A Driven Voltage-Controlled Reversible Electrooptic Effect in a Smectic A Phase II. PVA Anchoring

Abstract

The driven voltage-controlled reversible electrooptic effect in a smectic A phase with a preceding nematic phase twisted by SiO anchoring, previously discovered (cf. the previous paper) has been studied with polyvinyl alcohol (PVA) anchoring. The unique feature of this novel electrooptic effect in smectic A phases is that it reverses by relaxing when the electric field is removed. For the case of PVA anchoring the reversibility of the effect only occurs near the smectic A-nematic phase transition in the smectic A phase. The temperature region of the reversibility of this effect can be widened with a higher voltage. A comparison with the case of a reversible electrooptic effect in a smectic A with a preceding twisted nematic phase and SiO anchoring is made. The possible application of this electrooptic effect in liquid crystal displays with storage is discussed briefly.     

A Driven Voltage-Controlled Reversible Electrooptic Effect in a Smectic A Phase I. SiO Anchoring

A driven voltage-controlled reversible electrooptic effect in a smectic A phase with a preceding twisted nematic phase or a large-pitch cholesteric phase and SiO anchoring has been discovered. The unique feature of this novel electrooptic effect in a smectic A is that it reverses by relaxing when the electric field is removed. The conditions permitting the observation of such an effect are described in detail. On the basis of the smectic A textures observed a simple topological model is given. The influence of the polyvinyl alcohol anchoring is studied in the following paper. The main advantages and disadvantages of this novel electrooptic effect for a display device are briefly discussed.     

Surface memory effect in nematics with smectic C short-range order and hydrogen bonded molecules

A surface memory effect (SME) is found in the dimerized nematic with short range smectic C order of 4-n-octyloxybenzoic acid (OOBA) in a cell whose walls have been covered with an obliquely evaporated SiO or ITO layer. The recording and erasure processes are discussed and erasure in the nematic phase is realized. A mechanism for the surface memorization is presented which takes into account the specific character of dimerized nematics preceding cooling into a smectic C phase.     

Theory of nematic–smectic phase separation in thin twisted liquid crystal cells

Recently it has been shown experimentally by the authors that a highly twisted thin nematic cell at low temperatures can separate into a smectic A region in the middle of the cell surrounded by twisted nematic layers at the boundaries. In this case the twist is expelled into the nematic layers and the nematic–smectic A transition temperature is strongly depressed. We present a thermodynamic theory of such a phase transition in a twisted nematic cell, taking into account that the smectic A slab inside the nematic cell can be stable only if the decrease of free energy in the smectic region overcomes the increase in distortion energy of the twist deformation in the nematic layers plus the energy of the nematic–smectic A interface. In such a system the equilibrium thickness of the smectic A slab corresponds to the minimum of the total free energy of the whole cell, which includes all the bulk and surface contributions. Existing experimental data are at least qualitatively explained by the results of the present theory. This opens a unique possibility to study the properties of the nematic–smectic interface which is perpendicular to the smectic layers.     

Theory of nematic-smectic phase separation in thin twisted liquid crystal cells

Recently it has been shown experimentally by the authors that a highly twisted thin nematic cell at low temperatures can separate into a smectic A region in the middle of the cell surrounded by twisted nematic layers at the boundaries. In this case the twist is expelled into the nematic layers and the nematic-smectic A transition temperature is strongly depressed. We present a thermodynamic theory of such a phase transition in a twisted nematic cell, taking into account that the smectic A slab inside the nematic cell can be stable only if the decrease of free energy in the smectic region overcomes the increase in distortion energy of the twist deformation in the nematic layers plus the energy of the nematic-smectic A interface. In such a system the equilibrium thickness of the smectic A slab corresponds to the minimum of the total free energy of the whole cell, which includes all the bulk and surface contributions. Existing experimental data are at least qualitatively explained by the results of the present theory. This opens a unique possibility to study the properties of the nematic-smectic interface which is perpendicular to the smectic layers.     

The alignment of a liquid crystal in the smectic A phase in a high surface tilt cell

Liquid crystal alignment is studied using propagating optical mode techniques for a cell with a high surface tilt SiO alignment. The director configuration is determined for both the nematic and smectic A phases. In the nematic phase a uniform splay across the cell is demonstrated, as predicted by continuum theory. In the smectic A phase the structure is seen to be nearly uniform in the central region of the cell with large splay in boundary layers of about 0.5 μm thickness. The reason for this appears to be competition between the tilted surface alignment forces and internal forces within the bulk of the smectic A layer which would induce a homogeneous configuration.     

The influence of a bulk twist on the surface memorization in the liquid crystal HOBA

The surface-induced memorization of a smectic C liquid crystal texture in the temperature range of the nematic phase of 4- n -heptyloxybenzoic acid oriented by obliquely evaporated SiO on ITO was removed by imposing a bulk twist (Prevention of surface memorization was not observed when we used a simple ITO coating as the orienting surface.) A twist angle value ( Ωc ≈70°) above which the surface memorization is prevented was found. Using microtextural polarization analysis of the smectic C texture, a qualitative explanation of the phenomenon is suggested. The conditions for preventing the surface memorization were deduced in terms of the balance between the surface and bulk torques. These conditions provide surface anchoring breaking, so removing the surface memory effect.     

The influence of a bulk twist on the surface memorization in the liquid crystal HOBA

The surface-induced memorization of a smectic C liquid crystal texture in the temperature range of the nematic phase of 4-n-heptyloxybenzoic acid oriented by obliquely evaporated SiO on ITO was removed by imposing a bulk twist (Prevention of surface memorization was not observed when we used a simple ITO coating as the orienting surface.) A twist angle value (Ω_c ≈ 70°) above which the surface memorization is prevented was found. Using microtextural polarization analysis of the smectic C texture, a qualitative explanation of the phenomenon is suggested. The conditions for preventing the surface memorization were deduced in terms of the balance between the surface and bulk torques. These conditions provide surface anchoring breaking, so removing the surface memory effect.     

Twist deformation in anticlinic antiferroelectric structure in smectic B2 imposed by the surface anchoring

A simplified model is used to estimate the energies of observed structures in the smectic B₂ phase composed of bent-shaped molecules. An approximate twist solution connecting anticlinic antiferroelectric structure in the sample bulk with synclinic ferroelectric order induced at the sample surfaces by anchoring is proposed and the elastic and anchoring energies of this structure are determined. It is shown that uniform, twisted and partly twisted (mixed) anticlinic antiferroelectric structures can coexist with nearly the same energies.     

Low-birefringent, chiral banana phase below calamitic nematic and/or smectic C phases in oxadiazole derivatives

Bent-shaped molecules based on the oxadiazole central core with various side wings and terminal chain groups have been synthesized, and their liquid-crystalline behavior was investigated by optical microscopic, X-ray, and electrooptic measurements. These molecules exhibit liquid-crystal polymorphism including both the calamitic and banana phases. Such a characteristic polymorphism is attributable to the larger bend angle of the oxadiazole core compared to that of the resorcinol core used in conventional banana molecules. Only one type of banana phase, designated as the Bx phase, is formed. It appears upon cooling from the nematic and smectic liquid crystals and exhibits chiral domains with a very weak birefringence (apparently optically isotropic). By applying an electric field, the Bx phase is altered to a high-birefringence B2 phase with a homochiral SmC(A)P(A) structure that exhibits an antiferroelectric response. From detailed analyses of the optical texture and X-ray patterns through the transformation from well-oriented calamitic phases, the Bx phase was found to exhibit a helical structure, which arises as a frustration from the ground-state B2 phase in such a manner that the blocks of B2 layers are twisted with respect to each other in a direction parallel to the layer plane similarly to the twisted grain boundary (TGB) phase.     

Influence of the smectic A-nematic transitional order on the formation of the smectic phases of 4-n-hexyloxybenzylidene-4′-n-butyIaniline and 4-n-butyloxybenzylidene-4′-n-octyIaniline from an electrically deformed nematic phase

Abstract

The crucial role of the smectic A-nematic transitional order for the formation of the smectic A, B and G phases from an electrically deformed nematic phase of the liquid crystal 4-n-hexyloxy-benzylidene-4′-n-butylaniline (6O.4) with a typical smectic A-nematic first order transition and the formation of the smectic A and B phases from an electrically deformed nematic phase of the liquid crystal (4-n-butyloxy-benzylidene-4′-n-octylaniline (40.8) with a smectic A-nematic second order transition has been demonstrated. The nematic phase was deformed by an AC voltage of 2U,_th 5U _th and 10U _th, where U _th is the threshold voltage which causes the appearance of the Fréedericksz transition in the homeotropic nematic layer. The smectic textures have been observed on free cooling of the nematic phase or after the use of an oven. The smectic A phase of the liquid crystal 60.4 was observed with the formation of a clear smectic A-nematic phase boundary while the smectic A phase of the liquid crystal 40.8 has been formed from intermediate pretransitional stripes, observed by Cladis and Torza [1]. The homeotropic anchoring of the direction was crucial for the formation of the smectic phases of the liquid crystal 40.8 but not significant for the liquid crystal 60.4.     

Pretilt angle measurements on smectic A cells with chevron and tilted bookshelf layer structures

We have measured the pretilt angle induced by rubbed polymer films in a smectic A and in a nematic liquid crystalline medium using an optical phase retardation method. The pretilt angle was found to depend on the liquid crystalline phase (smectic A versus nematic) and on the smectic layer structure (chevron versus tilted-bookshelf). The occurrence of the different smectic layer structures was verified by X-ray diffraction measurements. The effect of the applied rubbing energy on the pretilt angle obtained is measured.     

Mesophase formation in a system of top-shaped hard molecules: density functional theory and Monte Carlo simulation

We present the phase diagram of a system of mesogenic top-shaped molecules based on the Parsons-Lee density functional theory and Monte Carlo simulation. The molecules are modeled as a hard spherocylinder with a hard sphere embedded in its center. The stability of five different phases is studied, namely, isotropic, nematic, smectic A, smectic C, and columnar phases. The positionally ordered phases are investigated only for the case of parallel alignment. It is found that the central spherical unit destabilizes the nematic with respect to the isotropic phase, while increasing the length of the cylinder has the opposite effect. Also, the central hard sphere has a strong destabilizing effect on the smectic A phase, due the inefficient packing of the molecules into layers. For large hard sphere units the smectic A phase is completely replaced by a smectic C structure. The columnar phase is first stabilized with increasing diameter of the central unit, but for very large hard sphere units it becomes less stable again. The density functional results are in good agreement with the simulations.     

Recent advances in nematic and smectic A anchoring on amorphous solid surfaces [1]

New anchoring properties of liquid crystals on amorphous solid surfaces are presented. In nematics (N), angular anchoring is usually described in terms of the Rapini-Papoular form, assuming constant surface order parameter. We generalize this expression, predicting a decrease of surface order for strong surface disorientation. Recent experiments on anchorings of varying strength confirm these predictions. Conjectures for the angular anchoring of smectic A on a solid amorphous surface explain the two easy layer orientations, normal to the surface or parallel, faceting inside a small critical angle. Roughness-induced surface transitions are discussed. For antagonistic nematic and smectic anchorings, we expect, below the N-S_A transition, a bent nematic surface boundary layer, recently observed by smectization under an electric field. Finally, the positional anchoring strength of smectics is introduced in terms of shear induced surface melting, and confirmed by a recent observation of oscillating shear stresses at the layer period.     

Oscillations in thin nematic layers

In the present paper a thin nematic liquid crystal layer between two identical boundary surfaces (solid walls or free surfaces in the case of a freely suspended film) is considered. In a mean field approximation it is shown that the interference between the boundary surface-induced smectic density waves results in oscillations of the free energy of the nematic layer and disjoining pressure acting on the boundary surfaces. Theoretical dependence of disjoining pressure on the nematic layer thickness is in qualitative agreement with experiment. Also we have considered a thin film of polar nematic in which in addition to an ordinary monolayer smectic A phase (S_A1) with the layer thickness d equal to the molecular length l the partial bilayer smectic A phase (S_Ad) occurs. It is shown that the variation of the distance between the boundary surfaces can result in the oscillatory S_A1-A_Ad phase transitions in this nematic film     

Studies of translational diffusion in the smectic A phase of a Gay-Berne mesogen using molecular dynamics computer simulation

Molecular dynamics computer simulations are used to determine the self-diffusion coefficients for a Gay-Berne model mesogen GB (4.4,20,1,1) in the isotropic, nematic and smectic A phases along two isobars. The values of the parallel and perpendicular diffusion coefficients, D(parallel) and D(perpendicular), are calculated and compared in the different phases. For the phase sequence isotropic-smectic A, D(perpendicular)*> or =D(parallel)* over the whole smectic A range with the ratio D(parallel)*/D(perpendicular)* decreasing with decreasing temperature. At a higher pressure, a nematic phase is observed between these two phases and we find that D(parallel)*>D(perpendicular)* throughout the nematic region and the inequality D(parallel)*>D(perpendicular)* remains on entering the smectic A phase. However, the ratio D(parallel)*/D(perpendicular)* decreases with decreasing temperature within the smectic A range and eventually this ratio inverts such that D(perpendicular)*>D(parallel)* at low temperatures. The temperature dependence of the parallel diffusion coefficient in the smectic A phase for this model mesogen is compared to that predicted by a theoretical model for diffusion subject to a cosine potential.     

Chirality, surface anchoring, and the cholesteric-smectic A phase transition

Magnetic Freedericksz transition measurements were performed on samples of chiral and racemic mixtures in the cholesteric/nematic phase. We found that the quantity Ф, which is defined as the threshold field H_th times the cell thickness, is larger for the chiral than for the racemic mixtures. This result is inconsistent with chiral contributions to the anchoring strength potential. Instead, the data support a model by Lubensky wherein the transition temperature to the smectic A phase is suppressed in a chiral liquid crystal.     

Communication: Experimental proof of symmetry breaking in tilted smectics composed of molecules with axial chirality

For the first time, domains with twisted structures have been established in planar samples of achiral compounds in tilted smectic C phase. This evidences separation of molecular conformers differing in the sense of axial chirality and confirms polar C(2) symmetry of these domains. A simple model considering polar surface anchoring energy and bulk energy of the twist can account for this finding. Conditions for coexistence of twisted and homogeneous domains are discussed.     

Helical twisting power and compatibility in twisted nematic phase of new chiral liquid crystalline dopants with various liquid crystalline matrices

ABSTRACT

Four chiral dopants exhibiting smectic LC phases themselves were prepared and their helical twisting power (HTP) and thermal phase behaviour in mixtures with four various LC hosts were studied. The influence of host liquid crystal on HTP was evaluated and generally higher values were found for hosts with high birefringence. Unexpectedly, high enhancement was found for an LC-chiral dopant pair, both having a similar aromatic core – biphenyl ring substituted with polar group. All studied chiral dopants exhibited limited compatibility with the LC hosts in twisted nematic phase at room temperature. For one of the studied mixtures, it was able to obtain single twisted nematic phase with selective light reflection band with maximum at wavelength about 1.0 µm. Carboxylic acid-type dopants exhibited total compatibility with the studied host in single twisted nematic phase at elevated temperatures, allowing preparation of mixtures with reflection band in the visible range. In case of the carboxylic acid dopants, blue phases for optimised compositions were observed. Intermolecular hydrogen bonding between carboxylic acid proton and pyridine nitrogen of chiral dopants was found. Doping the LC host with these dopants led to slight enhancement of HTP value and higher solubility in the LC host.     

Determination of elastic constants of a binary system (7CPB+9.CN) showing nematic,induced smectic Ad and re-entrant nematic phases

The temperature variation of the splay and bend elastic constants of a binary system exhibiting nematic-induced smectic A_d and re-entrant nematic phases measured by electric field-induced Freedericksz transition method has been reported. As bend deformation is not permitted in the smectic A phase, bend elastic constant (K₃₃) could only be determined in the nematic and re-entrant nematic phases. In both the nematic phases, the splay elastic constant has the same order of magnitude and does not show any pretransitional effect. However, in the induced smectic A_d phase, the value of K₁₁ is about one to two orders higher than that in the nematic phases. The bend elastic constant shows a strong pretransitional effect near the nematic–smectic and smectic–re-entrant nematic phase transitions. The influence of the presence of the induced smectic phase is observed even in those mixtures which have no induced smectic phases. As the smectic phase is approached, the ratio K₃₃/K₁₁ increases rapidly and diverges to infinity.