共查询到20条相似文献,搜索用时 31 毫秒
1.
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
2.
Free energy of semiflexible polymers and structure of interfaces 总被引:1,自引:0,他引:1
The free energy of semiflexible polymers is calculated as a functional of the compositional scalar order parameter and the orientational order parameter of second-rank tensor Sij on the basis of a microscopic model of wormlike chains with variable segment lengths. We use a density functional theory
and a gradient expansion to evaluate the entropic part of the free energy, which is given in a power series of .The interaction term of the free energy is derived with a random phase approximation. For the rigid rod limit, the nematic-isotropic
transition point is given by , N and w being the degree of polymerization and the anisotropic interaction parameter, respectively, and the degree of ordering at
the transition point is 0.33448. We also find that the contour length of polymer chains becomes larger in a nematic phase
than in an isotropic phase. Interface profiles are obtained numerically for some typical cases. In the neighborhood of isotropic-isotropic
interfaces, polymer chains tend to align parallel to the interface on the polymer-rich side and perpendicular on the poor
side. When an isotropic region and a nematic region coexist, orientational order parallel to the interface is preferred in
the nematic region.
Received: 28 May 1998 / Revised: 12 August 1998 / Accepted: 8 September 1998 相似文献
3.
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
4.
A. Albersdörfer E. Sackmann 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,10(4):663-672
In this paper we study the effect of monovalent and divalent ions on the swelling behavior and viscoelastic parameters of ultrathin layers of the natural polyelectrolyte hyaluronic acid
covalently coupled to glass substrates. A colloidal probe technique is applied for this purpose based on latex beads, hovering over the polymer cushion. By analyzing the vertical Brownian motion of these beads with reflection
interference contrast microscopy (RICM) we determined the equilibrium layer thickness (with 3 nm vertical resolution), the
interfacial interaction potential, and the characteristic mesh size limiting the hydrodynamic flow within the polyelectrolyte
film as a function of the ionic strength. The experimental results are interpreted in terms of three different theoretical
models: the polyelectrolyte brush approximation of Pincus [#!ref1!#], a modified polyelectrolyte brush approximation in the
high salt concentration limit of Ross and Pincus [#!ref2!#] and the simple scaling approximation for neutral adsorbed polymers
of de Gennes [#!ref3!#]. Within experimental error all of these different models fit our experimental data and yield comparable
results for the equilibrium layer thickness. Moreover we determine a thickness dependent, effective surface coverage from
both brush models. The hydrodynamic properties of the films are interpreted in terms of the Brinkmann model of elastic porous
media by assuming an effective mesh size, which depends linearly on the Debye screening length. The salt induced condensation
of the polyelectrolyte films can be described microscopically in terms of a progressive contraction of the mesh size with
increasing salt concentration.
Received 10 September 1998 and Received in final form 30 November 1998 相似文献
5.
Sommer JU 《The European physical journal. E, Soft matter》2006,19(4):413-422
The equilibrium state of polymer single crystals is considered by explicitly taking into account the amorphous fraction formed
by loops and tails of the chains using a statistical model introduced by Muthukumar (Philos. Trans. R. Soc. London, Ser. A
361, 539 (2003)). We show that under realistic conditions below the equilibrium melting temperature, tight loops and close re-entries
are favored, and that the amorphous fraction can be mapped into an excess surface free energy. The model is extended to many-chain
crystals where it is shown that the lamellar thickness increases with the number of chains in the crystal and extended-chain
conformations are thermodynamically favored if the number of chains in the crystal is sufficiently large. The number of chains
necessary to form an extended-chain crystal in thermodynamic equilibrium scales with the square of the degree of polymerization
of the chains. We discuss the temperature behavior of the equilibrium crystal thickness in the under-cooled state. 相似文献
6.
S. Villain-Guillot D. Andelman 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,4(1):95-101
We study interfacial behavior of a lamellar (stripe) phase coexisting with a disordered phase. Systematic analytical expansions
are obtained for the interfacial profile in the vicinity of a tricritical point. They are characterized by a wide interfacial
region involving a large number of lamellae. Our analytical results apply to systems with one dimensional symmetry in true
thermodynamical equilibrium and are of relevance to metastable interfaces between lamellar and disordered phases in two and
three dimensions. In addition, good agreement is found with numerical minimization schemes of the full free energy functional
having the same one dimensional symmetry. The interfacial energy for the lamellar to disordered transition is obtained in
accord with mean field scaling laws of tricritical points.
Received: 28 March 1997 / Revised: 6 February 1998 / Accepted: 16 February 1998 相似文献
7.
W. Michalke S. Kreitmeier 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,4(4):469-473
The diffusion Quantum-Monte-Carlo method of solving the Schr?dinger equation is applied to the vibrational ground state of
a polyethylene molecule. The results for the ground state energy show good agreement with normal mode analysis. In addition
to stretching, bending and torsional interaction van-der-Waals interaction is applied to a single chain showing a decrease
of the energy of 5%. The decrease for a polyethylene system of 5 chains with 10 atoms per molecule at the positions of a unit
cell is determined to be 4.8% per molecule. Finally first steps towards simulating excited states were performed.
Received: 9 February 1998 / Revised: 2 April 1998 / Accepted: 23 April 1998 相似文献
8.
M.C. Fauré P. Bassereau M.A. Carignano I. Szleifer Y. Gallot D. Andelman 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,3(3):365-375
We have studied both experimentally and theoretically the surface pressure isotherms of copolymers of polystyrene-polyethyleneoxide
(PS-PEO) at the air-water interface. The SCMF (single chain mean-field) theory provides a very good agreement with the experiments
for the entire range of surface densities and is consistent with the experiments if an adsorption energy per PEO monomer at
the air-water interface of about one kB T is taken. In addition, the chain density profile has been calculated for a variety of surface densities, from the dilute
to the very dense ones. The SCMF approach has been complemented by a mean-field approach in the low density regime, where
the PEO chains act as a two-dimensional layer. Both theoretical calculations agree with the experiments in this region.
Received: 19 June 1997 / Revised: 2 February 1998 / Accepted: 11 February 1998 相似文献
9.
J.-U. Sommer 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,10(3):537-542
We study the adsorption cross-over of ideal polymer chains in an environment of disordered traps. Starting from the assumption
of an optimal cluster size of traps (optimal fluctuation method) we derive a general scaling form of the free energy function
for arbitrary spatial dimensions. For small concentrations of traps we find a cross-over from localized (adsorbed) behavior
to delocalized behavior depending on the chain's length and on the depth of the traps; this is connected with the non-monotonic
behavior of the chain's extension. In terms of the free energy of the chain this cross-over resembles a first order transition
scenario, the chain gets localized at many traps at once.
Received 18 November 1998 相似文献
10.
We consider the adsorption of an isolated, Gaussian, random, and quenched copolymer chain at an interface. We first propose
a simple analytical method to obtain the adsorption/depletion transition, by averaging over the disorder the partition function
instead of the free energy. The adsorption thresholds obtained by previous authors at a solid/liquid and at a liquid/liquid
interface for multicopolymer chains can be rederived using this method. We also compare the adsorption thresholds obtained
for bimodal and for Gaussian disorder; they only agree for small disorder. We focus on the specific case of an ideally flat
asymmetric liquid/liquid interface, and consider the situation where the chain is composed of monomers of two different chemical
species A and B. The replica method is developed for this case. We show that the Hartree approximation, coupled to a replica symmetry assumption,
leads to the same adsorption thresholds as obtained from our general method. In order to describe the properties of the adsorbed
(or depleted) chain, we develop a new approximation for long chains, within the framework of the replica theory. In most cases,
the behavior of a random copolymer chain can be mapped onto that of a homopolymer chain at an asymmetric attractive interface.
The values of the effective adsorption energy are different for a random and a periodic copolymer chain. Finally, we consider
the case of uncorrelated annealed disorder. The behavior of an annealed chain can be mapped onto that of a homopolymer chain
at an asymmetric non attractive interface; hence, an annealed chain cannot adsorb at an asymmetric interface.
Received 21 January 1999 相似文献
11.
C.M. Papadakis K. Mortensen D. Posselt 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,4(3):325-332
Binary blends of compositionally symmetric diblock copolymers are investigated using small-angle neutron scattering. The study
focuses on the miscibility of blends of polystyrene-polybutadiene diblock copolymers as a function of chain length ratio and
blend composition, and the results are related to the theoretical phase diagram put forward by M.W. Matsen (J. Chem. Phys.
103, 3268 (1995)). Three different low molar mass copolymers were blended with a high molar mass copolymer. We find very good
coincidence with the theoretical phase diagram obtained. Only for blends having a chain length ratio of 0.06, theory predicts
that a larger amount of short copolymers can be dissolved in the matrix of long copolymers, and vice versa. With the latter blends and volume fractions of short chains between 0.11 and 0.70, the second-order Bragg-peaks do not vanish,
which indicates that the lamellae are asymmetric.
Received: 9 February 1998 / Revised: 20 April 1998 / Accepted: 24 April 1998 相似文献
12.
Zhulina EB Birshtein TM Borisov OV 《The European physical journal. E, Soft matter》2006,20(3):243-256
We revise the classical Daoud-Cotton (DC) model to describe conformations of polymer and polyelectrolyte chains end-grafted to convex spherical and cylindrical surfaces.
In the framework of the DC model, local stretching of chains in the brush does not depend on the degree of polymerization of grafted chains, and the
polymer density profile follows a single-exponent power law. This model, however, does not correspond to a minimum in free
energy of the curved brush. The nonlocal (NL) approximation exploited in the present paper implies the minimization of the overall free energy of the brush and predicts
that the polymer density profile does not follow a single-exponent power law. In the limit of large surface curvature the
NL approximation provides the same scaling laws for brush thickness and free energy as the local DC model. Numerical prefactors are however different. Extra extension of chains in the brush interior region leads to larger
equilibrium brush thickness and lower free energy per chain. A significant difference between outcomes of the two models is
found for brushes formed by ionic polymers, particularly for weakly dissociating (p H-sensitive) polyelectrolytes at low solution salinity. 相似文献
13.
The simulation of a two-dimensional, broadly polydisperse, living polymers system at high concentration reveals an unusual
conformational behaviour for the longer chains. Unlike in three dimensions, the longer chains are not swollen but are squeezed
by the smaller chains. This observation is discussed in terms of a two dimensional solvent- polymer mixture whose solvent
particules are larger than the polymer monomers.
Received: 13 December 1996 / Revised: 16 March 1998 / Accepted: 27 March 1998 相似文献
14.
J. Jimenez R. Rajagopalan 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,5(2):237-243
We introduce a new simulation method, which we call the contact-distribution method, for the determination of the Helmholtz potential for polymer/colloid systems from lattice Monte-Carlo simulations. This
method allows one to obtain forces between finite or semi-infinite objects of any arbitrary shape and dimensions in the presence
of polymer chains in solution or physisorbed or chemisorbed at interfaces. We illustrate the application of the method using
two examples: (i) the interaction between the tip of an atomic force microscope (AFM) and a single, end-grafted polymer chain
and (ii) the interaction between an AFM tip and a polymer brush. Numerical results for the first two cases illustrate how
the method can be used to confirm and extend scaling laws for forces and Helmholtz potentials, to examine the effects of the
shapes and sizes of the objects and to examine conformational transitions in the polymer chains.
Received: 15 May 1998 / Revised: 11 June 1998 / Accepted: 12 June 1998 相似文献
15.
Planar brushes formed by end-grafted semiflexible polyampholyte chains, each chain containing an equal number of positively
and negatively charged monomers, are studied using molecular dynamics simulations. Keeping the length of the chains fixed,
the dependences of the average brush thickness and equilibrium statistics of the brush conformations on the grafting density
and the salt concentration are obtained with various sequences of charged monomers. When similarly charged monomers of the
chains are arranged in longer blocks, the average brush thickness is smaller and the dependence of brush properties on the
grafting density and the salt concentration is stronger. With such long blocks of similarly charged monomers, the anchored
chains bond to each other in the vicinity of the grafting surface at low grafting densities and buckle toward the grafting
surface at high grafting densities. 相似文献
16.
G. Peng J.-U. Sommer A. Blumen 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,8(1):73-79
We investigate numerically, using the bond-fluctuation model, the adsorption of many random AB-copolymers with excluded volume
interactions at the interface between two solvents. We find two regimes, controlled by the total number of polymers. In the
first (dilute) regime, the copolymers near the interface extend parallel to it, while in the second regime they extend perpendicular
to it. The density at the interface and the density in the bulk depend differently on the total number of copolymers: In the
first regime the density at the interface increases more rapidly than in the bulk, whereas the opposite is true in the second
regime.
Received 4 March 1998 and Received in final form 22 September 1998 相似文献
17.
18.
L. Piraux S. Dubois A. Fert L. Belliard 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,4(4):413-420
A theory, based on earlier work by Valet and Fert, is first presented to describe the influence of temperature on the perpendicular
giant magnetoresistance (GMR) in multilayers. Then we present GMR measurements performed at T=77 K and at room temperature on Co/Cu multilayered nanowires with layer thicknesses ranging from a few nm to 1 μm. We use
our model to obtain a good quantitative fit to the experimental results in both the short spin diffusion length limit and
out of this limit. We discuss the temperature dependence of the bulk parameters, the scattering spin asymmetry coefficient
and spin diffusion length in the Co layers.
Received: 25 January 1998 / Accepted: 6 May 1998 相似文献
19.
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
20.
Bonet Avalos J Johner A Díez-Orrite S 《The European physical journal. E, Soft matter》2006,21(4):305-317
The structure of the layer formed after polymer adsorption onto a spherical particle is numerically studied by means of the
application of the Single-Chain Mean-Field theory. We have determined several overall layer properties including the monomer
volume fraction profiles, the layer thickness, adsorbances related to loops and to tails, as well as the variation of the
crossover distance between loops and tails for different particle radii and fixed polymer length. When the radius of the sphere
is small enough to affect the loop layer, one enters a single-adsorbed-chain regime, characterized by a critical sphere radius.
In this regime, structural changes in the adsorbed layer arise. For such small sphere, the loop layer is confined to a region
whose thickness is of the order of the radius of the adsorbing sphere, and two long tails dominate the outer layer and the
adsorbance due to tails dominates that due to loops. An analysis of the structure of the outer tail layer for this small sphere
case is also presented. 相似文献