共查询到20条相似文献,搜索用时 799 毫秒
1.
Maja Stojanović Alexej Bubnov Dusanka Ž. Obadović Vĕra Hamplová Miroslav Kašpar Miroslav Cvetinov 《Phase Transitions》2013,86(4):380-390
Studies of structural and phase properties obtained on several ferroelectric liquid crystalline materials with 2-alkoxypropionate group used as a chiral centre and without any lateral substitution are presented. In dependence on the chiral chain length these compounds exhibit the cholesteric N* phase, the ferroelectric smectic C* and a low-temperature SmX phase. Values of the spontaneous polarization and spontaneous tilt angle have been determined within the whole range of the SmC* phase. A low-temperature SmX phase has been identified as the orthogonal hexatic SmB* phase. The molecular parameters, namely the layer spacing in the SmC* and SmB* phases and the average intermolecular distances (D) between neighbouring parallel molecules in all investigated phases have been determined using the results of the X-ray diffraction obtained on non-oriented samples. The effect of the chiral chain length on mesomorphic, structural and physical properties of the studied ferroelectric liquid crystalline materials is discussed. 相似文献
2.
We investigated SmC* films sandwiched between silane coated glass plates and observed formation of textures exhibiting a uniform tilt of the smectic
layers with respect to the boundary plates. The layer tilt angle increases from zero to as the sample is cooled from the smectic A phase to room temperature. These films show linear electro-optical effects because
the permanent polarization can be aligned so that it has a component parallel to the applied field without changing the layer
structure. Our analysis indicates that mainly two effects determine the layer tilt. On the one hand, the surface tension tends
to minimise the layer tilt. On the other hand, the surface energy promotes the director to be normal to the boundary plates.
Received 17 July 1998 相似文献
3.
Molecular reorientations and internal conformational transitions of an aligned chiral liquid crystal (LC) 10B1M7 are studied by means of deuterium spin-lattice relaxation in its smectic A (SmA) and smectic C* (SmC*) phase. The motional model which is applicable to uniaxial phases of many LCs is found to be adequate even when the phase is a tilted SmC* phase. The deuterium NMR spectrum in this phase cannot discern rotations of the molecular director about the pitch axis. The basic assumption is that the phase biaxiality is practically unobservable. However, the relaxation rates can be accounted for by the tilt angle between the molecular director and the layer normal in the SmC* phase. The tumbling motion appears to show a higher activation energy upon entering from the uniaxial SmA into the SmC* phase. 相似文献
4.
L. S. Bezhanova Z. V. Bagdasaryan M. L. Baburyan 《Journal of Contemporary Physics (Armenian Academy of Sciences)》2007,42(3):120-124
We investigate thermodynamic properties and spontaneous polarization of ferroelectric liquid crystal (SmC*LC) systems: smectic C matrix + chiral adding (ChA). The fact of existence of an optimum concentration of ChA, for which the studied SmC*LC systems have the maximum value of spontaneous polarization, and a certain concentration, at which the phase transition of the chiral tilted smectic C phase to the smectic A phase occurs, is established. Temperature dependence curves for the free energy, configuration energy, specific heat, and the orientational order parameter are calculated in the temperature range of existence of liquid crystal mesophases. The curves are in good agreement with the experimentally observed trends of the SmC*LC properties. 相似文献
5.
The first experimental evidence for triclinic symmetry of bulk smectic liquid-crystal samples of achiral banana-shaped molecules is presented. This phase corresponds to the so-called Sm-CG phase consisting of biaxial molecules and characterized by two tilt directions with respect to the layer normal: tilt of the molecular plane (clinic) and tilt of the molecular kink direction (leaning). Each smectic layer has a polarization component normal to the smectic layers (C1 symmetry). The observations suggest that the phase tentatively labeled as B7 is identical with the Sm-CG phase. 相似文献
6.
Anamika Pramanik Malay Kumar Das Vĕra Hamplová Miroslav Kašpar Alexej Bubnov 《Phase Transitions》2015,88(7):745-757
The effect of several polar ester linkage groups incorporated in the molecular core of a chiral lactic acid derivative on self-assembling properties has been investigated by polarizing optical microscopy, small angle X-ray diffraction, differential scanning calorimetry, optical and electro--optical studies. The compound possesses the paraelectric smectic A* (SmA*) and ferroelectric smectic C* (SmC*) phases over a broad temperature range. Mesomorphic behaviour, spontaneous polarization, birefringence, optical transmission, dielectric anisotropy and structural properties of the self-assembled chiral material have been determined. The obtained results are discussed and compared with that of other liquid crystalline materials. Experimentally determined spontaneous polarization and tilt angle values are also used to elucidate the nature of SmC* to SmA* phase transition. The effect of polar ester linkages in the molecular core has also been discussed. 相似文献
7.
A. Carvalho P.J. Sebastião A. Ferraz A.C. Ribeiro H.T. Nguyen 《The European physical journal. E, Soft matter》2000,2(4):351-358
In this work we present a proton NMR comparative study of the molecular order in the smectic C2 (), smectic C^* () and anticlinic smectic C phases ( and ) of two liquid crystalline compounds. The second moments of the experimental proton NMR spectra are well explained taking
into account two different molecular orientations models. From the analysis of our experimental results it was possible to
estimate the tilt angles for all the smectic C phases studied in this work.
Received 15 September 1999 and Received in final form 18 January 2000 相似文献
8.
Marcerou JP Nguyen HT Bitri N Gharbi A Essid S Soltani T 《The European physical journal. E, Soft matter》2007,23(3):319-328
Usual ferroelectric compounds undergo a paraelectric-to-ferroelectric phase transition when the susceptibility of the electric polarization density changes its sign. The temperature is the only thermodynamic field that governs the phase transition. Chiral tilted smectics may also present an improper ferroelectricity when there is a tilt angle between the average long axis direction and the layer normal. The tilt angle is the order parameter of the phase transition which is governed by the temperature. Although the electric susceptibility remains positive, a polarization proportional to the tilt appears due to their linear coupling allowed by the chiral symmetry. Further complications come in when the chirality increases, as new phases are encountered with the same tilt inside the layers but a distribution of the azimuthal direction which is periodic with a unit cell of two (SmC(A)*, three (SmC(Fi1)*, four (SmC(Fi2)* or more (SmC(alpha)* layers. In most of these phases, the layer normal is a symmetry axis so there is no macroscopic polarization except for the SmC(Fi1)* in which the average long axis is tilted so the phase is ferrielectric. By studying a particular compound with only a SmC(Fi2)* and a SmC(alpha)* phase, we show that we recover the uniformly tilted ferroelectric SmC* when applying an electric field. We are thus led to build field-temperature phase diagrams for this class of compounds by combining different experimental techniques described here. 相似文献
9.
The SmA*–SmC* phase transition was studied by measuring the temperature and electric field dependences of the optical tilt angle, the electric polarisation and the dielectric spectra collected in a wide frequency range. Critical behaviour of the phase transition was analysed by varying the length of the fluorinated part of the alkyl terminal chain and by differing fluorine substitution in the terphenyl core. Both tilt and polarisation show tricritical mean-field behaviour for all homologues with n?>?2. Almost all coefficients that describe the SmA*–SmC* transition in the frame of the Landau theory were derived for homologue series. Double fluorine substitution in the central ring of the core seems to promote the ‘de Vries'-type smectic A*–C* phase transition with a little layer shrinkage. These well correspond with the lower tilt angle and smaller changes of the birefringence at the phase transition compared to the other homologues. 相似文献
10.
Giesselmann F Zugenmaier P Dierking I Lagerwall ST Stebler B Kaspar M Hamplová V Glogarová M 《Physical review. E, Statistical physics, plasmas, fluids, and related interdisciplinary topics》1999,60(1):598-602
The smectic layer spacing of a nonfluorinated ferroelectric liquid crystal (FLC) compound with almost no shrinkage and only minor tendency to form zigzag defects was characterized by small angle x-ray diffraction. The material lacks a nematic phase. The smectic-A*-smectic-C* phase transition was studied by measuring the thermal and electric field response of the optical tilt and the electric polarization. These properties are described very well by a Landau expansion even without introduction of a higher-order Theta(6) term. This result suggests a pure second-order phase transition far from tricriticality and differs considerably from the typical behavior of the A*-C* transition in most FLC materials. 相似文献
11.
12.
A correlation between spontaneous polarisation and the chirality index of the constituent molecules, multiplied by the molecular transverse dipole, has been considered for five series of ferroelectric smectic C* liquid crystals. A good correlation has been found within each series, and it has been shown that the scaled chirality index may be used to estimate the spontaneous polarisation in a series of smectic C* mesogens with small systematic changes of the molecular structure. At the same time one cannot expect any significant correlation between the polarisation and the chirality index for smectic C* materials of sufficiently different structure because the spontaneous polarisation may be significantly effected by some details of the molecule's structure including the orientation of the transverse dipole with respect to the primary short molecular axis. 相似文献
13.
Organic compounds exhibiting the smectic C phase are made of rod-like molecules that have dipolar groups with lateral components. We argue that the off-axis character of the lateral dipolar groups can account for tilt in layered smectics (SmC, SmC*, SmI etc.). We develop a mean-field
theory of the smectic C phase based on a single-particle potential of the form U
C ∝ sin(2θ)cosφ, consistent with the biaxial nature of the phase, where θ and φ are the polar and azimuthal angles, respectively.
The hard-rod interactions that favour the smectic A phase with zero tilt angle are also included. The theoretical phase diagrams
compare favourably with experimental trends. Our theory also leads to the following results: i) a first-order smectic C to
smectic A transition above some value of the McMillan parameter α, leading to a tricritical point on the smectic C to smectic
A transition line and ii) a first-order smectic C to smectic C transition over a very small range of values of the model parameters.
We have also extended the theory to include the next higher-order term in the tilting potential and to include the effect
of different tilt angles for the molecular core and the chain in the SmC phase.
Received 3 August 2002
RID="a"
ID="a"Present address: Department of Physics, Vijaya College, R. V. Road, Bangalore - 560 004, India.
RID="b"
ID="b"e-mail: nvmadhu@rri.res.in 相似文献
14.
Within the framework of Landau-de-Gennes formulation, we analyse the effect of pressure on electric-field-induced phase transitions
in a liquid crystal which shows spontaneously an isotropic-smectic A transition. Inferring from the experimental pressure
dependences on the layer spacing in smectic A phase, as well as the nematic-smectic A metastable temperature T*AN, we incorporated the pressure dependence in the free energy through (the surface energy term) and the coupling between the
quadrupolar nematic ordering Qij and the smectic order parameter ψ. From the S-T phase diagram, we found that the stability of field-induced nematic phase increases with pressure, whereas the discontinuity
of the transition decreases. Also, the region where paranematic phase transits directly to smectic A phase increases with
pressure. 相似文献
15.
R. Dąbrowski W. Drzewiński J. Dziaduszek J. Gąsowska P. A. Henderson P. Kula J. M. Otón N. Bennis 《Opto-Electronics Review》2007,15(1):32-36
Three homologous series of orthoconic (45 degree tilt) antiferroelectric liquid crystals containing either naphthyl or terphenyl
groups as mesogenic unit have been synthesized and their mesomorphic behaviour investigated by DSC and polarized light microscopy,
and their properties discussed. X-ray diffraction studies have shown the presence of a de Vries-type smectic A phase which
does not exhibit a layer shrinkage on transition to the ferroelectric smectic C phase. Mixtures of the new materials have
been formulated and their electro-optical properties investigated for their use in surface stabilized orthoconic antiferroelectric
liquid crystal (SSOAFLC) devices 相似文献
16.
D. Pociecha E. Gorecka J. Przedmojski 《The European physical journal. E, Soft matter》2000,1(2-3):137-140
Tilt-driven phase transitions between hexatic smectic phases: SmF-HexB and reversed HexB-SmF have been studied in compounds
belonging to two enaminoketone derivative homologue series. The tilt angle order parameter has been measured and its temperature
dependence near the phase transition point has been described by applying mean-field model. For both phase sequences the tricritical
points have been observed on phase transition lines in binary mixtures of respective materials having first and second order
phase transitions between hexatic phases.
Received 21 June 1999 相似文献
17.
A. V. Emelyanenko 《The European physical journal. E, Soft matter》2009,28(4):441-455
The fundamental theoretical approach derived in A.V. Emelyanenko et al., Phys. Rev. E 74, 011705 (2006) is complemented by a consideration of the influence of the homogeneous electric field on Sm- C
A
* , biaxial intermediate phases, and Sm-C
* . The crucial role of the induced polarization is investigated for the first time. The evolution of any tilted smectic phase
in the electric field is found to meet the two thresholds. The first threshold corresponds to the unwinding process, and the
second one corresponds to the phase transition into the bi-domain structure of Sm-C
* , where the tilt plane has some contribution either along or against the electric field, while the average direction may
still be perpendicular to the electric field. The tilt plane in the monodomain (conventional) structure preceding the second
threshold is the same in every unwound phase, and is perpendicular to the electric field. No 3D distortion in Sm- C
A
* is predicted on application of the electric field. The entire electric-field-temperature phase diagrams including the possibility
of existence of the maximal number of tilted smectic phases are plotted and compared with the experimental ones. The numerical
calculations in the framework of this fundamental study are done with help of AFLC Phase Diagram Plotter software developed
by the author and available at his web page. 相似文献
18.
Hunting for smectic C in calamitic azobenzene ionic liquid crystals with different cationic head groups 下载免费PDF全文
Nadia Kapernaum Eugen Wuckert Wolfgang Frey Sebastian Marino Manuel Wahl Frank Giesselmann Sabine Laschat 《Journal of Physical Organic Chemistry》2018,31(3)
The tilted smectic C phase is a rather uncommon phase in ionic liquid crystals (ILCs), whereas the orthogonal smectic A phase is the most common phase in ILCs. We now present 2 new groups of mesogens with an azobenzene core that exhibit smectic C as well as smectic A phases. Their phase sequences and tilt angles were studied by polarizing microscopy, and their temperature‐dependent layer spacings and orientational order parameters were investigated by X‐ray diffraction. We present 1 new amidinium azobenzene mesogen that forms enantiotropic smectic C and A phases and another amidinium as well as 2 new guanidinium azobenzene mesogens that exhibit monotropic smectic C and enantiotropic smectic A phases. With this study, we show that azobenzene is indeed an SmC‐promoting group in ILCs. Comparing these results with our earlier results on azobenzenes with an N‐methylimidazolium head group (N Kapernaum et al, ChemPhysChem 2016, 17, 4116‐4123), we show that the aromaticity of the imidazolium head group plays an important role in the formation of smectic C phases. 相似文献
19.
We predict the existence of a new defect-lattice phase near the nematic-smectic-C(NC) transition. This tilt-analogue of the
blue phase is a lattice of double-tilt cylinders which are disclination lines in the smectic layer normal as well as the c-field.
We discuss the structure and stability of the cone phase. We suggest that many ‘nematics’ exhibiting short range layering
and tilt order may in fact be in the molten cone phase, which is a line liquid. 相似文献
20.
Detection of phase biaxiality in liquid crystals by use of the quadrupole shift in 131Xe NMR spectra
An experimental method to unambiguously distinguish between uniaxial and biaxial liquid crystal phases is introduced. The method is based on the second order quadrupole shift (SOQS) observable in 131Xe NMR spectra of xenon dissolved in liquid crystals. It is shown that besides revealing the biaxiality, the 131Xe SOQS offers a novel method to determine the tilt angle in smectic C phases. As an example, the 131Xe SOQS in a ferroelectric liquid crystal is reported. It yields up a biaxial phase in between isotropic and smectic C phases. 相似文献