共查询到20条相似文献,搜索用时 15 毫秒
1.
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
2.
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
3.
We develop a minimal phenomenological model to describe the auxetic response recently observed in liquid crystal elastomers, and further determine by theoretical calculation the critical condition required for the auxetic response to occur. 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.
T. C. Lubensky 《Pramana》2005,64(5):727-742
This paper reviews the derivation of equations for slow dynamical processes in a variety of systems, including rotating rigid
rotors, crystalline solids, isotropic and nematic elastomers, gels in an isotropic fluid background, and nematic liquid crystals.
It presents a recent derivation of the Leslie-Ericksen equations for the dynamics of nematic liquid crystals that clarifies
the nature of the nonhydrodynamic modes in these equations. As a final example of the phenomenological approach to slow dynamical
processes, it discusses the dynamics of a driven nonequilibrium system: a two-dimensional gas of chiral ‘rattlebacks’ on a
vibrating substrate. 相似文献
6.
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
7.
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
8.
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
9.
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
10.
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
11.
We report a biaxial nematic liquid crystal in a simple thermotropic system. 相似文献
12.
One discusses phenomenologically the statics and dynamics of surface anchoring of nematic liquid crystals. In statics, for
symmetrical situations, one can induce, under the action of an external field, a surface breaking with a bifurcation between
two opposite orientations of the director. In dynamics, one shows that the surface director relaxation is intrinsically fast,
and accompanied by a transient surface shear flow. In thin cells with planar anchorings, filled by cyanobiphenyl materials
which orient normal to the plates under a strong electric field, there are now two effects which couple the relaxation of
the surface director: a static one, which favors parallel orientation and a uniform cell texture, and a transient dynamical
coupling from the surface flow, which forces π-twisted texture. With suitable electric signals, one can control the surface
bifurcations to give at will one of these two bistable textures. Results are presented which show that these mechanisms could
lead to a new class of surface controlled, fast nematic bistable displays.
Reproduced from ‘Liquid Crystals Today’ (Vol. 8, p. 1–7, 1998) by permission from Taylor and Francis. 相似文献
13.
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
14.
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
15.
Ferromagnetic nanoparticles insertions in nematic liquid crystals (NLC) in twisted configuration are studied and a theoretical model is proposed to explain the results. Experimental observation revealed that nanoparticles tend to overcrowd in long strings parallel to the rubbing direction of the alignment substrate of the LC cell. Their behavior under external field was studied and their interaction with their nematic host is described using elastic continuum theory. 相似文献
16.
A Gaussian distribution model was developed to examine the field-induced performance of anisotropic magnetorheological elastomers. The developed model was based on the assumption that the iron particles in magnetorheological elastomers aggregate into a large number of parallel body-centered tetragonal structure columns whose length obeys the Gaussian distribution. By using multi-pole approximation with local field effect and taking into account the nonlinearity and saturation of particle magnetization, the field-induced shear modulus was calculated as a function of distribution and dimension of the particle structures, the external magnetic field and the dynamic shear strain. Compared with other modes as well as the published experimental results, this model shows a remarkable improvement in accurately predicting the behavior of the magnetorheological elastomers. 相似文献
17.
18.
Thermomechanical mechanism of the director reorientation in twisted nematic liquid crystals caused by the one-dimensional longitudinal temperature gradient is predicted and studied theoretically. The calculated director reorientations are in the range that can be measured experimentally very easily. 相似文献
19.
A crystalline substrate such as mica or gypsum can orient a thin layer of nematic liquid crystal spread on top. By adsorbing one or more other kinds of molecules onto the surface, we have observed transitions in the orientation of the molecules on the surface. These “anchoring transitions” are analyzed via Landau-Ginzburg theory and may be first or second-order. The second-order anchoring transition is the first two-dimensional system where critical fluctuations may be observed optically. 相似文献