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

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 with 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 the remarkable fact that the conformations of chains in the melt at the interfaces are mostly unaffected by the strength of substrate/polymer attraction. Moreover, they practically coincide with the conformations of a single end-grafted chain at the critical point of adsorption, 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 that are found to be perfectly described by analytical expressions 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 Gamma proportional to root N regardless of adsorption strength. We also find that the pressure of the melt P-N decreases as P-N-P-infinity proportional to N-1 (where P-infinity is the extrapolated pressure for N - infinity ) with growing length N of the chains whereas the surface tension gamma at both walls is found to decline as gamma(N) proportional to 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 approximate to 2R(g) where R-g is the unperturbed radius of gyration of chains in the bulk.