Landau Theory of Transitions from the Drd Phase of Columnar Liquid Crystals

Possible symmetry patterns of disordered columnar phases associated with the continuous symmetry breaking of the D_rd (pgg) phase are given. The results may be used to predict new mesophases which are as yet undiscovered and should be of aid in identifying unconfirmed phases. The universality classes of the possible transitions are also given.     

Phase Diagrams of Cholesteric and Nematic Thermotropic Liquid-Crystalline Polymers

Linear mesogenic polyesters with flexible spacers (molecular weight ranging from 9,000 to 20,000) are studied by polarized microscopy. The nematic and cholesteric mesophases are identified by means of the contact method. Two enantiomers form perfect solid, cholesteric and liquid solutions. The sign of the twist of the cholesteric phases are determines.     

Mesophases de Derives du Cyclohexane

The mesophases of four trans 4-alkyl-cyclohexane carboxylic acids and of 1,4-bis(′-heptyl-cyclohexyl)-cyclohexane “all trans” were identified by isomorphy as nematic and normal smec-tic B phases. The existence of mesogenic molecules without a permanent dipolar moment or a delocalizated electron is thus confirmed.     

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.     

Alkyloxy azo-cinnamate ester based thermotropic liquid crystals and their photophysical investigations

Two series of rod shaped Schiff base containing azo-cinnamate thermotropic liquid crystalline compounds were synthesized and characterized. These molecules have four different alkyl spacers (n = 6, 8, 10, and 12) at one end and a dodecyloxy chain at another end (n = 12) which influence their liquid crystalline properties. Liquid crystallinity commence from six methylene spacer onwards in the series exhibiting with Nematic to Smectic-C mesophases. Analytical data confirm the molecular structures of homologues series of compounds. The characteristic texture of liquid crystalline phases obtained using polarizing‐light microscope with a heating stage. The structural transformation of these liquid crystals was confirmed by differential scanning calorimetry. All the compounds exhibited various calamitic mesophases with wide mesomorphic temperature ranges. The relationship between structure and mesomorphic properties was discussed in framework of geometrical configuration of central unit, linking part, and length of alkoxy chains. Photosensitive azobenzene group undergoes photoisomerization under UV light and monitored by UV-Visible spectroscopy.     

New Series of Biforked Mesogens Derived from 3,4-Dialkoxycinnamic Acid and 3-(3′4′-Dialkoxyphenyl)Propanoic Acid

Four series of biforked or polycatenar mesogens derived from 3,4-dialkoxycinnamic or 3-(3′,4′-dialkoxyphenyl)propanoic acids are described. These derivatives exhibit an extremely complex polymorphism including lamellar and columnar mesophases as well as nematic and cubic and some unidentified phases. X-Ray measurements have given the first evidence for a thermotropic rhombohedral mesophase. The relationships between the polymorphism and molecular structure are discussed.     

Calorimetric investigations in liquid crystalline terephthalylidene-bis-[4-n-alkylanilines] (TBAA)

By means of the calorimeter DSC-2 the temperatures and enthalpies of phase transitions were measured in the homologues of TBAA with the alkyl chain length n from 2 up to 10, in order to confirm the existence of nine types of liquid crystalline mesophases and of the polymorphism in the solid state within this series. Pre-and posttransitional phenomena, hysteresis effects in low temperature smectics and relations between enthalpies and chain lenght resp. kind of concerning phases could be observed.     

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.     

Density Studies in Polymorphic Liquid Crystal N- (p-n-Heptyloxybenzylidene) p-n-Pentylaniline

Density studies in polymorphic liquid crystal, N-(p-n-heptyloxybenzylidene) p-n-pentyl-aniline is carried out in isotropic liquid, nematic, smectic A, smectic C, smectic B and smectic G phases. The temperature variation of density in all the mesophases confirms that isotropic liquid to nematic, nematic to smectic A, smectic C to smectic B, smectic B to smectic G transitions are of first order while smectic A to smectic C transition is of second order. The density variation across smectic C to smectic B is greater than that of any other transition. Thermal expansion coefficients are also calculated.     

Phase Diagram Involving the Thermal Stability of TGB and Induced Smectic Phases in Binary Mixture of Thermotropic Liquid Crystals

We report the results of our studies on the optical and thermal properties of the mixture of terephthalidene-bis-4, n-alkylaniline and cholesteryl nonanoate, which exhibits very interesting liquid crystalline mesophases such as cholesteric, twisted grain boundary, smectic-A, smectic-C^*, smectic-C, smectic-I, and smectic-G, sequentially when the specimen is cooled from its isotropic phase. These phases have been characterized by using microscopic techniques. The temperature variations of optical anisotropy, electrical conductivity, ultrasonic velocity, and molar and adiabatic compressibility have also been discussed.     

Lyotropic Mesophase Behaviour of n-C16H33EO3 - Evidence for Two “V2” Phases

Abstract

Nonionic surfactants of the alkyl polyoxyethylene ether variety commonly form hexagonal (H₁) or lamellar (L_α) lyotropic mesophases with water¹. Less well-known is the frequent occurrence of I₁ or V₁ cubic mesophases.? No reports exist to date of the occurrence of reversed phases with these materials. While the surfactants are known to form several solid complexes with water, their melting points are below those of the anhydrous surfactants². In an optical microscope study of the liquid crystals formed by water penetration into n-hexadecyl trioxyethylene glycol ether (C₁₆EO₃) we have observed a solid complex with a melting point above that of the pure surfactant. We also observe a cubic region which we classify as a reversed structure (V₂). Moreover, the occurrence of a refractive index discontinuity within this region strongly suggests that two different phases are present. The cubic region coexists with water for part of the temperature range over which it occurs. When water droplets form within the cubic region on heating, they adopt angular shapes which suggest that a single, long-range structure exists, rather than a “powder” orientation of domains.     

New thermotropic cubic mesophases

1,2-Bis-[4-n-alkyloxybenzoyl]-hydrazines with the general formula possess smectic C phases and at lower temperatures additional optically isotropic phases. According to X-ray investigations these isotropic phases show cubic symmetry. The lattice units consist of aggregations with micellar dimensions. The substances were characterized by microscopic, dilatometric and calorimetric investigations. The new cubic mesophases do not exhibit complete miscibility with the “smectic D” phases.     

Optical Sign Determinations on Cholesteric Lyotropic Mesophases

Optical signs of type I and II micellar cholesteric lyotropic mesophases were for the first time determined. Lyomesophases were prepared from the following amphiphiles: potassium l-N-lauroyl-serinate, potassium laurate, sodium decylsulfate, cesium decylsulfate and decyl-ammonium chloride. Cholesterol was added to all mesophases in order to obtain the chevron pattern for magnetically oriented samples. The method involves the direct observation of the textures at the polarizing microscope. Using a quartz-wedge retardation plate the directions of the slow and fast light vibration components are determined. This method is simpler than the usual observation, under conoscopic illumination, of the basal interference figure which is hardly obtained in micellar lyotropic cholesteric mesophases. All type I cholesteric lyotropic mesophases here investigated exhibited a positive optical sign, whereas all type II cholesteric lyomesophases showed a negative birefringence.     

Alkoxy/halostyryl benzoic acids: Synthesis,crystal structure,and study of mesomorphic and photophysical properties

Alkoxy/halostyrylbenzoic acids were synthesized and evaluated for their mesomorphic and photophysical properties. The trans geometry of the molecules was established by the presence of large coupling constants in ¹H-NMR spectra and confirmed further by X-ray diffraction (XRD) analysis of two of the compounds. The straight-chain acids were found fairly stable showing first sign of decomposition above 290°C. The enantiotropic mesophases could only be studied in acids with seven or more methylene groups in the alkoxy chain. Textures of nematic and smectic phases were exhibited by these acids under polarizing optical microscope. Photophysical behavior of the compounds in solution was also investigated.     

Phase Diagrams of New Compounds with Nematic Reentrant Mesophases

Two new series of compounds exhibiting nematic and smectic reentrant mesophases are presented here; the first one is 4-n-alkoxy benzoyloxybenzylidene 4'-cyanoaniline series the second one the 1-(4-n-alkylbenzoyloxyphenyl) 2-(4'-cyanophenyl) ethane series. Isobaric binary phase diagrams are plotted for each compound by means of the contact method and the mesophases so identified.     

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.     

Preparation and Photoluminescent Properties of Hydrogen-bonded Discotic Liquid Crystals

Photoluminescent discotic liquid crystals (DLCs) have been prepared through intermolecular hydrogen bonding between core and peripheral units. 1,3,5-Trihydroxybenzene was employed as core molecule and peripheral stilbazole derivatives containing linearly conjugated moieties were complexed to form hydrogen-bonded DLCs. Their structures, mesophases, and optical properties were investigated. The complexation of core and peripherals made some discotic complexes form photoluminescent DLCs. The prepared luminescent DLCs exhibited columnar mesophases with strong blue or sea green photoluminescence. These findings may contribute to the development of luminescent supramolecular liquid crystals for emitting materials in light emitting devices.     

Induced Mesophases in Binary Systems of Non-Mesogens Where One of the Component is 1,2,4 Trisubstituted Benzene Derivatives

Abstract

Exhibition of mixed mesophases in binary systems where both the components are non-mesogens by themselves offer a very interesting study. In the present work four binary systems were studied where one of the components is non-mesogenic 1,2,4-trisubstituted benzene derivative while the other components are rod shaped non-mesogens. Induction of mesophases is observed in all the binary systems studied. Few compositions on cooling remain in liquid crystalline phase at the ambient temperatures for about 48 hours.     

X-Ray Diffraction by Liquid Crystals

Following a very brief historical review, x-ray diffraction by nematic and smectic liquid crystals is critically examined in terms of the data available and the structural models and molecular theories that have been proposed to explain it. Studiesof TBBA are used to illustrate some of the principal mesophases that have been distinguished. Other types of liquid crystals and their phase transitions are also briefly reviewed.

This analysis shows that careful experimental work is still needed to decide the kinds of models that best describe the various types of mesophases that have been shown to exist.