Predicting χ of Polymer Blends Using Atomistic Morphing Simulations

The Flory Huggins interaction parameter χ measures the compatibility of different species in mixtures and governs their phase behavior. We have previously used molecular dynamics (MD) simulations and thermodynamic integration along the path of transformation of one species to another (morphing), to determine χ in coarse-grained bead-spring models of polymer blends. In this work, we use united-atom (UA) MD simulations and morphing to calculate χ for real polymer blends: (1) poly(ethylene) and poly(ethylene oxide), (2) poly(styrene) and poly(2-vinyl pyridine), (3) poly(isoprene) and saturated poly(isoprene), and (4) poly(styrene) and poly(α-methyl styrene). These examples require different schemes for transforming chains: changing Lennard Jones parameters and partial charges (case 1 and 2), transforming double bonds to single bonds (case 3), and making atoms disappear (case 4). For the first three blends, χ predictions agree reasonably with experiments but are sensitive to the choice of force field parameters. For poly(styrene)/poly(α-methyl styrene), we reach the limits of the morphing method.