57 research outputs found
Weak violation of universality for Polyelectrolyte Chains: Variational Theory and Simulations
A variational approach is considered to calculate the free energy and the
conformational properties of a polyelectrolyte chain in dimensions. We
consider in detail the case of pure Coulombic interactions between the
monomers, when screening is not present, in order to compute the end-to-end
distance and the asymptotic properties of the chain as a function of the
polymer chain length . We find where
and is the exponent which characterize
the long-range interaction . The exponent is
shown to be non-universal, depending on the strength of the Coulomb
interaction. We check our findings, by a direct numerical minimization of the
variational energy for chains of increasing size . The
electrostatic blob picture, expected for small enough values of the interaction
strength, is quantitatively described by the variational approach. We perform a
Monte Carlo simulation for chains of length . The non universal
behavior of the exponent previously derived within the variational
method, is also confirmed by the simulation results. Non-universal behavior is
found for a polyelectrolyte chain in dimension. Particular attention is
devoted to the homopolymer chain problem, when short range contact interactions
are present.Comment: to appear in European Phys. Journal E (soft matter
Dynamics of polymeric manifolds in melts: Hartree approximation
The Martin-Siggia-Rose functional technique and the self-consistent Hartree
approximation is applied to the dynamics of a D-dimensional manifold in a melt
of similar manifolds.The generalized Rouse equation is derived and its static
and dynamic properties are studied. The static upper critical dimension
discriminate between Gaussian and non-Gaussian regimes, whereas its dynamic
counterpart discriminates between Rouse- and renormalized-Rouse behavior. The
dynamic exponents are calculated explicitly. The special case of linear chains
shows agreement with MD- and MC-simulations.Comment: 4 pages,1 figures, accepted by EPJB as a Rapid Not
Dynamics of a polymer test chain in a glass forming matrix: The Hartree Approximation
In this paper the Martin-Siggia-Rose formalism is used to derive a
generalized Rouse equation for a test chain in a matrix which can undergo the
glass transition. It is shown that the surrounding matrix renormalizes the
static properties of the test chain. Furthermore the freezing of the different
Rouse modes is investigated. This yields freezing temperatures which depend
from the Rouse mode index.Comment: to be published in Journal de Physique I
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 -
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 of the
binding energies of - and -segments in the detachment of an
-copolymer from adhesive surface strongly changes the - profile
whereby the -spikes vanish when .
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
The Hartree approximation in dynamics of polymeric manifolds in the melt
The Martin-Siggia-Rose (MSR) functional integral technique is applied to the
dynamics of a D - dimensional manifold in a melt of similar manifolds. The
integration over the collective variables of the melt can be simply implemented
in the framework of the dynamical random phase approximation (RPA). The
resulting effective action functional of the test manifold is treated by making
use of the selfconsistent Hartree approximation. As an outcome the generalized
Rouse equation (GRE) of the test manifold is derived and its static and dynamic
properties are studied. It was found that the static upper critical dimension,
, discriminates between Gaussian (or screened) and
non-Gaussian regimes, whereas its dynamical counterpart, , distinguishes between the simple Rouse and the
renormalized Rouse behavior. We have argued that the Rouse mode correlation
function has a stretched exponential form. The subdiffusional exponents for
this regime are calculated explicitly. The special case of linear chains, D=1,
shows good agreement with MD- and MC-simulations.Comment: 35 pages,3 figures, accepted by J.Chem.Phy
Polymer chain scission at constant tension - an example of force-induced collective behaviour
The breakage of a polymer chain of segments, coupled by anharmonic bonds with
applied constant external tensile force is studied by means of Molecular
Dynamics simulation. We show that the mean life time of the chain becomes
progressively independent of the number of bonds as the pulling force grows.
The latter affects also the rupture rates of individual bonds along the polymer
backbone manifesting the essential role of inertial effects in the
fragmentation process. The role of local defects, temperature and friction in
the scission kinetics is also examined.Comment: 6 pages, 7 page
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) of a regular block copolymer of chain length
and block size which is dragged through a selective liquid-liquid
interface by an external field . The theory predicts a non-Arrhenian
vs. relationship which depends strongly on the size of the blocks, , and
rather weakly on the total polymer length, . The overall behavior is
strongly influenced by the degree of selectivity between the two solvents
.
The variation of with and 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 - 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 with respect to
block length 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
Pulling an adsorbed polymer chain off a solid surface
The thermally assisted detachment of a self-avoiding polymer chain from an
adhesive surface by an external force applied to one of the chain ends is
investigated. We perform our study in the "fixed height" statistical ensemble
where one measures the fluctuating force, exerted by the chain on the last
monomer when a chain end is kept fixed at height over the solid plane at
different adsorption strength . The phase diagram in the plane is calculated both analytically and by Monte Carlo simulations.
We demonstrate that in the vicinity of the polymer desorption transition a
number of properties like fluctuations and probability distribution of various
quantities behave differently, if rather than is used as an independent
control parameter.Comment: 17 pages, 9 picture
Thermal Breakage and Self-Healing of a Polymer Chain under Tensile Stress
We consider the thermal breakage of a tethered polymer chain of discrete
segments coupled by Morse potentials under constant tensile stress. The chain
dynamics at the onset of fracture is studied analytically by Kramers-Langer
multidimensional theory and by extensive Molecular Dynamics simulations in 1D-
and 3D-space. Comparison with simulation data in one- and three dimensions
demonstrates that the Kramers-Langer theory provides good qualitative
description of the process of bond-scission as caused by a {\em collective}
unstable mode. We derive distributions of the probability for scission over the
successive bonds along the chain which reveal the influence of chain ends on
rupture in good agreement with theory. The breakage time distribution of an
individual bond is found to follow an exponential law as predicted by theory.
Special attention is focused on the recombination (self-healing) of broken
bonds. Theoretically derived expressions for the recombination time and
distance distributions comply with MD observations and indicate that the energy
barrier position crossing is not a good criterion for true rupture. It is shown
that the fraction of self-healing bonds increases with rising temperature and
friction.Comment: 25 pages, 13 picture
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