Surface Segregation in Miscible Blends of Polystyrene and Poly(vinylmethyl ether): Comparison of Theory and Experiment

The effect of the interplay between bulk and surface free energy terms on surface segregation in miscible blends is probed by comparing angle-dependent x-ray photoelectron spectroscopy (ADXPS) measurements for polystyrene/polyvinylmethylether (PS/PVME) blends of with those for perdeuteropolystyrene/polyvinylmethylether (dPS/PVME) blends. The magnitudes of the bulk interaction parameters for the two systems differ markedly while the surface interactions are essentially identical. Experimental concentration depth profiles are almost identical for the two systems indicating that their surface properties are little affected by bulk interactions and dominated by surface energy effects.These data and previous data from our group are compared to the predictions of the square gradient theory developed by Schmidt and Binder in order to gain a more quantitative understanding of the factors that control surface segregation in miscible blends. While there is general qualitative agreement between theory and experiment, predicted surface compositions fall significantly below experimental values and predicted composition depth profiles decay more gradually than what is observed experimentally, especially for low PVME contents. The use of the more appropriate Sanchez-Lacombe-Balazs equation of state does not yield any significant improvement over the use of the Flory-Huggins lattice model for representing the bulk free energy terms. Careful analysis of the experimental behavior suggests that configurational effects associated with the flattening of surface adsorbed chains and differences in mer-mer interaction parameters in the bulk and near surface regions are possible origins for the discrepancies between theory and experiment.