Electrical and Optical Properties of Ferroelectric Liquid Crystals and Influence of Applied Pressure

Much larger dielectric constants and spontaneous polarizations are evaluated in the smectic-C phase of p-alkoxybenzylidene-p'-amino-2-chloropropyl-cinnamate (HOBACPC, OOBACPC and DOBACPC) which have dipole moments at the chiral part compared with those of p-alkoxybenzylidene-p'-amino-2-methylbutyl-cinnamate. In the mixture of D-DOBAMBC and L-DOBACPC with inverse chirality, the pitch of the helical structure is longer compared with those of original compounds. On the other hand, the dielectric constant and the spontaneous polarization of the compounds are nearly linearly dependent on the concentration of L-DOBACPC.

The phase transition temperature from smectic-A to chiral smectic-C phases in DOBAMBC decreases remarkably with the mixing of organic molecules (PAA, eicosane, etc.) but increases with applied pressure.

The threshold field of electro-optical effects (transmission increase, colour switching, memory effects) in DOBACPC and HOBACPC etc. is lower than that in DOBAMBC etc. in accordance with the increase of the spontaneous polarization in the former.     

Dependence of the Tilt Angle on External Forces for Smectic-C And Chiral Smectic-C Liquid Crystals—Measurement of the Heat Capacity of DOBAMBC

A measurement of the specific heat of DOBAMBC is presented, and it is shown that a Landau expansion of the free-energy density with a sixth-order term is adequate to describe the measurement. The coefficients in the Landau expansion are estimated from this measurement.

The dependence of the tilt angle on applied electric and magnetic fields is discussed for both smectic-C and chiral (ferroelectric) smectic-C liquid crystals. Numerical estimates of the Landau coefficients are used to show that the effect of external forces on the tilt angle is negligible except when being less than approximately 0.2 K from the transition to the smectic-A phase. The effect on the smectic-C-smectic-A phase-transition temperature is also discussed.

Boundary effects can also affect the tilt angle, and an expression for the penetration depth of a boundary-induced disturbance is calculated. For thin, planar samples, a coupling between the two boundaries can induce a transition to the smectic-A phase. It is demonstrated that this transition is analogous to the ordinary Freederickz transition in nematic liquid crystals.     

Proton NMR studies of smectic-C phases

We present proton NMR second moments studies of some compounds exhibiting smectic-C phases. From the comparison of experimental and theoretical angular dependences of the second moments the tilt angles δ are derived. The ratio f of the second moments of samples with statistical distribution of domain orientations and of perfectly aligned samples is calculated and compared with experimental data. For certain substances with constant tilt here are significant deviations from the theoretically expected values suggesting that the underlying theoretical model has still to be modified.     

Phase Diagram Involving the Thermal Stability of TGB and Induced Smectic Phases in Binary Mixture of Thermotropic Liquid Crystals

We report the results of our studies on the optical and thermal properties of the mixture of terephthalidene-bis-4, n-alkylaniline and cholesteryl nonanoate, which exhibits very interesting liquid crystalline mesophases such as cholesteric, twisted grain boundary, smectic-A, smectic-C^*, smectic-C, smectic-I, and smectic-G, sequentially when the specimen is cooled from its isotropic phase. These phases have been characterized by using microscopic techniques. The temperature variations of optical anisotropy, electrical conductivity, ultrasonic velocity, and molar and adiabatic compressibility have also been discussed.     

Cholesteric Pitch Near the Smectic-C Phase

The temperature dependence of the cholesteric pitch of the chiral ester 4-n-hexyloxyphenyl-4'-(2”-methylbutyl)biphenyl-4-carboxylate has been measured in detail using the method of selective reflection. This compound has a cholesteric to smectic-C transition at 79°C. Because of the first-order nature of the transition, the pretransitional unwinding of the pitch is considerably less pronounced than that in cholesteryl esters near the smectic-A transition. The wavelength of maximum reflection increases from 440 nm at 110°C to 560 nm at the transition. When fitted to a power-law temperature dependence, the data imply an exponent of 0.82 ± 0.15, which is consistent with de Gennes’ theory in the meanfield approximation.     

Relaxation behavior of supra-molecular hydrogen-bonded liquid crystal phase structures: SA:11OBA

Low-frequency (20 Hz–10 MHz) dielectric response in liquid crystalline (LC) phases of hydrogen-bonded (HB) complex, SA:11OBA, is studied. Synthesis of SA:11OBA with non-mesogenic succinic acid (SA) and mesogenic p-n-undecyloxy benzoic acid (11OBA) and its spectroscopic confirmation are detailed. Phase transition temperatures (T_C) involving nematic, smectic-C (SmC), and smectic-G (SmG) phases by capacitance C(T) and loss tanΔ(T) anomaly agree with the microscopy (polarizing optical microscope) and calorimetry (differential scanning calorimetry) results. Low-frequency dispersion infers two types of reorientation processes, viz. higher frequency (～MHz) and lower frequency (kHz) processes. Distinct time scale mechanism are presented. Arrhenius behavior infers influence of HB on activation energy (E_a). Off-centered dispersion, viz. ?′(ω) with ?″(ω), analyzed through the Cole–Cole plots in N, SmC, and SmG phases infers a strong temperature trend for dielectric strength (Δ?) and distribution (α) parameters. Temperature dependence of α-parameter reveals on LC phases increasing fixture of molecular dipole in LC phase structure. Results are discussed in the wake of the body of data reported in LC phases exhibited by other LC compounds.     

On the Problem of Transient Hydrodynamic Instability of Nematics in the Magnetic Field. II: Linear Analysis of the Stability of Certain Models of Magnetohydrodynamic Domains

Crystallography Reports - Models describing the appearance of hydrodynamic domains in nematics as a result of their reorientation in magnetic field have been studied applying the linear stability...     

An Investigation of the Dynamic Structures of Ferrocene,Ruthenocene and Dibenzenechromium in the Solid State and in Solution

Pulse nuclear magnetic resonance measurements have been used to determine the activation enthalpies for the reorientation of the organic rings in ferrocene, ruthenocene and dibenzenechromium. The contribution from non-bonded interactions to the total potential for reorientation of the rings has been calculated and is compared to the experimental activation enthalpies. The results indicate that there is no contribution to the potential from the non-bonded interactions between the two rings on a single molecule and that the observed potential barrier is due to a combination of crystal packing forces and bonding forces within the molecule.

The crystal structure of ferrocene is thought to be disordered above 135[ddot]K, and a model is proposed for the order-disorder transition in the solid state. On this basis, dynamic models are proposed for the structures of the three metallocenes in solution.     

Thermal and phase behaviour studies of hydrogen-bonded compounds (SA:nOBA) using POM,DSC and image-processing techniques

A novel series of hydrogen-bonded liquid crystals are synthesised. The hydrogen bond is formed between non-mesogenic salicylic acid and the mesogenic p-n-alkoxy benzoic acids with n = 4–8, 10–12 and 16. The formation of hydrogen bond is confirmed from infrared studies. The transition temperatures and the phase identification are made with the use of polarising microscope (POM) attached with a hot stage. Further, the heats of transition and enthalpies are obtained by employing the differential scanning calorimeter (DSC). Along with POM and DSC, textural image analysis is done by image-processing techniques using MATLAB software and transition temperatures are identified. The results reveal that the compounds with the chain number n = 7, 8, 10–12 show the manifestation of smectic-B phase by quenching smectic-C phase.     

Study of Molecular Structure,Mesomorphism and Their Relation with Molecular Rigidity-Flexibility

Entirely smectogenic novel chalconyl homologue derivatives of thermotropic liquid crystals (LC) have been synthesized with a view to understanding and establishing the effects of molecular structure on liquid crystal properties and the degree of liquid crystallinity. The homologous series consists of 12 (C₁ to C₁₆) homologues with absence of nematogenic character throughout the present series. The textures of smectic phase are fan shaped or batonnet type of smectic-A or of smectic-C. Phase transition temperatures and thermotropic data were determined by an optical polarizing microscope equipped with a heating stage. The smectic–isotropic (Sm-I) phase transition curve and the Cr-Sm transition curve behave in normal manner in the phase diagram. Analytical and spectral data confirm molecular structures of novel homologues. The average thermal stability for smectic is 139.42°C and the mesogenic phase length ranges from 3.5°C to 79.6°C. Mesogenic properties are compared with the structurally similar known series.     

The Effect of Cooling Rate and Magnetic Field on the NMR Relaxation and Thermal Behavior of [N- (p-methoxybenzylidene) -p-n- butylaniline] (MBBA)

NMR relaxation time measurements and the dsc thermal behavior of the solid state of MBBA have been examined as a function of cooling rate and aligning magnetic field. The results indicate a complicated solid state morphology as a result of these effects. Aside from the normal crystal structure, anisotropies are introduced by the aligned magnetic field and the liquidcrystalline structure that is retained on quenching to the amorphous metastable state. These anisotropies are reflected in the occurance of additional endotherms in the thermal data, and by separate relaxations for methyl reorientation in the normal crystal and the metastable amorphous material in the nmr data. The activation energies for methyl reorientation are determined.     

UV spectral characterization of a smectic-C liquid crystal: Theoretical support to the experiment

In the present article, UV spectral characterization of a smectic-C liquid crystal 4,4′-bis(n-alkoxy)azoxybenzene (n = 14) (C₄₀H₆₆N₂O₃) has been carried out. Structure of the molecule has been optimized using the Density functional B3LYP with 6-31+G (d) basis set using crystallographic geometry as input. The absorption spectra have been estimated in the UV region by employing the DFT method, semiempirical CNDO/S and INDO/S parameterizations. The oscillator strength (f) and vertical transition energy (E_V) have been reported corresponding to absorption wavelength (λ_max). These values have been compared with the experimental value reported in the literature to offer theoretical support to the experimental value. Further, some electrochemical properties have been reported for the molecule.     

High Frequency Dielectric Relaxation of a Ferroelectric Liquid Crystal

Dielectric relaxation measurements on a optically active thiadiazole derivative in the frequency range from 1 MHz to 1 GHz gave relaxation frequencies between 100 MHz and 1 GHz in the smectic C* phase. This relaxation which is related to molecular reorientation processes around the long molecular axis is connected with a mean activation energy of 57 kJmol⁻¹. A simple model of the energetic behaviour of this reorientation including the production of the spontaneous polarization is discussed.     

EFFECT OF AZIMUTHAL SURFACE LAYER ANCHORING ON FIELD-INDUCED LAYER REORIENTATION IN A SHORT PITCH FERROELECTRIC LIQUID CRYSTAL

Abstract

The concept of the azimuthal surface layer anchoring (ASLA) is introduced to describe the field-induced layer reorientation in a short pitch ferroelectric liquid crystal (FLC). It was found that for the larger ASLA, the smaller layer reorientation occurs and saturates in the high field regime, especially near both surfaces. The ASLA tends to renormalize the surface interactions which produce the asymmetric director switching. It is suggested that the ASLA acts as a hindered restoring force for the layer reorientation.     

Tilt Angle Determination of a Smectic C Phase by Field-Induced Freedericksz Transition and x ray Investigations

A smectic C liquid crystal exhibiting positive dielectric anisotropy was oriented with the director parallel to the glass substrates. When an electric field was applied normal to the director direction a dielectric reorientation (Freedericksz transition) occurred. Dissolving a dichroic dye in the smectic C phase and applying an electric field, a guest host effect similar to nematic phases was observed. From the dielectric reorientation the tilt angle of the smectic C phase was determined at different temperatures. The tilt angles obtained by the electrooptical measurements agree sufficiently with the values measured by x ray investigations.     

Image enhancement of nano-dispersed N-(p-n-decyloxybenzylidene)-p-n-hexyloxy aniline using combined unsharp masking

The main objective of the image enhancement is to process an image with suitable technique to produce better visibility for a specific application. To identify key features like transition temperatures, clear phase identification in the liquid crystalline images, we require some novel image processing techniques. Characterisation and mesomorphic behaviour in pure and 1% ZnO nano-dispersed liquid crystalline N-(p-n-decyloxybenzylidene)-p-n-hexyloxy anilines, 10O.O6 compounds are carried out using a polarising microscope and images are preserved for enhancement. Both the compounds exhibits NACIG (nematic, smectic-A, smectic-C, smectic-I, smectic-G) phases and the transition temperatures of the 1% ZnO nano-dispersed 10O.O6 are reduced compared with pure 10O.O6. Further, in this paper, a novel image enhancement technique of combined unsharp masking is proposed on pure and 1% ZnO nano-dispersed 10O.O6 liquid crystalline compounds for better visibility of phases at transition temperatures. The proposed method is used to identify the uniform regions and to detect the defects which may not be clearly observed from the textures that are recorded by polarising microscope.     

Thermal expansion of Sm-Tb orthoferrites in the region of the spin reorientation transition

A new model of spin reorientation is discussed on the basis of the analysis of experimental data on thermal expansion of samarium-terbium orthoferrite and the earlier studies of its magnetic and crystallooptical properties. During the orientational phase transitions, the zero values of the thermal and optical parameters of rare earth orthoferrites correspond to the alignment of the magnetization vector along the [110] crystallographic directions. For orthorhombic Sm-Tb orthoferrite crystals, these directions coincide with the twofold axes in the plane.     

Dielectric relaxation investigations of the isotropic and nematic phases of 4-n-Butyloxyphenyl-4-n-hexylbenzoate (BOHB)

Experimental values of the complex dielectric permittivity in a wide frequency range are analized for the isotropic and nematic phase of BOHB. The results are discussed from the point of view of different of molecular reorientation mechanisms which are related to the structure of the molecule. It can be concluded that the molecules in the nematic phase exhibit a stronger tendency to reorient as “rigid bodies”.     

Domain structure in main crystallographic planes of orthoferrites at spin reorientation

The behaviour peculiarities of the domain structure of the single crystal samarium-terbium-erbium and samarium-terbium-thulium orthoferrites in the spin reorientation range in three crystallographic planes (001), (110) and (100) are investigated. In the (001)-plane the temperature-induced bubble domain lattice formation is observed. The formation kinetics of these domains is similar to the process of nucleation and that of growth with spontaneous crystallization. In the (110) plane the inclined domains formation is noted. The canting angle depends on temperature at spin reorientation. The values of remagnetization fields are measured as a function of temperature in the indicated crystallographic planes. From the measured changes of the domain structure the temperature dependences of the first anisotropy constant and the domain wall density are determined.