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2001
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Gebremichael Y, Schroder TB, Starr FW, Glotzer SC
Spatially correlated dynamics in a simulated glass-forming polymer melt: Analysis of clustering phenomena
PHYSICAL REVIEW E 64 (5): Art. No. 051503 Part 1, NOV 2001
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View Abstract
In recent years. experimental and computational studies have demonstrated that the dynamics of glass-forming liquids are spatially heterogeneous, exhibiting regions of temporarily enhanced or diminished mobility. Here we present a detailed analysis of dynamical heterogeneity in a simulated "bead-spring" model of a low-molecular-weight polymer melt. We investigate the transient nature and size distribution of clusters of "mobile" chain segments (monomers) 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 subset 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 time.-rows with decreasing temperature T. We show that for each T a particular fraction of particles maximizes the mean cluster size at some characteristic time, and this fraction depends on T, The growing size of the clusters demonstrates the growing range of correlated motion, previously reported for this same system [C. Beneman et al. Nature (London) 399, 246 (1999)]. The distribution of cluster sizes approaches a power law near the mode-coupling temperature, similar to behavior reported for a simulated binary mixture and a dense colloidal suspension, but with a different exponent, We calculate the correlation length of the clusters, and show that it exhibits similar 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 spatially heterogeneous dynamics and MCT.
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Starr FW, Schroder TB, Glotzer SC
Effects of a nanoscopic filler on the structure and dynamics of a simulated polymer melt and the relationship to ultrathin films
PHYSICAL REVIEW E 64 (2): Art. No. 021802 Part 1, AUG 2001
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View Abstract
We perform molecular dynamics simulations of an idealized polymer melt surrounding a nanoscopic filler particle. We show that the glass transition temperature T-g of the melt can be shifted to either higher or lower temperatures by tuning the interactions between polymer and filler. A gradual change of the polymer dynamics approaching the filler surface causes the change in the glass transition. We also find that polymers close to the surface tend to be elongated and flattened. Our findings show a strong similarity to those obtained for ultrathin polymer films.
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Glotzer SC, Warren JA
Computational materials science and industrial R=D: Accelerating progress
COMPUTING IN SCIENCE = ENGINEERING 3 (1): 67-71 JAN-FEB 2001
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