NEUTRON REFLECTOMETRY STUDY OF SURFACE SEGREGATION IN AN ISOTOPIC POLY(ETHYLENEPROPYLENE) BLEND - DEVIATION FROM MEAN-FIELD THEORY

Neutron reflectometry (NR) was used to measure the concentration profile of a symmetric PEP/dPEP polymer blend with nanometer-scale resolution in the one-phase region. The shape of the profile near the surface was found to differ significantly from mean-field theory predictions for all bulk concentrations and temperatures surveyed. The profile shape is flattened at the surface over a distance about half the bulk correlation length, a length scale far greater than that of the dispersive forces that are the dominant microscopic interaction in such blends. We introduce a new method for extracting the surface energy difference that drives segregation based on the Gibbs adsorption equation (GAE) and compare it to a previous method based on mean-field theory. While both methods produce qualitatively similar results (the surface energy of pure dPEP is similar to 0.21 mJ/m(2) smaller than pure PEP), the GAE method is much less model dependent, requiring knowledge of only the free energy of mixing in the bulk and the measured integral excess and surface composition. The surface energy difference between dPEP and PEP is similar to 2.6 times larger than that for dPS and PS, a factor in good agreement with the expectation that the surface energy difference between the isotopically different polymers arises from the same van der Waals interaction that leads to a positive Flory segment-segment interaction parameter chi in these blends.