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1.
The elastic anisotropy of nematic elastomers 总被引:2,自引:0,他引:2
We examine the robustness of order in nematic elastomers under mechanical strains imposed along and perpendicularly to the
director when director rotation is prohibited. In contrast to electric and magnetic fields applied to conventional nematics,
mechanical fields are shown theoretically and experimentally to greatly affect the degree of nematic order and related quantities.
Unlike in liquid nematics, one can impose fields perpendicular to the director, thereby inducing biaxial order which should
be susceptible to experimental detection. Nematic elastomers with unchanging director and degree of order should theoretically
have the same elastic moduli for longitudinal and transverse extensions. This is violated when nematic order is permitted
to relax in response to strains. Near the transition we predict the longitudinal modulus to be smaller than the transverse
modulus; at lower temperatures the converse is true, with a crossover a few degrees below the transition. The differences
are ascribed to the different temperature dependence of the stiffness of uniaxial and biaxial order. We synthesised side chain
single-crystal nematic polymer networks, performed DSC, X-ray, birefringence, and thermo-mechanical characterisations, and
then obtained linear moduli from stress-strain measurements.
Received 29 September 2000 相似文献
2.
3.
We calculate the ratio of moduli for extension parallel and perpendicular to the director of nematic elastomers. Elastomers
in practice are not ideal nematics with a jump from a finite to zero-order parameter with increasing temperature. Some classes
behave as if under effective external fields. These are really internal in origin and leave the system subcritical (still
with jumps) or supercritical. We give expressions for general non-ideality and memory of formation conditions, along with
its translation into values of the “fields". For the case of supercritical fields, we find that the modulus ratio deviates
more from unity, in accord with experiment, than in the subcritical case.
Received 29 September 2000 相似文献
4.
We develop a continuum theory of linear viscoelastic response in oriented monodomain nematic elastomers. The expression for
the dissipation function is analogous to the Leslie-Ericksen version of anisotropic nematic viscosity; we propose the relations
between the anisotropic rubber moduli and new viscous coefficients. A new dimensionless number is introduced, which describes
the relative magnitude of viscous and rubber-elastic torques. In an elastic medium with an independently mobile internal degree
of freedom, the nematic director with its own relaxation dynamics, the model shows a dramatic decrease in the dynamic modulus
in certain deformation geometries. The degree to which the storage modulus does not altogether drop to zero is shown to be
both dependent on frequency and to be proportional to the semi-softness, the non-ideality of a nematic network. We consider
the most interesting geometry for the implementation of the theory, calculating the dynamic response to an imposed simple
shear and making predictions for effective moduli and (exceptionally high) loss factors.
Received 16 October 2000 and Received in final form 10 December 2000 相似文献
5.
We report a combined theoretical and experimental study of linear viscoelastic response in oriented monodomain nematic elastomers. The model predicts a dramatic decrease in the dynamic modulus in certain deformation geometries in an elastic medium with an independently mobile internal degree of freedom, the nematic director with its own relaxation dynamics. Dynamic mechanical measurements on monodomain nematic elastomers confirm our predictions of dependence on shear geometry and on nematic order, and also show a very substantial mechanical loss clearly associated with director relaxation. 相似文献
6.
We discuss several elastic energies for nematic elastomers and their small strain expansions both in the regime of large director
rotations, and in the case that director changes are small. We propose two fully non-linear model anisotropic energies and
compare the behavior they predict with the currently available experimental evidence. 相似文献
7.
The diffusion of azo-dye (DR-1) in a planar liquid crystals host (5CB) at various temperatures has been investigated by laser-induced
holographic grating relaxation technique. The decay of the diffraction intensity provides information about the diffusion
of photoexcited azo-dye molecules. The relaxation time constants can be derived from the time dependence of the diffraction
intensity fitted by a single exponential function. Thus, the diffusion coefficients parallel and perpendicular to the director
of liquid crystals at various temperatures can be obtained from the plots of the reciprocal of the relaxation time versus
the square of the grating vector. From the analysis of the holographic grating relaxation, the diffusion is faster along the
molecular director than for the perpendicular case, and the diffusion increases with rising temperature either parallel or
perpendicular to the nematic director of liquid crystals.
PACS 42.40.Eq; 42.40.Lx 相似文献
8.
W. Stille 《The European physical journal. E, Soft matter》2009,28(1):57-71
The deformation of cholesteric elastomers by mechanical stress applied parallel to the helix axis is studied by calculation
of the free-energy density. The Frank-elasticity contribution is taken into account. A chiral solvent, present at cross-linking
time, is in general considered to be replaced after cross-linking by a solvent with different chirality. Two special cases
considered are zero and unchanged solvent chirality, the first known as that of imprinted cholesteric elastomers, the latter
equivalent to intrinsic cholesteric elastomers with chemically attached chiral groups. Depending on material parameters and
imposed strain, the director can show a tilt towards the helix axis up to the maximum tilt, corresponding to a nematic state.
In case of intrinsic elastomers with low conformation anisotropy, direct transitions from untilted to nematic states can be
induced by straining. The helix structure of the director field is coarsened with an average wave number different to that
of the information inscribed in the network at cross-linking time, if this lowers the average free-energy density. Switching
between different states can be achieved with electric fields of reasonable values applied parallel to the helix axis. Spectra
of the reflection of polarized light are calculated. 相似文献
9.
The nematic liquid crystal film composed of n molecular layers is studied based upon a spatially anisotropic pair potential, which reproduces approximately the elastic free energy density. On condition that the system has perfect nematic order, as in the Lebwohl—Lasher model, the director in the film is isotropic. The effect of the temperature is investigated by means of molecular field theory. Some new results are obtained. Firstly, symmetry breaking takes place when taking account of the temperature, and the state with the director along the normal of the film has the lowest free energy. Secondly, the N—I phase transition temperature increases as an effect of finite sizes instead of decreasing as in the Lebwohl—Lasher model. Thirdly, the nematic order is induced in the layers near the surface in the isotropic phase. 相似文献
10.
A. M. Menzel H. Pleiner H. R. Brand 《The European physical journal. E, Soft matter》2009,30(4):371-377
We investigate the response of prestretched nematic side-chain liquid single-crystal elastomers to superimposed external shear,
electric, and magnetic fields of small amplitude. The prestretching direction is oriented perpendicular to the initial nematic
director orientation, which enforces director reorientation. Furthermore, the shear plane contains the direction of prestretch.
In this case, we obtain a strongly decreased effective shear modulus in the vicinity of the onset and the completion of the
enforced director rotation. For the same regions, we find that it becomes comparatively easy to reorient the director by external
electric and magnetic fields. These results were derived using conventional elasticity theory and its coupling to relative
director-network rotations. 相似文献
11.
12.
S. P. Palto A. R. Geivandov I. V. Kasyanova V. V. Artemov M. V. Gorkunov 《JETP Letters》2017,105(3):174-178
Micro- and nanostructured surfaces creating spatially periodic boundary conditions of the alignment of nematic liquid crystals in two mutually orthogonal directions perpendicular and parallel to the surface are obtained by focused ion beam milling. It is shown that ion milling provides an easy axis along the normal and sufficiently strong anchoring energy. The value of this energy can noticeably exceed the energy of the planar anchoring of liquid crystals with typical orienting surfaces on the basis of polymer films. Using the numerical simulation, the anchoring energy values necessary for an implementation of a deep modulation of the director field with a spatial period of hundreds of nanometers are determined, which is important for creation of photonic liquid-crystal systems. 相似文献
13.
M. R. Hakobyan R. S. Hakobyan 《Journal of Contemporary Physics (Armenian Academy of Sciences)》2017,52(3):295-302
The general approach to study the properties of the mechanical deformations of solid nematics, which are the macroscopic homogeneous elastic media having the rotational symmetry of the nematic liquid crystals is proposed. The stress tensor, the Young modulus and the Poisson ratios for the parallel and perpendicular homogeneous orientations of nematic molecules relative to the axis of external forces influence are obtained by the varying of the free energy of mechanical deformation. It is shown that these constants have the anisotropic character and the experiments for the direct measurement of five elasticity coefficients entering the free energy expression are proposed. 相似文献
14.
E. I. Kats 《JETP Letters》2017,105(4):246-249
A macroscopic counterpart to the microscopic mechanism of the straightening dimer mesogens conformations, proposed recently by S.M. Saliti, M.G. Tamba, S.N. Sprunt, C. Welch, G.H. Mehl, A. Jakli, and J.T. Gleeson [Phys. Rev. Lett. 116, 217801 (2016)] to explain their experimental observation of the unprecedentedly large shift of the nematic–isotropic transition temperature is discussed. The proposed interpretation is based on singular longitudinal fluctuations of the nematic order parameter. Since these fluctuations are governed by the Goldstone director fluctuations, they exist only in the nematic state. External magnetic field suppresses the singular longitudinal fluctuations of the order parameter (similarly as is the case for the transverse director fluctuations, although with a different scaling over the magnetic field). The reduction of the fluctuations changes the equilibrium value of the magnitude of the order parameter in the nematic state. Therefore, it leads to additional (with respect to the mean field contribution) fluctuation shift of the nematic–isotropic transition temperature. Our mechanism works for any nematic liquid crystals, however the magnitude of the fluctuation shift increases with decrease in the Frank elastic moduli. Since some of these moduli supposed to be anomalously small for so-called bent-core or dimer nematic liquid crystals, just these liquid crystals are promising candidates for the observation of the predicted fluctuation shift of the phase transition temperature. 相似文献
15.
We model the elastic properties of main chain liquid crystalline
elastomers, formed by
cross linking chains in a strongly nematic state, when they have
hairpin defects. We study the response of the elastomer to imposed uniaxial
extension
along the nematic direction, and employ a microscopic model of how
the deformation is distributed non-affinely amongst the hairpin
and straight chain populations. The rubber shows a
plateau in the stress as a function of the elongation imposed
along the director. It is a consequence of the depletion of the
actively stretching population of hairpin chains and should not be
confused with soft elasticity effects associated with director rotation. 相似文献
16.
We study the linear dynamic-mechanical response of monodomain nematic liquid crystalline elastomers under shear in the geometry that allows the director rotation. The aspects of time-temperature superposition are discussed at some length and Master Curves are obtained between the glassy state and the nematic transition temperature Tni. However, the time-temperature superposition did not work through the clearing point Tni, due to the transition from the "soft-elasticity" nematic regime to the ordinary isotropic rubber response. We focus on the low-frequency region of the Master Curves and establish the power law dependence of the modulus G' alpha w(a). This law agrees very well with the results of the static stress relaxation, where each relaxation curve obeys the analogous power law G' alpha t(-a) in the corresponding region of long times and temperatures. 相似文献
17.
S. B. Chernyshuk B. I. Lev H. Yokoyama 《Journal of Experimental and Theoretical Physics》2001,93(4):760-770
We studied the collective elastic interaction in a system of many macroparticles embedded in a nematic liquid crystal. A theoretical approach to the interaction of macroparticles via deformation of the director field [1] is developed. It is found that the director field distortion induced by many particles leads to the screening of the elastic pair interaction potential. This screening strongly depends on the shape of the embedded particles: it exists for anisotropic particles and is absent for spherical ones. Our results are valid for the homeotropic and the planar anchoring on the particle surface and for different Frank constants. We apply our results to cylindrical particles in a nematic liquid crystal. In a system of magnetic cylindrical grains suspended in a nematic liquid crystal, the external magnetic field perpendicular to the grain orientation results in inclining the grains to the director and induces an elastic Yukawa-law attraction between the grains. The appearance of this elastic attraction can explain the cellular texture in magnetically doped liquid crystals in the presence of the magnetic field [2]. 相似文献
18.
Theoretical analysis of the influence of flexoelectric effect on the defect site in nematic inversion walls
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Based on the experimental phenomena of flexoelectric response at defect sites in nematic inversion walls conducted by Kumar et al., we gave the theoretical analysis using the Frank elastic theory. When a direct-current electric field normal to the plane of the substrate is applied to the parallel aligned nematic liquid crystal cell with weak anchoring, the rotation of ± 1 defects in the narrow inversion walls can be exhibited. The free energy of liquid crystal molecules around the +1 and-1 defect sites in the nematic inversion walls under the electric field was formulated and the electric-field-driven structural changes at the defect site characterized by polar and azimuthal angles of the local director were simulated. The results reveal that the deviation of azimuthal angle induced by flexoelectric effect are consistent with the switching of extinction brushes at the +1 and-1 defects obtained in the experiment conducted by Kumar et al. 相似文献
19.
We have measured the Stokes drag on magnetic nanowires suspended in the nematic liquid crystal 4-cyano-4'-pentylbiphenyl (5CB). The effective drag viscosity for wires moving perpendicular to the nematic director differs from that for motion parallel to the director by factors of 0.88 to 2.4, depending on the orientation of the wires and their surface anchoring. When the force on the wires is applied at an oblique angle to the director, the wires move at an angle to the force, demonstrating the existence of a lift force on particles moving in a nematic. This dynamic lift is significantly larger for wires with homeotropic anchoring than with longitudinal anchoring in the experiments, suggesting the lift force as a mechanism for sorting particles according to their surface properties. 相似文献
20.
Summary Close to the interface between a nematic liquid crystal (NLC) and another medium, the elastic constants become functions of
distancez from the interface and of angle θ between the directorn and the unit vectork orthogonal to the interface. Furthermore, due to the breaking of the translation symmetry at the interface, a lot of new
subsurface elastic contributions can appear. In a previous paper we investigated these subsurface anomalies by using a simple
molecular model based on induced-dipole-induced-dipole interactions and by making numerical calculations in the special case
of a planar director distortion. In this way, only the numerical values of some effective subsurface elastic constants that
characterise planar director distortions could be obtained. In this paper we make a more complete analytical calculation of
all the subsurface elastic constants by using a microscopic model and a more general theoretical procedure. The microscopic
interaction energy is written in a general form that allows us to investigate different kinds of intermolecular interactions
(induced diple-induced dipole, quadrupole-quadrupole and so on). Both thez-dependence and the θ-dependence of the subsurface elastic constants are obtained in a closed analytical form. In the special
case of induced-dipole-induced-dipole interactions and for planar director distortions, our analytical results are shown to
agree with the previous numerical results. The important macroscopic effect of these elastic subsurface anomalies is discussed. 相似文献