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

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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.

Section snippets

Introduction and overview

Many recent studies deal with the influence of confinement on the dynamic behavior of glass forming liquids [1], [2]. Besides the practical importance of such systems (polymer films as protective coatings, flow through porous materials, etc.) one motivation for this research is that it might also provide a better understanding of the glass transition in the bulk [1], [3], [4], [5]. If the glass transition was driven by an underlying correlation length ξ which grows with progressive supercooling

Coarse-grained lattice simulations of polymer films

The present approach uses a lattice model: the bond-fluctuation model [53], [54], [55], [56]. This model is intermediate between a highly flexible continuum treatment and standard lattice models of polymers [57], [58], [59]. With the latter it has in common the simple lattice structure which is very efficient from a computational point of view [60], [61]. However, it differs from them in that the set of bond vectors is not limited by the coordination number of the lattice, but much larger. In

Static properties: gyration tensor and density profiles

Structural properties represent an important input for analyzing the dynamics of a polymer melt (and of any other system in general). Close to a solid interface the structure of the melt markedly deviates from the behavior of the unconstrained bulk [78], [79]. This is pointed out by analytical approaches [80], [81], [82], [83], [84], [85], [86], [87], [88], [89], [90], [91], computer simulations of lattice [92], [93], [94], [95], [96], [97], [98] and continuum models [37], [81], [82], [84], [85]

Dynamic properties of the polymer films

Previous work on the dynamics of polymer films focused on the behavior of mean–square displacements and related quantities, such as the monomer mobility and the chain's diffusion coefficient parallel to the walls [36], [97], [118]. Another important means to study dynamic properties are time–displaced correlation functions. The present section discusses two different kinds of these functions: the incoherent intermediate scattering function which probes density fluctuations and the correlation

Summary

This paper reports simulation results for a simple model of glassy polymer films. The model consists of short (non-entangled) monodisperse chains. The monomers of the chains solely interact by excluded volume forces with each other and with two completely smooth walls that define the thin film geometry. The glassy behavior is brought about by a competition between the internal energy of a chain, which tries to make the chain expand at low temperatures and the dense arrangement of all monomers

Acknowledgements

We are indebted to S. Dasgupta, F. Eurich, T. Kreer, P. Maass, M. Müller, W. Paul and F. Varnik for helpful discussions on various aspects of this work and to the unknown referees for their valuable comments. This study would not have been possible without a generous grant of simulation time by the HLRZ Jülich, the RHRK Kaiserslautern and the computer center at the University of Mainz. Financial support by the ESF Programme on ‘Experimental and Theoretical Investigation of Complex Polymer

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