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1.
Shear mechanical anisotropy of side chain liquid-crystal elastomers: Influence of sample preparation
Rogez D Francius G Finkelmann H Martinoty P 《The European physical journal. E, Soft matter》2006,20(4):369-378
We study the mechanical anisotropy of a series of uniaxial side chain nematic elastomers prepared with the same chemical composition
but with different preparation protocols. For all the compounds, the experiments performed as a function of temperature show
no discontinuity in both G'
// and G'
⊥ (the labels // and ⊥ stand for the director parallel, respectively perpendicular to the shear displacement) around the nematic-isotropic
(N-I) phase transition temperature determined by DSC. They also all show a small decrease in G'
// starting at temperatures well above this temperature (from ∼ 4°
C to ∼ 20°
C depending on the compound studied) and leading to a small hydrodynamic value of the G'
⊥/G'
// ratio. The measurements taken as a function of frequency show that the second plateau in G'
// and the associated dip in G
//” expected from dynamic semi-soft elasticity are not observed. These results can be described by the de Gennes model, which
predicts small elastic anisotropy in the hydrodynamic and linear regimes. They correspond to the behavior expected for compounds
beyond the mechanical critical point, which is consistent with the NMR and specific heat measurements taken on similar compounds.
We also show that a reduction in the cross-linking density does not change the non-soft character of the mechanical response.
From the measurements taken as a function of frequency at several temperatures we deduce that the time-temperature superposition
method does not apply. From these measurements, we also determine the temperature dependence of the longest relaxation time
τE of the network for the situations where the director is either parallel or perpendicular to the shear velocity. Finally,
we discuss the influence on the measurements of the mechanical constraint associated with the fact that the samples cannot
change their shape around the pseudo phase transition, because of their strong adherence on the sample-bearing glass slides. 相似文献
2.
V. Fourmaux-Demange A. Brûlet F. Boué P. Davidson P. Keller J.P. Cotton 《The European physical journal. E, Soft matter》2000,1(4):301-317
We have studied the rheology and the conformation of stretched comb-like liquid-crystalline polymers. Both the influence of
the comb-like structure and the specific effect of the nematic interaction on the dynamics are investigated. For this purpose,
two isomers of a comb-like polymetacrylate polymer, of well-defined molecular weights, were synthesized: one displays a nematic
phase over a wide range of temperature, the other one has only an isotropic phase. Even with high degrees of polymerization
N, between 40 and 1000, the polymer chains studied were not entangled. The stress-strain curves during the stretching and relaxation
processes show differences between the isotropic and nematic comb-like polymers. They suggest that, in the nematic phase,
the chain dynamics is more cooperative than for a usual linear polymer. Small-angle neutron scattering has been used in order
to determine the evolution of the chain conformation after stretching, as a function of the duration of relaxation t
r. The conformation can be described with two parameters only: , the global deformation of the polymer chain, and p, the number of statistical units of locally relaxed sub-chains. For the comb-like polymer, the chain deformation is pseudo-affine:
is always smaller than (the deformation ratio of the whole sample). In the isotropic phase, has a constant value, while pincreases as tr. This latter behavior is not that expected for non-entangled chains, in which p varies as t
r
1/2 (Rouse model). In the nematic phase, decreases as a stretched exponential function of t
r, while p remains constant. The dynamics of the comb-like polymers is discussed in terms of living clusters from which junctions are
produced by interactions between side chains. The nematic interaction increases the lifetime of these junctions and, strikingly,
the relaxation is the same at all scales of the whole polymer chain.
Received 5 May 1999 and Received in final form 18 October 1999 相似文献
3.
J. Dunkel W. Ebeling U. Erdmann 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,24(4):511-524
We investigate the stochastic dynamics of an one-dimensional ring with N self-driven Brownian particles. In this model neighboring particles interact via conservative Morse potentials. The influence of the surrounding heat bath is modeled by Langevin-forces (white noise) and
a constant viscous friction coefficient γ. The Brownian particles are provided with internal energy depots which may lead to active motions of the particles. The depots
are realized by an additional nonlinearly velocity-dependent friction coefficient γ
1(v) in the equations of motions. In the first part of the paper we study the partition functions of time averages and thermodynamical
quantities (e.g. pressure) characterizing the stationary physical system. Numerically calculated non-equilibrium phase diagrams are represented.
The last part is dedicated to transport phenomena by including a homogeneous external force field that breaks the symmetry
of the model. Here we find enhanced mobility of the particles at low temperatures.
Received 21 July 2001 相似文献
4.
E. Orlandini T. Garel 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,6(1):101-110
We study a single self avoiding hydrophilic hydrophobic polymer chain, through Monte-Carlo lattice simulations. The affinity
of monomer i for water is characterized by a (scalar) charge , and the monomer-water interaction is short-ranged. Assuming incompressibility yields an effective short ranged interaction
between monomer pairs (i,j), proportional to . In this article, we take (resp. ()) for hydrophilic (resp. hydrophobic) monomers and consider a chain with (i) an equal number of hydro-philic and -phobic
monomers (ii) a periodic distribution of the along the chain, with periodicity 2p. The simulations are done for various chain lengths N, in d=2 (square lattice) and d=3 (cubic lattice). There is a critical value p
c
(d,N) of the periodicity, which distinguishes between different low temperature structures. For p >p
c
, the ground state corresponds to a macroscopic phase separation between a dense hydrophobic core and hydrophilic loops. For
p <p
c
(but not too small), one gets a microscopic (finite scale) phase separation, and the ground state corresponds to a chain
or network of hydrophobic droplets, coated by hydrophilic monomers. We restrict our study to two extreme cases, and to illustrate the physics of the various phase transitions. A tentative variational approach is also presented.
Received: 10 March 1998 / Received in final form: 25 June 1998 / Accepted: 1st July 1998 相似文献
5.
A.S. Sabino G.P. De Sousa C. Luz-Lima P.T.C. Freire F.E.A. Melo J. Mendes Filho 《Solid State Communications》2009,149(37-38):1553-1556
Raman spectroscopy investigations of L-isoleucine crystals under high pressures have been carried out up to 7.3 GPa. From this study it was possible to observe modifications on bands associated to both rocking vibrations of r(NH3+) and r(CO2−) as well as to lattice modes at about 2.3 and 5.0 GPa. These modifications were correlated to either conformational change of molecules or to a solid–solid phase transition undergone by the crystals involving the hydrogen bonds that maintain the molecules held in the unit cell. A comparison with a few results on other amino acid crystals is also given. 相似文献
6.
E. Orignac Y. Suzumura 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,23(1):57-67
We study the interplay of Anderson localization and interaction in a two chain Hubbard ladder allowing for arbitrary ratio
of disorder strength to interchain coupling. We obtain three different types of spin gapped localized phases depending on
the strength of disorder: a pinned 4k
F Charge Density Wave (CDW) for weak disorder, a pinned 2k
F CDWπ for intermediate disorder and two independently pinned single chain 2k
F CDW for strong disorder. Confinement of electrons can be obtained as a result of strong disorder or strong attraction. We
give the full phase diagram as a function of disorder, interaction strength and interchain hopping. We also study the influence
of interchain hopping on localization length and show that localization is enhanced by a small interchain hopping but suppressed
by a large interchain hopping.
Received 6 April 2001 相似文献
7.
S. Nonose S. Iwaoka H. Tanaka N. Okai T. Shibakusa K. Fuke 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2003,24(1-3):335-338
Photo-induced reaction of
[Fe(III)-protoporphyrin]+
(hemin+) ions solvated with
dimethylsulfoxide (DMSO) is investigated by using a tandem mass
spectrometer with electrospray ionization. We measure the
photodissociation yields of mass-selected
hemin+(DMSO)n
clusters for n = 0-3. The
mass spectra of the fragment ions show the -cleavage of
carboxymethyl groups in addition to the evaporation of solvent
molecules. Yield of the -cleavage reaction is found to depend
strongly on the excitation energy and the number of solvent
molecules. We also examine photo-induced reactions of
multiply-charged cytochrome c ions, (M + nH)n + (
n = 9-17). Photoionization is
found to be the dominant process for the lower charged states (
n = 9-12) and its yield
decreases rapidly with increasing the charge. The
photoionization is ascribed to the emission of electron by
multiphoton excitation of heme under the influence of Coulomb
attractive potential arising from the charges in the polypeptide
chain. Model calculations of the Coulomb potential suggest that
the structure of the polypeptide chain is completely elongated. 相似文献
8.
M. Block O. Rehm P. Seibert G. Werth 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》1999,7(3):461-465
We have measured the lifetime of the metastable 3D
5/2
level in Ca+ using the “quantum jump" technique on a single stored and laser cooled ion in a linear Paul trap. We found a linear dependence
of the measured decay rate on the power of the laser which repumps the ions from the long lived 3D
3/2
level. This can be explained by off-resonant depletion of the 3D
5/2
level. The proper lifetime of this level is obtained by a linear extrapolation of the measured lifetime to zero laser power.
We obtain 1100(18) ms in agreement with theoretical calculations. The observed systematic change of the decay rate resolves
discrepancies between earlier experiments in which this effect had not been considered. Measurements on a linear chain of
10 laser cooled ions showed unexpected frequent coincidences of quantum jumps within our observation time of 20 ms. This indicates
a so far unexplained correlation between the ions in the chain at large distances.
Received 3 March 1999 相似文献
9.
O. Pelletier C. Bourgaux O. Diat P. Davidson J. Livage 《The European physical journal. E, Soft matter》2000,2(2):191-198
We report small-angle X-ray scattering experiments performed in both the isotropic and nematic phases of aqueous V2O5 suspensions. We show that the scattering in the isotropic phase can be well described in the whole accessible q-range by only considering the form factor of non-interacting ribbons. We investigate the influence of concentration and pH on the dimensions of V2O5 ribbons and show that these parameters do not have any significant effect, as long as the system stays well within the chemical
stability domain of the ribbons. We then show that nematic single domains display an anisotropic small-angle scattering pattern,
even at scattering vectors small compared to that at which a characteristic correlation peak is observed. This feature is
expected for a nematic phase, but was rarely observed. We finally try to describe this scattering within the framework of
theories developed for the structure factor of a nematic polymer, and we reach the conclusion that chain ends are certainly
important to understand this pattern.
Received 21 July 1999 and Received in final form 17 December 1999 相似文献
10.
The depolarisation ratio for the Raman-active phenyl stretching mode has been measured over the whole of the mesophase range,
and the orientational order parameters deduced, in the uniaxial nematic liquid crystal octylcyanobiphenyl (8CB). Linearly
polarised light was incident normally on a homogeneously aligned sample and a χ2 minimisation routine performed on the 360° depolarisation ratio profile. The order parameters 〈P
200〉 and 〈P
400〉 , together with the differential polarisability ratio, r , are used as fitting parameters and measured as a function of temperature. Interestingly, we show that the value for r , conventionally measured in the isotropic phase and assumed to remain constant, has a clear temperature dependence, ranging
from -0.032±0.008 in the isotropic phase through to -0.245±0.015 at the nematic-to-smectic A phase transition. The measured
order parameters 〈P
200〉 and 〈P
400〉 varied from 0.35- 0.55±0.02 and 0.180- 0.245±0.02 , respectively, across the 8 ° C wide nematic phase range. The values of both 〈P
200〉 and 〈P
400〉 are in excellent agreement with theory, but it is noteworthy that 〈P
400〉 shows a much better quantitative match than has been reported in previous work. Crucially the temperature dependence of
r is shown to be a contributing factor in the low 〈P
400〉 values that have been conventionally reported from Raman scattering measurements. The potential for fitting the entire angular
depolarisation ratio distribution in liquid crystalline systems that are described by more order parameters, specifically
biaxial materials, is discussed. 相似文献
11.
12.
V. Rusanov H. Winkler C. Ober A.X. Trautwein 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,12(2):191-198
M?ssbauer studies were performed on single crystals of guanidinium nitroprusside with different orientations of their principal
crystallographic axes (a, b, c) with respect to the incident radiation. The markedly anisotropic Lamb-M?ssbauer factor f
LM
, i.e.
f
LM
(a) = 0.118(8), f
LM
(b) = 0.174(8), f
LM
(c) = 0.202(8) is in contrast to that of nitroprussides with inorganic anions. The observed anisotropy is ascribed to the anisotropic
vibrational mean-square displacement of the nitroprusside anions as a whole which is due to the specific packing of both,
anions and cations, as well as the very weak chemical bonding between the ions, typical only for guanidinium nitroprusside.
The vibrational anisotropy of iron atoms in barium nitroprusside that has been observed by X-ray structural investigations
has a different origin and therefore does not result in an anisotropic Lamb-M?ssbauer factor. We have also investigated metastable
states in guanidinium nitroprusside that have been populated by means of incoherent irradiation from light-emitting diodes.
With a specific orientation of the guanidinium nitroprusside single crystal a population of the metastable states up to 26%
could be achieved. Populations of comparable size on lithium, sodium and potassium nitroprussides have only been reached using
coherent laser irradiation.
Received 15 December 1998 and Received in final form 3 March 1999 相似文献
13.
V. Yamakov A. Milchev G.M. Foo R.B. Pandey D. Stauffer 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,9(4):659-667
A coarse-grained off-lattice bead-spring model is used to reveal the complex dynamics of a polymer chain in a quenched porous
medium in the presence of an external field B. The behavior of the mean square displacement (MSD) of the center chain bead and that of the center of mass of the chain
as a function of time is studied at different values of the barrier concentration C, the field strength B and the chain length N. In a field, important information on the way in which chains move between obstacles and overcome them is gained from the
MSD vs. time analysis in the directions parallel and perpendicular to the flow. Instead of a steady approach to uniform drift-like
motion at low C, for sufficiently strong field B we observe logarithmic oscillations in the effective exponents describing the time dependence of the MSD along and perpendicular to field. A common nature of
this phenomenon with oscillatory behavior, observed earlier for biased diffusion of tracers on random lattices, is suggested.
Received 7 August 1998 相似文献
14.
S.W.H. Eijt R. Currat J.E. Lorenzo P. Saint-Grégoire B. Hennion Y.M. Vysochanskii 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,5(2):169-178
The low frequency lattice dynamics and its relationship to the second order paraelectric-to-ferroelectric transition in Sn2P2S6 is studied. The dispersion branches of the acoustic and lowest lying optical phonons in the a*-c* plane have been obtained in the ferroelectric phase, for x-polarized phonons. Close to the phase transition a considerable softening is found for the lowest optical mode (Px), comparable to the behaviour observed in previous Raman investigations. As found previously in Sn2P2Se6, a strong coupling between the TO(Px) and TA(uxz) phonons is observed, although, apparently, not strong enough to lead to an incommensurate phase. The soft TO(Px) mode at the zone center is observed. The temperature dependence of its frequency and damping shows that the transition is
not entirely displacive. At low temperatures an unusual apparent negative LO-TO splitting is observed which is shown to arise
from the coupling of the x-polarized soft mode to the nearby z-polarized optical phonon. For comparison, the soft TO(Px) dispersion in the a*-b* plane is measured in both the paraelectric and ferroelectric phases. Consistent frequency changes and LO-TO splitting are
observed, revealing a significant interaction between the TA(uyx) and LA(uxx) acoustics branches and the TO and LO soft optic branches, respectively. In contrast, the nearby y-polarized optic branch shows almost no temperature dependence. Finally, the influence of piezoelectric effects on the limiting
acoustic slopes in the ferroelectric phase is discussed.
Received: 11 May 1998 / Revised and Accepted: 15 June 1998 相似文献
15.
G. Migliorini V.G. Rostiashvili T.A. Vilgis 《The European Physical Journal B - Condensed Matter and Complex Systems》2003,33(1):61-73
The Langevin dynamics of a self-interacting chain embedded in a quenched random medium is investigated by making use of the
generating functional method and one-loop (Hartree) approximation. We have shown how this intrinsic disorder causes different
dynamical regimes. Namely, within the Rouse characteristic time interval the anomalous diffusion shows up. The corresponding
subdiffusional dynamical exponents have been explicitly calculated and thoroughly discussed. For the larger time interval
the disorder drives the center of mass of the chain to a trap or frozen state provided that the Harris parameter, (Δ/b
d)N
2 - νd≥1, where Δ is a disorder strength, b is a Kuhnian segment length, N is a chain length and ν is the Flory exponent. We have derived the general equation for the non-ergodicity function f (p) which characterizes the amplitude of frozen Rouse modes with an index p = 2πj/N. The numerical solution of this equation has been implemented and shown that the different Rouse modes freeze up at the same
critical disorder strength Δ
c ∼ N
- γ where the exponent γ ≈ 0.25 and does not depend from the solvent quality.
Received 17 December 2002 Published online 23 May 2003
RID="a"
ID="a"e-mail: vilgis@mpip-mainz.mpg.de 相似文献
16.
We describe a chiral axial next nearest neighbour xy-model to account for the various subphases exhibited by antiferroelectric liquid crystals made of chiral rod-like molecules.
The assumed form of the interlayer interactions is based on physical processes which are discussed. Using a discrete model,
the predicted sequence of transitions is SmA-SmC-SmC-FIH-FII-FIL-SmC
*
A, where FI stands for a ferriphase, as seen in many compounds. The ferri and SmC phases are characterized by relatively large angles between the c-vectors of successive layers and occur only when the compounds have high optical purity. The calculated field induced structures
exhibit a plateau of the apparent tilt angle at , where is the tilt angle of the molecules in the ferriphase. The conoscopic figures in the presence of a field and ellipsometric
parameters in the absence of a field have also been generated, which agree extremely well with the experimental results. Recent
anomalous X-ray scattering studies prove the xy-character of the configurations, though the commensurate structures that are found in the ferriphase require an extension
of the model to include lock-in terms.
Received 23 August 1999 相似文献
17.
A. Qayyum M.N. Akhtar 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2000,12(1):181-184
Light emission during sputtering of graphite targets with 1–10 keV Ne+, Kr+ and Xe+ beams has been investigated in the 180-600 nm wavelength range. Beside the characteristic lines of sputtered C1 and C1
+, a continuum superimposed with a number of broad structures was observed in the 250-520 nm range, and having a maximum at
386 nm. Mass analysis of the sputtered flux confirmed the presence of negative carbon clusters C
m
-
, C2
- being the dominant one. Ion beam parameters i.e. ion mass, energy, current density and ion dose were varied to identify the origin of the continuum emission. On the basis
of the experimental results, it is suggested that the continuum is predominantly due to the overlapping of various band systems
of sputtered C2 with a small contribution from the heavier sputtered carbon clusters Cm (m>2).
Received 24 September 1999 and Received in final form 11 February 2000 相似文献
18.
M. Bertolus V. Brenner P. Millié 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2000,11(3):387-394
An ab initio study of the Nan(OH)n, Nan(OH)n-1
+, Agn(OH)n, and Agn(OH)n-1
+ clusters with n up to four is presented. The results of this study show that, in accordance with experimental observations, the sodium hydroxide
clusters are almost purely ionic, while the Ag-O bond exhibits a significant covalent character. The perturbation caused by
the non-spherical OH- group relatively to an atomic anion, as well as the influence on structures and energies of the covalent character of the
metal-oxygen bond are determined. The appearance of metal-metal bonds in the silver hydroxide clusters is also discussed.
Finally, the theoretical results obtained on the Na-OH clusters are compared to experimental results available on the dissociation
of the Nan(OH)n-1
+ clusters.
Received 9 August 1999 and Received in final form 1st December 1999 相似文献
19.
20.
Summary Spin labelling ESR, calorimetry and31P NMR have been used to investigate dipalmitoyl phosphatidylcholine dispersions incorporated with propranolol which is a β-adrenoceptor
blocking drug, widely used in the treatment of arrhythmy. The drug appears to enter the lipid bilayer also at the lowest concentrations
and to induce a progressive fluidification of multilamellar vesicles. In fact, the presence of drug reduces the temperature
of chain melting transition and the corresponding cooperation number. Furthermore the phase transitionL
β′
−P
β′
is not observable also at the lowest drug concentrations.
Work presented at the First USSR-Italy Bilateral Meeting on Liquid Crystals held in Portonovo, Ancona (Italy), September 30–October
2, 1987. 相似文献