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

The present paper reviews recent attempts to study the development of glass y behavior in thin polymer films by means of Monte Carlo simulations. The s imulations employ a version of the bond-fluctuation lattice model, in which the glass transition is driven by the competition between an increase of t he local volume requirement of a bond, caused by a stiffening of the polyme r backbone and the dense packing of the chains in the melt. The melt is geo metrically confined between two impenetrable walls separated by distances t hat range from once to approximately fifteen times the bulk radius of gyrat ion. The confinement influences static and dynamic properties of the films: Chains close to the walls preferentially orient parallel to it. This orien tation tendency propagates through the film and leads to a layer structure at low temperatures and small thicknesses. The layer structure strongly sup presses out-of-plane reorientations of the chains. In-plane reorientations have to take place in a high density environment which gives rise to an inc rease in the corresponding relaxation times. However, local density fluctua tions are enhanced if the film thickness and the temperature decrease. This implies a reduction of the glass transition temperature with decreasing fi lm thickness. (C) 2001 Elsevier Science B.V. All rights reserved.