Molecular Dynamic Simulation Studies on the Structure and Dynamic Properties of Confined Polyethylene Thin Films in Melts.
Cheol Jeong, Do Y. Yoon, and Christopher Soles
The conformation and related dynamic properties of linear polyethylene (PE) films in melts confined between graphite surfaces have been investigated by atomistic molecular dynamic simulations with backbone carbon atom number N = 100 and 300 and film thickness d up to 113 Å. Due to the attractive interaction between graphite surface and monomers, the interfacial layering with persistent length of 2 nm has been observed with enhanced ordering of backbone chains along graphite surface directions. Monomer and center-of-mass (COM) of chain displacements have been analyzed as a function of time and distance from graphite surface, exhibiting the anisotropic slow-down of monomer translation but faster COM motion of interfacial chains than in bulk at short time scale. Diffusion coefficients D of COM measured as a function of d suggest that interfacial attraction plays a dominant role in the slower diffusion of PE chains with N=100 as d decreases while disentanglement effect becomes important in the enhanced D for thin PE film with N=300, which has been supported by the analysis of power-law behavior of monomer displacements as a function of time. In addition, gas transport properties in the PE films in melts have been investigated to gain insight into the role of polymer dynamics in small molecules diffusion at different time and length scale.