Dynamics of polymer melts above the glass transition: Monte Carlo studies of the bond fluctuation model

The bond fluctuation model on the simple cubic lattice with a bond-length dependent potential energy favoring long bonds exhibits a glassy freezing in as the temperature is lowered. many properties being qualitatively similar to experiment. The present paper studies the dynamical properties of the model (as they result from the ''random hopping'' algorithm), using configurations of undercooled polymer melts that have been carefully equilibrated by the ''slithering snake'' algorithm. In this way quantitatively reliable data can be obtained for distinctly lower temperatures than in the previous work on the dynamics of this model that used the random hopping algorithm for equilibration as well. If various mean-square displacements, the self-diffusion constant, autocorrelation functions of the end-to-end vector, intermediate incoherent scattering functions, and single chain dynamic structure factors are analyzed, it is found that the slowing down of relaxation times occurs according to the Vogel-Fulcher law. Within the uncertainties of the analysis the Vogel-Fulcher temperature of all these times is the same, 0.12 less than or equal to T-0 less than or equal to 0.13. Particularly illuminating is an analysis of the Rouse modes, which supports this conclusion and furthermore yields compelling evidence for the time-temperature superposition principle. It is shown that no static cross-correlations develop between different Rouse modes in the undercooled melt.