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ORAL PRESENTATION
Session: 2 Multiscale modeling and molecular simulations
(scheduled: Monday, 08:00 )
Atomistic simulation of polymers with a non-linear molecular architecture: Calculation of branch point friction and chain reptation time of an H-shaped polyethylene melt
N.Ch. Karayiannis1,2, V.G. Mavrantzas1,2
1 Department of Chemical Engineering, University of Patras, GR 26504, Greece
2 Institute of Chemical Engineering and High-Temperature Chemical Processes (FORTH-ICE/HT), Patras GR 26504, Greece
A hierarchical simulation strategy is presented for simulating structure and dynamics in polymers characterized by a non-linear molecular architecture, such as the H-shaped macromolecules [1]. First, a novel Monte Carlo (MC) algorithm is employed to generate well equilibrated atomistic configurations of these highly non-linear chain structures. The new algorithm is built around state-of-the-art chain connectivity altering moves, like the end-bridging and double-bridging, and is many orders of magnitude more efficient than Molecular Dynamics (MD) in equilibrating these systems, even if multi-step time integration schemes are incorporated in the MD algorithm. In a second step, the equilibrated structures generated by the new MC algorithm are used as initial configurations in detailed NPT MD simulations of H-shaped polyethylene (PE) melts for very long times, on the order of microseconds. In our simulations, the average number of carbon atoms in the backbone has ranged from 48 up to 300 corresponding to both unentangled and entangled crossbars, while the average branch length was kept relatively small (it ranged from 24 up to 50 carbon atoms) corresponding always to unentangled arms. The MD simulation results provide convincing evidence for the different relaxation mechanisms exhibited by an H-polymer melt: the fast relaxation due solely to arm breathing (on the order of a few ns, for an H_300_50 melt), and the slow branch point diffusion which is accompanied by a sluggish backbone diffusion due to reptation (on the order of a few μs, for an H_300_50 melt). They have further demonstrated that the center-of-mass diffusivity in an H-polymer follows faithfully that of branch points, thus validating from first-principles the main assumption of the McLeish-Larson pom-pom theory that all friction in an H-molecule is concentrated at the branch points. For the longest H-polymers studied, logarithmic plots of the msd of the inner crossbar segments against time were seen to exhibit the four different regimes predicted by the reptation theory of Doi-Edwards for entangled linear polymer melts, with corresponding exponents remarkably close to those of the theory. This allowed us to extract the characteristic relaxation times τe, τR, and τd for each one of the simulated systems and their effective tube diameter. © IWNET 2006
[1] N.Ch. Karayiannis and V.G. Mavrantzas, Macromolecules 38, 8583 (2005).
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and Kleanthi for IWNET 2006