Molecular Dynamics Study of Polyethylene under Extreme Confinement

[摘要] We present results concerning the dynamics and the structure of adsorbed layers of molten polyethylene (PE) between two graphite surfaces. The molecular weight of the monodisperse PE chains reaches the entanglement regime. We study three cases of interwall distances, equal to two, three and four times the unperturbed radius of gyration (Rg) of PE chains. The confined system is equilibrated by use of efficient Monte Carlo (MC) algorithms. Conducting molecular dynamics (MD) simulations, we reveal the distribution of relaxation times as a function of distance from the graphite walls at the temperature of 450 K. From the atomic-level stresses we calculate a realistic estimate of the adhesion tension, which is not affected significantly by the width of the pore. Although the distance between the two walls is comparable to the width of the adsorbed layer, we do not record the formation of 'glassy bridges' under the studied conditions. The diffusion of polymer chains in the middle layer is not inhibited by the existence of the two adsorbed layers. Extreme confinement conditions imposed by the long range wall potentials bring about an increase in both the adsorption and desorption rates of chains. The presented results seem to cohere with a reduction in the calorimetric (heat capacity step) glass transition temperature (Tg).

[发布机构] School of Chemical Engineering, Aristotle University of Thessaloniki, Thessaloniki; GR-54124, Greece^1;School of Chemical Engineering, National Technical University of Athens, Zografou Campus, Athens; GR-15780, Greece^2

[效力级别] 数学

[关键词] Adhesion tensions;Adsorption and desorptions;Atomic level stress;Confined systems;Graphite surfaces;Molecular dynamics simulations;Molten polyethylene;Radius of gyration