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1.
E.M. Terentjev M. Warner 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,8(4):595-601
A molecular model of freely jointed chains of chiral monomers is developed to describe the piezoelectric effect in chiral
nematic elastomers. The model, an extension of the neo-classical theory of nematic polymer networks, takes into account a
chiral biasing of molecular alignment under shear which leads to induced polarisation if the monomers contain a transverse
dipole moment. The resulting theory is fully non-linear in elastic deformations, in the spirit of ordinary rubber elasticity.
The expansion to the highest order in small strains gives the three linear piezoelectric coefficients predicted by phenomenological
models.
Received 7 September 1998 and Received in final form
19 October 1998 相似文献
2.
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
3.
4.
Spontaneous thermal expansion of nematic elastomers 总被引:1,自引:0,他引:1
We study the monodomain (single-crystal) nematic elastomer materials, all side-chain siloxane polymers with the same mesogenic
groups and crosslinking density, but differing in the type of crosslinking. Increasing the proportion of long di-functional
segments of main-chain nematic polymer, acting as network crosslinking, results in dramatic changes in the uniaxial equilibrium
thermal expansion on cooling from the isotropic phase. At higher concentration of main chains their behaviour dominates the
elastomer properties. At low concentration of main-chain material, we detect two distinct transitions at different temperatures,
one attributed to the main-chain, the other to the side-chain component. The effective uniaxial anisotropy of nematic rubber,
r(T) = / proportional to the effective nematic order parameter Q(T), is given by an average of the two components and thus reflects the two-transition nature of thermal expansion. The experimental
data is compared with the theoretical model of ideal nematic elastomers; applications in high-amplitude thermal actuators
are discussed in the end.
Received 25 June 2001 and Received in final form 29 September 2001 相似文献
5.
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
6.
The biaxial stretching of sheets of liquid crystalline neo-Hookean elastomer has been studied in the isotropic case. The results
suggest two types of laminate structures in the process of quasiconvexification of the free energy, a fact that implies the
appearance of several shear terms in the deformation gradient matrix. More that one decomposition of the deformation gradient
is possible, which is consistent with a bifurcation in the undeformed configuration (l \lambda = 1) . This situation is similar to the well-known Rivlin’s problem of the triaxial symmetric traction of a neo-Hookean cube.
The problem can easily be generalized for an anisotropic material by introducing a semisoft term in the free-energy expression.
In this case, the horizontal plateau corresponding to the minimal energy, characteristic of the soft elasticity, disappears,
and only an equilibrium condition is obtained. 相似文献
7.
Teixeira PI Warner M 《Physical review. E, Statistical physics, plasmas, fluids, and related interdisciplinary topics》1999,60(1):603-609
We analyze analytically and numerically the dynamics of how a nematic elastomer-an anisotropic rubber-responds elastically and orientationally to an imposed strain. Because positional and orientational degrees of freedom are coupled, the response is not the simple exponential one might expect for a viscous system. Indeed, as a result of this nonlinear coupling, the different modes decay in two qualitatively different ways: with either two distinct or with the same exponential laws, depending, respectively, on whether there is or there is not complete reorientation of the molecular long axes. In addition, at the special values of the strain that form the boundaries between different equilibrium behaviors, relaxation is much slower, i.e., it follows a power law. 相似文献
8.
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
9.
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
10.
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
11.
12.
Martinoty P Stein P Finkelmann H Pleiner H Brand HR 《The European physical journal. E, Soft matter》2004,14(4):311-321
We investigate the behavior of the shear rigidity modulus G = G + iG of three mono-domain side chain liquid-crystalline elastomers composed of side chain polysiloxanes cross-linked by either flexible or rigid cross-linkers. The measurements were taken in a frequency domain ranging form
Hz to
Hz applying the shear in a plane perpendicular to or containing the director. The measurements as a function of temperature show an anisotropy of G which appears around
, when decreasing the temperature, and which is due to the expected lowering of
coming from the coupling between the shear and the director. The measurements as a function of frequency show that G has two components for both geometries, in both the isotropic phase and in the nematic phase around the phase transition. One reflects the network behavior in its hydrodynamic regime (G is constant and
, where f is the frequency), the other which appears at higher frequencies is characterized by a scaling law behavior (
) of the Rouse type. We discuss the results in the framework of available theories and show that the three elastomers present a non-soft behavior, even for the elastomer for which the contrary was claimed, and that there is no separation of time scales between the director and the network. We also present data on a poly-domain sample and a non-mesomorphic one which complement these results.Received: 19 February 2004, Published online: 24 August 2004PACS:
83.80.Va Elastomeric polymers - 61.30.-v Liquid crystals - 83.60.Bc Linear viscoelasticityPresented at the First World Congress on Biomimetics and Artificial Muscles, 9-11 December 2002, Albuquerque, New Mexico, USA. Some aspects of this work have been published as an abridged version in the Proceedings of the Conference. 相似文献
13.
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
14.
15.
Changing the orientational order in liquid-crystal
elastomers leads to internal stresses and changes of the sample
shape. When this effect is induced by light, due to
photoisomerization of constituent molecular moieties, the
photomechanical actuation results. We investigate quantitatively
how the intensity and the polarization of light affect
photoactuation. By studying dissolved, as well as covalently
bonded azo-dyes, we determine the changes in absorption and the
response kinetics. For the first time we compare the response of
aligned monodomain, and randomly disordered polydomain nematic
elastomers, and demonstrate that both have a comparable
photoresponse, strongly dependent on the polarization of light.
Polarization-dependent photoactuation in polydomain elastomers
gives an unambiguous proof of its mechanism since it is the only
experiment that distinguishes from the associated thermal
effects. 相似文献
16.
17.
In this paper we study the macroscopic behavior of nematic side-chain liquid single crystal elastomers exposed to an external electric or magnetic field. For this purpose we use the framework of a continuum model. The geometries investigated comprise the bend and the twist geometry known from the classical Frederiks transition in low molecular weight liquid crystals. For the bend geometry we find a laterally homogeneous and a two-dimensional undulatory instability, which may compete at onset. In the case of the twist geometry three instabilities can occur at onset, two of which are two dimensional and clearly show undulations. As a major result we propose how the values of the twist coefficient K(2) and the values of the material parameters D(1) and D(2) connected to relative rotations between the director field and the polymer network can be determined from experimental observations. In addition, we explain why a twist experiment is probably the most suitable set-up in order to measure the material parameter D(1). 相似文献
18.
19.
We present a Monte Carlo study of external field-induced switching in nematic elastomers, employing a coarse-grained shearable
lattice model. In large enough systems a full-wavelength Fréedericksz effect is observed --as opposed to the half-wavelength
effect seen in ordinary nematics-- that clearly reflects in simulated polarized light textures, as well as in deuterium magnetic
resonance spectra. The reorientation of mesogenic units is accompanied by pronounced shear deformations. 相似文献
20.
Nematic elastomers with a locked-in anisotropy direction exhibit semisoft elastic response characterized by a plateau in the stress-strain curve in which stress does not change with strain. We calculate the global phase diagram for a minimal model, which is equivalent to one describing a nematic in crossed electric and magnetic fields, and show that semisoft behavior is associated with a broken symmetry biaxial phase and that it persists well into the supercritical regime. We also consider generalizations beyond the minimal model and find similar results. 相似文献