Molecular Dynamic Study of the Structure and Dynamics of Polymer Melt at Solid Surfaces

Abstract

We investigate the dynamic and static properties of a polymer melt near solid surfaces. The melt, composed of linear chains, is confined between two solid walls which one of the walls being repulsive whereas the opposite, attractive wall, is characterized by different degrees of roughness caused by an array of short perpendicular pillars with variable grafting density. We demonstrate that the conformations of chains in the melt at the interfaces do not depend on substrate/polymer interactions and practically coincide with the conformations of a single end-grafted chain under critical adsorption conditions, in agreement with Silberberg?s hypothesis. This agreement is corroborated by the analysis of the size distributions of trains, loops and tails of melt chains at the walls which are found to be perfectly described by distributions pertaining to end-grafted single chains at critical adsorption. The adsorbed amount at the attractive bottom surface is found to scale with √macromolecule length as Γ ∝ N regardless of adsorption strength. We also find that the pressure of the melt PN decreases as PN − P∞ ∝ N −1 (where P∞ is the extrapolated pressure for N → ∞) with growing length of the chains N whereas the surface tension γ at both walls is found to decline as γN ∝ N −2/3 . Eventually, a study of the polymer dynamics at the rough interface reveals that surface roughness leads to dramatic drop of the coefficient for lateral diffusion whenever the separation between obstacles (neighboring pillars) becomes less than ≈ 2Rg where Rg is the radius of gyration of chains in the bulk.