Block copolymers in homopolymer blends: Interface vs micelles

The location of poly(styrene-b-poly(methyl methacrylate)) (PS-b-PMMA) block copolymers in PMMA/poly(cyclohexyl methacrylate) (PCHMA) melt mixed blends was determined. Among all these components, only the PS portion of the block copolymer can be stained by ruthenium tetroxide and appears dark in transmission electron micrographs. Then the locations of block copolymer in the blends, i.e., interfaces and micelles, can be easily identified. Morphology and the aggregation of the PS-b-PMMA were studied as a function of molecular weight and molecular weight fraction. At low molecular weight PS-b-PMMA, a PMMA/PS-b-PMMA macrophase formed. At higher M_n, small (<1 μm) minor phase (PMMA) drops coated with copolymer formed, but these drops also contained micelles even at low PS-b-PMMA concentration. Increasing the molecular weight of the PMMA first caused the drop size to increase, and then the copolymer micelles to relocated from the PMMA to the PCHMA phase. Increasing block copolymer concentration caused PMMA drop size to decrease roughly in proportion to interfacial coverage. The results are discussed qualitatively in terms of Leibler's wet-dry brush theory.