In the first part, the results of molecular dynamics simulations of linear and ring polyethylene (PE) melts and blends of linear/ring PE melts will be discussed, together with results of lattice-model simulations, focusing on the fundamental effects of molecular topology on conformations and dynamics of polymer melts. For entangled ring PE melts, we observe the time dependence of mean-square monomer displacements and the chain-length dependence of diffusion coefficients to be very similar to entangled linear chains. However, the diffusion coefficients remain larger for the entangled rings than the corresponding entangled linear chains, due to the larger chain length for the onset of entanglement. Severe topological constants on diffusion coefficients are found only for the rings in the linear chain matrix.<br/>In the second part, the interface characteristics of polystyrene melts in free-standing thin films will be discussed from the molecular dynamics simulations employing an explicit atom force field. The calculated surface tension is in good agreement with experiment, which provides a support for the force field parameters. In the surface region the phenyl-plane normal axis and chain backbone axis are both preferentially oriented along the film surface, in agreement with available experiments. The density profile exhibits an enrichment of phenyls relative to the alkyls at the top low-density surface, but this top surface is followed by a layer of relatively depleted phenyl density of ca. 1 nm thickness. The presence of phenyl depleted region near the free surface of polystyrene films may be related to the most pronounced decrease of glass transition temperatures in thin films of polystyrene as compared with other polymers.

Effects of molecular topology and interface on conformations and dynamics of polymer melts from atomistic molecular dynamics simulations

253rd National Meeting (2017)

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