Molecular Dynamics Simulation of a Polymer Melt/Solid Interface:  Local Dynamics and Chain Mobility in a Thin Film of Polyethylene Melt Adsorbed on Graphite

Abstract

Molecular dynamics (MD) simulations have been performed on a dense polymer melt adsorbed on a solid substrate on the one side and exposed to vacuum on the other. As a model system, a thin film of polyethylene (PE) melt supported by a crystalline graphite phase on its one side (the other surface of the film is free) has been examined. Most simulations have been carried out with unentangled PE melt systems, such as C78 and C156, in the NPT statistical ensemble at T = 450 K and P = 0 atm for times up to 100 ns, using a multiple-time step MD algorithm and by incorporating the correct dependence of the long-range contribution to the energy and stress tensor on the density profile. To increase the statistical accuracy of the results, large systems have been employed in the MD simulations, such as a 200-chain C78 melt consisting of 15 600 carbon atoms. The MD simulation data have been analyzed to provide information about the spatial dependence of the short-time dynamical properties (conformational relaxation) of the melt and the long-time segmental motion and mobility in the film (transport and diffusion). Local mobility near the graphite phase is predicted to be highly anisotropic: although it remains practically unaltered in the directions x and y parallel to the surface, it is dramatically reduced in the direction z perpendicular to it. To calculate the long time self-diffusion coefficient of adsorbed segments in the direction perpendicular to the graphite plane, MD trajectories have been mapped onto the (numerical) solution of a macroscopic, continuum diffusion equation describing the temporal and spatial evolution of the concentration of adsorbed atoms in the polymer matrix. Our calculations prove that the diffusive motion of segments remains inhomogeneous along the z direction of the adsorbed film for distances up to approximately 5−6 times the root-mean-square of the radius of gyration, R g, of the bulk, unconstrained melt.

A hierarchical dualscale study of bisphenol-A-polycarbonate on a silicon surface: structure, dynamics and impurity diffusion

“…There are several peaks and troughs near the surface and the density does not become constant and bulk-like until around 20 A. This value is similar to the value found for other polymer melts in contact with surfaces, for example, polystyrene on graphite 21 and poly(ethylene oxide) on TiO 2 .…”

Graphitic nanofillers in PMMA nanocomposites—An investigation of particle size and dispersion and their influence on nanocomposite properties

“…Thus, the true effective interparticle distance may be significantly smaller. Our results for T g suggest this is the case as for all wt % loading the T g of ARG systems is $30 8C above baseline PMMA, which should occur when nano-particle spacing on the order of several radii of gyration of the polymer chains [63][64][65][66] (for our PMMA this would be on the order of 100 nm).…”

Dynamics of Polystyrene Melts through Hierarchical Multiscale Simulations

“…88,89 This is important since for the study of the longtime dynamics of polymers in such systems an atomistic description needed very close to the surface whereas a mesoscopic description can be used for length scales far from the surface. [90][91][92] …”