Theoretical Reconstruction of the Structure and Dynamics of Polymer Melts from Their Coarse-Grained Description

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Title: Theoretical Reconstruction of the Structure and Dynamics of Polymer Melts from Their Coarse-Grained Description
Author: Lyubimov, Ivan
Abstract: A theoretical formalism to reconstruct structural and dynamical properties of polymer liquids from their coarse-grained description is developed. This formalism relies on established earlier analytical coarse-graining of polymers derived from the first principles of liquid theory. The polymer chain is represented at a mesoscale level as a soft particle. Coarse-grained computer simulations provide input data to the reconstruction formalism and allow one to achieve the most gain in computational efficiency. The structure of polymer systems is reconstructed by combining global information from mesoscale simulations and local information from small united-atom simulations. The obtained monomer total correlation function is tested for a number of systems including polyethylene melts of different degrees of polymerization as well as melts with different local chemical structure. The agreement with full united-atom simulations is quantitative, and the procedure remains advantageous in computational time. The dynamics in mesoscale simulations is artificially accelerated due to the coarse-graining procedure and needs to be rescaled. The proposed formalism addresses two rescalings of the dynamics. First, the internal degrees of freedom averaged out during coarse-graining procedure are reintroduced in "a posteriori" manner, rescaling the simulation time. The second rescaling takes into account the change in friction when switching from a monomer level description to mesoscopic. Both friction coefficients for monomer and soft particle are calculated analytically and their ratio provides the rescaling factor for the diffusion coefficient. The formalism is extensively tested against the united-atom molecular dynamic simulations and experimental data. The reconstructed diffusive dynamics of the center-of-mass for polyethylene and polybutadiene melts of increasing degrees of polymerization show a quantitative agreement, supporting the foundation of the approach. Finally, from the center-of-mass diffusion the monomer friction coefficient is obtained and used as an input into Cooperative Dynamics theory. The dynamics of polymer chains at any length scale of interest is described through a Langevin equation. In summary, the proposed formalism reconstructs the structure and dynamics of polymer melts enhancing computational efficiency of molecular dynamic simulations. This dissertation includes previously published and unpublished co-authored material.
Date: 2012

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