Glass transition of polymer melts: test of theoretical concepts by computer simulation

Polymers are good glass formers and allow for the study of melts near the glass transition in (meta-)stable equilibrium. Theories of the glass transition imply such an equilibrium and can, hence, be tested by the study of polymer melts. After a brief summary of the basic experimental facts about the glass transition in polymers, the main theoretical concepts are reviewed: mode coupling theory (MCT), entropy theory, free-volume theory, the idea of a growing length describing the size of cooperative regions, etc. Then, two basic coarse-grained models of polymers are described, which have been developed aiming at a test of these concepts. The first model is the bond-fluctuation model on the simple cubic lattice; the second is a bead-spring model in the continuum. While the first model is studied by kinetic Monte Carlo methods, the second is studied by molecular dynamics simulation: the issue is addressed which aspects of the results are model-dependent and which aspects are generic and should universally apply, including real materials. Attempts to include chemical detail in order to describe real materials are discussed, too. It is shown that idealized MCT is a good description for the onset of slow relaxation ('cage effect') in the discussed model polymer melts, but the singularities at the critical temperature of MCT are rounded, and the correct description of the dynamics close to the calorimetric glass transition remains controversial. While the configurational entropy of the polymer melt does decrease strongly when the glass transition is approached, evidence is presented that the 'entropy catastrophe' of the theory of Gibbs and Di Marzio is an artifact of inaccurate approximations. Simulations of polymer melts confined in a thin film geometry are also reviewed, emphasizing the question to which extent they shed light on the issue of a growing glass correlation length. Finally, we discuss the implications of the glass transition on the motion of polymers at larger length scales. It is shown that for short (non-entangled) chains the Rouse model stays essentially valid, the friction coefficient reflecting the slowing down as described by the alpha-relaxation, while for small-scale motions (e.g. beta-relaxation regime of MCT) the connectivity of the polymer is not very relevant. We conclude that the theories describe some aspects of vitrification phenomena correctly, but a unified description that puts all these phenomena into one coherent framework, does not yet exist. (C) 2002 Elsevier Science Ltd. All rights reserved.