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 profi le of a symmetric PEP/dPEP polymer blend with nanometer-scale resoluti on in the one-phase region. The shape of the profile near the surface was found to differ significantly from mean-field theory predictions f or all bulk concentrations and temperatures surveyed. The profile shap e is flattened at the surface over a distance about half the bulk corr elation 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 G AE method is much less model dependent, requiring knowledge of only th e free energy of mixing in the bulk and the measured integral excess a nd 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 differ ence between the isotopically different polymers arises from the same van der Waals interaction that leads to a positive Flory segment-segme nt interaction parameter chi in these blends.