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1.
We present a simple coarse-grained lattice model for monodomain biaxial liquid-crystal elastomers and perform large-scale
Monte Carlo simulations in the proposed model system. Orientational ordering --uniaxial or biaxial-- reflects in sample deformations
on cooling the system. The simulation output is used to predict calorimetry data and deuterium magnetic resonance spectra. 相似文献
2.
We present a model of the dynamics of director rotation in nematic gels under combined electro-mechanical loading. Focusing
on a model specimen, we describe the critical voltages that must be exceeded to achieve director reorientation, and the blocking
stresses that prevent alignment of the nematic director with the applied electric field. The corresponding phase diagram shows
that the dynamic thresholds defined above are different from those predicted on the sole basis of energetics. Multistep loading
programs are used to explore the energy landscape of our model specimen, showing the existence of multiple local minima under
the same voltage and applied stress. This leads us to conclude that hysteresis should be expected in the electro-mechanical
response of nematic gels. 相似文献
3.
P.I.C. Teixeira 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,9(3):471-477
A model is proposed for the reorientation dynamics of a confined nematic liquid crystal elastomer, where the effect of crosslinks
is to couple the director to deformations of the elastic matrix. The model combines the (equilibrium) `neo-classical' theory
of liquid crystal rubber elasticity with the simplest time evolution equations for a system described by two coupled, non-conserved
order parameters. Relaxation from an orientation imposed by an electric field is studied as a function of elastic softness,
starting angle, surface pretilt, and the relative mobilities of director and strain. Most importantly, the absence of a `semi-soft'
elastic threshold changes the long-time behaviour of the effective refractive index of the medium from exponential to inverse
power law decay. Predictions are compatible with recent experimental results by Chang, Chien and Meyer [Phys. Rev. E 56, 595 (1997)].
Received 22 June 1998 相似文献
4.
We investigate nematic-isotropic transition in liquid crystal elastomers employing a variant of Wang-Landau sampling. This technique facilitates calculation of the density of states from which other thermodynamic properties can be obtained. We consider a lattice model of a liquid crystal elastomer and a Hamiltonian which accounts for interactions among liquid crystalline units and interaction of local nematics with global strain. We investigate the effect of varying the strength of coupling between nematic and orientational degrees of freedom. When the local director is coupled strongly to the global strain, the transition is strongly first order. When the strength of the coupling decreases the transition becomes weakly first order. The transition temperature decreases when the coupling becomes weaker. We also report for the first time results on variation of free energy as a function of average energy at different temperatures and coupling constants. 相似文献
5.
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
6.
7.
Dense rubbery networks are highly entangled polymer systems, with significant topological restrictions for the mobility of
neighbouring chains and crosslinks preventing the reptation constraint release. In a mean-field approach, entanglements are
treated within the famous reptation approach, since they effectively confine each individual chain in a tube-like geometry.
We apply these classical ideas to calculate the effective rubber-elastic free energy of anisotropic networks, nematic liquid
crystal elastomers, and present the first theory of entanglements for such a material.
Received 25 June 2001 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.
J.J. Zanna H.T. Nguyen J.P. Parneix G. Ruffié M. Mauzac 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,10(2):345-351
Dielectric response, from 1 Hz to 10 MHz, of liquid crystalline side chain elastomers has been compared to the one of analogous
uncrosslinked materials. Results are discussed in terms of mobility of the mesogens. Two different systems have been investigated:
smectic A elastomers allow to determine the influence of crosslinking on the relaxation (i.e. the reorientation of the whole mesogen around the chain), a S
C
* elastomer shows the drastic influence of the polymer network on the Goldstone mode.
Received 12 October 1998 相似文献
10.
We present a dissipative particle dynamics (DPD) approach for simulating the realignment of a nematic nanodroplet suspended
in an isotropic fluid following a switch in the direction of an applied external magnetic field. The interaction of the mesogens
with the external field is weak relative to the inter-molecular interactions. The simulations were used to investigate the
way orientational equilibrium is re-established. The results reveal that the realignment process of the nanodroplet is consistent
with its fluid structure. The reorientation of the nanodroplet as a whole is found to be caused by an internal structural
rearrangement rather than a coherent rotation of the centres of mass of the mesogens about the centre of the nanodroplet.
The switch in the field direction furthermore is found to induce a transient spatial variation in the orientational order
of the long axes of the mesogens: the orientational order parameters decreases on moving from the core of the nanodroplet
to the surface in contact with the isotropic environment. The results highlight differences between the time evolution of
the orientation of the long molecular axes in the field and the rotations of the centres of mass of the mesogens about the
centre of the nanodroplet. 相似文献
11.
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
12.
The annihilation of the nematic hedgehog and anti-hedgehog within an infinite cylinder of radius R is studied. The semi-microscopic lattice-type model and Brownian molecular dynamics are used. We distinguish among the i)
early pre-collision, ii) late pre-collision, iii) early post-collision, and iv) late post-collision stages. In the pre-collision stage our results agree qualitatively with the existing experimental observations and also continuum-type simulations. The
core of each defect exhibits a ring-like structure, where the ring axis is set perpendicular to the cylinder symmetry axis. For ξ(0)d/(2R) > 1 the interaction between defects is negligible, where ξ(0)d describes the initial separation of defects. Consequently, the defects annihilate within the simulation time window for ξ(0)d/(2R) < 1. For close enough defects their separation scales as ξd
(tc - t)0.4±0.1, where tc stands for the collision time. In elastically anisotropic medium the hedgehog is faster than the anti-hedgehog. In the early pre-collision stage the defects can be treated as point-like particles, possessing inherent core structure, that interact via the nematic director field. In the late pre-collision stage the cores reflect the interaction between defects. After the collision a charge-less ring structure is first formed. In the early post-collision stage the ring adopts an essentially untwisted circular structure of the radius ξr. In the late post-collision stage we observe two qualitatively different scenarios. For μ = ξr/R < μc ∼ 0.25 the ring collapses leading to the escaped radial equilibrium structure. For μ > μc the chargeless ring triggers the nucleation growth into the planar polar structure with line defects. 相似文献
13.
The flexoelectric coefficients e1 and e3 for polar liquid crystals, such as 4-n-pentyl- 4′-cyanobiphenyl, are investigated theoretically by means of an integral equation approach, which takes into account translational, orientational
correlations as well as their coupling. e1 and e3 are evaluated from microscopic expressions derived on the basis of a density-functional method.
Received 27 April 2001 and Received in final form 27 August 2001 相似文献
14.
Segmental order in end-linked monomodal and bimodal polymer networks is investigated by means of bond-fluctuation Monte Carlo
simulations. The tensor order parameter, which is a central observable in NMR experiments, is not uniquely related to simple
vectorial order. The relaxation of NMR-detected tensorial interactions towards their finite long-time limit is best described
by a power law and occurs over much longer time scales than the relaxation of vectorial order. The well-known prediction for
the segmental order of Gaussian chains as a simple function of the segment number between constraints is not straightforwardly
obeyed, neither in dry nor in swollen networks. Excluded-volume interactions tend to significantly reduce the tensorial order,
as is clearly observed in single-chain simulations. A distribution extends along the chain, where order is increased in a
region of 30-40 bonds around the cross-links in networks. The dominating contribution to the order parameter distribution
arises from the frozen-in distribution of end-to-end separations. We find strong deviations from the Gamma distribution, which
has so far been implicitly used in most NMR works, as it is a straightforward consequence of a Gaussian distribution of end
separations. Specifically, we find narrower distributions, as small values of the tensor order parameter are strongly suppressed,
most probably as a result of trapped entanglements. The markedly subaffine behavior of the average order parameter and the
changes in its distribution on swelling are assigned to orientation processes of strands which compensate for the non-affine
local deformation. Our central observations and interpretations are well supported by our previous experimental and theoretical
work. 相似文献
15.
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
16.
A nematic liquid crystal confined between two identical flat solid substrates, with an alternating stripe pattern of planar
and homeotropic anchoring, is studied in the framework of the Frank-Oseen theory. By means of numerical minimization of the
free energy functional we study the effect of the sample thickness D on the location of the phase transition between a uniform alignment, either planar or homeotropic, and a distorted nematic
texture. The solvation force f due to distortions of the nematic director is also studied. It is found that f is always attractive, and for D small compared to the periodicity of the surface structure it exhibits two distinct asymptotic behaviors: f ∼ - D
-1/2 or f ∼ - D
-1, depending on the relation between D and the extrapolation lengths.
Received 12 November 2002 Published online: 16 April 2003 相似文献
17.
We have used the density functional theory to study the effect of molecular elongation on the isotropic-nematic, isotropic-smectic A and nematic-smectic A phase transitions of a fluid of molecules interacting via the Gay-Berne intermolecular potential. We have considered a range of length-to-width parameter 3.0 ⩽ x0 ⩽ 4.0 in steps of 0.2 at different densities and temperatures. Pair correlation functions needed as input information in density functional theory are calculated using the Percus-Yevick integral equation theory. Within the small range of elongation, the phase diagram shows significant changes. The fluid at low temperature is found to freeze directly from isotropic to smectic A phase for all the values of x0 considered by us on increasing the density while the nematic phase stabilizes in between isotropic and smectic A phases only at high temperatures and densities. Both isotropic-nematic and nematic-smectic A transition density and pressure are found to decrease as we increase x0. The phase diagram obtained is compared with computer simulation result of the same model potential and is found to be in good qualitative agreement. 相似文献
18.
The Frank elasticity constants which describe splay (K
1), twist (K
2), and bend (K
3) distortion modes are investigated for 4-n-pentyl-4'-cyanobiphenyl (5CB) in the nematic liquid crystal. The calculations
rest on statistical-mechanical approaches where the absolute values of K
i
(i=1,2,3) are dependent on the direct correlation function (DCF) of the corresponding nematic state. The DCF was determined
using the pair correlation function by solving the Ornstein-Zernike equation. The pair correlation function, in turn, was
obtained from molecular dynamics (MD) trajectory. Three different approaches for calculations of the elasticity constants
were employed based on different level of approximation about the orientational order and molecular correlations. The best
agreement with experimental values of elasticity constants was obtained in a model where the full orientational distribution
function was used. In addition we have investigated the approximation about spherical distribution of the intermolecular vectors
in the nematic phase, often used in derivation of various mean-field theories and employed here for the construction of the
DCF. We found that this assumption is not strictly valid, in particular a strong deviation from the isotropic distribution
is observed for short intermolecular distances.
Received 22 March 2000 and Received in final form 9 June 2000 相似文献
19.
The effect of silica aerosils on the kinetics of the first-order nematic-isotropic (NI) phase transition is phenomenologically described in the framework of the time-dependent Landau-Ginzburg equation. A steady-state solution to the equation is presented such that the NI interface may propagate with a solitary-like wave profile under constant quenching. The results provide a plausible basis for the interpretation of the dynamical effects of quenched disorder in the liquid-crystal systems, caused by randomly interconnected porous media, such as aerosils. In the low silica aerosil ρs ( ≤0.1 g/cm^3) regime, the calculated values of the interface velocity v(T,ρs), the interface thickness κ(T,ρs), and the critical radius of a spherical nucleus of new nematic phase in a bulk isotropic environment, composed of polar molecules, such as 4-n-octyl- 4′- cyanobiphenyl and 4-n-heptyl- 4′- cyanobiphenyl shows that the effect of silica aerosils on the kinetics is reflected in a shifting of the set of temperature-dependent curves to lower temperature values.-1 相似文献
20.
H. Stark J. Stelzer R. Bernhard 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,10(3):515-523
Recently, it was observed that water droplets suspended in a nematic liquid crystal form linear chains [Poulin et al., Science 275, 1770 (1997)]. The chaining occurs, e.g., in a large nematic drop with homeotropic boundary conditions at all the surfaces. Between each pair of water droplets a
point defect in the liquid crystalline order was found in accordance with topological constraints. This point defect causes
a repulsion between the water droplets. In our numerical investigation we limit ourselves to a chain of two droplets. For
such a complex geometry we use the method of finite elements to minimize the Frank free energy. We confirm an experimental
observation that the distance d of the point defect from the surface of a water droplet scales with the radius r of the droplet like .When the water droplets are moved apart, we find that the point defect does not stay in the middle between the droplets,
but rather forms a dipole with one of them. This confirms a theoretical model for the chaining. Analogies to a second order
phase transition are drawn. We also find the dipole when one water droplet is suspended in a bipolar nematic drop with two
boojums, i.e., surface defects at the outer boundary. Finally, we present a configuration where two droplets repel each other without a
defect between them.
Received 11 December 1998 相似文献