Conoscopic image of a biaxial nematic phase in a sodium decyl sulfate – decanol – D2O mixture

ABSTRACT

The nematic phases of a lyotropic system NadS/ decanol/ heavy water are investigated using optical conoscopy and image processing. The phase diagram obtained from these lyotropic materials predicts the occurrence of a direct phase transition, which does not present the biaxial nematic phase, between the discotic (N_D) and calamitic (N_C) nematic phases. A biaxial nematic (N_B) phase is optically characterized and confirmed through conoscopic image, inside the biaxial range, between the two uniaxial nematic phases. Also, their respective transition points are determined by means of image processing. The N_B phase observed here is discussed as part of the nature of the micellar configuration of lyotropic materials which exhibit uniaxial nematic phases.     

Effects of Nematic Compounds with High Clearing Points on the Multiplexing Performance of Twisted Nematic Displays

The addition of nematic compounds with high clearing points (N_h) to multiplexable nematic liquid crystals decreases the temperature dependence of the threshold voltage of twisted nematic displays. This effect depends on the dielectric anisotropy of the nematic liquid crystals containing N_h, as well as on the increase in T _c. In the case of 4-n-butylphenyl 4'-(4”-butyibenzoyloxy) benzoate, the low threshold voltage was exceptionally compatible with a small temperature dependence of threshold voltage, and this result was attributed to the large temperature dependence of the dielectric anisotropy in comparison with that of nematic liquid crystals containing other N_h's.     

Nematic Thiophenes for STN-LCDS and Flexoelectric LCDS

Abstract

We report anew class of liquid crystals incorporating a 2,5-disubstituted thiophene ring and one or two conjugated trans-carbon-carbon double bonds in the terminal chain for use as components of nematic mixtures for TN-LCDs and STN-LCDs. The combination of these molecular elements contribute towards generating a high birefringence, a high nematic clearing point as well as a low viscosity for these materials. Several of these compounds exhibit a melting point below room temperature. This is the first time a room temperature nematic phase has been reported for phenyl thiophene derivatives. The new thiophene compounds can be used to induce a high birefringence in nematic mixtures for LCDs with short response times.     

Mesogenic 3,6-bis(4-hydroxyphenyl)-1,2,4,5-tetrazine alkanoate esters

A novel series of 3,6-bis(4-hdroxyphenyl)-1,2,4,5-tetrazine alkanoate esters were synthesized and their mesogenic properties were studied using differential scanning calorimetry (DSC) and polarizing optical microscopy (POM). The impact of changing the tail-core linkage from alkyl or alkoxy to ester is profound. Compared to the alkyl or alkoxy linkages, the ester linkage reduced mesogenic properties. Short-tailed compounds are non mesogenic (4a-4e), while long-tailed compounds (4f-4r) exhibit nematic phases. Unlike the alkyl or alkoxy tail series, none of the 18 presented esters in this series exhibits a smectic phase.     

The Effect of Trifluoromethyl and Difluoromethoxy Groups on Mesomorphic Properties

Abstract

This paper describes the effect of terminal substituent on the mesomorphic properties in 3-and 4-(4-alkoxyphenoxycarbonyl)phenyl 3- and 4-R-benzoate and 4-(4-alkoxybenzoyloxy)phenyl 2-, 3-, and 4-R-benzoates (R = CF₃, OCF₃, OCF₂H). The meta-trifluoromethoxy and para-difluoromethoxy compounds tend to show a smectic C phase, as well as smectic A and nematic ones, and introduction of a chiral carbon in the alkoxyl group induces the chiral smectic C one.     

Liquid Crystalline Phases in 4-Acetyl-4′-n-Alkanoyloxy-Azobenzenes

Abstract

The preparation and melting behaviour of the homologous series of 4-acetyl-4′-n-alkanoylox-azobenzenes, CH₃-CO-C₆H₄-N?N-C₆H₄-O-CO-(CH₂) _n -CH₃, (with n ranging from zero to 16) is reported. The melting behaviour has been investigated by means of thermal microscopy and differential scanning calorimetry. The experimental results indicate for almost all the compounds the presence of solid state polymorphism and a type A smectic; for one compound a type B smectic phase is also present. Some homologs present, in addition, a nematic phase.     

Optical Studies on Williams Domains

The paper discusses the interference studies carried out on Williams domains in the case of N-(p-propoxybenzylidene) p-pentylaniline. Measurements of the periodic variation of the orientation of the director in the nematic medium are reported.     

Mesophase design: I. Molecular structure and crystal packing motifs of 4-alkylcyanobiphenyls

A comparative study of the crystal and molecular structures of 4-alkylcyanobiphenyls (ACB-n) C _n H_{2n + 1}-C₆H₄-C₆H₄-CN (n = 2, 4–12) belonging to liquid-crystal compounds revealed no direct relationship between the molecular geometry of these compounds and their liquid-crystalline properties. Mesogenic properties are determined by the alternation of the loosely packed aliphatic and closely packed aromatic regions in the crystals. Graphs describing the crystals and mesophases were designed for ACB-n. The graph for nematic mesogens of ACB-n (n = 5−7) has one structure-forming element. The graph for smectic-nematic (n = 8 and 9) and smectic (n = 10−12) mesogens have more than one structure-forming element. If different types of secondary bonds in the smectic mesophase have equal energies, the disruption of these interactions caused by the temperature rise occurs simultaneously and the system undergoes a transition from the smectic phase to the isotropic liquid. If the energies of different types of secondary bonds are different, the destruction of the mesophase occurs in steps and the phase transitions are more complicated (smectic-nematic-isotropic transitions).     

Synthesis,Characterization and Mesomorphic Properties of New Rod-like Thiophene Based Liquid Crystals

Two new mesogenic homologous series of Schiff base esters, 2-[4-(4′-n-Alkoxy benzoyloxy) benzylidenamino] 3-cyno thiophine (Series-A) and Schiff base cinnamates, 2-[4-(4′-n-alkoxy cinnamoyloxy) benzylidenamino] 3-cyano thiophene (Series-B), comprising a thiophene moiety were synthesized. Structural elucidation was carried out using elemental analysis and spectroscopic techniques such as FT-IR, ¹H-NMR and ¹³C-NMR, and mass spectrometry. The mesomorphic properties and thermal stabilities of the title compounds were studied by using differential scanning calorimetry and optical polarizing microscopy. All the derivatives are mesomorphic in nature showing the nematic phase, and the higher members of Series-A show a smectic C phase whereas Series-B exhibits only the nematic mesophase. The mesomorphic properties of the present series are compared with other structurally related compounds.     

Market Trends for Flat Panel Displays

Abstract

Arecent report from Stanford Resources entitled ‘Flat Panel Monitor Market Trends 1988’ presents a realistic forecast for the FPD market, which is expected to reach a shipment value of $4 billion in the year 2000. The Flat Panel Display market covers active matrix twisted nematic displays (TFT-LCD) and matrix-addressed super-twisted nematic displays (STN-LCD), and the following are highlights from the market analysis and forecasts.     

Low-Frequency Dielectric Relaxations in Nematics and Dual-Frequency Addressing of Field Effects

Abstract

Novel low threshold nematic mixtures are presented exhibiting very low dielectric crossover frequencies f_c ? 1 kHz at 20°C and unusually large, low- as well as high-frequency dielectric anisotropies ΔEL > 4 and ΔEH < 4 respectively. ΔEL and ΔEH are shown to be independently adjustable. The frequency and temperature dependence of the dielectric constants as well as the optical, elastic and viscous material constants are measured. Approximations are derived which quantitatively describe the influence of the LC material properties on the static electro-optical performance of dual-frequency addressed twisted nematic displays. It is shown that the number of multiplexable lines of dual-frequency addressed TN-LCDs comprising the new materials can be increased by more than a factor of 30 compared with conventional addressing. Moreover, very short turn-off times are reported.     

An optical microscopy study of the biaxial ‐ calamitic nematic lyotropic phase transition

Lyotropic nematic textures are investigated, using optical microscopy, near the reentrant isotropic (I_RE) ‐ discotic nematic (N_D^‐) ‐ biaxial nematic (N_B) ‐ calamitic nematic (N_C⁺) ‐ isotropic (I) phase transitions in a lyotropic mixture of potassium laurate, decanol and D₂O. The N_B, intermediate phase between the two uniaxial nematic ones, is characterized by optical birefringence measurements. In this way, using a polarizing microscope and a colour CCD digital camera, the N_B – N_C⁺ phase transition is identified as well as the domain of the N_B phase and N_D^‐ – N_B transition point in accordance with optical birefringence data (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Liquid Crystals: A Bibliography for 1975

Abstract

The static dielectric constants of biphenyl-4-p-n-alkoxybenzoates have been measured as functions of temperature. The dielectric anisotropy (δ?) for the successive homologues (hexyl to undecyl) shows an alternation similar to that exhibited by the nematic-isotropic transition temperature (T _NI). The dispersion of ?_∥ in the frequency range of 0.1-15 MHz has been studied for four successive members of biphenyl-4-p-n-alkoxybenzoates (heptyl to decyl derivatives) as well as of trans-p-n-alkoxy-α-methyl-p'-cyanophenyl cinnamates (eighth to eleventh members). For both series the relaxation frequency (f _R) and the frequency of dielectric isotropy (f _o) evaluated at a common relative temperature show an alternation similar to T _NI. For any given compound the Arrhenius plots for f _R and f _o give the same value of the activation energy (W). In the case of the biphenyl esters the vaue of W is the same for the seventh and eighth members, but increases for the higher members which exhibit a smectic phase at lower temperatures. On the contrary for cinnamic acid esters W is found to be the same for all homologues irrespective of whether the nematic phase is preceded by a smectic phase or not.     

Electro-Optical Properties in Polymer-Stabilized Blue Phase of n-OCB Homologue Chiral Nematic Liquid Crystal Mixtures

In this study, the chemical structures and physical property dependencies of the electro-optical properties were investigated in polymer-stabilized blue phase (BP) using the four kinds of 4-n-alkyloxy-4′-cyanobiphenyl (n-OCB) homologue chiral nematic LCs. It was confirmed that the optical response time and Kerr coefficient in polymer-stabilized BP was varied with the molecular parity as determined by the alkyl chain length of the n-OCB homologue chiral nematic LC mixtures.     

Phase transition of nematic to smectic C through an intermediate boundary induced smectic A structure

An intermediate phase has been observed at the transition nematic to smectic C where a smectic A structure induced by the boundaries and a nematic phase exist simultaneously. The temperature interval of existence of the intermediate phase is of the order of one tenth of degree. Several conditions are necessary for its appearance: the latent heat L of the phase transition nematic-smectic C of the liquid crystal should be sufficiently small (L < k_BT_c); the nematic phase should undergo a bend deformation by the boundaries (possibly a twist one as well) and the transition nematic-smectic C should be carried out as slower as possible.     

CROSSOVER BETWEEN FLEXOELECTRIC STRIPE PATTERNS AND ELECTROCONVECTION IN HYBRID ALIGNED NEMATICS

Abstract

We report experimental and theoretical results on the flexoelectric instability and crossover between flexoelectric domains and electroconvection in a hybrid aligned nematic MBBA under d.c. voltage. At threshold a spatially periodic flexoelectric deformation in the form of longitudinal domains (along the planar director) was observed. With increasing voltage the director deviation out of the initial plane becomes clearly detectable. Observations of tracers show that hydrodynamic flow develops inside the flexoelectric stripe pattern in the form of circular orbits perpendicular to the domain stripes. With further increasing d.c. voltage electroconvection sets in in the form of drifting oblique rolls inside the flexoelectric domains.     

ENHANCED FLEXOELECTRIC SWITCHING BEHAVIOUR IN THE CHIRAL NEMATIC PHASE

Abstract

In order to develop materials that exhibit enhanced flexoelectric switching in the chiral nematic phase we have identified mesogenic units that display inherently strong flexoelectric coupling capabilities. Here we examine the oxycyanobiphenyl (OCB) moiety: homologues from the nOCB series exhibit significant electro-optic switching effects when doped with a highly chiral additive. Here we have examined lower dielectric anisotropy materials, since they allow the flexoelectric response to be extended to high field amplitudes. We show that dielectric coupling strength can be low in symmetric bimesogenic molecules. The flexoelectric response of such a molecular structure is tested by doping a homologue from the series CBOnOCB with a chiral additive: very significantly we find that the optic axis is rotated through 2φ = 45° in < 50 μs on reversing the polarity of the field (amplitude E = ±6 V μm^?1). Subsequently we have synthesized room temperature chiral nematic materials that exhibit 2φ > 90° at E = 10 V μ^?1.     

A Study of 4 Nonanoate, 4’ Cyanobiphenyl Using Dielectric Relaxation Method

In this paper, the results of a dielectric absorption study for 4 nonanoate 4’ cyanobiphenyl are presented. For electric field E perpendicular to director n, a very distributed domain can be observed. It can be interpreted as resulting from the superposition of rotation and libration movements of the molecule and of intramolecular movements. For E parallel to n, the reorientational movement of the molecule is observed. The activation energy of this mechanism is lower in SmA phase at T > 30°C than in nematic phase. Moreover, if T < 25°C the activation energy increases. These variations seem to be characteristic of a partially bilayer SmA phase with the appearing of monolayer S_c domains at T < 25°C.     

Dielectric Anisotropy and the Order Parameter of HXBBA

Abstract

Measurements of dielectric anisotropy (Δ?), refractive indices (n _e , n _o), birefringence (Δn) and density (ρ) have been made in the nematic and smectic phases of N(-p-hexyloxybenzylidene)-p-butylaniline (HXBBA). The results indicate that the various transitions are of the first order type except smectic B-smectic G, which may be a second order transition. The order parameter S has been determined using the isotropic internal field model (Vuks approach) and the anisotropic internal field model (Neugebauer's approach) and both the values agree fairly well. The dielectric anisotropy (Δ?) increases strongly in the smectic phases whle S increases only slowly. It is interpreted by an increase of dipole-dipole correlations.     

Phase transitions from density and order parameter from polarisabilities in symmetric dimeric liquid crystals

As a part of our systematic studies on liquid crystal dimers, we present in this article the nature of phase transitions across isotropic–nematic and nematic–smectic-A exhibited by DLCs, α,ω-bis-(4-n-alkylaniline benzylidene-4?-oxy) alkanes. Further, the orientational order parameter in the nematic phase of these DLCs are estimated from the molecular polarisabilities calculated using the experimental refractive indices and density results. The molecular polarisabilities α_e and α_o are obtained for the compounds using the above results for both Vuks and Neugebauer local field models applicable to nematic liquid crystal. α_e and α_o calculated in this way are used to obtain Δα. The polarisability anisotropy in the perfect order (absolute K) is calculated semi-empirically using the δ-function model developed by Lippincott et al. and molecular vibration method. The values of polarisability anisotropy for both local electric field models differ significantly. No criterion is known to decide which value is correct. To avoid the determination of uncertain α and Δα values considering different local field models, a simple procedure developed by Kuczynski et al. was used for evaluation of S, based solely on birefringence δn = (n_e-n_o) and this value of S is compared with those obtained from field models.