共查询到20条相似文献,搜索用时 15 毫秒
1.
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
2.
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
3.
N. Lee D. Thirumalai 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,12(4):599-605
The effect of electrostatic interactions on the stretching of DNA is investigated using a simple worm like chain model. In
the limit of small force there are large conformational fluctuations which are treated using a self-consistent variational
approach. For small values of the external force f, we find the extension scales as where is the Debye screening length. In the limit of large force the electrostatic effects can be accounted for within the semiflexible
chain model of DNA by assuming that only small excursions from rod-like conformations are possible. In this regime the extension
approaches the contour length as where f is the magnitude of the external force. The theory is used to analyze experiments that have measured the extension of double-stranded
DNA subject to tension at various salt concentrations. The theory reproduces nearly quantitatively the elastic response of
DNA at small and large values of f and for all concentration of the monovalent counterions. The limitations of the theory are also pointed out.
Received 13 October 1998 and Received in final form 9 June 1999 相似文献
4.
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
5.
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
6.
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
7.
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
8.
Pavlov GM 《The European physical journal. E, Soft matter》2007,22(2):171-180
It is proposed to normalize the Mark-Kuhn-Houwink-Sakurada type of equation
relating the hydrodynamic characteristics, such as intrinsic viscosity,
velocity sedimentation coefficient and translational diffusion coefficient
of linear macromolecules to their molecular masses for the values of linear
density ML and the statistical segment length A. When the set of data covering virtually all known
experimental information is normalized for ML, it is
presented as a size spectrum of linear polymer molecules. Further
normalization for the A value reduces all data to two
regions: namely the region exhibiting volume interactions and that showing
hydrodynamic draining. For chains without intachain excluded volume effects
these results may be reproduced using the Yamakawa-Fujii theory of wormlike
cylinders. Data analyzed here cover a range of contour lengths of linear
chains varying by three orders of magnitude, with the range of statistical
segment lengths varying approximately 500 times. The plot of the dependence
of [η]M on M represents the
spectrum of average specific volumes occupied by linear and branched
macromolecules. Dendrimers and globular proteins for which the volume
occupied by the molecule in solution is directly proportional to
M have the lowest specific volume. The homologous series
of macromolecules in these plots are arranged following their fractal
dimensionality. 相似文献
9.
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
10.
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
11.
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
12.
M. Bellour A. Knaebel J.P. Munch S.J. Candau 《The European physical journal. E, Soft matter》2000,3(2):111-121
Static and dynamic light scattering and conductivity experiments were carried out on salt-free aqueous micellar solutions
of cetyltrimethylammonium n-hexane sulfonate (CTAC_6SO_3) and cetyltrimethylammonium n-heptane sulfonate (CTAC_7SO_3) as a function of surfactant concentration. This study confirms the analogy between the behavior
in the semi-dilute regime of elongated micellar systems and “classical” polyelectrolyte solutions. Time-resolved scattering
experiments performed after a variation of concentration from about twice the overlap volume fraction to less than half of it revealed the existence of a structural relaxation with a characteristic time of several hours.
Received 21 December 1999 相似文献
13.
We construct a model for a slippage plane in a sheared melt, based on a balance between reptation bridging and shear debonding.
The resulting state could show up at rather low shear rates and be locally stable. But it is not easy to nucleate: the conventional
entangled state is also locally stable. We propose that slippage occurs on solid walls: either at the container surface, or
on dust particles floating in the melt. Slippage at solid/melt interfaces was studied (experimentally and theoretically) long
ago. There is a critical stress
for slippage: our estimate (for strong adsorption of melt chains on the solid) gives
(plateau modulus) for typical cases. Thus, melt fracture is expected at moderate stresses, in agreement with observations
by S.Q. Wang and coworkers. 相似文献
14.
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
15.
16.
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
17.
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
18.
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
19.
E. Freyssingeas D. Antelmi P. Kékicheff P. Richetti A.-M. Bellocq 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,9(1):123-136
The compressibility modulus of a lamellar phase containing a neutral polymer guest molecule was measured directly using a
surface force apparatus. The system studied consisted of sodium dodecyl sulphate (SDS), pentanol, water and polyethylene glycol
(PEG) . The lamellar phase was induced from a micellar phase in situ via a confinement induced isotropic to lamellar phase transition. This avoided problems resulting from the viscosity and turbidity
normally characteristic of these lamellar phase samples. Increasing the amount of PEG resulted in a marked decrease in the
layer compressibility modulus indicating a decrease in the repulsive forces between the lamellae. The origin of such a phenomenon is discussed in terms
of different mechanisms including depletion interactions, bridging interactions and modification of the electrostatic interaction
between the lamellae by the polymer.
Received 2 February 1998 相似文献
20.
M.M. Britton P.T. Callaghan 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,7(2):237-249
Using NMR velocimetry and mechanical measurements we study the flow dynamics, within a cone-and-plate rheometer, of the wormlike
micelle system, cetylpyridinium chloride/sodium salicylate (CPyCl/NaSal) at 100 mM/60 mM concentration in distilled water.
Depending on precise conditions within the system, two classes of behaviour are observed, one in which the boundary between
different shear rate phases fluctuates rapidly (on the order of tens of milliseconds) and one in which it migrates slowly
with a time constant of many seconds. These modes of behaviour may depend on minor solution impurities, which presumably affect
the detailed constitutive properties, but also on the externally applied shear rate within a given system. We argue that the
slow migrations are governed by stress relaxation effects while the rapid migrations are flow driven and arise from interfacial
instability.
Received: 2 June 1998 / Received in final form and Accepted: 27 July 1998 相似文献