Reversible Hierarchical Phase Separation of a Poly(methyl methacrylate) and Poly(n-nonyl acrylate) Blend in a Langmuir Monolayer

Mixed Langmuir monolayers composed of poly(methyl methacrylate) (PMMA) and poly(n-nonyl acrylate) (PNA) were studied by surface pressure area isotherms and atomic force microscopy (AFM). The mixed monolayer was miscible at a low surface pressure but underwent a reversible hierarchical phase separation at a high surface pressure, with the monolayer of the major component being spread on the water surface, on top of which the minor component separated out. The phase separation grew with time, but upon reduction of the surface pressure into the miscible region, the phase separation immediately disappeared. A subsequent increase of the surface pressure into phase separation region again caused a phase separation similar to that from the miscible state, indicating that the blend monolayer converted to the original miscible state immediately upon reduction of the surface pressure. Thus, this hierarchical phase separation is completely reversible and a true thermodynamic transition. A PMMA-b-PNA block copolymer also showed a similar reversible hierarchical phase separation, with the major component being spread on the water surface, on top of which the minor component again separated out. The domain size was regular, corresponding reasonably well to the chemical structure of the block copolymer, and did not grow with elapsed time.