Segregation and stability in ultra-thin film polymer-polymer blends
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It is well known that the phase behavior of thin film polymer-polymer blends differs appreciably from bulk analogs. This difference is due largely to the influence of interfacial interactions (polymer-substrate and polymer free surface) and to entropic effects associated with confinement. Interfacial interactions, for example, “drive” preferential segregation of the constituents of the mixture to the interfaces. This influences the symmetry of the phase diagram. Beginning with the work of Schmidt and Binder in the late 1980s, a number of authors examined the influence of a single interface (the so-called semi-infinite case) on the “local” concentration profiles of polymer-polymer mixtures. The most complete work to date involve simulations of Flebbe and Binder, who examined a situation in which one component of the mixture preferentially segregated to both interfaces, the so-called symmetrically adsorbing case. They showed how the phase diagram would be altered under varying degrees of confinement. Polymer-polymer thin film blends of poly(styrene-co-acrylonitrile) (SAN) and Poly(methylmethacrylate) (PMMA) in the thickness range h<5Rg, where Rg is the radius of gyration of one component, were examined. The phase behavior of this system was observed to phase separate over temperature ranges where the bulk mixtures were miscible. In fact, the PMMA component segregated to both interfaces (symmetric adsorption). Structural instabilities were observed to accompany phase segregation. Specifically, surface patterns (topographies), nucleation and growth and spinodal patterns, were observed over a range of film thicknesses. Such patterns are usually due to the antagonistic action of long and short-range intermolecular forces that influence stability of films in the thickness range of h<10 nm. We observed similar patterns over a wider range of film thickness, depending on the composition of the SAN copolymer. These observations can not be rationalized in terms of the current theory. A series of numerical calculations were performed to explain these observations. It was shown that in this thickness range, segmental asymmetry effects (entropic), not accounted for by Binder and coworkers, have a strong influence on the phase behavior of these systems. These numerical calculations further enable a calculation of the interfacial free energy of the system as a function of film thickness. This is significant since the topographical instabilities that we observe in these films can now be rationalized.