共查询到20条相似文献,搜索用时 28 毫秒
1.
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
2.
P.G. de Gennes 《The European physical journal. E, Soft matter》2002,7(1):31-34
When a glassy polymer film is formed by evaporation, the region near the free surface is polymer rich and becomes glassy first,
as noticed long ago by Scriven et al. We discuss the thickness of this “crust” and the time interval where it is present --before freezing of the whole film. We
argue that the crust is under mechanical tension, and should form some cracks. This may be the source of the roughness observed
on the final, dry films, when the solvent vapor pressure is high (and leads to thin crusts).
Received 13 November 2001 相似文献
3.
L.V. Fil'kov V.L. Kashevarov 《The European Physical Journal A - Hadrons and Nuclei》1999,5(3):285-292
The Compton scattering on a charged pion and the process γγ→π0π0 are studied using the dispersion relations. Unknown parameters of the S-wave ππ interaction and a sum and a difference of the π0 meson polarizabilities are found from a fit to the experimental data for the γγ→π0π0 process. The found parameters of the ππ interaction are used for the calculation of the cross section of the elastic γπ± scattering. The analysis of the obtained results shows that the experimental data for the elastic γπ± scattering in the energy region up to 1 GeV together with the data for the γγ→π0π0 process could be used both for a determination of the pion polarizability values and for study of the S-wave ππ interaction.
Received: 20 November 1998 / Revised version: 22 January 1999 相似文献
4.
The dynamic and static properties of a supercooled (non-entangled) polymer melt are investigated via molecular-dynamics (MD) simulations. The system is confined between two completely smooth and purely repulsive walls. The
wall-to-wall separation (film thickness), D, is varied from about 3 to about 14 times the bulk radius of gyration. Despite the geometric confinement, the supercooled
films exhibit many qualitative features which were also observed in the bulk and could be analyzed in terms of mode-coupling
theory (MCT). Examples are the two-step relaxation of the incoherent intermediate scattering function, the time-temperature
superposition property of the late time α-process and the space-time factorization of the scattering function on the intermediate
time scale of the MCT β-process. An analysis of the temperature dependence of the α-relaxation time suggests that the critical
temperature, T
c, of MCT decreases with D. If the confinement is not too strong ( D≥10monomer diameter), the static structure factor of the film coincides with that of the bulk when compared for the same distance,
T - T
c(D), to the critical temperature. This suggests that T - T
c(D) is an important temperature scale of our model both in the bulk and in the films.
Received 12 September 2001 相似文献
5.
D. Langevin 《The European physical journal. E, Soft matter》2001,5(1):81-85
Studies of thin liquid films, made from semidilute polyelectrolyte solutions, are presented. The disjoining pressure variation
with film thickness exhibits oscillations, corresponding to film stratification. The oscillations become sharper as the polymer
concentration c increases, and disappear when salt is added. The period of the oscillations scales as c
-1/2. The observed stratification is related to the polymer network and the size of the steps to the mesh size ξ.
Received 25 April 2000 and Received in final form 3 October 2000 相似文献
6.
W. Cassing Ye.S. Golubeva L.A. Kondratyuk 《The European Physical Journal A - Hadrons and Nuclei》2000,7(2):279-285
We study the possibility to measure the elastic ΦN (Φ≡J/ψ,ψ(2S), ψ(3770), χ2c) scattering cross section in the reaction ˉp+d→Φ+n
sp and the elastic D(ˉD)N scattering cross section in the reaction ˉp+d→D
−
D
0
p
sp. Our studies indicate that the elastic scattering cross sections can be determined for Φ momenta about 4–6 GeV/c and D/ˉD momenta 2–5 GeV/c by selecting events with p
t≥ 0.4 GeV/c for Φ's and p
t(p
sp) ≥ 0.5 GeV/c for D/ˉD-meson production.
Received: 8 November 1999 相似文献
7.
K.-M. Källman M. Brenner V.Z. Goldberg T. Lönnroth P. Manngård A.E. Pakhomov V.V. Pankratov 《The European Physical Journal A - Hadrons and Nuclei》2003,16(2):159-169
The excitation function and angular distributions of elastic α-particle scattering on 28Si have been measured in the laboratory energy range 6-28 MeV using a backscattering technique on a thick target, yielding
a continuous energy distribution. More than 200 narrow states are observed, with widths in the range ∼ 30-100 keV at excitation
energies E
* = 13-32 MeV. Angular distributions at backward angles were measured, and angular momentum values of more than 83 states have
been deduced. The analysis gives spin-parities J
π, α-partial widths Γα and reduced widths of the narrow high-lying resonant states in 32S. The experimentally observed states display both the negative- and the positive-parity rotational-like sequences with seemingly
no parity splitting, a finding which is at variance with most potential-model predictions. The deduced effective moment of
inertia indicates a more extended structure than the ground-state configuration. The observed strength of each ℓ-value is
analyzed in terms of an underlying split doorway state of Lorentzian form, which yields an interpretation as fragmented rotational
α + 28Si states.
Received: 26 June 2000 / Accepted: 16 September 2002 / Published online: 4 February 2003
RID="a"
ID="a"e-mail: kkallman@abo.fi
RID="b"
ID="b"Present address: Swedish Polytechnic, FIN-65200 Vasa, Finland.
Communicated by D. Guerreau 相似文献
8.
S. Kutter E.M. Terentjev 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,21(3):455-462
Twist-storing polymers respond with elastic energy penalty to coherent or random twisting along the local chain axis away
from its equilibrium, which can be straight (as in “ribbons”) or helical (as in DNA and other biopolymers). Here we study
the equilibrium conformation of such polymers, focusing on the thermodynamic balance between twist and writhe, resulting from
the competition between the random coil entropy and the potential energy stored in superhelical portions of the polymer chain.
Two macroscopic variables characterise such a chain, the end-to-end distance R and the link number Lk, which is a topological invariant of a given polymer with clamped ends. We find that with increasing link number Lk, the chain accommodates its excess twist in growing plectonemes, unless forced out of this state by stretching its end-to-end
distance R. We calculate the force-extension relation, which exhibits crossovers between different deformation regimes.
Received 16 November 2000 and Received in final form 6 February 2001 相似文献
9.
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
10.
G. Savona D.V. Livanov A.A. Varlamov 《The European Physical Journal B - Condensed Matter and Complex Systems》2003,32(4):415-420
We study the fluctuation-induced magnetotransport of a two-dimensional superconductor in the quasiballistic regime, where
ξ
GL(T) ≪ ℓ (ℓ is the electron mean free path and ξ
GL(T) is the Ginzburg-Landau coherence length). The magnetoconductivity is evaluated in the nonlocal fluctuation regime thereby
extending the existing theory valid in the local limit. We show that the Maki-Thompson (MT) and density-of-states (DOS) contributions
strongly compensate each other and their sum is negligible in comparison with the Aslamazov-Larkin (AL) term. The hierarchy
of the fluctuation contributions to the magnetoconductivity in the high-field limit is also qualitatively discussed.
Received 10 July 2002 / Received in final form 21 November 2002 Published online 7 May 2003 相似文献
11.
12.
J. Becker H.G. Andresen J.R.M. Annand K. Aulenbacher K. Beuchel J. Blume–Werry Th. Dombo P. Drescher M. Ebert D. Eyl A. Frey P. Grabmayr T. Großmann P. Hartmann T. Hehl W. Heil C. Herberg J. Hoffmann J.D. Kellie F. Klein K. Livingston M. Leduc M. Meyerhoff H. Möller Ch. Nachtigall A. Natter M. Ostrick E.W. Otten R.O. Owens S. Plützer E. Reichert D. Rohe M. Schäfer H. Schmieden R. Sprengard M. Steigerwald K.-H. Steffens R. Surkau Th. Walcher R. Watson E. Wilms 《The European Physical Journal A - Hadrons and Nuclei》1999,6(3):329-344
The electric form factor of the neutron GEn has been determined in double polarized exclusive 3
He(e,e'n) scattering in quasi–elastic kinematics by measuring asymmetries A
⊥, A
∥ of the cross section with respect to helicity reversal of the electron, with the nuclear spin being oriented perpendicular
to the momentum transfer q in case of A⊥ and parallel in case of A∥. The experiment was performed at the 855 MeV c. w. microtron MAMI at Mainz. The degree of polarization of the electron beam
and of the gaseous 3
He target were each about 50%. Scattered electrons and neutrons were detected in coincidence by detector arrays covering large
solid angles. Quasi–elastic scattering events were reconstructed from the measured electron scattering angles ϑe, φe and the neutron momentum vector p
n
′ in the plane wave impulse approximation. We obtain the result <G
En>(0.27 < Q2c2/GeV2 < 0.5)= 0.0334 ± 0.0033stat± 0.0028syst which is averaged over the indicated range of Q
2, the squared momentum transfer. This G
En value is significantly smaller than measured from the D(e,e'n) reaction under similar kinematical conditions. To what extent final state interactions in 3He quench the G
En result is subject of calculations currently in progress elsewhere.
Received: 29 April 1999 相似文献
13.
Verneuil E Clain J Buguin A Brochard-Wyart F 《The European physical journal. E, Soft matter》2003,10(4):345-353
A soft bead (radius R
b) is pressed with a force F against a hydrophobic glass plate through a water drop (“wet” JKR set-up). We observe with a fast camera the growth of the
contact zone bridging the rubber bead to the glass. Depending on the approach velocity V, two regimes are observed : i) at large V a liquid film is squeezed at the interface and dewets by nucleation and growth of a dry contact; ii) at low velocities, the
bead remains nearly spherical. As it comes into contact, the rubber bead spreads on the glass with a characteristic time (in
the range of one millisecond) τ ≈ ηR
b
2/F, where η is the liquid viscosity. The laws of spreading are interpreted by a balance of global mechanical and viscous forces.
Received: 22 December 2002 / Accepted: 24 March 2003 / Published online: 29 April 2003
RID="a"
ID="a"e-mail: brochard@curie.fr 相似文献
14.
Th. Pospischil P. Bartsch D. Baumann R. Böhm K. Bohinc M. Ding S. Derber M.O. Distler D. Elsner I. Ewald J. Friedrich J.M. Friedrich S. Grözinger S. Hedicke P. Jennewein J. Jourdan M. Kahrau F. Klein K.W. Krygier J. Lac A. Liesenfeld S. Malov J. McIntyre H. Merkel P. Merle U. Müller R. Neuhausen M. Potokar R.D. Ransome D. Rohe G. Rosner J. Sanner H. Schmieden M. Seimetz S. Širca I. Sick O. Strähle A. Süle A. Wagner Th. Walcher M. Weis 《The European Physical Journal A - Hadrons and Nuclei》2001,12(1):125-127
The polarization transfer from longitudinally polarized electrons to protons in the elastic scattering p(
e, e'
p) has been measured around Q2 = 0.4 (GeV/c)2 with the three-spectrometer facility at the Mainz microtron MAMI. From this polarization transfer the ratio G
Ep/(G
Mp/μp) has been determined. The ratio is found to be slightly less than unity in agreement with recent results from other laboratories
and from the Rosenbluth separation of cross-sections measured with unpolarized electrons.
Received: 26 July 2001 / Accepted: 25 September 2001 相似文献
15.
Higgins AM Sferrazza M Jones RA Jukes PC Sharp JS Dryden LE Webster J 《The European physical journal. E, Soft matter》2002,8(2):137-143
We investigate the dynamics of spinodal dewetting in liquid-liquid polymer systems. Dewetting of poly(methyl-methacrylate)
(PMMA) thin films on polystyrene (PS) “substrates” is followed in situ using neutron reflectivity. By following the development of roughness at the PS/PMMA interface and the PMMA surface we extract
characteristic growth times for the dewetting process. These characteristic growth times are measured as a function of the
molecular weight of the two polymers. By also carrying out experiments in the regime where the dynamics are independent of
the PS molecular weight, we are able to use dewetting to probe the scaling of the PMMA thin film viscosity with temperature
and molecular weight. We find that this scaling reflects bulk behaviour. However, absolute values are low compared to bulk
viscosities, which we suggest may be due in part to slippage at the polymer/polymer interface.
Received 25 June 2001 and Received in final form 5 December 2001 相似文献
16.
E.T. Seppälä M.J. Alava 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,21(3):407-424
We study the effect of an external field on (1 + 1) and (2 + 1) dimensional elastic manifolds, at zero temperature and with
random bond disorder. Due to the glassy energy landscape the configuration of a manifold changes often in abrupt, “first order”-type
of large jumps when the field is applied. First the scaling behavior of the energy gap between the global energy minimum and
the next lowest minimum of the manifold is considered, by employing exact ground state calculations and an extreme statistics
argument. The scaling has a logarithmic prefactor originating from the number of the minima in the landscape, and reads ΔE
1∼L
θ[ln(L
z
L
- ζ)]-1/2, where ζ is the roughness exponent and θ is the energy fluctuation exponent of the manifold, L is the linear size of the manifold, and Lz is the system height. The gap scaling is extended to the case of a finite external field and yields for the susceptibility
of the manifolds ∼L
2D + 1 - θ[(1 - ζ)ln(L)]1/2. We also present a mean field argument for the finite size scaling of the first jump field, h
1∼L
d - θ. The implications to wetting in random systems, to finite-temperature behavior and the relation to Kardar-Parisi-Zhang non-equilibrium
surface growth are discussed.
Received December 2000 and Received in final form April 2001 相似文献
17.
J. Horbach W. Kob K. Binder 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,19(4):531-543
We present the results of extensive molecular dynamics computer simulations in which the high frequency dynamics of silica,
i.e. for frequencies ν > 0.5 THz, is investigated in the viscous liquid state as well as in the glass state. We characterize the
properties of high frequency sound modes by analyzing J
l(q,ν) and J
t(q,ν), the longitudinal and transverse current correlation function, respectively. For wave-vectors q > 0.4 ?-1 the spectra are sitting on top of a flat background. The dynamic structure factor S(q,ν) exhibits for q > 0.23 ?-1 a boson peak which is located nearly independent of q around 1.7 THz and for which the intensity scales approximately linearly with temperature. We show that the low frequency
part of the boson peak is mainly due to the elastic scattering of transverse acoustic modes with frequencies around 1 THz.
The strength of this scattering depends on q and is largest around q = 1.7 ?-1, the location of the first sharp diffraction peak in the static structure factor. By studying S(q,ν) for different system sizes we show that strong finite size effects are present in the low frequency part of the boson
peak in that for small systems part of its intensity is missing. We discuss the consequences of these finite size effects
for the structural relaxation.
Received 27 June 2000 and Received in final form 9 January 2001 相似文献
18.
Hartmann L Gorbatschow W Hauwede J Kremer F 《The European physical journal. E, Soft matter》2002,8(2):145-154
The molecular dynamics in thin films (18 nm-137 nm) of isotactic poly(methyl methacrylate) (i-PMMA) of two molecular weights
embedded between aluminium electrodes are measured by means of dielectric spectroscopy in the frequency range from 50 mHz
to 10 MHz at temperatures between 273 K and 392 K. The observed dynamics is characterized by two relaxation processes: the
dynamic glass transition (α-relaxation) and a (local) secondary β-relaxation. While the latter does not depend on the dimensions
of the sample, the dynamic glass transition becomes faster (≤2 decades) with decreasing film thickness. This results in a
shift of the glass transition temperature T
g to lower values compared to the bulk. With decreasing film thickness a broadening of the relaxation time distribution and
a decrease of the dielectric strength is observed for the α-relaxation. This enables to deduce a model based on immobilized
boundary layers and on a region displaying a dynamics faster than in the bulk. Additionally, T
g was determined by temperature-dependent ellipsometric measurements of the thickness of films prepared on silica. These measurements
yield a gradual increase of T
g with decreasing film thickness. The findings concerning the different thickness dependences of T
g are explained by changes of the interaction between the polymer and the substrates. A quantitative analysis of the T
g shifts incorporates recently developed models to describe the glass transition in thin polymer films.
Received 12 August 2001 and Received in final form 16 November 2001 相似文献
19.
We consider a low-density assembly of spherical colloids, such that each is clothed by L end-grafted chemically incompatible polymer chains either of types A or B. These are assumed to be dissolved in a good common solvent. We assume that colloids are of small size to be considered as
star-polymers. Two adjacent star-polymers A and B interact through a force F originating from both excluded-volume effects and chemical mismatch between unlike monomers. Using a method developed by
Witten and Pincus (Macromolecules 19, 2509 (1986)) in the context of star-polymers of the same chemical nature, we determine exactly the force F as a function of the center-to-center distance h. We find that this force is the sum of two contributions F
e and F
s. The former, that results from the excluded volume, decays as F
e∼A
L
h
-1, with the L -dependent universal amplitude A
L∼L
3/2. While the second, which comes from the chemical mismatch, decays more slowly as F
s∼χB
L
h
-1 - τ, where τ is a critical exponent whose value is found to be τ 0.40, and χ is the standard Flory interaction parameter. We find that the corresponding L-dependent universal amplitude is B
L∼L
3 + τ
/2. Theses forces are comparable near the cores of two adjacent star-polymers, i.e. for h∼h
c∼a
(a is the monomer size). Finally, for two star-polymers of the same chemical nature (A or B), the force F that simply results from excluded-volume effects coincides exactly with F
e, and then the known result is recovered.
Received 2 October 2000 and Received in final form 24 January 2001 相似文献
20.
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