Structure and dynamics of a polymer melt at adsorbing surfaces

A. DE VIRGILIIS; A. MILCHEV

Dresden

Workshop; Workshop of the SPP 1369 program "Polymer-Solid Contacts"; 2011

DFG

A polymer melt made of linear, non-entangled chains is studied by means of molecular dynamics simulations of a bead-spring model. The system is put in contact with a solid wall consisting of a layer of atoms occupying the sites of a triangular lattice. This wall exerts an attractive potential on every monomer in such a way that a polymer film limited by the wall on the bottom and a free interface on the top is equilibrated.Both structural and dynamical properties of the polymer film are obtained, as a function of the distance to the wall, z. We demonstrate that the structure of the melt is affected by the substrate in an adjacent region with thickness ~Rg. Within this region the polymers are partially adsorbed and their conformations are characterized by a composition of different buiding blocks: trains, loops and tails.We determine the probability distribution of trains, loops and tails from the simulation and find that they agree well with theoretical predictions for adsorbed linear chains. The adsorbed amount Г(M) is found to scale as Г(M) ~ M1/2 with chain length M regardless of the adsorption strength εw.The polymer melt dynamics in the vicinity of the substrate is characterized by relaxation times of the chains and mobilities of the individual monomers that change with distance z from the plane. Different properties like polymer end-to-end relaxation, bond-bond relaxation and monomer mobilities display a steady slowdown with increasing proximity to the substrate. Moreover, the mobilities of adsorbed monomers are strongly influenced by the strength of adsorption indicating the onset of confined dynamics (“cage” effects). At strong adsorption a typical crossover to glassy dynamics is observed in the layers that are in the immediate vicinity of the substrate.It is found that the exchange dynamics of adsorbed monomers with those in the bulk is well described by stretched exponential behavior.