Mesoscopic Simulations of Free Surfaces of Molten Polyethylene: Brownian Dynamics/Kinetic Monte Carlo Coupled with Square Gradient Theory and Compared to Atomistic Calculations and Experiment

AP Sgouros and AT Lakkas and G Megariotis and DN Theodorou, MACROMOLECULES, 51, 9798-9815 (2018).

DOI: 10.1021/acs.macromol.8b01873

A mesoscopic simulation approach is developed for liquid-gas interfaces of weakly and strongly entangled polymer melts and implemented for linear polyethylene at 450 K. A combined particle and field-theoretic treatment is adopted based on aggressive coarse-graining, each polymer bead representing similar to 50 carbon atoms, with effective bonded interactions extracted from atomistic simulations. Nonbonded interactions in the mesoscopic model are dictated by an equation of state (here the Sanchez-Lacombe) in conjunction with a variant of gradient theory-the discrete square gradient theory. The dynamics of free films is examined in the presence and in the absence of topological constraints (modeled by slip-springs) to unveil the impact of the latter on chain self-diffusion, to assess their contribution to the interfacial free energy, and to explore how this contribution can be removed by invoking a compensating potential. The molar mass dependence of surface tension-which arises from bonded contributions among beads in the mesoscopic chains-is extracted over a broad range of molar masses (10(3)-10(6) g/mol), in excellent agreement with experiment. Two approaches for computing the surface tension are adopted, based on stress profiles and based on changes in free energy with interfacial area, leading to consistent results. The predicted density profiles, conformations, and orientational tendencies of the mesoscopic chains are retrieved from the simulations and shown to reproduce very well the corresponding results from atomistic simulations. An annealing scheme is developed with the purpose of accelerating transitions of metastable states into more stable biphasic states such as spherical and cylindrical droplets, free films, and spherical and cylindrical bubbles, which minimize the free energy of the periodic model system. Results for the phase diagram as a function of polymer volume fraction conform to the predictions of atomistic simulations of simpler systems.

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