The film thickness dependence of surface structure for immiscible polystyrene/poly(methyl methacrylate) (PS/PMMA) films was investigated on the basis of atomic force microscopic observation and X-ray photoelectron spectroscopic measurement. In the case of the PS/PMMA film of 25 μm thickness, the air - polymer interfacial region was covered with a PS rich overlayer due to its lower surface free energy compared with that of PMMA and a well-defined macroscopic phase-separated structure was formed in the bulk phase. Also, in the case of the PS/PMMA thin film of 100 nm thickness, the phase-separated structure, in which the PMMA rich domains separated out of the PS rich matrix, formed at the film surface. The formation of the surface structure for the PS/PMMA thin film can be attributed to either the chain conformation or chain aggregation structure being frozen at the air - polymer interfacial region before the formation of a PS rich overlayer due to the fairly fast evaporation of solvent molecules. On the other hand, the two-dimensional PS/PMMA ultrathin film of 10.2 nm thickness did not show distinct phase-separated structure. When the film thickness became thinner than 10.2 nm, the two-dimensional PS/PMMA ultrathin film of 6.7 nm thickness showed fine and distinct phase-separated structure with the domain size of a few hundred nanometers. This structure can be designated as "mesoscopic phase-separated structure". The surface phase state for the two-dimensional PS/PMMA ultrathin films can be explained by the film thickness dependence of both the interaction parameter and the degree of entanglement among polymer chains.
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