共查询到20条相似文献,搜索用时 15 毫秒
1.
O.V. Borisov E.B. Zhulina 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,4(2):205-217
We have developed a scaling theory that describes the conformations of weak star-branched polyelectrolytes in dilute solutions.
The dependences of the overall star size on the number of branches and on the ionic strength of the solution (tuned by the
addition of low molecular weight salt) are analyzed. The intrinsic structure of the polyelectrolyte stars in salt-free and
salt-added solutions is discussed in terms of concentration and elastic blobs. In contrast to neutral stars, the swollen corona
of the polyelectrolyte star consists of blobs which are not closely packed. We have shown that the size of star polyelectrolytes
is less sensitive to the variation in the ionic strength than the size of linear polyelectrolytes. The effects of the ionization-recombination
balance in the star polyelectrolyte were taken into account. For polyelectrolytes with small ionization constant, the size
of the star depends nonmonotonically on the number of branches and on the ionic strength of the solution due to recombination
of counterions with charged monomers.
Received: 10 November 1997 / Revised: 16 February 1998 / Accepted: 1st April 1998 相似文献
2.
O.V. Borisov A. Halperin 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,9(2):251-259
A theoretical model of the extension and confinement of globular polysoaps predicts novel force laws. Polysoaps are polymers
comprising of a flexible hydrophilic backbone incorporating, at intervals, amphiphilic monomers. The equilibrium configuration
of long polysoaps, that form numerous spherical intrachain micelles, is a spherical globule of close packed micelles. The
coupling of the deformation to the hierarchical self organization of the chain gives rise to a distinctive force law involving,
for extension, two plateau regimes. When the chain is stretched by extensional flow the two regimes merge and the polysoap
exhibits a single globule-stretch transition.
Received 16 June 1998 and Received in final form 19 November 1998 相似文献
3.
E.P.K. Currie G.J. Fleer M.A. Cohen Stuart O.V. Borisov 《The European physical journal. E, Soft matter》2000,1(1):27-40
We present an analytical self-consistent-field (SCF) theory for a neutral polymer brush (a layer of long polymer chains end-grafted
to a surface) with annealed excluded volume interactions between the monomer units. This model mimics the reversible adsorption
of solute molecules or aggregates, such as small globular proteins or surfactant micelles, on the grafted chains. The equilibrium
structural properties of the brush (the brush thickness, the monomer density profile, the distribution of the end segments
of the grafted chains) as well as the overall adsorbed amount and the adsorbate density profile are analyzed as a function
of the grafting density, the excluded volume parameters and the chemical potential (the concentration) of the adsorbate in
the solution. We demonstrate that, when the grafting density is varied, the overall adsorbed amount always exhibits a maximum,
whereas the root-mean-square brush thickness either increases monotonically or passes through a (local) minimum. At high grafting
densities the chains are loaded by adsorbed aggregates preferentially in the distal region of the brush, whereas in the region
proximal to the grafting surface depletion of aggregates occurs and the polymer brush retains an unperturbed structure. Depending
on the relative strength of the excluded volume interactions between unloaded and loaded monomers both the degree of loading
of the chains and the polymer density profile are either continuous or they exhibit a discontinuity as a function of the distance
from the grafting surface. In the latter case intrinsic phase separation occurs in the brush: the dense phase consists of
unloaded and weakly extended chains and occupies the region proximal to the surface, whereas a more dilute phase consisting
of highly loaded and strongly extended chains forms the periphery of the brush.
Received 26 November 1998 and Received in final form 2 April 1999 相似文献
4.
N. Lee S. Obukhov 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,1(3):371-376
We study the configurational statistics of a ring polyampholyte chain made of N randomly charged monomers with elementary charge .To a large extent, the overall structure of a polyampholyte is controlled by a total sum Q of all charges. When the total charge is smaller than , the polyampholyte has a compact globular structure. At charges larger than , the configuration has the form of a ring of small globules (beads) connected by strings. Between Q1 and Q2 we find a remarkable diversity of meta-stable configurations having the shapes of irregular clusters of small globules connected
by the strings. We estimate the number of these configurations and the energy barriers between them. Between Q1 and Q2, the minimum energy configurations are completely controlled by randomness in the charge distribution along the chain. There
are hysteresis effects in the shapes of the clusters. As the total charge increases, the linearly extended configurations
become dynamically more preferable. When the charge decreases, the circular shapes are preferred. We remark on the probable
connection with the multiple phase transitions observed in polyampholyte gels.
Received: 10 July 1997 / Accepted: 13 October 1997 相似文献
5.
A.N. Semenov 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,10(3):497-507
Secondary domain superstructures in correlated random block copolymers are considered theoretically using the concept of the
second order parameter related to fluctuations of the local mean block length. It is shown that the size of secondary domains,
, is much larger than the primary domain size, L: , while , where is a small parameter defining the composition asymmetry. Different secondary morphologies are characterized. It is also shown
that separation of the system in two macroscopic phases with different primary morphologies predicted earlier using the free
energy expansion up to ( is the usual order parameter related to local composition) is an artifact of this widely accepted theoretical model.
Received 15 July 1998 and Received in final form 18
January 1999 相似文献
6.
A. Halperin 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,3(3):359-364
The compression of brushes of terminally anchored chain within the de Gennes n-cluster model is analysed. This model was developed for Poly(ethylene oxide) in water but may apply to other systems. Brushes
described by this model exhibit discontinuous concentration profile associated with the coexistence of an inner dense “phase”
and an outer, dilute, one. The compression induces growth of the dense, weakly compressible region. This, in turn, gives rise
to distinctive force profiles associated with changes of slope. When the dilute region disappears, the compression of two
brushes can give rise to a transient attraction.
Received: 4 November 1997 / Revised: 29 January 1998 / Accepted: 24 March 1998 相似文献
7.
The mean area of a two-dimensional Gaussian ring of N monomers is known to diverge when the ring is subject to a critical pressure differential, p
c ∼ N
-1. In a recent publication (Eur. Phys. J. E 19, 461 (2006)) we have shown that for an inextensible freely jointed ring this divergence turns into a second-order transition
from a crumpled state, where the mean area scales as 〈A〉 ∼ N, to a smooth state with 〈A〉 ∼ N
2. In the current work we extend these two models to the case where the swelling of the ring is caused by trapped ideal-gas
particles. The Gaussian model is solved exactly, and the freely jointed one is treated using a Flory argument, mean-field
theory, and Monte Carlo simulations. For a fixed number Q of trapped particles the criticality disappears in both models through an unusual mechanism, arising from the absence of
an area constraint. In the Gaussian case the ring swells to such a mean area, 〈A〉 ∼ NQ, that the pressure exerted by the particles is at p
c for any Q. In the freely jointed model the mean area is such that the particle pressure is always higher than p
c, and 〈A〉 consequently follows a single scaling law, 〈A〉 ∼ N
2
f (Q/N), for any Q. By contrast, when the particles are in contact with a reservoir of fixed chemical potential, the criticality is retained.
Thus, the two ensembles are manifestly inequivalent in these systems.
An erratum to this article is available at . 相似文献
8.
M. Castelnovo P. Sens J.-F. Joanny 《The European physical journal. E, Soft matter》2000,1(2-3):115-125
We investigate the equilibrium charge distribution along a single annealed polyelectrolyte chain under different conditions.
The coupling between the conformation of the chain and the local charge distribution is described for various solvent qualities
and salt concentration. In salt free solution, we find a slight charge depletion in the central part of the chain: the charges
accumulate at the ends. The effect is less important if salt is added to the solution since the charge inhomogeneity is localized
close to the chain ends over a distance of order of the Debye length. In the case of poor solvent conditions we find a different
charge per monomer in the beads and in the strings in the framework of the necklace model. This inhomogeneity leads to a charge
instability and a first order transition between spherical globules and elongated chains.
Received 19 March 1999 and Received in final form 2 August 1999 相似文献
9.
We discuss theoretically the relaxation of charge fluctuations in polyampholyte solutions. It has been shown previously by
some of us (J. Wittmer et al. Europhys. Lett. 24, 263 (1993)) that the charge distribution along the polyampholyte backbone has a dramatic influence on the polarization energy
and hence on the solubility. Here it is demonstrated that a similar effect exists for the charge relaxation. The charge relaxation
mechanism qualitatively depends on the statistics: for alternating polyampholytes the relaxation is mainly due to local dipole
inversion and is not primarily driven by electrostatic interactions, whereas for random polyampholytes it is driven by electrostatic
interactions. Intermediate statistics (with short-ranged (exponential) correlations) appear as a combination of these two
limiting cases: short-wavelength modes are insensitive to the loss of correlations along the backbone, whereas long-wavelength
modes correspond to a random statistics with renormalized charges. The relaxation of the dielectric constant is also calculated.
Received: 20 December 2002 / Accepted: 13 March 2003 / Published online: 24 April 2003
RID="a"
ID="a"e-mail: johner@ics.u-strasbg.fr 相似文献
10.
Jarkova E Johner A Maresov EA Semenov AN 《The European physical journal. E, Soft matter》2006,21(4):371-386
A mean-field theory of globules of random amphiphilic copolymers in selective solvents is developed for the case of an annealed
copolymer sequence: each unit can be in one of two states, H (insoluble) or P (soluble or less insoluble). The study is focussed
on the regime when H and P units tend to form long blocks, and when P units dominate in the dilute phase, but are rare in
the globule core. A first-order coil-to-globule transition is predicted at some T = T
cg. The globule core density at the transition point increases as the affinity of P units to the solvent, ˜, is increased. Two
collapse transitions, coil → “loose” globule and “loose” globule → “dense” globule, are predicted if ˜ is high enough and
P units are marginally soluble or weakly insoluble. H and P concentration profiles near the globule surface are obtained and
analyzed in detail. It is shown that the surface excess of P units rises as ˜ is increased. The surface tension decreases
in parallel. Considering the interaction between close enough surfaces of two globules, we show that they always attract each
other at a complete equilibrium. It is pointed out, however, that such equilibrium may be difficult to reach, so that partially equilibrium structures (defined by the condition that a chain forming one globule does not penetrate into the core of the other globule)
are relevant. It is shown that at such partial equilibrium the interaction is repulsive, so the globules may be stabilized
from aggregation. The strongest repulsion is predicted at the coil-to-globule transition point T
cg: the repulsion force decreases with the distance between the surfaces according to a power law. In the general case (apart
from T
cg) the force vs. distance decay becomes exponential; the decay length ξ diverges as T → T
cg. The developed theory explains certain anomalous properties observed for globules of amphiphilic homopolymers. 相似文献
11.
Khokhlov AR Semenov AN Subbotin AV 《The European physical journal. E, Soft matter》2005,17(3):283-306
Shapes of globules formed by amphiphilic multi-block-copolymers in a selective solvent are considered theoretically. We focus on copolymers consisting mostly of insoluble H-units forming large core surrounded by a shell of soluble P-blocks. It is shown that the globule becomes non-spherical when the effective shell tension is low enough. The resultant shape depends on the shell bending energy: it is prolate if this energy is larger than the elastic energy of the core, and oblate in the opposite case. The central result is the prediction of the formation of a surface pattern of fingers accompanying or even preempting the shape transition mentioned above. We elucidate and discuss the following finger morphologies: 1) nearly spherical knob; 2) a necklace of spherical beads extending away from the surface; 3) mostly cylindrical fingers; 4) large thorn-like fingers. The first 3 morphologies develop at equilibrium as the shell area increases (or, equivalently, the shell tension decreases). Considering the relevant kinetical aspects we show that formation of fingers is a nucleation and growth process, and that the energy of their equilibrium nucleation is likely to be high. Therefore, the finger formation may be delayed, and may actually occur in the regime where the plain spherical surface is metastable. It is the last morphology (thorn-like fingers) that characterizes the metastable regimes when the finger formation is controlled by a high activation energy. The universal features of the above predictions inviting experimental tests are discussed. 相似文献
12.
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
13.
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
14.
Y.O. Popov A.N. Semenov 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,6(2):245-256
Nematic ordering in anisotropic non-Gaussian elastomers is considered theoretically using mean field approximation. We focus on the effect of anisotropy during network
cross-linking on the system elasticity and, in particular, on the so-called soft deformation mode. As the main result, we
calculate the dependence of the elastomer free energy on the angle between the axis of “frozen” anisotropy and the nematic
director. The dependence of the isotropic-nematic transition point on the orientational field acting on the monomers during
the cross-linking process is also calculated.
Received: 5 November 1997 / Revised and Accepted: 29 June 1998 相似文献
15.
We study isotropic-isotropic and isotropic-nematic phase transitions of fluid mixtures containing hard spherocylinders (HSC)
and added non-adsorbing ideal polymer chains using scaled particle theory (SPT). First, we investigate isotropic-nematic (I -N phase coexistence using SPT in the absence of polymer. We compare the results obtained using a Gaussian form of the orientational
distribution function (ODF) to minimize the free energy versus minimizing numerically. We find that formal numerical minimization gives results that are much closer to computer simulation
results. In order to describe mixtures of HSC plus ideal chains we studied the depletion of ideal chains around a HSC. We
analyze the density profiles of ideal chains near a hard cylinder and find the depletion thickness δ is a function of the
ratio of the polymer's radius of gyration Rg and the cylinder radius Rc. Our results are compared with a common approximation in which the depletion thickness is taken equal to the radius of gyration
of the polymer chain. We incorporate the correct depletion thickness into SPT and find that for R
g/R
c < 1.56 using ideal chains gives phase transitions at smaller polymer concentrations, whereas for R
g/R
c > 1.56 , which is a common experimental situation, the phase transitions are found at larger polymer concentrations with
respect to δ = R
g . The differences are significant, especially for R
g ≫ R
c , so we can conclude it is essential to take into account the properties of ideal polymer chains and the resulting depletion
near a cylinder. Finally, we present phase diagrams for rod-polymer mixtures which could be realized under experimental conditions. 相似文献
16.
P. Dommersnes S. Straumsnes E.G. Flekkøy T. Nicolai S. Roux 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,9(4):669-677
We study the hydrodynamic properties of polymers and more generally self-similar structures using a new recursion model. The
hydrodynamic interaction between monomers is modeled by the standard Green's function of Stokes flow in which an ultrametric
distance is substituted for the usual Euclidean distance. This leads to a model where the long-range hydrodynamic interactions
and the long-range correlations of the polymer conformation can both be accounted for and yet allow for analytical solutions.
We explore the asymptotic as well as the finite size corrections to the scaling behavior with this model. In order to compare
the results of the present scheme with more conventional techniques a generalized version of the existing mean field results
by Kirkwood and Riseman for the hydrodynamic drag is introduced.
Received 26 August 1998 and Received in final form 7 December 1998 相似文献
17.
I. Sabbagh M. Delsanti P. Lesieur 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,12(2):253-260
Anomalous small angle X-ray scattering experiments show that before demixion in sodium polyacrylate/cobalt and sodium polyacrylate/calcium
mixtures all the divalent counterions are in the close vicinity of the polyacrylate chain. The present results are consistent
with previous UV/VIS spectroscopy, which have shown that all cobalt ions are chemically associated with acrylate groups. The
chemical association dehydrates the acrylate monomers. However, the hydrophobicity of the complexed monomers is not strong
enough to induce a collapse of the polymer chain at small spatial scale before the demixion. Indeed, the scattered intensity
(X-ray and neutron scattering) decreases with the scattering vector q as q-x with for q > 0.1 nm
-1 which indicates that the local conformation of the chain is Gaussian.
Received 21 January 1999 相似文献
18.
A.N. Semenov M. Rubinstein 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,1(1):87-94
The effect of excluded-volume interactions on the reptation dynamics of long polymer chains is considered theoretically. It
is shown that interactions give rise to an exponential increase of the reptation time, , if polymer chains are long enough: , where is the number of monomers per entanglement. We propose a novel dynamical mechanism of activated reptation implying that neighboring
chains exchange conformations of their terminal fragments. It is shown that the exchange mechanism is compatible with the
equilibrium polymer chain statistics and that it provides a bridge between the previous theories.
Received: 25 July 1997 / Accepted: 8 October 1997 相似文献
19.
V.G. Rostiashvili M. Rehkopf T.A. Vilgis 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,6(2):233-243
In this paper the Martin-Siggia-Rose (MSR) functional integral representation is used for the study of the Langevin dynamics
of a polymer melt in terms of collective variables: mass density and response field density. The resulting generating functional
(GF) takes into account fluctuations around the random phase approximation (RPA) up to an arbitrary order. The set of equations
for the correlation and response functions is derived. It is generally shown that for cases whenever the fluctuation-dissipation
theorem (FDT) holds we arrive at equations similar to those derived by Mori-Zwanzig. The case when FDT in the glassy phase
is violated is also qualitatively considered and it is shown that this results in a smearing out of the ideal glass transition.
The memory kernel is specified for the ideal glass transition as a sum of all “water-melon” diagrams. For the Gaussian chain
model the explicit expression for the memory kernel was obtained and discussed in a qualitative link to the mode-coupling
equation.
Received: 9 January 1998 / Revised: 24 April 1998 / Accepted: 2 July 1998 相似文献
20.
J.J. Arenzon J.F. Stilck Y. Levin 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,12(1):79-82
We present a simple model for the possible mechanism of appearance of attraction between like charged polyions inside a polyelectrolyte
solution. The attraction is found to be short ranged, and exists only in the presence of multivalent counterions. It is produced
by the correlations in layers of condensed counterions surrounding each polyion and is only weakly temperature dependent.
We find the attraction to be maximum at zero temperature and dimish as the temperature is raised. The attraction is only possible
if the number of condensed counterions exceeds the threshold, , where is the valence of counterions and Z is the polyion charge.
Received 10 March 1999 and Received in final form 20 April 1999 相似文献