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.
A tight-binding model coupled with Marcus rate accurately predicts polaron hopping rates in various crystalline and amorphous poly(3-hexylthiophene) materials.
Classic
experiments show that polybutadiene oligomers align in
a network of stretched chains. Furthermore, the oligomers orient almost
as strongly as the network, which suggests a large nematic coupling,
despite polybutadiene being a flexible polymer. Here, we combine self-consistent
field theory (SCFT) and atomistic molecular dynamics (MD) simulations
of polymer chains under tension to obtain the nematic coupling constant
α in polybutadiene. Using α, we compute the ratio of orientation
of free chains and stretched chains of polybutadiene in a melt of
stretched chains. We show that nematic coupling in polybutadiene,
though not quite enough to induce a nematic phase, is surprisingly
strong. When extrapolated to the experimental temperature, we find
an orientation ratio of 0.8, consistent with the experimental value
of 0.9.
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