Polymer films in the normal-liquid and supercooled state: a review of recent Monte Carlo simulation results

Abstract

The present paper reviews recent attempts to study the development of glassy behavior in thin polymer films by means of Monte Carlo simulations. The simulations employ a version of the bond-fluctuation lattice model, in which the glass transition is driven by the competition between an increase of the local volume requirement of a bond, caused by a stiffening of the polymer backbone and the dense packing of the chains in the melt. The melt is geometrically confined between two impenetrable walls separated by distances that range from once to approximately fifteen times the bulk radius of gyration. The confinement influences static and dynamic properties of the films: Chains close to the walls preferentially orient parallel to it. This orientation tendency propagates through the film and leads to a layer structure at low temperatures and small thicknesses. The layer structure strongly suppresses out-of-plane reorientations of the chains. In-plane reorientations have to take place in a high density environment which gives rise to an increase in the corresponding relaxation times. However, local density fluctuations are enhanced if the film thickness and the temperature decrease. This implies a reduction of the glass transition temperature with decreasing film thickness.