The Flory-Huggins χ parameter describes the excess free energy of mixing and governs phase behavior for polymer blends and block copolymers. For chemically distinct polymers, the value of χ is dominated by the mismatch in cohesive energy densities of the monomers. For blends of chemically similar polymers, the entropic portion of χ, arising from non-ideal local packing, becomes more significant. Using polymer field theory, Fredrickson et al. predicted that a difference in backbone stiffness can result in a positive χ for chains consisting of chemically identical monomers. To quantitatively investigate this phenomenon, we perform molecular dynamic (MD) simulations for bead-spring chains which differ only in stiffness. From the simulations, we apply a novel thermodynamic integration to extract χ as low as 10^(-4) per monomer for blends with stiffness mismatch. To compare with experiments, we introduce a standardized effective monomer, which allows us to redefine the backbone stiffness for real polymers. By mapping polymers with effective stiffness to our bead-spring chains, we show that the predicted χ agree with experimental data.
