The effects of confinement and surface interactions on coexistence in a binary polymer mixture

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Abstract
We have measured the composition‐distance profile across a film consisting of two thin layers (200–600 nm) of a model binary isotopic mixture of deuterated polystyrene (dPS) and protonated polystyrene (hPS), coexisting with each other near their equilibrium compositions below the critical temperature for phase demixing for this pair. Profiles were determined normal to the silicon wafer on which the bilayer is mounted using nuclear reaction analysis, both for an uncoated silicon surface and for one coated with a gold layer. Measurements reveal that when both layers are thick relative to the characteristic width w (∼100 nm) of the interfacial region between them, the coexisting compositions about the interface are close to their bulk values as determined earlier for this system. When the dimensions of the layers are made comparable with w, however, interactions with the confining surfaces may significantly modify the composition profile of the coexisting layers about the interface. This effect is marked at the polymer/silicon interface as a result of its interactions with one of the components (dPS), but is absent for a gold‐coated surface in an identical geometry due to the much weaker influence of the surface. Our results are discussed in detail in terms of mean‐field models of mixing in polymeric mixtures, and enable quantitative determination (using a Cahn construction approach) of the interaction parameters both at the polymer–air and polymer–silicon interfaces. Though we are not able to calculate in a completely a priori fashion the coexistence profiles as a function of the film thickness, we propose an approximate approach which provides good agreement of calculated composition profiles with those determined experimentally over the range of parameters in our experiments.