Tilted Smectic Phases Research Articles

X-ray Structural Analysis of a Mesogenic Compound N,N′-Bis-(p-Butoxybenzylidene)-α,α′-bi-p-Toluidine

Abstract Crystal and molecular structure of a mesogenic compound N,N′-Bis-p-(Butoxybenzylidene)-α,α′-bi-p-Toluidine has been determined by direct methods. The compound crystallizes in triclinic system with space group P1. Cell parameters are a = 6.116(3), b = 7.916(3), c = 31.421 (8)Å, α = 92.39(2), β = 92.35(3), y = 96.59(2)° and z = 2. The two symmetrical fractions of the molecule are almost planar but the angle between these planes is 63.5°. The molecules are arranged in tilted layers which is precursor to the tilted smectic phase. Crystal-mesophase transition is discussed.

The synthesis and liquid crystal properties of some 4-n-nonylphenyl esters of 3-(4′-n-alkoxybiphenylyl)- and 3-(4′-n-alkoxybiphenylyl)-3-methyl-propanoic acids and their laterally fluorinated analogues

Abstract A number of 4-n-nonylphenyl esters and mono- and di-fluorinated 4-n-nonylphenyl esters derived from 3-(4′-n-alkoxybiphenylyl)propanoic acids (II) have been synthesized and their thermotropic liquid crystal properties assessed with a view to obtaining tilted smectic phases for possible use in ferroelectric display devices. Many of these compounds exhibit wide temperature range SC and SI/F phases, but none gave the ideal phase sequence SC[sbnd]SA[sbnd]N[sbnd]I mainly because the structure of these compounds was not conducive to the formation of the nematic phase. To try to alleviate this problem a series of 4-n-nonylphenyl esters based on 3-(4′-n-alkoxybiphenylyl)-3-methylpropanoic acid was prepared, where a lateral methyl group was incorporated in the β-position of the [sbnd]CH2CH2CO2[sbnd] linkage. Incorporation of the lateral methyl group encouraged the formation of a nematic phase at the expense of both the smectic A phase and tilted smectic phases.

Ferroelectric liquid crystalline polymers

Abstract The recent state and perspectives in the field of synthesis, phase behaviour and properties of ferroelectric liquid crystalline polymers are reviewed. Spontaneous polarization, pyroelectric effect and various relaxation processes in chiral tilted smectic phases of both side chain and combined (side group and main chain) liquid crystalline polymers are discussed.

One-dimensional correlations in ordered smectic phases of comb-like polymers

The one-dimensional ordering of the ordered tilted (F) and normal (B) smectic phases of comb-like polymers is discussed. The one-dimensional correlation functions are calculated and the damping behaviour of one-dimensional correlations is characterized. The density distribution profiles along the normal to the plane of a smectic layer are determined. Finally, the variations of one-dimensional ordering during tilted-normal smectic phase transitions are examined.

The structural basis of transitions between highly ordered smectic phases in semifluorinated alkanes

Abstract The semifluorinated alkanes F(CF2)10(CH2)9H and F(CF2)10(CH2)11H both undergo a reversible transition between tilted smectic phases above room temperature as demonstrated previously for F(CF2)10(CH2)10H. The transformation on cooling is achieved in two stages: an increase in molecular tilt relative to the layer normal, followed by a change in the molecular interdigitation within each layer. The extent of complete transformation is sensitive to temperature and time, due to strains occasioned by the accompanying volume change, and because the second stage requires significant molecular translations. Characterization of the transformation is possible by transmitted light microscopy, supplemented by X-ray diffraction.

The ferroelectric polarization of liquid crystals and its measurements

Abstract In liquid crystals, the chiral tilted smectic phases only present, a spontaneous (ferroelectric) polarisation, which is due to the molecular rotations cramped by the structure in these phases. The amplitude of the polarisation given by a polar group on the molecule varies with the distance of this group from the chiral part. This spontaneous polarization is a very important parameter because its linear coupling with an applied electrical field is the basis of all applications of these compounds. The measurement of the polarization is obtained from the current induced when it is reversed. I comment on different methods to obtain a good measurement by polarization reversal in the whole sample and separation of the reversal current from the ionic and displacement currents.

The soft-mode ferroelectric effect

Abstract In this paper a presentation is given of some of the basic physics of the soft-mode ferroelectric effect, along with its experimental background. Further, possible applications in electrooptic devices are elucidated. A fast (sub-microsecond) electrooptic switching can be achieved in the A* phase, and in similar orthogonal smectic phases made up of chiral molecules. Instead of using the phase variable Φ, it uses the tilt angle θ, which in principle is a “hard” variable, but is expected to soften on approaching the transition to a lower-lying adjacent tilted smectic phase. However, the switching is efficient in the whole range of the orthogonal phase (in fact, less efficient near the tilting transition) and is observed whether an adjacent tilted phase is present or not. As compared to the surface-stabilized (SSFLC) electrooptic mode, this soft mode (SMFLC) is based on the electro clinic, effect of essentially ferroelectric nature very closely related to the presently more investigated ferroelectric, effect in tilted phases. It uses the same bookshelf geometry and has the same symmetry in its ON-OFF switching characteristics. It possesses no bistability but can generate a continuous grey-scale or even a continuous colour-scan controlled by an applied voltage. Depending on polarizer and retarder arrangement, it may be used for modulating a light signal with a transmitted intensity linear in the applied electric field, or it may be used as a tunable colour filter. One of the obvious potential applications is in active matrices to replace the present nematics; another one is in spatial light modulators.

Solitary waves in ferroelectric liquid crystals.

Since their discovery by Meyer et al. in 1975,1 ferroelectric liquid crystals (FLCs) in the smectic C phase have been the focus of a considerable research effort. It is now well established thay any tilted smectic liquid crystalline phase composed of chiral molecules should possess a permanent electric polarization P which is oriented perpendicular to the molecular director n and parallel to the smectic layer plane (See Figure 5.1). The presence of permanent dipoles fundamentally alters the nature of the interactions between the molecules themselves and with any cell walls or applied electric fields. This is evidenced by the existence of a whole family of director orientational structures which result in the distinctive textures peculiar to ferroelectric liquid crystals (FLCs)2

Textures and structure of the low temperature liquid crystal phases of HOBACPC [(R-) hexyloxybenzylidene p'-amino-2-chloropropyl cinnamate

HOBACPC is known to exhibit three helielectric phases identified as C*, I* and (G')* or J*. Here we present evidence that a truly ferroelectric phase, X, exists at lower temperatures than these phases. Owing to their helicoidal structure, the spontaneous polarization of helielectric phases globally averages to zero. In bulk samples they are not usually bistable (by bistable we mean that the field induced configuration remains when the field is removed) so that when the field E is turned off, the helicoidal structure returns. In contrast, long range inter-layer correlations suppress the helicoidal structure in smectic X and it is bistable. We argue that the cross-over from helielectric to ferroelectric behaviour takes place in the (G')* phase since it is characterized by finite inter-layer correlations resulting in a non-uniform helicoidal structure. Because many tilted smectic phases have been discovered recently and there have been numerous changes in nomenclature, we find it useful to review our current understanding of the structural features of these phases as they may apply to HOBACPC in an appendix Mise en evidence d'une phase intrinsequement ferroelectrique. Determination de sa texture et de sa reponse electrooptique

Spontaneous Polarisation Measurements on CE8 and CE3, two Commercially Available Ferroelectric Liquid Crystals

Abstract Measurements of the spontaneous polarisation are presented on two commercially available chiral esters, both of which have a smectic ‘C’ phase. Experimental results for the spontaneous polarisation in chiral tilted smectic phases other than the SmC* are also included for one of the materials. It is shown that a dramatic difference exists in the ferroelectric properties of the two structurally very similar materials.

Pyroelectric properties of liquid crystals

Abstract Possibilities are discussed of observing the pyroelectric effect in ferroelectric and nonferroelectric (under special conditions) liquid crystals. The temperature dependencies of the volume pyroelectric coefficient γ(T) and spontaneous polarization P(T) are demonstrated for the chiral smectic C* and H* phases. The relationship between the P value and the location of an electric dipolar moment in a molecule of the liquid crystals has been established. The dynamics of the spontaneous polarization have been investigated and the critical decrease of P detected near the smectic C* ↔ smectic H* and smectic C* ↔ smectic A phase transitions. The effect of impurities, particularly conductive ones, on pyroelectric properties and polarization of liquid crystals is discussed in detail. For the first time non-helicoidal liquid crystalline ferroelectrics have been prepared using certain mixtures of left- and right-hand rotating chiral smectic phases. High values of the spontaneous polarization have been detected in tilted smectic phases with ferroelectricity induced by chiral impurities.