It is usually assumed that the Flory−Huggins interaction parameter χ in polymer blends can be represented by a structureless, pointlike interaction. On a local scale, however, the Flory−Huggins parameter must show a spatial dependence as it basically reflects the segment−segment interaction potential. We show that SANS can be used to access this spatial dependence and that even in the SANS regime, a pronounced Q dependence of χ is found. The polymer blend used in the experiments was a polystyrene/poly(p-methylstyrene) blend. As we did not want to rely on literature values for the segment lengths of PS and PPMS, respectively, we measured the isotope blends of h-PS/d-PS and h-PPMS/d-PPMS separately. A modified random phase formula based on the PRISM theory of Schweizer and Chandler is used to evaluate the data and fit a simple model to χ(Q). We find that the range of a Yukawa-like segment−segment interaction potential is about 9 Å.
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