Theory for the chi parameter of polymer blends: Effect of attractive interactions

Using an integral equation theory for polymer blends, we have calculated the effective chi parameter for a binary blend of Gaussian polymers interacting with identical hard core repulsive and dissimiliar attractive interactions between chain segments. Our results predict a parabolic composition dependence for χ in qualitative accordance with recent neutron scattering experiments and Monte Carlo simulations. We also find that the attractive interactions induce structural correlations which tend to significantly reduce χ and thus stabilize the miscible mixture.