Heat capacities and phase transitions of the antiferroelectric liquid crystals MHPOBC and MHPOCBC

Heat capacities of the antiferroelectric liquid crystals 4-(1-methylheptyloxycarbonyl)phenyl4-octyloxybiphenyl-4-carboxylate (MHPOBC) and 4-(1-methylheptyloxycarbonyl)phenyl4-octylcarboxybiphenyl-4-carboxylate (MHPOCBC), have been measured with an adiabatic calorimeter between 350 and 460 K. MHPOBC showed three smectic subphases (ferrielectric C*, ferroelectric C* and a fancy phase C*) between antiferroelectric smectic C* and paraelectric gamma alpha smectic A, while MHPOCBC exhibited only one subphase (smectic C*). These phases are clearly discriminated by the existence of phase transitions. The enthalpies and entropies gained at the respective phase transitions were very small. A much larger phase transition from smectic A to isotropic liquid was also observed in both compounds. A alpha     

Surface tension of some liquid crystals in the cyanobiphenyl series

The temperature dependence of the surface tension was measured by the pendant drop method for four compounds from the homologous series of alkylcyanobiphenyls (nCB), in the nematic liquid crystal and isotropic phases. For 8CB (octylcyanobiphenyl) the temperature dependence was also measured in the smectic range. Not very close to the isotropic transition temperature, and with the exception of 8CB, the surface tension decreases with increase in temperature in the nematic range. A downward jump at the transition temperature was observed for all liquid crystals studied. The shape of the drop in the smectic A phase of 8CB gives indications of stratification in a system of terraces.     

Synthesis and mesomorphic properties of several series of fluorinated ester liquid crystals

Eight series of fluorosubstituted three-ring ester liquid crystals have been synthesized. Polarizing microscopic textural observations and DSC measurements of their phase transitions show that most are thermotropic liquid crystals with nematic and smectic A phases; furthermore, several show monotropic high order smectic phases. The results showed that the SmA phase is enhanced with the increasing number of fluoro-substituents at the para - and meta -positions of the terminal phenyl groups. The mesomorphic properties of these compounds are also affected by the direction of the ester bonds. The effect of the triple bond is also discussed.     

Odd-even effects in the phase transition behaviour of novel U-shaped liquid crystals

A homologous series of U-shaped dimeric liquid crystals in which two mesogenic groups are connected via catechol has been prepared and the effects of terminal alkyl chains, alkyl spacers and core structures on the transition properties investigated by means of optical microscopy and differential scanning calorimetry. The phase sequence exhibits a pronounced odd-even effect as the length and parity of the spacers is varied, in which the even members favour the nematic and smectic C phases, whereas the odd members favour the nematic and smectic A phases. We discuss the transition behaviour of the U-shaped compounds in terms of molecular shape.     

Surface tension of some liquid crystals in the cyanobiphenyl series

The temperature dependence of the surface tension was measured by the pendant drop method for four compounds from the homologous series of alkylcyanobiphenyls (nCB), in the nematic liquid crystal and isotropic phases. For 8CB (octylcyanobiphenyl) the temperature dependence was also measured in the smectic range. Not very close to the isotropic transition temperature, and with the exception of 8CB, the surface tension decreases with increase in temperature in the nematic range. A downward jump at the transition temperature was observed for all liquid crystals studied. The shape of the drop in the smectic A phase of 8CB gives indications of stratification in a system of terraces.     

Induced smectic phases in phase diagrams of binary nematic liquid crystal mixtures

To elucidate induced smectic A and smectic B phases in binary nematic liquid crystal mixtures, a generalized thermodynamic model has been developed in the framework of a combined Flory-Huggins free energy for isotropic mixing, Maier-Saupe free energy for orientational ordering, McMillan free energy for smectic ordering, Chandrasekhar-Clark free energy for hexagonal ordering, and phase field free energy for crystal solidification. Although nematic constituents have no smectic phase, the complexation between these constituent liquid crystal molecules in their mixture resulted in a more stable ordered phase such as smectic A or B phases. Various phase transitions of crystal-smectic, smectic-nematic, and nematic-isotropic phases have been determined by minimizing the above combined free energies with respect to each order parameter of these mesophases. By changing the strengths of anisotropic interaction and hexagonal interaction parameters, the present model captures the induced smectic A or smectic B phases of the binary nematic mixtures. Of particular importance is the fact that the calculated phase diagrams show remarkable agreement with the experimental phase diagrams of binary nematic liquid crystal mixtures involving induced smectic A or induced smectic B phase.     

Supramolecular liquid‐crystalline polyurethane

A novel supramolecular liquid‐crystalline polyurethane was prepared by mixing the polyurethane containing a pendant pyridyl unit as H‐bond acceptor and 4‐dodecyloxybenzoic acid as H‐bond donor. Intermolecular hydrogen bond formation was confirmed by Fourier transform infrared spectroscopy. The liquid‐crystalline behaviour of the complex formed was established by differential scanning calorimetry, polarizing optical microscopy and X‐ray diffractometry. The complex exhibited highly ordered smectic and nematic phases. The smectic‐nematic and nematic‐isotropic transitions were observed at 89°C and 120°C, respectively.     

Anticlinic smectic C phase in new and novel five-ring hockey stick-shaped compounds

The synthesis of a new homologous series of novel five-ring compounds composed of hockey stick-shaped molecules derived from 4-hydroxybiphenyl-3-carboxylic acid which exhibit an interesting sequence of phase transitions is presented. Uniaxial nematic and smectic A phases, anticlinic smectic C phase and an unidentified smectic phase at a lower temperature than the latter have been observed. This is perhaps the first example of hockey stick-shaped compounds exhibiting anticlinic smectic C phase directly from the isotropic phase. Polarised light optical microscopy, differential scanning calorimetry, X-ray diffraction measurements and electro-optical switching studies were carried out to identify the mesophases.     

Mesomorphic phase transitions of 4,4'-bis(ω-hydroxyalkoxy)-azoxybenzenes. Appearance of isotropic mesophase

Mesomorphic phase transitions of 4,4'-bis(ω-hydroxyalkoxy)-azoxybenzenes (number of carbons in the alkoxy group n = 2,3,6,8,11 and 12) have been studied by differential scanning calorimetry (DSC) and polarized optical microscopy. For the n = 8 compound, an optically isotropic (01) mesophase reminiscent of the smectic D phase was observed in the temperature range of 4 0 M 1 1 K between smectic C (below 400 K) and nematic (above 41 1 K) phases. On the other hand, the 0 1 mesophase was not observed in the other homologues used here; the n = 2,3 and 6 compounds had only a nematic phase and the n= 11 and 12 compounds had only a smectic C phase. This preliminary work points out that the thermal and optical properties of the 01 phase for the n = 8 compound are very similar to those of the smectic D phase.     

Synthesis and mesomorphic behaviour of wedge-shaped nematic liquid crystals with flexoelectric properties

The synthesis and mesomorphic behaviour of a series of wedge-shaped liquid crystals and some reference compounds are reported. These unusual liquid crystals possess smectic C, smectic A and nematic phases. These new wedge-shaped materials with a high degree of shape anisotropy and a large dipole moment can be used to induce an increase in the flexoelectric effects of nematic guest-host mixtures as dopants at low concentrations.     

Molecular symmetry effects on the stability of highly ordered smectic phases

In an effort to control the phase ranges of highly ordered smectic phases, we examined the impact of molecular symmetry on phase behaviour of a series of 12 symmetrical and unsymmetrical 4,4′-dialkanoyloxybiphenyl derivatives. Combined differential scanning calorimetry, polarised optical microscopy, and X-ray diffraction studies indicated that the compounds studied formed smectic F liquid crystals, and in some cases, G phases at lower temperatures. Although the clearing temperatures were largely unaffected by molecular symmetry, the transitions from the SmF liquid crystals to more ordered phases were consistently lowered upon reducing the molecular symmetry. As a result, unsymmetrical molecules had broader mesophases than their higher symmetry isomers, suggesting a strategy for tuning the phase behaviour of these highly ordered lamellar phases, which have been widely targeted for organic semiconductors.     

High pressure study on induced SA phases in binary liquid crystal mixtures

Binary mixtures of terminal polar and non-polar liquid crystals exhibiting induced smectic phases are studied under high pressure. For terminal polar compounds with smectic phases, there are two types of T, x phase diagrams known up to now. Diagrams with a nematic gap between the induced phase and the smectic phase of the terminal polar compound and diagrams with an uninterrupted miscibility of the smectic phases. We find a continuous transformation between these phase diagrams with pressure. At a certain pressure, the phase transition lines form a cross separating two nematic and two smectic phases.     

Mesomorphic phase transitions of 4,4′-bis(ω-hydroxyalkoxy)-azoxybenzenes. Appearance of isotropic mesophase

Abstract

Mesomorphic phase transitions of 4,4′-bis(ω-hydroxyalkoxy)-azoxybenzenes (number of carbons in the alkoxy group n = 2,3,6,8,11 and 12) have been studied by differential scanning calorimetry (DSC) and polarized optical microscopy. For the n = 8 compound, an optically isotropic (01) mesophase reminiscent of the smectic D phase was observed in the temperature range of 4 0 M 1 1 K between smectic C (below 400 K) and nematic (above 41 1 K) phases. On the other hand, the 0 1 mesophase was not observed in the other homologues used here; the n = 2,3 and 6 compounds had only a nematic phase and the n= 11 and 12 compounds had only a smectic C phase. This preliminary work points out that the thermal and optical properties of the 01 phase for the n = 8 compound are very similar to those of the smectic D phase.     

Synthesis and mesomorphic properties of some fluorinated phenyl 4-[(4-n-alkoxyphenyl)ethynyl]benzoates

Eight homologous series of fluorinated phenyl 4-[(4-n-alkoxyphenyl)ethynyl]benzoates have been synthesized. Textural observations by polarizing microscopy and DSC measurements of the phase transitions show that most of these compounds are thermotropic liquid crystals with nematic and smectic A phases; furthermore, several show monotropic high order smectic phases. The results showed that the SmA phase is enhanced with increasing degree of fluorosubstitution on the para- and meta-positions of the terminal phenyl groups. The mesomorphic properties of these compounds are also affected by the direction of ester bonds. The effect of triple bonds is also discussed.     

Morphology of polymer networks polymerized in highly ordered liquid crystalline phases

The morphology and optical properties of polymer stabilized liquid crystals formed in a more highly ordered low molecular weight liquid crystal solvent were studied. Tetrafunctional, mesogenic monomers (with and without flexible spacers) were polymerized in isotropic, nematic and smectic phases of the LC solvent (4′-octyl-4-cyanobiphenyl) and studied with scanning electron microscopy and cross-polarized light microscopy. The network morphology of the nematic and isotropic phase polymerizations showed strong similarities with the corresponding polymerizations in other solvents. Polymerization in the smectic phase, however, resulted in marked increases in network order and directionality. Most dramatically, even the polymer without flexible spacer formed a fibrous network of rodlike units, in contrast to the random, beaded texture formed by the same polymer in nematic or isotropic conditions. Correspondingly, a large increase in birefringence demonstrated significant polymer orientation and more effective orientational interaction with the liquid crystalline solvent.     

Preorganised effects of a tetramesogenic supermolecule on supramolecular assembly in the liquid crystalline phases

After preparing a homologous series of tetrameric mesogenic compounds in which two U-shaped molecules were connected via a rigid benzene derivative or a flexible alkyl chain, we investigated their phase transition behaviour using optical microscopy, differential scanning calorimetry and X-ray diffraction analysis. The compounds possessing an alkyl spacer as the central group exhibited nematic and smectic A phases just as the corresponding U-shaped molecule did. The compound possessing a 1,2-benzene unit as the connecting group showed nematic and smectic A phases, although the compound possessing a 1,3-benzene unit exhibited only an anticlinic smectic C phase. Structure–property relations of the liquid crystalline tetramers are interpreted in terms of preorganised effects of the four mesogenic units.     

Studies of first- and second-order phase transitions in LC methacrylic monomers based on the ω-hexyloxysalicylaldimine group

The synthesis and phase characterization of two liquid crystalline methacrylic monomers based on the ω-hexyloxysalicylaldimine group with octyl and decyloxy chain substitutions is presented. The liquid crystalline materials were characterized by polarizing optical microscopy, differential thermal analysis and X-ray diffractometry. Nematic and tilted smectic C phases were observed in the compounds. Their liquid crystalline properties where compared with previously studied samples of homologous compounds. In contrast to previous results, this structural modification induces pronounced enantiotropic first-order phase transitions between nematic and smectic C mesophases. A correlation between the phase transition behaviour and structural features of the sample is included.     

Synthesis and mesomorphic behaviour of wedge‐shaped nematic liquid crystals with flexoelectric properties

The synthesis and mesomorphic behaviour of a series of wedge‐shaped liquid crystals and some reference compounds are reported. These unusual liquid crystals possess smectic C, smectic A and nematic phases. These new wedge‐shaped materials with a high degree of shape anisotropy and a large dipole moment can be used to induce an increase in the flexoelectric effects of nematic guest–host mixtures as dopants at low concentrations.     

Phase transitions,optical, dielectric and viscoelastic properties of colloidal suspensions of BaTiO3 nanoparticles and cyanobiphenyl liquid crystals

We report experimental studies on the phase transitions and physical properties of colloidal suspensions of BaTiO₃ nanoparticles and two cyanobiphenyl liquid crystals (4-pentyl-4?-cyanobiphenyl and 4-octyl-4?-cyanobiphenyl). From the differential scanning calorimetric measurements, we show that the nanoparticles have antagonistic effect on the isotropic to nematic and nematic to smectic-A phase transitions. The birefringence, dielectric anisotropy and splay elastic constant remain almost unchanged, whereas the bend elastic constant and rotational viscosity decrease considerably. The experimental results are discussed based on the possible contribution of BaTiO₃ nanoparticles and free surfactant molecules in the suspensions.     

Adiabatic scanning calorimetry of the nematic-smectic A1 and the smectic A1–smectic A transitions in frustrated smectic mixtures at low temperatures

Abstract

Phase diagram results and high resolution adiabatic scanning calorimetric data are presented for several binary mixtures of 4-cyanocyclohexyl-4'-n-butyl-cyclohexane and 1 amino-9-fluorenone showing smectic A₁ and smectic à phases at low temperatures. Contrary to present theoretical expectations and experimental evidence for other systems, we find strongly first order nematic to smectic A₁ transitions with a latent heat of about 2 kJ/mol for all the mixtures investigated. The enthalpy change related to the smectic A₁ to smectic à transitions is two orders of magnitude smaller and corresponds most likely to a fluctuation induced first order transition.