Tunable ordered droplets induced by convection in phase-separating P2VP/PS blend film

Ordered hexagonal droplets patterns in phase-separating polymeric blend films of polystyrene and poly(2-vinylpyridine) (PS/PVP) formed due to the convection effect by solvent evaporation. The influences of PS molecular weight, solvent evaporation rate, and the weight ratio of PS to PVP on the PVP-rich domains pattern formation and distributions were investigated by atomic force microscope (AFM). Only in an appropriate range of molecular weight of PS, can the ordered pattern form. Too low or too high molecular weight of PS led no ordered pattern due to the viscosity effects. The increase of solvent evaporation rate decreased the mean radius of the PVP-rich domains and the intervals between the centers of the domains due to the enhancement of the viscosity on the top layer of the fluid film. The increase of the weight ratio of PS to PVP decreased mean radius of the PVP-rich domains whereas the intervals between the centers of droplets remained constant. Therefore, the size and the distributions of ordered patterns can be tuned by the polymer molecular weight, the weight ratio of the two components and the solvent evaporation rate.