Predicting χ of polymer blends using atomistic "morphing" simulations

The Flory Huggins interaction parameter χ governs phase behavior in polymer blends and block copolymers. We have previously used molecular dynamic (MD) simulations and thermodynamic integration during the morphing of one species to another, to compute the excess free energy of mixing in coarse-grained, bead-spring model of polymer blends. Using this method, we have studied the effect on χ of factors like stiffness mismatch, chain architecture and Lennard Jones interactions. In this work, we use united-atom MD simulations and the morphing method to calculate χ for real polymer blends: (1) poly(ethylene) - poly(ethylene oxide), (2) poly(styrene) - poly(2-vinyl pyridine), (3) poly(isoprene) - saturated (polyisoprene) and (4) poly(styrene) - poly(α-methyl styrene). These examples require different kinds of morphing- LJ interactions and partial charges of atoms (cases 1 and 2), transform double bond to single bond (case 3) and disappearance of atoms (case 4). All simulations used force field parameters adopted from TRAPPE. Our χ values from simulations are in reasonable agreement with experiment, but are sensitive to force field parameters used in the simulation.