Surface Enrichment in a Miscible Polymer Blend: An Experimental Test of Self-Consistent Field and Long-Wavelength Approximation Models

Neutron reflectivity (NR) and low-energy forward recoil spectrometry (LE-FRES) were used to study surface enrichment in miscible blends of deuterated polystyrene, d-PS, and poly(styrene- co -4-bromostyrene), PBr 0.049 S, having a 0.049 mole fraction of 4-bromostyrene units. The d-PS component was found to segregate preferentially to the polymer blend/air interface, whereas no enrichment of either component was detected at the polymer blend/silicon interface. The experimental values of the surface concentration, φ 1, and the surface excess, z *, of d-PS were interpreted using both the theory of Schmidt and Binder (SB) ( J . Phys . II (Paris) 1985, 46, 1631) and the self-consistent field (SCF) approach of Genzer et al . ( Phys . Rev . E 1994, 50, 2373). Although both SB and SCF models were found to be in good qualitative agreement with the experimentally measured values of φ 1 and z *, we demonstrate that the latter proves to be in better quantitative agreement with the experimental results. Moreover, a comparison of the SB and SCF volume fraction profiles of d-PS revealed that the SCF model described more accurately the experimental profile. We also demonstrated that adding long-range interactions to the surface potential in the SCF model produced just minor changes in the shape of the d-PS profile near the surface.