The effect of the preferential affinity of the solvent on the microstructure of solution-cast block copolymer thin films.

We have studied the microstructures of the solvent-cast films of polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) diblock copolymer with PMMA cylinder-forming composition. The casting solvents, 1,1,2-trichloroethane (TCE), toluene (Tol) and their binary mixtures, were controlled to evaporate at a certain rate R approximately 0.01 mL/h, and the effect of the preferential affinity of the solvents for a certain block P2VPng microstructures is investigated. 1,1,2-Trichloroethane and toluene, with similar volatility, are good solvents for both PS and PMMA blocks but having opposite preferential affinity for each block (TCE has preferential affinity for the minority PMMA block and Tol has preferential affinity for the majority PS block), and the preferential affinity of the solvents is modulated by mixing the two solvents. As the preferential affinity of the solvents for the majority PS block increases, a series of microstructures including perforated lamella with ringlike morphologies, PMMA cylinders, PMMA spheres, and PMMA cylinders covered with a layer of PS microdomain have been observed. The results are discussed in view of the effective volume fraction of each block induced by the solvent preferential affinity and the special mechanical strain field brought by the solvent evaporation.