Studies on Anisotropic Molecular Orientation and Mesophase Stability in a Ternary Mixture of Liquid Crystalline Materials

The multicomponent system of cholesteryl linolenate (CL), sodium oleate (Naol), and orthophosphoric acid (H₃PO₄) exhibits liquid crystalline mesophases, like cholesteric and smectic phases, such as SmA, SmC, SmB, and SmG sequentially when the specimen is cooled from its isotropic phase. These phases have been observed by using microscopic technique. Helfrich potential and elastic moduli have also been estimated in the smectic phase using Helfrich model. Optical transmittance and electrical conductivity have also been discussed.     

Phase transition behavior and structure of the thermotropic liquid crystal 6‐{[(4′‐{[(undecyl)carbonyl]oxy}biphenyl‐4yl)carbonyl]oxy}‐1‐hexyne

The phase transition behaviors and corresponding structures of 6‐{[(4′‐{[(undecyl)carbonyl]oxy}biphenyl‐4yl)carbonyl]oxy}‐1‐hexyne (A4EE11) were investigated using differential scanning calorimetry (DSC), polarizing optical microscopy (POM) and wide angle X‐ray diffraction (WAXD). In comparison with the published homologues, 5‐{[(4′‐heptoxy‐biphenyl‐4‐yl)carbonyl]oxy}‐1‐pentyne (A3EO7) which shows a monotropic smectic A (SmA) phase and a metastable monotropic smectic C (SmC) phase; 5‐{[(4′‐heptoxy‐biphenyl‐4‐yl)oxy]carbonyl}‐ 1‐pentyne (A3E'O7) that exhibits three enantiotropic stable liquid crystalline (LC) phases, SmA phase, SmC phase and smectic X (SmX) phase; 5‐{[(4′‐heptoxy‐biphenyl‐4‐yl)carbonyl]oxy}‐1‐undecyne (A9EO7) which has a monotropic SmA phase and a metastable crystal phase, A4EE11 integrates the enantiotropy, monotropy and metastability of the LC phases of those three compounds. Upon cooling from isotropic state to room temperature, in the temperature range of 62.0 to 58.5°C, A4EE11 shows an enantiotropic smectic A (SmA) phase with a layer spacing d=32.69Å. Further lowering the temperature, it enters into a metastable monotropic smectic B (SmB) phase with a longer layer spacing d=34.22Å which has a tendency towards crystallization. The metastability of the liquid crystalline phase may associate to the linkage order of the ester bridge between the mesogenic core and the flexible spacer. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Nano Molecular Self Assembly in the Formation of Induced Mesomorphism in Mixture of ChCl,DTAC, and EG

The multi-component system of cholesteryl chloride (ChCl), dodecyl trimethylammonium chloride (DTAC), and ethylene glycol (EG) exhibits very interesting liquid crystalline mesophases like cholesteric and SmA, SmC, and SmB phases sequentially when the specimen is cooled from its isotropic phase. These phases have been characterized by employing optical and X-ray studies. Pitch of the cholesteric phase has been calculated and discussed. Variation of pitch from the cholesteric phase to smectic phase is smooth and continuous. The temperature variation of optical anisotropy and electrical-conductivity has also been discussed. It has been found that wherever there is a phase transition, the value of electrical susceptibility changes appreciably.     

Anisotropic Molecular Orientation of Micellar Nematic Phase in a Binary Mixture of Two Nonmesogenic Compounds

We report the results of our studies on the optical and thermal properties of two nonmesogenic compounds, namely, Cetyl-dimethyl-ethylammonium bromide (CDEAB) and Glacial acetic acid. The mixture exhibits very interesting schilieren texture of micellar nematic phase, SmA, SmC*, and SmE phases, respectively, at different concentrations of CDEAB sequentially when the specimen is cooled from its isotropic phase. The order parameter (S) of the micellar nematic phase is estimated with the help of temperature dependence of optical anisotropy from the measured values refractive index and density data. The temperature variation of order parameter of the experimental curve is very well fitted with the Mayer–Saupe theoretical curve. X-ray studies have been discussed. The formation of above phases has been confirmed by optical and DSC studies.     

Pressure Study of a Hexatic B and Crystal B Phase

Pressure temperature phase diagrams of two compounds exhibiting smectic B phases are presented. In one compound, the smectic B phase is liquid (no long range positional correlations) and is called a hexatic B phase, whereas in the other the smectic B phase is crystalline in nature. “Transition bars”, which are often observed at a smectic A to crystal B transition are observed above 1.5 kbar (and not below) in the former compound and the smectic A phase appears quite piezo-resistant. In contrast, for the latter compound, the smectic A phase disappears above 1.94 kbars. These results suggest that the two smectic phases are stabilized by different forces and we suggest a model.     

Synthesis of four liquid crystals and study of alkyl chain effect on the nematic range for application in GC

Four liquid crystals (LCs) compounds which contain the 1,3,4-oxadiazole group were synthesized and characterized with spectroscopic techniques (IR, ¹H- and ¹³C NMR), their thermal properties were analyzed by the Differential Scanning Calorimetry (DSC) and the polarizing microscope (POM). A comparative study of the mesomorphic properties of these LCs and three other compounds which have already been used as a stationary phase in gas chromatography (GC) was carried out. These compounds have the same main nucleus. LCs V1, LC1, LC2 and LC3 gave a nematic (N) phase to the heating, LCs V2 and V3 recorded smectic A (SmA) and N phases. However, the range (N) has disappeared in V4.     

Dielectric reorientation of smectic C,F and I phases

As shown for 4 liquid crystalline substances also smectic F and smectic I phases may be reoriented by an electric field similar to a smectic C phase. The threshold voltage of the effect was determined approximately. For one substance the tilt angle of the smectic C and I phase could be measured by means of the dielectric reorientation.     

Synthesis and Mesomorphic Behavior of Calamitic Liquid Crystals with a Biphenyl Moiety

A new mesogenic homologous series having a biphenyl moiety has been synthesized by condensing 4-hydroxy-4′-nitrobiphenyl- and methoxy-substituted 4-n-alkoxy cinnamoyl chlorides, and their liquid crystalline properties have been studied. All the members of the series are enantiotropic liquid crystals. The methyl to n-pentyl derivatives exhibit both Smectic A (SmA) and Nematic (N) phases; the higher members, starting with the n-hexyl derivative show only a SmA phase. The plot of transition temperatures versus number of carbon atoms in the alkoxy chain exhibits zig-zag pattern for Sm-N and N-Isotropic (Iso) transition temperature curves. The average thermal stability is compared with other related homologous series. The introduction of polar nitro group increases significant intermolecular force of attraction which stabilizes the molecular orientation. This results into the increase in the thermal stability of the system.     

Liquid crystals,fractals and art

We discuss the aesthetic appeal of liquid crystal textures and take a brief polarising microscopic tour through the wealth of the appearances of nematic, cholesteric, smectic and banana phases. Some examples are given on how these textures may be used in a creative process employing digital image processing to produce ‘computer art’, including images exhibiting self-similarity. A discussion of the ‘art’ of computer-generated mathematical fractals leads us to examples of fractal structures observed in the phase-ordering process of some liquid crystalline phases. This includes a first brief report of dilatation invariance observed for aggregates of a conventional SmC phase and percolation simulations to quantitatively explain the obtained textures. The circle between liquid crystals, fractals and arts is closed by a discussion of self-similarity in modern arts, especially works of the movement known as abstract expressionism. More than 100 paintings of the artists Wassily Kandinsky, Jackson Pollock, Mark Tobey and Franz Kline were analysed, placing an emphasis of the discussion on the ‘drip-paintings’ of Pollock in relation to recent reports.     

First Truly Ferroelectric Liquid Crystal

The first faroelectric liquid crystalline phase (in 8SI* (ΔS-(+)-(4-(2′methylbuty1) phenyl-4′-n-octylbiphenyl-4 carboxylate) is reported and its physical properties are discussed. The close connection between the phase sequences of 8SI* and its racemate are established. Both compounds show a number of smectic phases above the newly discovered ferroelectric phase and its analogue in the racemate. These phases (CI and G') are tilted in the racemate 8SI and tilted with a helicoidal structure in 8SI*. In the truly ferroelectric phase, X, the director of the molecules can be oriented in a bistable way in an electric field Furthermore when the field is turned off the director orientation remains unchanged. This behaviar is fundamentally different from that in bulk smectic phases which support the helix structure in the absence of external forces. A model far ‘racemates’ in higher smectic phases like G, G', H’ etc as well as a the newly discovered ferroelectric phase is presented. The response of cholesteric blue phases of 8SI* to an external field is discussed in an appendix.     

Computer Simulation of Biphenyl-Phenyl Ester Liquid Crystals

Abstract

Molecular orbital (MO) calculation and molecular dynamics (MD) simulation were carried out for a set of smectic liquid crystalline molecules, 8O-O8 (4-octyloxyphenyl-4′-octyloxybi-phenyl-4-carboxylate) and 8O-8 (4-octylphenyl-4′-octyloxybiphenyl-4-carboxylate), to understand the molecular origin of different crystalline structure formation and to predict their conformational property in liquid crystalline phases. The results of MO and MD analyses indicate that the structure of 8O-O8 and 8O-8 in crystalline phase is essentially determined by intramolecular interactions and the experimentally observed structure is chosen as a consequence of intermolecular packing constraints. This paper also reports the first application of two-molecular ab-initio calculation to liquid crystalline systems.     

Thermal Microscopy and DSC Studies of 4-[4-n-pentyloxybenzylideneamino] Azobenzene

Phase transition studies of the liquid crystal, 4-[4-n-pentyloxybenzylidene–amino]–azobenzene (PBAAB) have been investigated by thermal microscopy and Differential Scanning Calorimetry (DSC) studies. From these studies it is confirmed that the compound, PBAAB exhibits nematic, smectic A and smectic B phases. The textures exhibited by different phases are presented. The enthalpies and entropies at various phase transitions are also reported.     

New chalcone based liquid crystals with allylidene amino linking unit: Synthesis and characterization

In this study, the synthesis, structural characterization and mesomorphic properties of newly thirteen calamitic shaped compounds derived from allylidene amino chalcone and 4-n-alkoxy benzoyloxy benzoic acid. Comp.H₁ to H₃ exhibited nonliquid crystalline nature, while comp.H₄ to comp.H₁₈ displayed enantiotropical smectic C phase. Phase transition temperatures of present synthesised compounds were determined by optical polarising microscopy (POM), differential scanning calorimetric (DSC) and powder X-ray diffraction (PXRD) techniques. Thermal stabilities of smectic to isotropic phase are 150.0 °C and temperature range of mesophase is in decreasing order from comp.H₃ to comp.H₁₈ respectively. The presences of SmC phase are the type of broken fan and needle type in present synthesized series. It is also shown that presence of chalcone amino allylidene central linking group favors a calamitic liquid crystalline behaviour in molecules with lower member to higher member aliphatic side chain in alkoxy group (-OR).     

Structure parameters in mixed liquid crystals (I). Layer thickness of the smectic mixed phases B in the binary system of ethyl-4-(4-ethoxy-benzylidene-amino)-cinnamate and terephthylidene-bis-(4-n-butyl-aniline)

The thickness of the smectic layers in binary liquid crystalline mixed phases of the phase type B are measured in dependence on the concentration. Both components have different B-phase structures. In the part of the binary system with a smectic polymorphism BA the long molecular axes are perpendicular to the layer planes. If the polymorphism is changed by the appearance of a mixed phase of the type C, i.e. the smectic polymorphism BCA exists, there occurs a transition into the B-phase structures with tilted axes, the tilt angle of which is changed continuously with the concentration.     

Terminal Effect of the Semi-fluorocarbon Chain on the Liquid Crystalline Behaviors of Some Compounds

Two series of chiral compounds with semi-perfluorocarbon chains and alkoxy ones, nBP4F and nBP6F, are synthesized. Enantiotropic SmA phase is identified in all of these compounds. When the alkoxy chain length is short, enantiotropic SmB is also identified. When the alkoxy chain length is longer than 6 carbons, enantiotropic SmC* phase is formed. Their clearing points increase with increasing the perfluorocarbon chain length. Comparison of compounds with -CF₃, -CHF₂ and -CF₂Cl groups at the terminals of the fluorocarbon chains, the clearing point sequence is -CHF₂ < -CF₂Cl < -CF₃; and the melting point sequence is -CF₃ < -CHF₂ < -CF₂Cl. Therefore, the compounds with semi-perfluorocarbon chains show the widest liquid crystalline temperature range. Moreover, the SmC* phase of the compounds with -CF₃ at the terminals shows the highest thermostability.     

Resonant scattering from liquid crystal devices: an in-situ structural probe for the smectic phases

Resonant X-ray scattering is a relatively new technique in the field of liquid crystals and in recent years has been used to investigate the structures of the SmC* sub-phases. This work has been carried out on several materials which include either a sulfur or selenium atom and has revealed the detailed biaxial structure of the four layer intermediate (ferrielectric) phase. Resonant X-ray scattering has also been used to produce the first in-situ measurements of the chiral smectic sub-phases in a liquid crystal device. Both the antiferroelectric and the four-layer intermediate phases have been observed via this technique in a device geometry. Electric field studies have also been carried out in these phases whilst monitoring phase structure simultaneously. In this paper I will summarize the most up-to-date results from resonant scattering in free-standing films and discuss resonant scattering experiments on liquid crystal devices.     

Synthesis and mesomorphic properties of 2,4-bis(4′-n-pentyloxybenzoyloxy)- benzylidine-4″- n-alkoxyaniline

The synthesis and mesomorphic properties of a new series of 2,4-bis(4′-npentyloxybenzoyloxy)- benzylidine-4″ -n-alkoxyaniline (DC5An) are reported. The molecular structure of compounds was confirmed by FTIR, ¹H-NMR, ¹³C-NMR, mass spectroscopy and elemental analysis. The mesomorphic properties were studied by differential scanning calorimetry (DSC) and polarizing optical microscopy (POM) measurements. All compounds of the series exhibit nematic (N) and smectic C (SmC) phases. The first four homologues (DC5A1-DC5A4) display a N mesophase, whereas the highest homologues (DC5A5-DC5A10) exhibit an enantiotropic dimorphism N and SmC phases. The mesomorphic properties of the present series are compared and discussed with other structurally related series.     

Investigation of the Homologous Series of 4-Propionyl–4′-n-Alkanoyloxyazobenzenes by X-Ray Diffraction

A study of the homologous series of liquid cristalline 4-propionyl-4′-n-alkanoyloxyazobenzenes was performed by x-ray diffraction. Two indipendent parame ters were used, namely the temperature and the paraffinic chain length, in order to obtain some structural in formation. In particular the molecular layer thickness was obtained as a function of the temperature for the different phases and different homologous. A linear de pendence of the molecular layer thickness as a function of chain length was found for the crystalline phase as it was previously observed for the smectic A phase. By comparison of the present data with those of the series 4-acetyl-4′-n-alkanoyloxyazobenzenes, a bilayer molecular organization similar to that of the smectic A phase appears as very probable also for the crystalline phase.     

Synthesis and Mesomorphic Characterisation of Chiral Homologues

Abstract

Chiral liquid crystals have attracted considerable interest as they exhibit a good variety of modulated phases. We have synthesised a homologous series viz., 4-(4^!-n-alkoxy benzoyloxy) benzylidene-4^!!?1-(s)-methyl propoxy anilines, incorporating a terminal chiral centre, inorder to obtain better understanding of the relationship between molecular structure and appearance of SmC* phase in the molecules. It is observed that, in the present series the lower members upto butyl are pure nematogens, while pentyl to hexadecyl derivatives exhibit classical smectic as well as nematic mesophases. An additional smectic C* phase is observed in the middle octyl to dodecyl homologues. The homologues have been characterised by IR, NMR and DSC. Their mesomorphic properties have been compared with structurally related homologous series.     

ALIGNMENT INSTABILITY INDUCED BY IRRADIATION OF VISIBLE LIGHT IN FRUSTOELECTRIC LIQUID CRYSTALLINE CELLS SHOWING THE V-SHAPED SWITCHING

Abstract

Irradiation of the powerful visible light from the laser expedites the deterioration of the frustoelectric liquid crystalline cells (Inui mixture) showing the V-shaped switching. This deterioration is caused by the light absorption in the aligning layer, which strongly influences the molecule-surface polar interaction; this cannot be observed in the cell with the transparent aligning material in the used visible light region. This is observed just in the tilted smectic X* phase, not in the SmA phase. Irradiation during the switching leads to hysteresis in the V-shaped pattern and changes even the surface molecular alignment. These results can be explained by the shielding of the surface charge due to the alignment of the spontaneous polarization and the disturbed polar anchoring.