Computer simulations of undercooled fluids and the glass transition

Two model studies are presented that attempt to describe the static and dynamic properties of glass-forming fluids via molecular dynamics simulations: The first model is an atomistically realistic model of SiO(2), the second model provides a coarse-grained description of polymer liquids, i.e., typical 'fragile' glassformers, while SiO(2) is the prototype of a 'strong glassformer'. For both models, attention is given to the questions as to which range of temperatures are properties in equilibrium, and whether such simulations can help to interpret experiments and/or check theoretical predictions. While in the simulation of SiO(2) using the potential of van Beest, Kramer and van Santen ('BKS potential') only temperatures T greater than or equal to 2750 K can be equilibrated, nevertheless many properties can be extracted that are in almost quantitative agreement with experiment. Sodium silicate liquids investigated at temperatures T greater than or equal to 1900 K are in agreement to within a few percent with experimental data, too. At very high temperatures (T greater than or equal to 2750 K), even for SiO(2) a temperature range is found where the mode coupling theory (MCT) of the glass transition is applicable (with T(c) = 3330 K). The polymer melt is described by flexible polymer chains represented by a continuum bead-spring model with a Lennard-Jones interaction between the effective monomers. The typical features of undercooled macromolecular fluids are reproduced qualitatively, with the properties at small length scales (much less than 1 nm) described by MCT, while at larger length scales a crossover to Rouse-like dynamics occurs. (C) 2000 Elsevier Science B.V. All rights reserved.