Homologous Series Of Liquid Crystals Research Articles

Dramatic effect of an additional CH2 group on the temperature variation of the Sm-C(*)(alpha) short helical pitch.

High-resolution differential optical reflectivity was used to study the temperature evolution of the short helical pitch in the Sm-C(*)(alpha) phase of successive members from two liquid-crystal homologous series. With the addition of one CH2 group, the magnitude and temperature evolution of the pitch change dramatically, and the molecular arrangements between consecutive surface layers found in free-standing films change from being anticlinic to synclinic.

Mesomorphic Properties of a Homologous Series of Liquid Crystals with Short Fluorocarbon Chain

A homologous series of short fluorocarbon chain-containing liquid crystals has been synthesized. Several of them show the phase sequence N→SmA→SmC in the cooling process from the isotropic liquid. The mesomorphic properties have been studied by polarizing microscopy and DSC measurements. The thermostability of SmC phases was enhanced along with the lengthening of fluorocarbon chains.

Electro-Optic Kerr Effect Investigations in a New Homologous Series of Liquid Crystal

Measurements are reported for the first time for the D.C. Kerr response and the pretransitional behaviour of a new homologous series of liquid crystal 4′-(trans-4′-alkylcyclohexyl)-benzonitrile(for n=5 to 9) at and above the nematic-isotropic phase transition temperatures. The real part of the third order nonlinear susceptibilities(χ(3)) have been determined for this homologous series of liquid crystals from the Electro-optic Kerr effect (EOKE) experiments at and above the nematic-isotropic phase transition temperatures. The variation of the Kerr constant with temperature has been discussed on the basis of the Landau-de Gennes model of nematic-isotropic phase transition. The Kerr relaxation time and other statistical parameters concerned with a nematic-isotropic phase transition like the latent heat of transition have also been determined using the Landau-de Gennes' theory of phase transition. The variation of the Kerr constant, the values of the third order nonlinear susceptibilities, the Kerr relaxation time and the latent heat of transition with the number of carbon atoms in the alkyl chain within the series have been discussed on the basis of an interesting effect called the “ODD-EVEN” effect. The results and discussions are presented.

Liquid Crystal Wetting in Cylindrical Pores

We have performed a calorimetry and deuterium nuclear magnetic resonance study for a member of the liquid crystal homologous series, n.O m, in bulk and confined to the 0.2 μm cylindrical pores of aluminum oxide Anopore membranes. It was shown in an ellipsometry study by Lucht and coworkers that this material shows a prewetting nematic transition at an air interface. Thermodynamically, at the Anopore solid pore walls these liquid crystals also exhibit unique wetting properties: a distinct signature in the specific heat, a “bump” at a temperature approximately 0.5 K above T NI is found. We believe that such signature is related to the manner that the nematic liquid crystal wets the solid surfaces.

Cotton-Mouton constants and pretransitional phenomena in the isotropic phase of liquid crystals

The Cotton-Mouton constants for three homologous series of liquid crystals were measured in the isotropic phase. Some of these measurements were performed with high temperature resolution. The results are compared with theories on pretransitional phenomena. The Landau-de Gennes theory proves to be inadequate to describe the data.

Thermodynamics of Solution of Nonmesomorphic Solutes at Infinite Dilution in the Isotropic and Nematic Phases of p-Cyanophenyl p-n-Alkylbenzoates

Infinite-dilution activity coefficients (γ2∞) and partial molar excess enthalpies and entropies obtained using gas−liquid chromatography are reported for 10 solutes, varying in molecular size, shape and flexibility, in the isotropic and the nematic phases of the ethyl, propyl, butyl, pentyl, hexyl, and heptyl members of the p-cyanophenyl p-n-alkybenzoate homologous series of liquid crystals. General trends in the six solvents are identified. The results, which corroborate earlier results, demonstrate the effects that the solute structure and the length of the flexible alkyl tail on the solvent molecule have on the solution process.

Anchoring Phase Transition and Bulk Phase Sequence of Liquid Crystals Studied by Scanning Tunneling Microscopy

Abstract On the basis of the highly reproducible scanning tunneling microscopy (STM) images, the anchoring structures of a homologous series of liquid crystals, n-alkylcyanobiphenyls (mCBs) are clearly divided into two categories; a single-row type and a double-row type. We have proposed that the single-row is the anchoring structure of a nematic phase, while the double-row is the one of a smectic phase. Furthermore, a new phase transition only available at the boundary has been confirmed for the first time in the binary mixtures. The correlation between those anchoring structures and the bulk phase diagrams is discussed from the viewpoint of anchoring phase formation at a molecular level.

The Structure Property Relationship of a Homologous Series of Mesogenic Compounds

Abstract In this paper efforts have been made to investigate a relation between the molecular structure, packing coefficient and phase transition behaviour for a homologous series of liquid crystals. We have undertaken the studies of the twelve members of the series (nOCB) The molecules belonging to this series are asymmetric. The orientational order parameters in the nematic phase have been calculated for the molecules (n=5 to 9) introducing asymmetry and the variation of order parameter with chain length near the transition temperature is discussed.

Sign inversion of the helical pitch in carbohydrate-based liquid crystals

A four-step synthesis starting from tri-O-acetyl-D-glucal leads to chiral trioxadecalin compounds containing a cyano group. The mesogenic properties of a homologous series of liquid crystals are reported; for two derivatives a helix inversion of the cholesteric phase is observed.

Novel Series of Enaminoketone Liquid Crystals Having Hexatic Smectic B Phase

Abstract Several homologous series of liquid crystals having an aminoketone mesogenic core (Liq. Cryst., 10, 593 (1991)), have been synthesized to derive some new wide-temperature untilted smectics. Compounds belonging to three of six novel series have the hexatic smectic B phase between smectic A and enantiotropic crystal smectic B phases. Eleven of them exhibit first order phase transition between smectic B phases, whereas for other compounds this phase transition is not detectable neither by DSC nor texture studies. Molecular modifications leading to further untitled hexatic compounds are suggested.

Correlation Between Bulk Orderings and Anchoring Structures of Liquid Crystals Studied by Scanning Tunneling Microscopy

The anchoring structures of a homologous series of liquid crystals, n-alkylcyanobiphenyls (mCBs: m=7-12), on molybdenum disulfide (MoS2) are directly observed by scanning tunneling microscopy (STM) in order to analyze the alignment mechanism of liquid crystals. Based on highly reproducible STM images, the anchoring structures of mCBs are clearly divided into two categories, a monolayer (single-row) type (7CB, 8CB, 9CB and 11CB) and a bilayer (double-row) type (10CB and 12CB). Here we propose the monolayer is the anchoring structure of a nematic phase, while the bilayer is the anchoring structure of a smectic phase. The correlation between bulk phase orderings and anchoring structures of liquid crystals is discussed from the viewpoint of surface-phase formation for the mCBs and different solid substrates.

Liquid Crystals With a Chiral Core: Cyclohexene Carboxylates

Abstract A series of new liquid crystals containing a cyclohexene ring have been synthesized and characterized. The cyclohexene ring is formed by the Diels-Alder reaction between myrcene and methyl acrylate, and has a chiral center. Hydrolysis of the resulting methyl ester into the corresponding acid and esterification of the latter with 4-hydroxy-4′-n-alkoxybiphenyl yields a homologous series of liquid crystals with the structure: The compounds thus prepared are racemic mixtures. The lower members of this series (n = 1 and 2) have a large nematic range, and the higher members (n = 3–10) have multiple smectic phases in addition to the nematic phase. When (1R,2S,5R)-(-)-menthyl acrylate was used instead of methyl acrylate in the Diels-Alder synthesis, optically active compounds with 40% enantiomeric excess were obtained.

Surface-Induced Ordering in a Homologous Series of Liquid Crystals—Orientational Wetting

Abstract We have studied the orientational wetting behavior of a liquid crystal homologous series. A change over from partial wetting to complete wetting has been observed as the length of the alkyl chain increases.

Etude dilatométrique et mesure des paramètres volumiques de la série des acétyl-4 n-alkanoyloxy-4′ azobenzènes

In this paper, we present a dilatometric study of the even members of the following homologous series of liquid crystals: 4-acetyl-4′-n-alkanoyloxyazobenzene. The nature of all the transitions occurring as a function of temperature is shown to be of first order. At the transition from the crystal to the mesomorphic state (smectic A or nematic), the aliphatic chains of the molecules melt in one step. At the transition between the smectic A phase and the isotropic phase, only the aromatic stems play a role in the melting process.

Mesomorphic properties and molecular structure. I. Structure of the smectic A phase in the 4-acetyl-4′-n-alkanoyloxy-azo-benzene series

This paper reports an investigation by dilatometry and x-ray diffraction of the structure at the molecular level of the smectic A phases occuring in homologous series of liquid crystals, namely the 4-acetyl-4′-n alkanoyloxyazobenzene series. First, several structural parameters referring to the aromatic part and to the paraffinic chain were obtained; then, on the basis of these parameters, a model was proposed concerning the structural organization of the molecules inside the smectic A layers. According to this model, a bilayer organization of the molecules was found to occur with a strong segregation between paraffinic and polar parts of the molecules, along with a pronounced spreading of the aliphatic chains at the interface.

High-pressure measurements in a homologous series of liquid crystals

Pressure-volume-temperature measurements throughout the nematic phase for the first six members of the homologous series of di-alkoxyazoxybenzenes are reported. These data represent the first investigation of thermodynamic parameters in an homologous series, thus demonstrating the role of the end chains in a specific molecular geometry. Measurements at constant volume permit the conversion of previous orientational order studies at atmospheric pressure to constant-volume conditions. Measurements at constant temperature at the nematic-isotropic phase transition reveal the behavior of the specific volume, volume discontinuity, and volume dependence of the mean-field potential in this series. Finally, the implication of these data on several liquid-crystal theories is explored.

Thermodynamic data for nonmesomorphic solutes at infinite dilution in the nematic and isotropic phases of hexylcyanobiphenyl

Infinite dilution solute activity coefficients γ o 2 , partial molar excess enthalpies $$\overline H ^e$$ and entropies $$\overline S ^e$$ , and partial molar enthalpies ( $$(\Delta \overline H ^e )$$ ) and entropies ( $$(\Delta \overline S )$$ ) of solution, obtained using gas-liquid chromatography (GLC), are reported for thirty nonmesomorphic solutes in the nematic and isotropic phases of p-n-hexyl-p′-cyanobiphenyl (6CB). The solutes studied include normal and branched alkanes, alkenes and hexadienes (with some cis and trans isomers), and benzene. The results corroborate earlier studies on other members of the p-n-alkyl-p′-cyanobiphenyl homologous series of liquid crystals. The results demonstrate the effect that solute structure (size, shape, flexibility, polarizability and polarity) has on the solution process. Thermodynamic data for the cis and trans isomers of 2-pentene and 2-hexene are examined. A method for the simultaneous examination of the effects of both solute and solvent structures on the solution process is suggested.

I-N-A Phase Diagrams of Liquid Crystals

The isotropic liquid (I) ⊥ nematic (N) ⊥ smectic A(A) phase diagrams of homologous series of liquid crystals obtained experimentally may be divided into at least 4 types (Fig. 1 a-d) according to the characteristic behavior of TNI.

A Theoretical Investigation of the Lattice of Symmetric and Asymmetric Rigid Rods with Anisotropic Dispersion Forces and Rigid Body Repulsions

Abstract The method of symmmetry breaking potential and first order cluster expansion technique for the partition functions adopted for the theory of ordering in liquid crystals has been extended to symmetric asymmetric molecules. The order parameter is calculated as a function of temperature and packing coefficient as a function of chainlength for homologous series of liquid crystals having symmetric and asymmetric molecules. The theoretical calculations account fairly well for the gradient differences in the order parameters of symmetric molecules and packing coefficients.

Synthesis and Study of New Mesogens II: p-(p′-n-Alkoxycinnamoyloxy)-benzylidene p″ -anisidines (1) and p-(p′-n-Alkoxycinnamoyloxy)- benzylidene anilines (2)

Abstract Two new homologous series of liquid crystals viz. p-(p'-n-alkoxycinnamoyloxy) benzylidene p”-anisidines (1) and p-(p'-n-alkoxycinnamoyloxy) benzylidene anilines (2) comprising twenty four mesogens have been synthesized. Both are high melting series of mesomorphs. In the homologous series (1) the nematic mesophase is exhibited from the very first member; however, the first five compounds decompose at high temperatures before passing into isotropic liquid condition. Polymesomorphism is exhibited with the appearance of the smectic mesophase at the octyl derivative and remains up to the last viz. octadecyl homologue. Within polymesomorphic region the nematic mesophase is homeotropic; however, it shows a clear threaded texture where it is the only mesophase shown i. e. in the first seven members of the homologous series (1). The smectic mesophase shows focal conic fan shape texture belonging to smectic-A variety. The nematic-isotropic curve shows descending tendency as the series is ascended-a charact...