Spatially correlated dynamics in a simulated glass-forming polymer melt: Analysis of clustering phenomena - art. no. 051503

In recent years. experimental and computational studies have demonstrated t hat the dynamics of glass-forming liquids are spatially heterogeneous, exhi biting regions of temporarily enhanced or diminished mobility. Here we pres ent a detailed analysis of dynamical heterogeneity in a simulated "bead-spr ing" model of a low-molecular-weight polymer melt. We investigate the trans ient nature and size distribution of clusters of "mobile" chain segments (m onomers) as the polymer melt is cooled toward its glass transition. We also explore the dependence of this clustering on the way in which the mobile s ubset is defined, We show that the mean cluster size is time dependent with a peak at intermediate time, and that the mean cluster size at the peak ti me.-rows with decreasing temperature T. We show that for each T a particula r fraction of particles maximizes the mean cluster size at some characteris tic time, and this fraction depends on T, The growing size of the clusters demonstrates the growing range of correlated motion, previously reported fo r this same system [C. Beneman et al. Nature (London) 399, 246 (1999)]. The distribution of cluster sizes approaches a power law near the mode-couplin g temperature, similar to behavior reported for a simulated binary mixture and a dense colloidal suspension, but with a different exponent, We calcula te the correlation length of the clusters, and show that it exhibits simila r temperature- and time-dependent behavior as the mean cluster size, with a maximum at intermediate time. We show that the characteristic time of the maximum cluster size follows the scaling predicted by mode-coupling theory (MCT) for the beta time scale, revealing a possible connection between spat ially heterogeneous dynamics and MCT.