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 SiO2, the second model provides a coarse-grained description of polymer liquids, i.e., typical `fragileʹ glassformers, while SiO2 is the prototype of a `strong glassformerʹ. 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 SiO2 using the potential of van Beest, Kramer and van Santen (`BKS potentialʹ) only temperatures T⩾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⩾1900 K are in agreement to within a few percent with experimental data, too. At very high temperatures (T⩾2750 K), even for SiO2 a temperature range is found where the mode coupling theory (MCT) of the glass transition is applicable (with Tc=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 (≪1 nm) described by MCT, while at larger length scales a crossover to Rouse-like dynamics occurs.