS cylinders. While when the film thickness was 55 nm, only P4VP cylinder morphology was obtained under the same annealing condition and solvent removal rate. We consider that the low swollen rate will reduce solvation effect when the film is thick, in the mean time the effect of solvent removal rate on the final morphology is weakened.