Molecular-dynamics simulation of a glassy polymer melt: Rouse model and cage effect

We report results of molecular-dynamics simulations for a glassy polymer melt consisting of short, linear bead-spring chains. The model does not crystallize upon cooling, but exhibits a glassy slowing down. The onset of this slowing down is brought about by the dense packing in the melt. It was shown in an earlier work that this onset is compatible with the predictions of the mode coupling theory of the glass transition. The physical process of "caging" of a monomer by its spatial neighbors leads to a distinct two step behavior in scattering functions and particle mean square displacements. In this work, we analyze the effects of this caging process on the Rouse description of polymer melt dynamics. The Rouse model is known, both from experimental and simulational work, to be a reasonable description of the dynamics of short chains in the melt. We show that the Rouse description is applicable for length and time scales above the typical scales for the caging process, and that the typical time scale of the Rouse model reflects the onset of freezing as described by mode coupling theory. The Rouse modes are eigenmodes of the chains in the supercooled state, and the relaxation times of the modes exhibit the same temperature dependence as the diffusion coefficient of the chains. The decay of the mode correlation functions is stretched and depends on the mode index. Therefore, there is no time-mode superposition of the correlation functions. However, they exhibit a time-temperature superposition at late times. At intermediate times, they decay in two steps for temperatures close to the dynamical critical temperature of mode coupling theory. The monomer displacement is compared with simulation results for a binary LJ-mixture to illustrate the differences which are introduced by the connectivity of the particles. (C) 1999 Elsevier Science Ltd. All rights reserved.