415 research outputs found

### The Electrostatic Persistence Length of Polymers beyond the OSF Limit

We use large scale Monte Carlo simulations to test scaling theories for the
electrostatic persistence length $l_e$ of isolated, uniformly charged polymers
with \DH intrachain interactions in the limit where the screening length
$\kappa^{-1}$ exceeds the intrinsic persistence length of the chains. Our
simulations cover a significantly larger part of the parameter space than
previous studies. We observe no significant deviations from the prediction
$l_e\propto\kappa^{-2}$ by Khokhlov and Khachaturian which is based on applying
the Odijk-Skolnick-Fixman theory to the stretched de
Gennes-Pincus-Velasco-Brochard polyelectrolyte blob chain. A linear or
sublinear dependence of the persistence length on the screening length can be
ruled out. We argue that previous numerical results pointing into this
direction are probably due to a combination of excluded volume and finite chain
length effects. The paper emphasizes the role of scaling arguments in the
development of useful representations for experimental and simulation data.Comment: 11 pages, 7 figure

### Dynamic Compression of in situ Grown Living Polymer Brush: Simulation and Experiment

A comparative dynamic Monte Carlo simulation study of polydisperse living
polymer brushes, created by surface initiated living polymerization, and
conventional polymer monodisperse brush, comprising linear polymer chains,
grafted to a planar substrate under good solvent conditions, is presented. The
living brush is created by end-monomer (de)polymerization reaction after
placing an array of initiators on a grafting plane in contact with a solution
of initially non-bonded segments (monomers). At equilibrium, the monomer
density profile \phi(z) of the LPB is found to decline as \phi(z) ~ z^{-\alpha}
with the distance from the grafting plane z, while the distribution of chain
lengths in the brush scales as c(N) ~ N^{-\tau}. The measured values \alpha =
0.64 and \tau = 1.70 are very close to those, predicted within the framework of
the Diffusion-Limited Aggregation theory, \alpha = 2/3 and \tau = 7/4. At
varying mean degree of polymerization (from L = 28 to L = 170) and effective
grafting density (from \sigma_g = 0.0625 to \sigma_g = 1.0), we observe a
nearly perfect agreement in the force-distance behavior of the simulated LPB
with own experimental data obtained from colloidal probe AFM analysis on
PNIPAAm brush and with data obtained by Plunkett et. al., [Langmuir 2006, 22,
4259] from SFA measurements on same polymer

### Detachment of semiflexible polymer chains from a substrate - a Molecular Dynamics investigation

Using Molecular Dynamics simulations, we study the force-induced detachment
of a coarse-grained model polymer chain from an adhesive substrate. One of the
chain ends is thereby pulled at constant speed off the attractive substrate and
the resulting saw-tooth profile of the measured mean force vs height
$D$ of the end-segment over the plane is analyzed for a broad variety of
parameters. It is shown that the observed characteristic oscillations in the $<
f >$-$D$ profile depend on the bending and not on the torsional stiffness of
the detached chains. Allowing for the presence of hydrodynamic interactions
(HI) in a setup with explicit solvent and DPD-thermostat, rather than the case
of Langevin thermostat, one finds that HI have little effect on the -$D$
profile. Also the change of substrate affinity with respect to the solvent from
solvophilic to solvophobic is found to play negligible role in the desorption
process. In contrast, a changing ratio $\epsilon_s^A / \epsilon_s^B$ of the
binding energies of $A$- and $B$-segments in the detachment of an
$AB$-copolymer from adhesive surface strongly changes the $$-$D$ profile
whereby the $B$-spikes vanish when $\epsilon_s^A / \epsilon_s^B < 0.15$.
Eventually, performing an atomistic simulation of a (bio)-polymer {\it
polyglycine}, we demonstrate that the simulation results, derived from our
coarse-grained model, comply favorably with those from the all-atom simulation.Comment: Latex, 12 pages, 8 figures, to appear in JC

### Star Polymers Confined in a Nanoslit: A Simulation Test of Scaling and Self-Consistent Field Theories

The free energy cost of confining a star polymer where $f$ flexible polymer
chains containing $N$ monomeric units are tethered to a central unit in a slit
with two parallel repulsive walls a distance $D$ apart is considered, for good
solvent conditions. Also the parallel and perpendicular components of the
gyration radius of the star polymer, and the monomer density profile across the
slit are obtained. Theoretical descriptions via Flory theory and scaling
treatments are outlined, and compared to numerical self-consistent field
calculations (applying the Scheutjens-Fleer lattice theory) and to Molecular
Dynamics results for a bead-spring model. It is shown that Flory theory and
self-consistent field (SCF) theory yield the correct scaling of the parallel
linear dimension of the star with $N$, $f$ and $D$, but cannot be used for
estimating the free energy cost reliably. We demonstrate that the same problem
occurs already for the confinement of chains in cylindrical tubes. We also
briefly discuss the problem of a free or grafted star polymer interacting with
a single wall, and show that the dependence of confining force on the
functionality of the star is different for a star confined in a nanoslit and a
star interacting with a single wall, which is due to the absence of a symmetry
plane in the latter case.Comment: 15 pages, 9 figures, LaTeX, to appear in Soft Matte

### Kinetics of copolymer localization at a selective liquid-liquid interface

The localization kinetics of a regular block-copolymer of total length $N$
and block size $M$ at a selective liquid-liquid interface is studied in the
limit of strong segregation between hydrophobic and polar segments in the
chain. We propose a simple analytic theory based on scaling arguments which
describes the relaxation of the initial coil into a flat-shaped layer for the
cases of both Rouse and Zimm dynamics. For Rouse dynamics the characteristic
times for attaining equilibrium values of the gyration radius components
perpendicular and parallel to the interface are predicted to scale with block
length $M$ and chain length $N$ as $\tau_{\perp} \propto M^{1+2\nu}$ (here
$\nu\approx 0.6$ is the Flory exponent) and as $\tau_{\parallel} \propto N^2$,
although initially the characteristic coil flattening time is predicted to
scale with block size as $\propto M$. Since typically $N\gg M$ for multiblock
copolymers, our results suggest that the flattening dynamics proceeds faster
perpendicular rather than parallel to the interface, in contrast to the case of
Zimm dynamics where the two components relax with comparable rate, and proceed
considerably slower than in the Rouse case.
We also demonstrate that, in the case of Rouse dynamics, these scaling
predictions agree well with the results of Monte Carlo simulations of the
localization dynamics. A comparison to the localization dynamics of {\em
random} copolymers is also carried out.Comment: 11 pages, 15 figure

### Field - Driven Translocation of Regular Block Copolymers through a Selective Liquid - Liquid Interface

We propose a simple scaling theory describing the variation of the mean first
passage time (MFPT) $\tau(N,M)$ of a regular block copolymer of chain length
$N$ and block size $M$ which is dragged through a selective liquid-liquid
interface by an external field $B$. The theory predicts a non-Arrhenian $\tau$
vs. $B$ relationship which depends strongly on the size of the blocks, $M$, and
rather weakly on the total polymer length, $N$. The overall behavior is
strongly influenced by the degree of selectivity between the two solvents
$\chi$.
The variation of $\tau(N,M)$ with $N$ and $M$ in the regimes of weak and
strong selectivity of the interface is also studied by means of computer
simulations using a dynamic Monte Carlo coarse-grained model. Good qualitative
agreement with theoretical predictions is found. The MFPT distribution is found
to be well described by a $\Gamma$ - distribution. Transition dynamics of ring-
and telechelic polymers is also examined and compared to that of the linear
chains.
The strong sensitivity of the ``capture'' time $\tau(N,M)$ with respect to
block length $M$ suggests a possible application as a new type of
chromatography designed to separate and purify complex mixtures with different
block sizes of the individual macromolecules.Comment: 20 pages, 10 figure

### Controlling the Interactions between Soft Colloids via Surface Adsorption

By employing monomer-resolved computer simulations and analytical
considerations based on polymer scaling theory, we analyze the conformations
and interactions of multiarm star polymers strongly adsorbed on a smooth,
two-dimensional plane. We find a stronger stretching of the arms as well as a
stronger repulsive, effective interaction than in the three dimensional case.
In particular, the star size scales with the number of arms $f$ as $\sim
f^{1/4}$ and the effective interaction as $\sim f^{2}$, as opposed to $\sim
f^{1/5}$ and $\sim f^{3/2}$, respectively, in three dimensions. Our results
demonstrate the dramatic effect that geometric confinement can have on the
effective interactions and the subsequent correlations of soft colloids in
general, for which the conformation can be altered as a result of geometrical
constraints imposed on them.Comment: 17 pages, LaTeX, 5 figures, to appear in Macromolecule

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