Nematic Temperature Range Research Articles

Liquid-crystal-anchoring transitions at surfaces created by polymerization-induced phase separation

A surface anchoring transition of a nematic at polymer surfaces created by polymerization-induced phase separation is presented. This transition is unusual in that it occurs far from bulk nematic phase transitions and it is tunable across nearly the entire nematic temperature range by modification of the polymer side group. Anchoring behavior is qualitatively understood by considerating enthalpic and entropic contributions to surface energy. Interesting behavior of some polymer-dispersed liquid-crystal films is explained, and observations provide a pathway to control properties. @S1063-651X~98!50308-X# PACS number~s!: 64.70.Md, 64.75.1g Surface energetics of liquid crystals is intriguing and crucial for technologies that use liquid crystals, such as flatpanel displays and optical components. The fact that liquid crystals exhibit preferential orientation at interfaces, a phenomenon called surface anchoring, has been known for over 80 years, but its molecular origins are still elusive. Whether a liquid-crystal material prefers to orient with the director n normal to the interface ~homeotropic anchoring!, in the plane of the interface ~homogeneous anchoring!, or along a tilted direction or directions depends upon subtle interactions that are not well understood. Unidirectional homogeneous alignment can be produced by rubbing a substrate @20# and homeotropic alignment by treating the bounding surfaces with amphiphilic alkyl compounds such as surfactants @1‐4#. The less common tilted anchoring has been seen at a glass surface exposed to smoke soot @19# or coated with a commercial fluorinated surfactant @5#. Transitions between surface anchoring states are particularly exciting because they arise from a competition between opposing components of surface anchoring. A number of nematic surface anchoring transitions have been observed. Most of them occur close ~within ;10 K! to bulk phase transitions of the nematic to isotropic, smectic, or crystalline phases. Noting the sensitivity of several contributions to surface anchoring on the bulk nematic order parameter S, surface anchoring transitions near the nematic-isotropic transition have been attributed to the rapid decrease in S upon heating close to the nematic-isotropic transition @6#. Near the transition to smectic or crystalline phases, pretransitional layering near surfaces has been cited to explain transitions to homeotropic anchoring @7#. Surface anchoring transitions very far from bulk phase transitions are rare. Volovik and Lavrentovich @8# reported a transition at a surface composed of two fluid components known to induce opposing anchoring tendencies. This could perhaps arise because of a changing surface composition or because of different temperature dependencies of the anchoring strength of the two components. Recently, Drzaic @9# reported a surface anchoring transition far from bulk liquidcrystal transitions at poly~vinyl alcohol! surfaces modified by

The surface memory effect at conducting and dielectric surface coatings

The surface memory effect (SME) storage of smectic C textures in the nematic temperature range has been investigated at various conducting and dielectric orienting coatings. The surface memorization for p-n-heptyl-,-octyl- and-nonyl-oxybenzoic acids (HOBA, OOBA, NOBA) consisting of dimer molecules is relatively strong and is observed only on conducting coatings (ITO, SiO/ITO) while dielectric surfaces depress the effect. p-n-Decyloxybenzoic acid (DOBA) exhibits a SME neither for conducting nor for dielectric coatings. The basic role of adsorption in the surface memorization is discussed in combination with the influence of surface polarization and of certain mechanical properties of the liquid crystal molecules. It is demonstrated that the erasure activation energy V hyperbolically decreases with increasing cell thickness d and reaches saturation values at d 250mum. Probably this dependence indicates the role of the dielectric state of the coatings in the surface memorization process. The problem of the possible biaxial character of the memorized smectic C-like texture in the temperature range of the uniaxial nematic phase is also discussed.

Influence of the lateral alkoxy chain length on the gas chromatographic properties of five liquid crystals

AbstractThe gas Chromatographic properties of five laterally substituted liquid crystals are the subject of a comparative study. These liquid crystals belong to the homologous series: 2‐alkoxy‐3‐methyl‐4‐(4‐ethoxybenzoyl‐oxy)‐4′‐(4‐trans‐n‐pentyclcyclohexanecarboxyloxy) azobenzene, referred to as ALn, where n, the carbon number in the lateral alkoxy chain, is equal to 4, 6, 8, 10, or 12 carbon atoms. Their thermal properties were established by differential scanning calorimetry (DSC). It appears that the introduction of a lateral alkoxy chain affects the nematic temperature range which decreases when the carbon number increases. The chromatographic separation abilities of AL4, AL6, AL8, AL10, and AL12 were studied using capillary glass columns. The five liquid cystals are efficient before and after solid‐nematic or nematic‐liquid transitions; however, the plate numbers are higher in the nematic state. Interesting analytical properties were noted in various fields: iisomeric separation of alkanes, aromatics, polyaromatics, volatile aroma compounds, and cis and trans isomers. However, the chain length does not exert any great influence on the separation abilities.

Advances in Liquid Crystals for Am-Lcd

AbstractPhysical properties of mono-, di- and tri- fluorobenzenc derivatives with different core structures consisting of 3-ring system were measured. By introduction of fluorine atoms, phase change telnperaturcs decreased, nematic temperature range narrowed, optical anisotropy and elastic constant decreased, and dielectric anisotropy increased. From a calculation with MOPAC ver. 6.0, it is clear that elastic constants depend upon shapes of molecules under a restricted condition.By blending the investigated compounds as main components, different nematic mixtures, having low threshold voltage low viscosity, large optical anisotropy or wide nematic range, which are suitable for specific applications for AM-LCD, were obtained

On the refractive indices, polarizabilities and order parameter of a nematic discogenic mixture

Abstract The phase diagram of a mixture of the discogens triphenylene hexa-n-heptanoate (HET7) and triphenylene hexa-(2-methyl-4-n-decyloxy)benzoate is presented. This mixture provides a more accessible nematic temperature range than was previously possible. The refractive indices of one mixture composition are measured and a rather small, negative birefringence found. The two-parameter Haller extrapolation method is used to calculate the molecular polarizability ratio Δα/α. This has a rather small magnitude compared with calamitic mesogens, although the individual polarizabilities are much larger, due to the greater molecular mass of these compounds. The calculated value of Δα/α is used to give the uniaxial orientational order parameter S, whose value is comparable to those of calamitic examples. Tabulated bond polarizability data are used to predict a value for Δα/α, which is found to be similar to the experimental result.

Electric dipole interactions of chain molecules in nematics: The analysis of segmental ordering in α, ω-dibromoalkanes

The residual electric dipole interactions recently detected in the deuterium NMR spectra of α,ω-dibromoalkanes dissolved in nematic liquid crystalline solvents [J. Phys. Chem. 96, 8176 (1992)] are analyzed in the context of the modular formulation of flexible molecule ordering. The chord model realization of this formulation, previously applied to the study of n-alkanes, is minimally extended to include the additional coupling of the dipole carrying segments of the solute molecules to the dipolar bias produced by the solvent. The ensuing simple model successfully reproduces the observed segmental order parameter profiles of the solute over the entire nematic temperature range of the solvents. Three types of solvents are considered (Phase V, 5CB, 5OCB) differing in their molecular dipole moment constitutions. It is found that in all solvent cases the orientational ordering is suppressed at the dipole moment carrying ends of the solute molecules whereas the ordering in the main body could either be reduced or elevated depending on the configuration of the dipole moments on the solvent molecules. Quantitative results for the effects of the dipolar bias on the potential of mean torque are presented and the mechanism of electric dipole driven ordering in these solvent–solute systems is discussed.

Flexo-electric polarization and anchoring energy for a nematic liquid crystal in a hybrid cell

The flexo-electric polarization of hybrid nematic cells filled with 5 CB was measured as a function of cell thickness. To this aim the pyroelectric response of a nematic layer with small amount of light absorbing dye dissolved was detected in a nematic temperature range. From the experimental data the angle of the director deviation at the homeotropic boundary was calculated as a function of cell thickness. This dependence allows the qualitative determination of the shape of the potential well for the director deviation which is inconsistent with Rapini’s sinesquared form. The «apparent» Rapini’s anchoring energy varies with a surface director angle (at the homeotropic boundary) from 5·10−3 to 3.5·10−2 erg·cm−2.

Liquid crystals of some new tolane derivatives containing a 1,2-ethylene linkage

Abstract Three types of nematogenic compound, 4-[2-(trans-4-alkylcyclohexyl)ethyl]-4′-alkyltolanes, 2-fluoro-4-[2-(trans-4-alkylcyclohexyl)ethyl]-4′-alkyltolanes and 4-[2-(trans-4-alkylcyclohexyl)ethyl]-4′-halogenotolanes, have been synthesized and their transition temperatures and enthalpies determined. Their bulk viscosity and birefringence have been estimated from those of mixtures with other liquid-crystalline compounds. These compounds have been found to be excellent materials for twisted nematic displays, because of their wide nematic temperature range, high birefringence (Δn = 0·26), and low viscosity (η = 21 cP at 20°C). Several related compounds having other central linkages, namely -CH2O-, a single bond or -COO-, in place of the 1,2-ethylene group, have also been studied.

Thermotropic polyesters: structure of oriented poly( p-oxybenzoate- co- p-phenylene isophthalate)s

Previous work has shown that poly( p-oxybenzoate- co- p-phenylene isophthalate)s are nematogenic within the composition range 33 67 to 75 25 . X-ray investigations of oriented fibres showed that, with respect to the physical structure, they differ from the corresponding unoriented materials. The main emphasis has been placed on structural characterization of fibres from 50 50 and 67 33 copolymers, which have a broad nematic temperature range. The fibres spun from the nematic mesophase were highly oriented but crystalline order was absent. Annealing at temperatures ranging from 240 to 300°C causes crystallization. The structure of annealed fibres can be regarded as a mosaic of thin crystalline regions containing p-phenylene isophthalate sequences, together with paracrystalline regions.

Smectic-A1-to-nematic phase transition: X-ray diffraction of the bulk and comparison with surface structure.

We report x-ray scattering measurements around the smectic-${A}_{1}$-to-nematic phase transition of ${\mathrm{C}}_{8}$${\mathrm{H}}_{17}$O-\ensuremath{\varphi}-OOC-\ensuremath{\varphi}-${\mathrm{OCH}}_{2}$-\ensuremath{\varphi}-CN, where \ensuremath{\varphi} is the phenylene functional group. Critical fluctuations occur at the smectic-${A}_{1}$ monolayer wave vector ${\mathrm{q}}_{2}$ and noncritical bilayer fluctuations at the incommensurate wave vector ${\mathrm{q}}_{1}$\ensuremath{\approxeq}${0.56\mathrm{q}}_{2}$. This is different from the situation at the surface where the two wave vectors are commensurate with ${\mathrm{q}}_{1}$${=(1/2\mathrm{q}}_{2}$. For both types of layering the correlation lengths for the bulk were found to be equal to the penetration depths at the surface. The critical behavior of the monolayer fluctuations is described by the critical exponents \ensuremath{\gamma} for the susceptibility and ${\ensuremath{\nu}}_{?}$ for the correlation length, which are both close to their tricritical values. This is unexpected for the large nematic temperature range involved.

Thermotropic polyesters: Synthesis and properties of poly(chloro-1,4-phenylene terephthalate-co-4,4′-oxybisbenzoate)s

Poly(chloro-1,4-phenylene terephthalate), poly(chloro-1,4-phenylene 4,4′-oxybisbenzoate), and their copolymers are synthesized and the effect of composition on thermal transitions is evaluated. All the materials are nematogenic; copolymer compositions from 75 25 to 25 75 display a nematic temperature range in excess of 100°C which is sufficient for melt processing. The crystal-nematic transition temperature of the 50 50 copolymer is strongly affected by the molecular weight, but the glass transition, although low, is molecular weight independent. Brief annealing at temperatures below the crystal-nematic transition does not reduce the nematic temperature range of this copolymer.

On the Even-Odd Effect of Volume Changes at I/N and N/SmA Transitions

Abstract The temperature dependences of specific volumes have been measured by dilatometer for N-(4-n-pentyloxybenzylidene)-4-n-alkylaniline, 50.n (from n = 4 to 14). The compounds with odd-numbered carbons in the alkyl chain exhibit larger volume changes than those with the next even-numbered carbons at the isotropic to nematic transition, whereas at the nematic to smectic A transition an opposite alternation is found in the magnitude of the volume change. The even-odd effect at the nematic-smectic A transition is attributed to the width in the nematic temperature range.

Inter and Intramolecular Contributions to the Proton Dipolar and Rotating Frame Spin-Lattice Relaxation Times in the Nematic Phase of the Liquid Crystal 5CB-d 11

Abstract An isotopic dilution study of the proton dipolar and rotating frame spin-lattice relaxation times (T 1 D and T 1 p) in the nematic liquid crystal 4-cyano-4′-n-pentyl-d 11-biphenyl (5CB-d 11) at 13.8 MHz over the nematic phase temperature range was used to separate the intermolecular and intramolecular contributions to the ring proton relaxation rates. This is the first such study to be reported. Combined with our earlier proton study of inter/intra Zeeman spin-lattice relaxation (T 1) in 5CB-d 11, these data permit a critical study of the soft-mode dynamics in the system. A qualitative explanation of the T 1 D> results is presented, but a complete explanation of the T 1 p results is not possible with existing theory.

Glass Capillary Columns with High Temperature Nematogenic Liquid Crystals as Stationary Phase for Polycyclic Aromatic Hydrocarbon Separations

Abstract In view of the interest that has arisen during recent years about the determination of polycyclic aromatic hydrocarbons (PAH) we have carried out a study to prepare a capillary column with a special phase: a liquid crystal. Most of the workers in the field of the analysis of PAH in environmental samples use techniques such as high performance liquid chromatography and gas chromatography with packed columns. Utilization of nematogenic liquid crystals of the type N,N′-Bis(p-methoxy-benzy1idene)-α,α′-bi-p-toluidme as stationary phase in glass capillary columns made it possible to achieve faster and more complete separation of three-, four- and five-ring PAH in comparison with packed columns. The separation properties of three liquid crystals in 20 m glass capillaries within the nematic temperature range in terms of the quality of the column and the separation system were investigated. The columns were tested with three-, four- and five-ring PAH and methylchrysene isomers and the quality of the colum...

Visco-Elastic Properties of Some Nematic Liquid Crystals

We report on some temperature dependent visco-elastic properties of di-butyl-azoxy-benzene (Dibab) and 4 methoxy-4′-azoxy-benzene (N4) which have their nematic temperature range from 19.5 to 31.5[d...

Broadening of the Nematic Temperature Range by a Non-Mesogenic Solute in a Nematic Liquid Crystal

Abstract The general rule that addition of a non-mesogenic solute causes a sharp decrease in the nematic-isotropic transition temperature (TN→ I) of a nematic solvent is not obeyed when the solute and solvent can enter into a donor-acceptor interaction. Addition of 4-aminobiphenyl to the nematic liquid crystal 4-cyano-4′-pentylbiphenyl (nematic range ∼ 25–35°) leads to an increase in TN→ I and a decrease in the crystal-nematic transition temperature. The maximum nematic range (21–38°) is achieved at ∼ 7 mole % solute.

Optical properties of a nematic twist cell

We show that many of the complicated static and dynamic optical transmission characteristics of a Schadt-Helfrich twist cell, including the “bounce” in transmission that is observed at negative oblique angles of incidence after switching it off, can be predicted quite well from a very simple dynamic model. The model ignores flow and has only a viscous resistance to angular reorientation, in addition to the Ossen-Frank elastic properties. The bounce observed in some cases with normally incident light cannot be predicted by the model. As expected, the model yields equilibrium configurations at any applied voltage that give the observed transmission very accurately. Dynamic observations in the upper part of the nematic temperature range, where the observed bounce at normal incidence is small, are well reproduced by the model. At lower temperatures the discrepancies are larger. When interpreted with the model, both dynamic and static measurements concur in indicating that the directors of the liquid crystal adjacent to the surfaces assumed a more or less permanent tilt of a few degrees in our cell. Alignment was obtained by rubbing the conducting electrodes in one direction with linen cloth.

Nematic liquid-crystal displays: Some properties and limitations

The objectives of this study were to determine the feasibility of utilizing nematic liquid crystals (LC) as media for aircraft displays from the point of view of their temperature range, response time, gray scale, and color control. According to this objective, the dynamic scattering and tunable birefringence mode of operation were investigated in some detail. Experiments, in agreement with derived mathematical models, led to the following conclusions : 1) The nematic temperature range can be made to fit any reasonable specification by forming multiple eutectic mixtures, provided the ingredients are compatible. 2) Write or contact times less than 2 µs have been (and less than 0.1 probably can be) realized with the help of blocking diodes. However, true rise times become generally longer than 1 s at temperatures below -20° C and electric fields at or below 3 V/µm. 3) The optical properties of electricallyinduced (dynamic) scattering can, in a first order approximation, be described by specifying the liquid crystal birefringence ratio n_{0}/n_{e} their average refractive index \bar{n} , and the applied voltage. 4) Of several possible color display approaches, electrically tunable birefringence was selected as the most promising one. The widest viewing angle, |α| ≤30°, is achieved with reflective displays and best color definition (resolution Δλ/λ≃0.2) with retardations of Γ≃3/2 in single stage displays, as long as the overall cell thickness fluctuation \Delta d . In addition, nematic liquid scattering and color display features are compared to those of PLZT, an electrooptic ceramic.

Liquid Crystalline Properties of Substituted Azo- and Azoxy-benzenes. I

Abstract In order to obtain low-temperature mesomorphic materials, thirty-two p-alkyl-p′-alkoxy-azo- and -azoxybenzenes have been synthesized. The former have been made by the condensation of p-n-alkylnitrosobenzenes and p-n-alkoxyanilines, and the latter, by the oxidation of the former. Twenty-nine of the compounds show nematic behavior. In the majority of cases, the nematic temperature range of the azoxy compounds, which have been considered to consist of two isomers synthesized simultaneously, is appreciably wide.

Elastic Constants of the Nematic Liquid Crystalline Phase of p-Methoxybenzylidene-p-n-Butylaniline (MBBA)

The bend and splay elastic constants of MBBA were measured over its nematic temperature range. The ratio of the two constants is 1.25± 0.05. The temperature dependence of the elastic constants is found to be adequately described by the simple mean-field models. A comparison is made between viscosity coefficients obtained from the elastic constants and light scattering spectra and from classical viscosity measurements. A method proposed for the measurement of twist elastic constants is evaluated and an upper limit of this constant of MBBA as a function of temperature is reported.