4,362 research outputs found
Thermally activated breakdown in a simple polymer model
We consider the thermally activated fragmentation of a homopolymer chain. In
our simple model the dynamics of the intact chain is a Rouse one until a bond
breaks and bond breakdown is considered as a first passage problem over a
barrier to an absorbing boundary. Using the framework of the Wilemski-Fixman
approximation we calculate activation times of individual bonds for free and
grafted chains. We show that these times crucially depend on the length of the
chain and the location of the bond yielding a minimum at the free chain ends.
Theoretical findings are qualitatively confirmed by Brownian dynamics
simulations
A practical density functional for polydisperse polymers
The Flory Huggins equation of state for monodisperse polymers can be turned
into a density functional by adding a square gradient term, with a coefficient
fixed by appeal to RPA (random phase approximation). We present instead a model
nonlocal functional in which each polymer is replaced by a deterministic,
penetrable particle of known shape. This reproduces the RPA and square gradient
theories in the small deviation and/or weak gradient limits, and can readily be
extended to polydisperse chains. The utility of the new functional is shown for
the case of a polydisperse polymer solution at coexistence in a poor solvent.Comment: 9 pages, 3 figure
Classification of graph C*-algebras with no more than four primitive ideals
We describe the status quo of the classification problem of graph C*-algebras
with four primitive ideals or less
Slow plasmon modes in polymeric salt solutions
The dynamics of polymeric salt solutions are presented. The salt consists of
chains and , which are chemically different and interact with a
Flory-interaction parameter , the chain ends carry a positive
charge whereas the chain ends are modified by negative charges. The
static structure factor shows a peak corresponding to a micro phase separation.
At low momentum transfer, the interdiffusion mode is driven by electrostatics
and is of the plasmon-type, but with an unusually low frequency, easily
accessible by experiments. This is due to the polymer connectivity that
introduces high friction and amplifies the charge scattering thus allowing for
low charge densities. The interdiffusion mode shows a minimum (critical slowing
down) at finite when the interaction parameter increases we find then a low
frequency quasi-plateau.Comment: accepted in Europhys. Let
Analog of Astrophysical Magnetorotational Instability in a Couette-Taylor Flow of Polymer Fluids
We report experimental observation of an instability in a Couette-Taylor flow
of a polymer fluid in a thin gap between two coaxially rotating cylinders in a
regime where their angular velocity decreases with the radius while the
specific angular momentum increases with the radius. In the considered regime,
neither the inertial Rayleigh instability nor the purely elastic instability
are possible. We propose that the observed "elasto-rotational" instability is
an analog of the magnetorotational instability which plays a fundamental role
in astrophysical Keplerian accretion disks.Comment: 4 pages, 1 figur
Accurate statistics of a flexible polymer chain in shear flow
We present exact and analytically accurate results for the problem of a
flexible polymer chain in shear flow. Under such a flow the polymer tumbles,
and the probability distribution of the tumbling times of the polymer
decays exponentially as (where is the
longest relaxation time). We show that for a Rouse chain, this nontrivial
constant can be calculated in the limit of large Weissenberg number
(high shear rate) and is in excellent agreement with our simulation result of
. We also derive exactly the distribution functions for
the length and the orientational angles of the end-to-end vector of the
polymer.Comment: 4 pages, 2 figures. Minor changes. Texts differ slightly from the PRL
published versio
Confinement and Viscoelastic effects on Chain Closure Dynamics
Chemical reactions inside cells are typically subject to the effects both of
the cell's confining surfaces and of the viscoelastic behavior of its contents.
In this paper, we show how the outcome of one particular reaction of relevance
to cellular biochemistry - the diffusion-limited cyclization of long chain
polymers - is influenced by such confinement and crowding effects. More
specifically, starting from the Rouse model of polymer dynamics, and invoking
the Wilemski-Fixman approximation, we determine the scaling relationship
between the mean closure time t_{c} of a flexible chain (no excluded volume or
hydrodynamic interactions) and the length N of its contour under the following
separate conditions: (a) confinement of the chain to a sphere of radius D, and
(b) modulation of its dynamics by colored Gaussian noise. Among other results,
we find that in case (a) when D is much smaller than the size of the chain,
t_{c}\simND^{2}, and that in case (b), t_{c}\simN^{2/(2-2H)}, H being a number
between 1/2 and 1 that characterizes the decay of the noise correlations. H is
not known \`a priori, but values of about 0.7 have been used in the successful
characterization of protein conformational dynamics. At this value of H
(selected for purposes of illustration), t_{c}\simN^3.4, the high scaling
exponent reflecting the slow relaxation of the chain in a viscoelastic medium
Dynamics of polydisperse irreversible adsorption: a pharmacological example
Many drug delivery systems suffer from undesirable interactions with the host immune system. It has been experimentally established that covalent attachment (irreversible adsorption) of suitable macromolecules to the surface of the drug carrier can reduce such undesirable interactions. A fundamental understanding of the adsorption process is still lacking. In this paper, the classical random irreversible adsorption model is generalized to capture certain essential processes involved in pharmacological applications, allowing for macromolecules of different sizes, partial overlapping of the tails of macromolecules, and the influence of reactions with the solvent on the adsorption process. Working in one dimension, an integro-differential evolution equation for the adsorption process is derived, and the asymptotic behavior of the surface area covered and the number of molecules attached to the surface are studied. Finally, equation-free dynamic renormalization tools are applied to study the asymptotically self-similar behavior of the adsorption statistics
Hydrodynamic induced deformation and orientation of a microscopic elastic filament
We describe simulations of a microscopic elastic filament immersed in a fluid
and subject to a uniform external force. Our method accounts for the
hydrodynamic coupling between the flow generated by the filament and the
friction force it experiences. While models that neglect this coupling predict
a drift in a straight configuration, our findings are very different. Notably,
a force with a component perpendicular to the filament axis induces bending and
perpendicular alignment. Moreover, with increasing force we observe four shape
regimes, ranging from slight distortion to a state of tumbling motion that
lacks a steady state. We also identify the appearance of marginally stable
structures. Both the instability of these shapes and the observed alignment can
be explained by the combined action of induced bending and non-local
hydrodynamic interactions. Most of these effects should be experimentally
relevant for stiff micro-filaments, such as microtubules.Comment: three figures. To appear in Phys Rev Let
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