Thomas B. Schrøder

Effects of a Nanoscopic Filler on the Structure and Dynamics of a Simulated Polymer Melt and the Relationship to Ultrathin Films

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.

Crossover to potential energy landscape dominated dynamics in a model glass-forming liquid

An equilibrated model glass-forming liquid is studied by mapping successive configurations produced by molecular dynamics simulation onto a time series of inherent structures (local minima in the potential energy). Using this “inherent dynamics” approach we find direct numerical evidence for the long held view that below a crossover temperature, Tx, the liquid’s dynamics can be separated into (i) vibrations around inherent structures and (ii) transitions between inherent structures [M. Goldstein, J. Chem. Phys. 51, 3728 (1969)], i.e., the dynamics become “dominated” by the potential energy landscape. In agreement with previous proposals, we find that Tx is within the vicinity of the mode-coupling critical temperature Tc. We further find that near Tx, transitions between inherent structures occur via cooperative, stringlike rearrangements of groups of particles moving distances substantially smaller than the average interparticle distance.

Time-Dependent, Four-Point Density Correlation Function Description of Dynamical Heterogeneity and Decoupling in Supercooled Liquids

by neighboring particles. We evaluate these quantities for a cold Lennard-Jones ~LJ! liquid, and show that in this system x 4(t) is dominated by growing spatial correlations between localized particles. We then compare the behavior of x 4(t) with a generalized time-dependent susceptibility related to a correlation function of squared particle displacements. From these two quantities we find twodifferent characteristic time scales: the time scale on which temporarily localized particles are most spatially correlated scales with temperature like the structural relaxation time, while the time scale on which the correlation between squared particle displacements is strongest scales like the inverse diffusion coefficient. In this way, we demonstrate that the decoupling of diffusion and relaxation in this model liquid arises from dynamical heterogeneity. Consider a liquid of N particles in a volume V, with density r(r,t)5S i51 N d(r2ri(t)). The simplest density correlation function that contains information on correlated particle motion is fourth-order. We write this function in terms of the deviations of r(r,t) from its average value, Dr(r,t) 5r(r,t)2r 0 , where r 05^r&5N/V, and ^fl& denotes an ensemble average:

Growing correlation length on cooling below the onset of caging in a simulated glass-forming liquid

We present a calculation of a fourth-order, time-dependent density correlation function that measures higher-order spatiotemporal correlations of the density of a liquid. From molecular dynamics simulations of a glass-forming Lennard-Jones liquid, we find that the characteristic length scale of this function has a maximum as a function of time which increases steadily beyond the characteristic length of the static pair correlation function g(r) in the temperature range approaching the mode coupling temperature from above. This length scale provides a measure of the spatially heterogeneous nature of the dynamics of the liquid in the alpha-relaxation regime.

POTENTIAL ENERGY LANDSCAPE SIGNATURES OF SLOW DYNAMICS IN GLASS FORMING LIQUIDS

We study the properties of local potential energy minima (‘inherent structures’) sampled by liquids at low temperatures as an approach to elucidating the mechanisms of the observed dynamical slowing down observed as the glass transition temperature is approached. This onset of slow dynamics is accompanied by the sampling of progressively deeper potential energy minima. Further, evidence is found in support of a qualitative change in the inherent structures sampled in a temperature range that includes the mode coupling critical temperature Tc, such that a separation of vibrational relaxation within inherent structure basins from that due to inter-basin transitions becomes valid at temperatures T<Tc. Average inherent structure energies do not show any qualitatively significant system size dependence.