528 research outputs found

### Monte Carlo simulations of infinitely dilute solutions of amphiphilic diblock star copolymers

Single-chain Monte Carlo simulations of amphiphilic diblock star copolymers
were carried out in continuous space using implicit solvents. Two distinct
architectures were studied: stars with the hydrophobic blocks attached to the
core, and stars with the polar blocks attached to the core, with all arms being
of equal length. The ratio of the lengths of the hydrophobic block to the
length of the polar block was varied from 0 to 1. Stars with 3, 6, 9 or 12
arms, each of length 10, 15, 25, 50, 75 and 100 Kuhn segments were analysed.
Four distinct types of conformations were observed for these systems. These,
apart from studying the snapshots from the simulations, have been
quantitatively characterised in terms of the mean-squared radii of gyration,
mean-squared distances of monomers from the centre-of-mass, asphericity
indices, static scattering form factors in the Kratky representation as well as
the intra-chain monomer-monomer radial distribution functions.Comment: 12 pages, 11 ps figures. Accepted for publication in J. Chem. Phy

### Stresses in Smooth Flows of Dense Granular Media

The form of the stress tensor is investigated in smooth, dense granular flows
which are generated in split-bottom shear geometries. We find that, within a
fluctuation fluidized spatial region, the form of the stress tensor is directly
dictated by the flow field: The stress and strain-rate tensors are co-linear.
The effective friction, defined as the ratio between shear and normal stresses
acting on a shearing plane, is found not to be constant but to vary throughout
the flowing zone. This variation can not be explained by inertial effects, but
appears to be set by the local geometry of the flow field. This is in agreement
with a recent prediction, but in contrast with most models for slow grain
flows, and points to there being a subtle mechanism that selects the flow
profiles.Comment: 5 pages, 4 figure

### Are better conducting molecules more rigid?

We investigate the electronic origin of the bending stiffness of conducting
molecules. It is found that the bending stiffness associated with electronic
motion, which we refer to as electro-stiffness, $\kappa_{e}$, is governed by
the molecular orbital overlap $t$ and the gap width $u$ between HOMO and LUMO
levels, and behaves as $\kappa_{e}\sim t^{2}/\sqrt{u^2+t^{2}}$. To study the
effect of doping, we analyze the electron filling-fraction dependence on
$\kappa_{e}$ and show that doped molecules are more flexible. In addition, to
estimate the contribution of $\kappa_{e}$ to the total stiffness, we consider
molecules under a voltage bias, and study the length contraction ratio as a
function of the voltage. The molecules are shown to be contracted or dilated,
with $\kappa_{e}$ increasing nonlinearly with the applied bias

### Velocity Correlations in Dense Gravity Driven Granular Chute Flow

We report numerical results for velocity correlations in dense,
gravity-driven granular flow down an inclined plane. For the grains on the
surface layer, our results are consistent with experimental measurements
reported by Pouliquen. We show that the correlation structure within planes
parallel to the surface persists in the bulk. The two-point velocity
correlation function exhibits exponential decay for small to intermediate
values of the separation between spheres. The correlation lengths identified by
exponential fits to the data show nontrivial dependence on the averaging time
\dt used to determine grain velocities. We discuss the correlation length
dependence on averaging time, incline angle, pile height, depth of the layer,
system size and grain stiffness, and relate the results to other length scales
associated with the rheology of the system. We find that correlation lengths
are typically quite small, of the order of a particle diameter, and increase
approximately logarithmically with a minimum pile height for which flow is
possible, \hstop, contrary to the theoretical expectation of a proportional
relationship between the two length scales.Comment: 21 pages, 16 figure

### Cyclic motion and inversion of surface flow direction in a dense polymer brush under shear

Using molecular simulations, we study the properties of a polymer brush in
contact with an explicit solvent under Couette and Poiseuille flow. The solvent
is comprised of chemically identical chains. We present evidence that
individual, unentangled chains in the dense brush exhibit cyclic, tumbling
motion and non-Gaussian fluctuations of the molecular orientations similar to
the behaviour of isolated tethered chains in shear flow. The collective
molecular motion gives rise to an inversion of hydrodynamic flow direction in
the vicinity of the brush-coated surface. Utilising Couette and Poiseuille
flow, we investigate to what extend the effect of a brush-coated surface can be
described by a Navier slip condition.Comment: 6 pages, 6 figures, submitted for publicatio

### Structure of bottle-brush brushes under good solvent conditions. A molecular dynamics study

We report a simulation study for bottle-brush polymers grafted on a rigid
backbone. Using a standard coarse-grained bead-spring model extensive molecular
dynamics simulations for such macromolecules under good solvent conditions are
performed. We consider a broad range of parameters and present numerical
results for the monomer density profile, density of the untethered ends of the
grafted flexible backbones and the correlation function describing the range
that neighboring grafted bottle-brushes are affected by the presence of the
others due to the excluded volume interactions. The end beads of the flexible
backbones of the grafted bottle-brushes do not access the region close to the
rigid backbone due to the presence of the side chains of the grafted
bottle-brush polymers, which stretch further the chains in the radial
directions. Although a number of different correlation lengths exist as a
result of the complex structure of these macromolecules, their properties can
be tuned with high accuracy in good solvents. Moreover, qualitative differences
with "typical" bottle-brushes are discussed. Our results provide a first
approach to characterizing such complex macromolecules with a standard bead
spring model.Comment: To appear in Journal of Physics Condensed Matter (2011

### Effective interactions between star polymers and colloidal particles

Using monomer-resolved Molecular Dynamics simulations and theoretical
arguments based on the radial dependence of the osmotic pressure in the
interior of a star, we systematically investigate the effective interactions
between hard, colloidal particles and star polymers in a good solvent. The
relevant parameters are the size ratio q between the stars and the colloids, as
well as the number of polymeric arms f (functionality) attached to the common
center of the star. By covering a wide range of q's ranging from zero (star
against a flat wall) up to about 0.75, we establish analytical forms for the
star-colloid interaction which are in excellent agreement with simulation
results. A modified expression for the star-star interaction for low
functionalities, f < 10 is also introduced.Comment: 37 pages, 14 figures, preprint-versio

### Scaling of Selfavoiding Tethered Membranes: 2-Loop Renormalization Group Results

The scaling properties of selfavoiding polymerized membranes are studied
using renormalization group methods. The scaling exponent \nu is calculated for
the first time at two loop order. \nu is found to agree with the Gaussian
variational estimate for large space dimension d and to be close to the Flory
estimate for d=3.Comment: 4 pages, RevTeX + 20 .eps file

### Static and dynamic properties of the interface between a polymer brush and a melt of identical chains

Molecular dynamics simulations of a short-chain polymer melt between two
brush-covered surfaces under shear have been performed. The end-grafted
polymers which constitute the brush have the same chemical properties as the
free chains in the melt and provide a soft deformable substrate. Polymer chains
are described by a coarse-grained bead-spring model with Lennard-Jones
interactions between the beads and a FENE potential between nearest neighbors
along the backbone of the chains. The grafting density of the brush layer
offers a way of controlling the behavior of the surface without altering the
molecular interactions. We perform equilibrium and non-equilibrium Molecular
Dynamics simulations at constant temperature and volume using the Dissipative
Particle Dynamics thermostat. The equilibrium density profiles and the behavior
under shear are studied as well as the interdigitation of the melt into the
brush, the orientation on different length scales (bond vectors, radius of
gyration, and end-to-end vector) of free and grafted chains, and velocity
profiles. The viscosity and slippage at the interface are calculated as
functions of grafting density and shear velocity.Comment: 12 pages, submitted to J Chem Phy

### What is the Entanglement Length in a Polymer Melt ?

We present results of molecular dynamics simulations of very long model
polymer chains analyzed by various experimentally relevant techniques. The
segment motion of the chains is found to be in very good agreement with the
repatation model. We also calculated the plateau-modulus G_N. The predicitions
of the entanglement length N_e from G_N and from the mean square displacements
of the chains segments disagree by a factor of about 2.2(2), indicating an
error in the prefactor in the standard formula for G_N. We show that recent
neutron spin echo measurements were carried out for chain lengths which are too
small for a correct determination of N_e.Comment: 5 pages, 4 figures, RevTe

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