Molecular relaxations in dual-frequency nematic liquid crystals

The preparation of nematic liquid crystals mixtures results in changing of molecular relaxations in comparison to pure substances. Typical example is the creation of dual-frequency nematic liquid crystals using a base mixture and functional admixtures. In this paper, we present how dielectric properties of starting compounds change at mixture preparation. Three dual-frequency nematic mixtures of different composition were prepared and examined by means of dielectric spectroscopy in a wide frequency (100 Hz to 10 MHz) and temperature range (170°C to ?60°C). Parameters of detected modes for pure compounds and final mixtures were calculated and their relationships with crossover frequency are discussed.     

Ethylenedioxythiophene as a novel central unit for bent-core liquid crystals

We have discovered that ethylenedioxythiophene (EDOT) can be used as a central unit for the synthesis of bent-core liquid crystals (BC LCs). Two series of EDOT-based BC LCs are prepared via Sonogashira coupling reaction. The mesophase behaviour of all the compounds was characterised using a combination of polarising optical microscopy, differential scanning calorimetry and X-ray diffraction measurements. EDOT-based three-ring compounds were found to be nonliquid crystalline, while all the four derivatives of five-ring series, including a branched alkoxy chain derivative, display enantiotropic nematic phase over wide temperature range. The bent angle of these compounds is about 153°, which falls in between typical rod-like and banana liquid crystals. The transition temperature of branched alkoxy chain compound is lower than straight alkoxy chain compounds. The detailed XRD investigations of all the mesogens corroborate the presence of nematic phase.     

Thermotropic liquid crystals based on ferrocenylbiphenyl and ferrocenylterphenyl

4′‐Ferrocenyl‐1,1′‐biphenyl‐4‐yl 4‐alkoxybenzoates Fc–(C₆H₄)₂–OC(O)–C₆H₄–O–C _n H_2n+1 (n = 8, 10, 12) (3a–c), representing a new class of ferrocene‐containing thermotropic mesogens with nematogenic properties, were prepared. Two approaches were used for the construction of these mesogens: (i) reaction of 4′‐ferrocenyl‐1,1′‐biphenyl‐4‐ol with 4‐alkoxybenzoylchlorides, and (ii) crosscoupling of tris(4‐ferrocenylphenyl)boroxine with the corresponding halobenzenes. Crosscoupling was also applied for the synthesis of terphenyl‐containing mesogens Fc–(C₆H₄)₃–OC(O)–C₆H₄–O–C _n H_2n+1 (n = 10, 12) (6a,b) and (RC₅H₄)Fe‐[C₅H₄–(C₆H₄)₃–OC(O)–C₆H₄–O–C₁₀H₂₁] (11a, R = Et; 11b, R = n?Bu). The latter compounds also form nematic phases. Mesogens 6a,b form mesophases with wider temperature ranges than their biphenyl‐containing counterparts 3b,c. The most pronounced mesomorphism was displayed by compounds 11a and 11b, which have mesophases in the ranges 141–253°C and 120–238°C, respectively. The purity of compounds was established by ¹H NMR spectra and elemental analysis. Mesophases were identified by polarizing optical microscopy and differential scanning calorimetry.     

Thermal tuning band gap in cholesteric liquid crystals

The phase of a liquid crystal (LC) changing from a nematic phase to a cholesteric (Ch) mesophase is achieved by adding different ratios of chiral dopants S811. By studying the transmission spectrum, we are able to measure the helical pitch in cholesteric phase. The pitch in the mixtures of nematic E7 and chiral dopants S811 as a function of the concentration of the dopant and temperature is investigated. The sensitivity of the selective reflection notch of the cholesteric phase to the thermal tuning depends strongly on the ratios of the chiral dopants. It reveals that the influence of temperature is more profound for those cholesteric liquid crystals (CLCs) which exhibit smectic A (SmA) at lower temperatures. When fitted using Keating's formula, the helical pitch calculated from our experimental results lies on the predicted curve. Optimised ratios of the mixture CLCs for the optimised reflection band with the specified wavelength ranging from 467 nm to 2123 nm are suggested.     

Nitro substitution in achiral calamitic liquid crystals

This review examines in some detail the effect of terminal and lateral nitro substitutions in achiral calamitic liquid crystals on their physico-chemical properties. The results of this study are compared with those obtained for other groups, and are rationalized in terms of existing theories.     

Electroconvection characterization of guest-host nematic liquid crystals for dynamic scattering mode applications

ABSTRACT

The threshold voltage for electro-hydrodynamic convection (EHC) in nematic liquid crystals in the low-frequency convective regime is examined. The addition of dopant species is known to strongly effect the frequency response of this regime. In this work, we investigate the effect of adding optically anisotropic species on the low-frequency threshold, and, demonstrate that certain dichroic dyes have a marked effect. We have measured electrical transport properties of suitably doped liquid crystal mixtures in order to make a comparison with theoretical predictions based upon linear stability considerations. Our results show satisfactory agreement with these predictions. Our findings indicate future directions for new applications of dynamic scattering mode technologies with more accessible operating parameters previously known.     

Influence of the anisometry of magnetic particles on the isotropic–nematic phase transition

The influence of the shape anisotropy of magnetic particles on the isotropic–nematic phase transition was studied in ferronematics based on the nematic liquid crystal (LC) 4-(trans-4-n-hexylcyclohexyl)-isothiocyanato-benzene (6CHBT). The LC was doped with spherical or rod-like magnetic particles of different size and volume concentrations. The phase transition from isotropic to nematic phase was observed by polarising microscope as well as by capacitance measurements. The influence of the concentration and the shape anisotropy of the magnetic particles on the isotropic–nematic phase transition in LC are demonstrated here. The results are in a good agreement with recent theoretical predictions.     

Nanocomposites of ferroelectric liquid crystals and FeCo nanoparticles: towards a magnetic response via the application of a small electric field

ABSTRACT

We study a nanocomposite consisting of a ferroelectric liquid crystal and a magnetic nanoparticle in order to explore the possibility of using it as a magnetic resonant imaging contrast agent which will measure a field of 20 V/m. To achieve this we use the ferroic properties exhibited by the nanocomposite. We used the ferroelectric liquid crystal 2-(4-((2-fluorooctyl)oxy)phenyl)-5-(octyloxy)pyrimidine mixed with FeCo nanoparticles nominally 2–3 nm in diameter in concentrations of 0.56, 4.3 and 10.8 wt%. The 10.8 wt% sample was chosen for our study because the nanoparticles acted as a lubricant for the ferroelectric liquid crystal. This concentration yields nanoparticle clusters in about 5 ? 10 μm diameter spherulites. An electric field as low as 5V/cm is enough to turn and realign the spherulites where the particles are contained. We estimate the value of the magnetic in a spehrulite and associate it to the number of spherulites aligned as a function of electric field. We find thus that we can achieve low electric fields.     

Synthesis and characterisation of reactive liquid crystals containing an azo group

A new series of liquid crystal (LC) monomers – not only contain a double bond but also contain an azo group – were designed and synthesised. The length of side groups in the LC monomers containing azobenzene ester varied from 1 to 2 methylene units, and the length of the substituted groups in the main chain varied from 1 to 3 methylene units. The molecular structures of the intermediates and target compounds were confirmed by Fourier transform infrared, ultraviolet and visible spectrum and nuclear magnetic resonance (NMR) spectroscopy. The thermal phase behaviour of the LC monomers was investigated by polar optical microscopy coupled with hot stage and differential scanning calorimetry (DSC). In this paper, the effect on the LC zone with the substituents were investigated, and with the increased methylene of main chain, the melting point and the phase transition temperature of the substance will be lowered and LC regions will be narrower.     

Cholesteric star-shaped liquid crystals induced by a maltose core: synthesis and characteristics

Three esters of maltose (A1–A3) with 4–(4–(alkoxybenzoyloxy)phenoxy) –6–oxohexanoic acid (b1–b3) side-arms have been synthesised. All the maltose derivatives were laevo-rotationary, unlike their parent cores. The side arm b1 did not display liquid crystalline (LC) properties, and b2 and b3 displayed thermotropic nematic LC properties. The star-shaped compound (SSC) A1 with b1 side-arms did not display a mesomorphic phase. Unlike the nematic schlieren texture provided by side-arms b2 and b3, the star-shaped liquid crystals (SSLC) A2 and A3 displayed Grandjean and oily texture in the cholesteric phase. The results suggest that the LC properties of the side-arms have an important influence on the formation of LC properties in an SSC, and that the maltose core is important in determining the mesomorphic phase type. In other words, the SSC displayed LC properties only when the side-arms were also LC, and the maltose core induced a cholesteric phase in the SSLC with nematic side-arms. The mesomorphic regions for A2 and A3 were 39.1 and 53.7°C during the heating cycle and 63.8 and 107.0°C during the cooling cycle, respectively. The longer terminal chain rendered the mesomorphic region broader.     

Synthesis of achiral four-ring unsymmetrically substituted toluene derived liquid crystals with a polar end group

Achiral four-ring unsymmetrically substituted toluene-derived liquid crystals have been designed and synthesised by known and straightforward methods. All these compounds exhibit nematic phase over wide temperatures.     

The role of crosslinking and polarity in the dark relaxation of azobenzene-based,polymer-stabilised cholesteric liquid crystals

Prior examinations have reported that polymer stabilisation of azobenzene-based cholesteric liquid crystal (CLC) mixtures can reduce the time necessary for complete colour restoration in the dark from three days to as few as five minutes. This work extends upon these prior examinations by exploring and elucidating the role of crosslinker concentration and monomer polarity on the colour restoration of a representative CLC mixture composed of a high HTP bis(azo) binapthanyl chiral dopant (QL76) mixed into the cyanobiphenyl nematic liquid crystal host MDA-00-1444. The impact of these variables was unexpectedly convoluted. In all the formulations examined here, polymer stabilisation dramatically reduces the time for complete colour restoration of the starting reflection notch. In mixtures based on nonpolar liquid crystal monomers, increasing the crosslinker concentration reduces the time necessary for complete colour restoration. However, the dependence on crosslinker concentration reverses in mixtures composed from polar liquid crystal monomers in which increasing the crosslinker concentration serves to increase the time necessary for complete colour restoration.     

Self-assembled discotic nematic liquid crystals formed by simple hydrogen bonding between phenol and pyridine moieties

New discotic nematic liquid crystals have been prepared through intermolecular hydrogen bonding between the core of 1,3,5-trihydroxybenzene (phloroglucinol, PG) or 1,3,5-tris(4-hydroxyphenyl)benzene (THPB) and the peripheral molecules of stilbazole derivatives. The various nematic phases formed by new hydrogen bonding building blocks were investigated by differential scanning calorimetry, polarising optical microscopy and X-ray diffraction. The first discotic complexes of PG and trans-4-alkoxy-4′-stilbazoles exhibited nematic columnar (N_C) and hexagonal columnar phases depending on the length of alkyl chains, which were considered as the basic discotic structure. Several structural variations on the building blocks were attempted to examine their effects on the liquid crystalline properties of discotic complexes. The nematic lateral phase (N_L) with enhanced intercolumnar order was observed for the complexes of PG and trans-4-cyanoalkoxy-4′ stilbazoles due probably to the strong dipole interactions between cyano groups at the end of alkoxy chains. By introducing the nonlinear structure in three arms of supramolecular discotic mesogen, a discotic nematic phase (N_D) was observed for the complex of THPB and trans-4-octyloxy-4 -stilbazole. The single hydrogen bonding between phenol and pyridine moieties in this study provides a simple and effective method for preparing the rarely found discotic nematic liquid crystals.     

Synthesis and properties of reactive liquid crystal monomers and side-chain liquid crystalline polymers

Two nematic liquid crystal (LC) monomers containing double bonds in the side chain were designed and synthesised. Length of the side groups varied from 1 to 2 methylene units. The side-chain polymers were synthesised by hydrosilylation reaction. The molecular structures of the intermediates and the LC monomers were characterised by Fourier transform infrared, elemental analysis and nuclear magnetic resonance spectroscopy. The thermal phase behaviour of the monomers and polymers were investigated by differential scanning calorimetry and polar optical microscopy coupled with hot stage. The LC monomers showed only one nematic mesophase in the cooling process. And, the two polymers exhibit an enantiotroppic nematic mesophase either in the heating or in the cooling process.     

Synthesis and mesomorphic properties of four-ring fluorinated liquid crystals with trifluoromethyl group

The discovery of nematic phase in 3-fluoro-4-trifluoromethyl-4?-(4?-alkyl [trans, trans-1,1?-bicyclohexyl]-4-yl) 1,1?-biphenyls (n = 2, 3, 4, 5) is an accidental affair. A convenient method was used for the synthesis of these target compounds. After the preparation of the intermediate with trifluomethyl group, the target compounds were synthesised by Suzuki cross-coupling reaction. Then, these phase transition temperatures were measured by polarising textural observation, which confirmed the existence of single nematic phase. These temperatures were also measured by differential scanning calorimetry, which showed that the clearing point of compound (n = 3) is 267℃. The possible mechanism of occurrence of unique nematic phase was discussed on the basis of the principles of fluorine chemistry.     

Computer simulation of liquid crystals

Summary We review recent progress in the computer simulation of liquid crystals, with special emphasis on hard particle models. Surprisingly, the simplest molecular models, taking account only of molecular size and shape, are sufficient to generate a wide variety of liquid crystalline phases, closely analogous to those observed in real life. Thermodynamic stability of different phases is very sensitive to shape, and presumably will also be sensitive to further details of intermolecular interactions as they are incorporated into the model. Realistic atom-atom potential models of liquid crystals are available, but the associated simulations are quite expensive. Thus, while idealized models may be used to study quite general, fundamental properties of mesophases, the modelling of specific liquid crystal systems in a realistic way remains a great challenge. Progress continues to be made on both these fronts.     

Nematic liquid crystals in frequency and amplitude modulated electric fields

In this paper we study peculiarities of behaviour of a nematic liquid crystal (NLC) in an electric field consisting of two harmonics with different amplitudes and frequencies. The most interesting result is the experimental observation of high frequency, stabilization of a low frequency electrohydrodynamic instability in the NLC. A method of measurement of the one frequency instability threshold has been proposed. The beating regime of the two frequency electric field was also studied. The theory developed provides an explanation for all the experimental results.     

Effect of an electric field applied during the injection procedure on the pretilt angle of a nematic liquid crystal

We examined the effect of an electric field applied during the injection procedure on the polar pretilt angle of a nematic liquid crystal (LC). The pretilt angle of the sample injected at 25°C gradually increased as the electric field was increased. On the other hand, the pretilt angle of a sample injected at 90°C (which is above the nematic-isotropic phase transition temperature of LC) showed a sudden increase in the presence of the electric field and also increased with a greater electric field. We think the alignment layer might be swollen with LC molecules, and the rotation of the immersed LC molecules by the electric field induces a deformation of the alignment layer. These results imply LC and the alignment layer were coupled, and their cooperation had an influence on determining the bulk pretilt angle.