Simulation Studies of the Glass Transition of Polymers: Recent Progress

摘要

A comparative discussion of two coarse-grained models for flexible polymer chains in dense supercooled melts is presented. The first model is the bond-fluctuation model on a simple bubic lattice, where long bonds are energetically favored; this model is studied by Monte Carlo simulation. The second model is an off-lattice bead-spring model with Lennard-Jones interactions between the beads, which is studied by molecular dynamics methods (treating both constant volume and constant pressure ensembles). In both cases, glass formation is induced by an incompatibility between intra-and interchain interactions. Both models show a slowing down of the self-diffusion constant and of various relaxation times which is compatible with a (rounded) critical behavior, as it is described by the mode-coupling theory of the glass transition, but the simulation data can also be fitted to Vogel-Fulcher formulas. The Vogel-Fulcher temperature is considerably lower than the mode-coupling, critical temperature. Additionally, it is shown that both models exhibit Rouse-model-like behavior, if motions on length scales at least of the order of the distance between beads are considered. On smaller scales, model-dependent characteristics become dominant. Finally, we briefly discuss what consequences result from our simulations for a discussion of the validity of various theories of the glass transition.