5,409 research outputs found
Logarithmic Relaxation in a Kinetically Constrained Model
We present Monte Carlo simulations in a modification of the
north-or-east-or-front model recently investigated by Berthier and Garrahan [J.
Phys. Chem. B 109, 3578 (2005)]. In this coarse-grained model for relaxation in
supercooled liquids, the liquid structure is substituted by a three-dimensional
array of cells. A spin variable is assigned to each cell, with values 0 or 1
denoting respectively unexcited and excited local states in a mobility field.
Change in local mobility (spin flip) for a given cell is permitted according to
kinetic constraints determined by the mobilities of neighboring cells. In this
work we keep the same kinetic constraints of the original model, but we
introduce two types of cells (denoted as "fast'' and "slow'') with very
different rates for spin flip. As a consequence, fast and slow cells exhibit
very different relaxation times for spin correlators. While slow cells exhibit
standard relaxation, fast cells display anomalous relaxation, characterized by
a concave-to-convex crossover in spin correlators by changing temperature or
composition. At intermediate state points logarithmic relaxation is observed
over three time decades. These results display striking analogies with dynamic
correlators reported in recent simulations on a bead-spring model for polymer
blends.Comment: Major changes. To be published in Journal of Chemical Physic
Static and dynamic contributions to anomalous chain dynamics in polymer blends
By means of computer simulations, we investigate the relaxation of the Rouse
modes in a simple bead-spring model for non-entangled polymer blends. Two
different models are used for the fast component, namely fully-flexible and
semiflexible chains. The latter are semiflexible in the meaning that static
intrachain correlations are strongly non-gaussian at all length scales. The
dynamic asymmetry in the blend is strongly enhanced by decreasing temperature,
inducing confinement effects on the fast component. The dynamics of the Rouse
modes show very different trends for the two models of the fast component. For
the fully-flexible case, the relaxation times exhibit a progressive deviation
from Rouse scaling on increasing the dynamic asymmetry. This anomalous effect
has a dynamic origin. It is not related to particular static features of the
Rouse modes, which indeed are identical to those of the fully-flexible
homopolymer, and are not modified by the dynamic asymmetry in the blend. On the
contrary, in the semiflexible case the relaxation times exhibit approximately
the same scaling behaviour as the amplitudes of the modes. This suggests that
the origin of the anomalous dynamic scaling for semiflexible chains confined in
the blend is esentially of static nature. We discuss implications of these
observations for the applicability of theoretical approaches to chain dynamics
in polymer blends.Comment: 15 pages (single-column), 6 figure
Dynamic Arrest in Polymer Melts: Competition between Packing and Intramolecular Barriers
We present molecular dynamics simulations of a simple model for polymer melts
with intramolecular barriers. We investigate structural relaxation as a
function of the barrier strength. Dynamic correlators can be consistently
analyzed within the framework of the Mode Coupling Theory (MCT) of the glass
transition. Control parameters are tuned in order to induce a competition
between general packing effects and polymer-specific intramolecular barriers as
mechanisms for dynamic arrest. This competition yields unusually large values
of the so-called MCT exponent parameter and rationalize qualitatively different
observations for simple bead-spring and realistic polymers. The systematic
study of the effect of intramolecular barriers presented here also establishes
a fundamental difference between the nature of the glass transition in polymers
and in simple glass-formers.Comment: 4 pages, 3 figures, 2 table
Relaxation Scenarios in a Mixture of Large and Small Spheres: Dependence on the Size Disparity
We present a computational investigation on the slow dynamics of a mixture of
large and small soft spheres. By varying the size disparity at a moderate fixed
composition different relaxation scenarios are observed for the small
particles. For small disparity density-density correlators exhibit moderate
stretching. Only small quantitative differences are observed between dynamic
features for large and small particles. On the contrary, large disparity
induces a clear time scale separation between the large and the small
particles. Density-density correlators for the small particles become extremely
stretched, and display logarithmic relaxation by properly tuning the
temperature or the wavevector. Self-correlators decay much faster than
density-density correlators. For very large size disparity, a complete
separation between self- and collective dynamics is observed for the small
particles. Self-correlators decay to zero at temperatures where density-density
correlations are frozen. The dynamic picture obtained by varying the size
disparity resembles features associated to Mode Coupling transition lines of
the types B and A at, respectively, small and very large size disparity. Both
lines might merge, at some intermediate disparity, at a higher-order point, to
which logarithmic relaxation would be associated. This picture resembles
predictions of a recent Mode Coupling Theory for fluids confined in matrixes
with interconnected voids [V. Krakoviack, Phys. Rev. Lett. {\bf 94}, 065703
(2005)].Comment: Journal of Chemical Physics 125, 164507 (2006
Anomalous Dynamic Arrest in a Mixture of Big and Small Particles
We present molecular dynamics simulations on the slow dynamics of a mixture
of big and small soft-spheres with a large size disparity. Dynamics are
investigated in a broad range of temperature and mixture composition. As a
consequence of large size disparity, big and small particles exhibit very
different relaxation times. As previously reported for simple models of
short-ranged attractive colloids and polymer blends, several anomalous dynamic
features are observed: i) sublinear behavior for mean squared displacements,
ii) concave-to-convex crossover for density-density correlators, by varying
temperature or wavevector, iii) logarithmic decay for specific wavevectors of
density-density correlators. These anomalous features are observed over time
intervals extending up to four decades, and strongly resemble predictions of
the Mode Coupling Theory (MCT) for state points close to higher-order MCT
transitions, which originate from the competition between different mechanisms
for dynamic arrest. For the big particles we suggest competition between
soft-sphere repulsion and depletion effects induced by neighboring small
particles. For the small particles we suggest competition between bulk-like
dynamics and confinement, respectively induced by neighboring small particles
and by the slow matrix of big particles. By increasing the size disparity, a
new relaxation scenario arises for the small particles. Self-correlators decay
to zero at temperatures where density-density correlations are frozen. The
behavior of the latters resembles features characteristic of type-A MCT
transitions, defined by a zero value of the critical non-ergodicity parameter.Comment: Version 2. Added major new result
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