Small-angle neutron scattering investigation of the Q-dependence of the Flory-Huggins interaction parameter in a binary polymer blend

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.