共查询到20条相似文献,搜索用时 140 毫秒
1.
We use large-scale Monte Carlo simulations to test scaling theories for the electrostatic persistence length l
e of isolated, uniformly charged polymers with Debye-Hückel intrachain interactions in the limit where the screening length
κ-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∝κ-2 by Khokhlov and Khachaturian which is based on applying the Odijk-Skolnick-Fixman theories of electrostatic bending rigidity
and electrostatically excluded volume 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 show that previous results
pointing into this direction are 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.
Received 12 February 2002 相似文献
2.
G. Migliorini N. Lee V. Rostiashvili T.A. Vilgis 《The European physical journal. E, Soft matter》2001,6(3):259-270
We study the properties of polyelectrolyte chains under different solvent conditions, using a variational technique. The free
energy and the conformational properties of a polyelectrolyte chain are studied by minimizing the free energy FN, depending on N(N - 1)/2 trial probabilities that characterize the conformation of the chain. The Gaussian approximation is considered for
a ring of length 24 < N < 28 and for an open chain of length 50 < N < 200 in poor- and theta-solvent conditions, including a Coulomb repulsion between the monomers. In theta-solvent conditions
the blob size is measured and found in agreement with scaling theory, including charge depletion effects, expected for the
case of an open chain. In poor-solvent conditions, a globule instability, driven by electrostatic repulsion, is observed.
We notice also inhomogeneous behavior of the monomer-monomer correlation function, reminiscence of necklace formation in poor-solvent
polyelectrolyte solutions. A global phase diagram in terms of solvent quality and inverse Bjerrum length is presented.
Received 7 June 2001 and Received in final form 17 October 2001 相似文献
3.
Monte Carlo simulations are used to study the non-uniform equilibrium charge distribution along a single annealed polyelectrolyte
chain under θ solvent conditions and with added salt. Within a range of the order of the Debye length charge accumulates at
chain ends while a slight charge depletion appears in the central part of the chain. The simulation results are compared with
theoretical predictions recently given by Castelnovo et al. In the parameter range where the theory can be applied we find almost perfect quantitative agreement.
Received 7 March 2002 and Received in final form 28 May 2002 相似文献
4.
R.R. Netz H. Orland 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,8(1):81-98
Variational methods are applied to a single polyelectrolyte chain. The polymer is modeled as a Gaussian chain with screened
electrostatic repulsion between all monomers. As a variational Hamiltonian, the most general Gaussian kernel, including the
possibility of a classical or mean polymer path, is employed. The resulting self-consistent equations are systematically solved
both for large and small monomer-monomer separations along the chain. In the absence of screening, the polymer is stretched
on average. It is described by a straight classical path with Gaussian fluctuations around it. If the electrostatic repulsion
is screened, the polymer is isotropically swollen for large separations, and for small separations the polymer correlation
function is calculated as an analytic expansion in terms of the monomer-monomer separation along the chain. The electrostatic
persistence length and the electrostatic blobsize are inferred from the crossover between distinct scaling ranges. We perform
a global analysis of the scaling behavior as a function of the screening length and electrostatic interaction strength , where is the Bjerrum length and A is the distance of charges along the polymer chain. We find three different scaling regimes. i) A Gaussian-persistent regime with Gaussian behavior at small, persistent behavior at intermediate, and isotropically swollen behavior at large length
scales. This regime occurs for weakly charged polymers and only for intermediate values of the screening length. The electrostatic
persistence length is defined as the crossover length between the persistent and the asymptotically swollen behavior and is given by and thus disagrees with previous (restricted) variational treatments which predict a linear dependence on the screening length
.ii) A Gaussian regime with Gaussian behavior at small and isotropically swollen behavior at large length scales. This regime occurs for weakly
charged polymers and/or strong screening, and the electrostatic repulsion between monomers only leads to subfluent corrections
to Gaussian scaling at small separations. The concept of a persistence length is without meaning in this regime. iii) A persistent regime , where the chain resembles a stretched rod on intermediate and small scales. Here the persistence length is given by the
original Odijk prediction, , if the overstretching of the chain is avoided. We also investigate the effects of a finite polymer length and of an additional
excluded-volume interaction, which modify the resultant scaling behavior. Applications to experiments and computer simulations
are discussed.
Received 24 December 1997 相似文献
5.
Cocco S Marko JF Monasson R Sarkar A Yan J 《The European physical journal. E, Soft matter》2003,10(3):249-263
The elastic response of flexible polymers made of elements which can be either folded or unfolded, having different lengths
in these two states, is discussed. These situations are common for biopolymers as a result of folding interactions intrinsic
to the monomers, or as a result of binding of other smaller molecules along the polymer length. Using simple flexible-chain
models, we show that even when the energy ε associated with maintaining the folded state is comparable to k
B
T, the elastic response of such a chain can mimic usual polymer linear elasticity, but with a force scale enhanced above that
expected from the flexibility of the chain backbone. We discuss recent experiments on single-stranded DNA, chromatin fiber
and double-stranded DNA with proteins weakly absorbed along its length which show this effect. Effects of polymer semiflexiblity
and torsional stiffness relevant to experiments on proteins binding to dsDNA are analyzed. We finally discuss the competition
between electrostatic self-repulsion and folding interactions responsible for the complex elastic response of single-stranded
DNA.
Received 7 August 2002 and Received in final form 7 March 2003 / Published online: 15 April 2003
RID="a"
ID="a"e-mail: jmarko@uic.edu 相似文献
6.
G. Migliorini V.G. Rostiashvili T.A. Vilgis 《The European physical journal. E, Soft matter》2001,4(4):475-487
A variational approach is considered to calculate the free energy and the conformational properties of a polyelectrolyte chain
in d 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 N. We find R≃N
ν(log N)γ, where ν = and λ is the exponent which characterizes the long-range interaction U∝ 1/r
λ. The exponent γ 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 24 < N < 215. 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 24 < N < 210. The non-universal behavior of the exponent γ previously derived within the variational method is also confirmed by the simulation
results. Non-universal behavior is found for a polyelectrolyte chain in d = 3 dimension. Particular attention is devoted to the homopolymer chain problem, when short-range contact interactions are
present.
Received 8 August 2000 and Received in final form 19 December 2000 相似文献
7.
We study the effect of electrostatic interactions on the distribution function of the end-to-end distance of a single polyelectrolyte
chain in the rod-like limit. The extent to which the radial distribution function of a polyelectrolyte is reproduced by that
of a wormlike chain with an adjusted effective persistence length is investigated. Strong evidence is found for a universal
scaling formula connecting the effective persistence length of a polyelectrolyte with the strength of the electrostatic interaction
and the Debye screening length.
Received 4 March 2002 and Received in final form 1 July 2002
RID="a"
ID="a"e-mail: jrudnick@physics.ucla.edu 相似文献
8.
We investigate the drift of an end-labeled telehelic polymer chain in a frozen disordered medium under the action of a constant
force applied to the one end of the macromolecule by means of an off-lattice bead spring Monte Carlo model. The length of
the polymers N is varied in the range 8 < N < 128, and the obstacle concentration in the medium C is varied from zero up to the percolation threshold C≈ 0.75. For field intensities below a C-dependent critical field strength B
c, where jamming effects become dominant, we find that the conformational properties of the drifting chains can be interpreted
as described by a scaling theory based on Pincus blobs. The variation of drag velocity with C in this interval of field intensities is qualitatively described by the law of Mackie-Meares. The threshold field intensity
B
c itself is found to decrease linearly with C.
Received 20 August 2001 and Received in final form 19 November 2001 相似文献
9.
Castelnovo M Bowles RK Reiss H Gelbart WM 《The European physical journal. E, Soft matter》2003,10(2):191-197
We consider the problem of inserting a stiff chain into a colloidal suspension of particles that interact with it through
excluded volume forces. The free energy of insertion is associated with the work of creating a cavity devoid of colloid and
sufficiently large to accommodate the chain. The corresponding work per unit length is the force that resists the entry of
the chain into the colloidal suspension. In the case of a hard sphere fluid, this work can be calculated straightforwardly
within the scaled particle theory; for solutions of flexible polymers, on the other hand, we employ simple scaling arguments.
The forces computed in these ways are shown, for nanometer chain and colloid diameters, to be of the order of tens of pN for solution volume fractions of a few tenths. These magnitudes are argued to be important for biophysical processes such
as the ejection of DNA from viral capsids into the cell cytoplasm.
Received 18 December 2002 Published online: 16 April 2003
RID="a"
ID="a"e-mail: castel@chem.ucla.edu
RID="b"
ID="b"Present address: Courant Institute of Mathematical Sciences, NYU, New York, New York 10012, USA 相似文献
10.
Elastic rod model of a supercoiled DNA molecule 总被引:4,自引:0,他引:4
We study the elastic behaviour of a supercoiled DNA molecule. The simplest model is that of a rod-like chain, involving two
elastic constants, the bending and the twist rigidities. Writing this model in terms of Euler angles, we show that the corresponding
Hamiltonian is singular and needs a small distance cut-off, which is a natural length scale giving the limit of validity of
the model, of the order of the double-helix pitch. The rod-like chain in the presence of the cut-off is able to reproduce
quantitatively the experimentally observed effects of supercoiling on the elongation-force characteristics, in the small supercoiling
regime. An exact solution of the model, using both transfer matrix techniques and its mapping to a quantum mechanics problem,
allows to extract, from the experimental data, the value of the twist rigidity. We also analyse the variation of the torque
and the writhe-to-twist ratio versus supercoiling, showing analytically the existence of a rather sharp crossover regime which can be related to the excitation
of plectoneme-like structures. Finally we study the extension fluctuations of a stretched and supercoiled DNA molecule, both
at fixed torque and at fixed supercoiling angle, and we compare the theoretical predictions to some preliminary experimental
data.
Received 1 April 1999 and Received in final form 4 January 2000 相似文献
11.
We present a field-theoretic Renormalization Group (RG) analysis of the statistical mechanics of long flexible, screened polyelectrolyte
chains (Debye-Hückel chains) in polar solvents where the screening length is of the order of the chain size. A systematic
analysis of the resulting field theory shows that the system is one with two length-scales requiring the calculation of scaling functions as well as exponents to fully describe its physical behaviour. This means that care
must be taken to understand the interplay of the length-scales. Using the RG we identify the relevant scaling variables and
explicitly calculate the scaling behaviour of the end-to-end distance for single chains. In addition we consider the many-chain
system and calculate the scaling behaviour of the osmotic pressure of a dilute solution of chains.
Received 16 December 1999 and Received in final form 13 December 2000 相似文献
12.
We study the force-induced unfolding of random disordered RNA or single-stranded DNA polymers. The system undergoes a second-order
phase transition from a collapsed globular phase at low forces to an extensive necklace phase with a macroscopic end-to-end
distance at high forces. At low temperatures, the sequence inhomogeneities modify the critical behaviour. We provide numerical
evidence for the universality of the critical exponents which, by extrapolation of the scaling laws to zero force, contain
useful information on the ground-state (f = 0) properties. This provides a good method for quantitative studies of scaling exponents characterizing the collapsed globule.
In order to get rid of the blurring effect of thermal fluctuations, we restrict ourselves to the ground state at fixed external
force. We analyze the statistics of rearrangements, in particular below the critical force, and point out its implications
for force-extension experiments on single molecules.
Received 18 June 2002 and Received in final form 23 September 2002
RID="a"
ID="a"e-mail: muller@ipno.in2p3.fr 相似文献
13.
Ngai KL 《The European physical journal. E, Soft matter》2002,8(2):225-235
Large increases of mobility of local segmental relaxation observed in polymer films as the film thickness is decreased, as
evidenced by decreases of the glass temperature, are not found for relaxation mechanisms that have longer length scales including
the Rouse relaxation modes and the diffusion of entire polymer chains. We show that the coupling model predictions, when extended
to consider polymer thin films, are consistent with a large increase of the mobility of the local segmental motions and the
lack of such a change for the Rouse modes and the diffusion of entire polymer chains. There are two effects that can reduce
the coupling parameter of the local segmental relaxation in thin films. One is the chain orientation that is induced parallel
to the surface when the film thickness h becomes smaller than the end-to-end distance of the chains and the other is a finite-size effect when h is no longer large compared to the cooperative length scale. Extremely thin ( ≈ 1.5 nm) films obtained by intercalating a
polymer into layered silicates have thickness significantly less than the cooperative length scale near the bulk polymer glass
transition temperature. As a result, the coupling parameter of the local segmental relaxation in such thin films is reduced
almost to zero. With this plausible assumption, we show the coupling model can explain quantitatively the large decrease of
the local segmental relaxation time found experimentally.
Received 1 August 2001 and Received in final form 1 December 2001 相似文献
14.
G. Migliorini V.G. Rostiashvili T.A. Vilgis 《The European Physical Journal B - Condensed Matter and Complex Systems》2003,33(1):61-73
The Langevin dynamics of a self-interacting chain embedded in a quenched random medium is investigated by making use of the
generating functional method and one-loop (Hartree) approximation. We have shown how this intrinsic disorder causes different
dynamical regimes. Namely, within the Rouse characteristic time interval the anomalous diffusion shows up. The corresponding
subdiffusional dynamical exponents have been explicitly calculated and thoroughly discussed. For the larger time interval
the disorder drives the center of mass of the chain to a trap or frozen state provided that the Harris parameter, (Δ/b
d)N
2 - νd≥1, where Δ is a disorder strength, b is a Kuhnian segment length, N is a chain length and ν is the Flory exponent. We have derived the general equation for the non-ergodicity function f (p) which characterizes the amplitude of frozen Rouse modes with an index p = 2πj/N. The numerical solution of this equation has been implemented and shown that the different Rouse modes freeze up at the same
critical disorder strength Δ
c ∼ N
- γ where the exponent γ ≈ 0.25 and does not depend from the solvent quality.
Received 17 December 2002 Published online 23 May 2003
RID="a"
ID="a"e-mail: vilgis@mpip-mainz.mpg.de 相似文献
15.
We investigate, using numerical simulations and analytical arguments, a simple one-dimensional model for the swelling or the
collapse of a closed polymer chain of size N, representing the dynamical evolution of a polymer in a Θ-solvent that is rapidly changed into a good solvent (swelling)
or a bad solvent (collapse). In the case of swelling, the density profile for intermediate times is parabolic and expands
in space as t
1/3, as predicted by a Flory-like continuum theory. The dynamics slows down after a time ∝N
2 when the chain becomes stretched, and the polymer gets stuck in metastable “zig-zag” configurations, from which it escapes
through thermal activation. The size of the polymer in the final stages is found to grow as . In the case of collapse, the chain very quickly (after a time of order unity) breaks up into clusters of monomers (“pearls”).
The evolution of the chain then proceeds through a slow growth of the size of these metastable clusters, again evolving as
the logarithm of time. We enumerate the total number of metastable states as a function of the extension of the chain, and
deduce from this computation that the radius of the chain should decrease as 1/ln(ln t). We compute the total number of metastable states with a given value of the energy, and find that the complexity is non-zero
for arbitrary low energies. We also obtain the distribution of cluster sizes, that we compare to simple “cut-in-two” coalescence
models. Finally, we determine the aging properties of the dynamical structure. The subaging behaviour that we find is attributed
to the tail of the distribution at small cluster sizes, corresponding to anomalously “fast” clusters (as compared to the average).
We argue that this mechanism for subaging might hold in other slowly coarsening systems.
Received 23 October 2000 相似文献
16.
E. Zhulina A.V. Dobrynin M. Rubinstein 《The European physical journal. E, Soft matter》2001,5(1):41-49
We develop a scaling theory for a single polyampholyte chain adsorbed on a charged spherical particle in a theta-solvent.
Adsorption of a polyampholyte molecule is due to its polarization in the electrostatic field of the particle. For large particles
with sizes exceeding the thickness of the adsorbed layer, the conformations of the chain are similar to the one found for
polyampholyte adsorption on charged planar surface. However, an adsorbed polyampholyte chain forms a self-similar flower-like
structure near the particles with sizes smaller than its Gaussian size. These self-similar structures result from the balance
of the polarization energy of loops and the excluded volume interactions between monomers. The structure of an adsorbed polyampholyte
in the flower-like conformation is similar to that of a neutral star polymer.
Received 3 March 2000 and Received in final form 5 July 2000 相似文献
17.
Mattioni L Wittmer JP Baschnagel J Barrat JL Luijten E 《The European physical journal. E, Soft matter》2003,10(4):369-385
Correlations in the motion of reptating polymers in a melt are investigated by means of Monte Carlo simulations of the three-dimensional
slithering-snake version of the bond-fluctuation model. Surprisingly, the slithering-snake dynamics becomes inconsistent with
classical reptation predictions at high chain overlap (created either by chain length N or by the volume fraction φ of occupied lattice sites), where the relaxation times increase much faster than expected. This
is due to the anomalous curvilinear diffusion in a finite time window whose upper bound (N) is set by the density of chain ends φ/N. Density fluctuations created by passing chain ends allow a reference polymer to break out of the local cage of immobile
obstacles created by neighboring chains. The dynamics of dense solutions of “snakes” at t ≪ is identical to that of a benchmark system where all chains but one are frozen. We demonstrate that the subdiffusive dynamical
regime is caused by the slow creeping of a chain out of its correlation hole. Our results are in good qualitative agreement
with the activated-reptation scheme proposed recently by Semenov and Rubinstein (Eur. Phys. J. B, 1 (1998) 87). Additionally, we briefly comment on the relevance of local relaxation pathways within a slithering-snake scheme.
Our preliminary results suggest that a judicious choice of the ratio of local to slithering-snake moves is crucial to equilibrate
a melt of long chains efficiently.
Received: 18 December 2002 / Accepted: 3 April 2003 / Published online: 12 May 2003
RID="a"
ID="a"e-mail: jwittmer@dpm.univ-lyon1.fr
RID="b"
ID="b"Current address: University of Illinois at Urbana-Champaign. 相似文献
18.
F.B. Anders M. Huth 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,19(4):491-499
We present calculations of the electronic transport properties of heavy-fermion systems within a semi-phenomenological approach
to the dynamical mean field theory. In this approach the dynamics of the Hund's rules 4f (5f )-ionic multiplet split in a crystalline environment is taken into account. Within the scope of this calculation we use the
linear response theory to reproduce qualitative features of the temperature-dependent resistivity and hall conductivity, the
magneto-resistivity and the thermoelectric power typical for heavy-fermion systems. The model calculations are directly compared
with experimental results on CeCu
2
Si
2.
Received 30 June 2000 and Received in final form 15 December 2000 相似文献
19.
The subject of this paper is inspired by microphase-separated copolymer melts in which a small-scale structure is present
inside one of the phases of a large-scale structure. Such a situation can arise in a diblock copolymer melt, if one of the
blocks of the diblock is in itself a multiblock copolymer or a comb copolymer. Due to the presence of the large-scale structure,
the chains are stretched. The aim of this paper is to investigate the influence of this chain stretching on the formation
of the small-scale structure. To gain insight we study infinite melts of infinitely long copolymer chains that are subjected
to a stretching force. For melts of monodisperse multiblock copolymers we find that the stretching destabilizes the homogeneous
phase. If the stretching is strong, the lamellar structure is the only stable structure. The periodicity increases with the
degree of stretching. For melts of monodisperse comb copolymers the chain stretching has no influence on the stability of
the homogeneous phase. If the stretching is strong, the lamellar structure and the hexagonal structure are the only stable
structures. The periodicity is independent of the degree of stretching. For the multiblock copolymer we investigated the influence
of block length polydispersity. For small polydispersity the period of the structure increases monotonically with the degree
of stretching. For intermediate polydispersity, the period initially decreases before it starts to increase. For large polydispersity, the mean-field period at the spinodal is infinite, becoming finite
once the stretching force exceeds some critical value. For very large polydispersity the mean-field period at the spinodal
remains infinite for any value of the stretching force.
Received: 14 February 2002 / Accepted: 24 March 2003 / Published online: 29 April 2003
RID="a"
ID="a"e-mail: hindrik.angerman@abp.nl 相似文献
20.
A nematic liquid crystal confined between two identical flat solid substrates, with an alternating stripe pattern of planar
and homeotropic anchoring, is studied in the framework of the Frank-Oseen theory. By means of numerical minimization of the
free energy functional we study the effect of the sample thickness D on the location of the phase transition between a uniform alignment, either planar or homeotropic, and a distorted nematic
texture. The solvation force f due to distortions of the nematic director is also studied. It is found that f is always attractive, and for D small compared to the periodicity of the surface structure it exhibits two distinct asymptotic behaviors: f ∼ - D
-1/2 or f ∼ - D
-1, depending on the relation between D and the extrapolation lengths.
Received 12 November 2002 Published online: 16 April 2003 相似文献