Optical and static dielectric study of a terminally fluorinated liquid crystalline compound

Optical and static dielectric studies on a terminally fluorinated liquid crystalline compound have been carried out. Measurements of temperature variation of refractive indices of the compound are done by using thin prism method. A four-parameter model is validated by fitting the experimentally measured values of refractive indices, birefringence and average refractive indices of the compound with the theoretical ones. Refractive index and density data are utilized for determination of orientational order parameter. The temperature variation of dielectric permittivities of the compound are measured by a LCR meter.     

Optical and X-ray studies of a four component cyano liquid crystal mixture: Comparison of properties with that of a similar mixture

Optical and X-ray studies have been conducted on a four component liquid crystal mixture (code name 2013) with cyano–end groups in all the four components and a lateral substitution of fluorine atom in one of them. The variations in the optical birefringence with temperature, have been investigated and the thermal dependence of polarisability anisotropy and orientational order parameter have been evaluated. X-ray studies have been conducted to determine the effective molecular characteristics such as apparent molecular length and inter molecular distance and their variations with temperature. From nature of molecular alignment as envisaged from X-ray photographs the order parameter variation with temperature has been determined and compared with macroscopic order parameter variation as determined from birefringence studies. The results have been compared with these obtained for another structurally similar four-component liquid crystal mixture (code name 2014) with isothiocyanato end group in place of cyano.     

Orientational order parameter determination of the liquid crystal: 4 propyl paraethoxy phenyl cyclohexyl carboxylate by X-ray studies

The compound 4 propyl paraethoxy phenyl cyclohexyl carboxylate (code name D302) is a member of a homologous series and exhibits liquid crystalline (nematic) behavior in the temperature range 48°C to 78°C. An experiment has been set up for conducting X-ray studies of liquid crystal samples at various temperatures. From X-ray studies conducted at different temperatures on D302, the apparent molecular length and inter-molecular distance and their temperature dependence have been determined. The variation of the orientational-order parameter <P₂>_xray has been determined from the intensities of the scanned X-ray photographs taken at different temperatures. The results have been compared with the<P₂>_opt values obtained by us from birefringence studies.     

Optical and dielectric properties of an isothiocyanato liquid crystal compound and comparison of its behavior with that of a homologous member

The properties of an isothiocyanato liquid crystal compound (code name S1) has been studied with a view to compare its behavior with that of another member of its homologous series (code name S5). Optical properties, such as refractive index, polarisability, and their anisotropies, have been evaluated with varying temperature and the orientational order experimentally determined from the study of polarisability anisotropy and compared with theoretically computed values. Dielectric studies have been conducted to determine not only the dielectric anisotropy but also the effective molecular dipole moment and its temperature dependence. The temperature variation of the angle of inclination (β) of molecular dipole moment with the director direction has also been determined and compared with that of S5. In order to determine the nature of molecular association in the mesophase, the molecular correlation factor (g) has been estimated from the measurements of dipole moments of compound in solution and compared with the values obtained for S5.     

Piezo-optic properties of incommensurately modulated K<Subscript>2</Subscript>ZnCl<Subscript>4</Subscript> crystals

The effect that uniaxial pressures along the principal crystallophysical directions have on the dispersion and temperature dependences of the birefringence in incommensurate K₂ZnCl₄ crystals has been investigated in a wide temperature range. This parameter has been established to be fairly sensitive to uniaxial pressures. The spectral and temperature dependences of combined piezo-optic coefficients are analyzed. Significant anomalies of these coefficients are found at the incommensurate-commensurate phase transition. They are due to the change in the induced birefringence that results from spontaneous polarization and to the significant effect of uniaxial stress on the soliton structure of the crystal. The contributions that the electrooptic effect, the elasto-optic effect, and the order parameter make to the spontaneous changes in the piezooptic coefficients are separated.     

Temperature dependence of the crystal-optics characteristics of [N(CH<Subscript>3</Subscript>)<Subscript>4</Subscript>]<Subscript>2</Subscript>ZnCl<Subscript>4</Subscript>:Ni<Superscript>2+</Superscript> crystals with correlated motion of tetrahedral groups

The optical birefringence, optical indicatrix rotation, and residual intensity have been experimentally investigated in the parent and incommensurate phases of [N(CH₃)₄]₂ZnCl₄ crystals doped with Ni²⁺. The temperature dependences obtained are nonlinear in a wide temperature range (T _i − 360 K). It is shown that the nature of this nonlinearity is related to the presence of local spatial regions of the correlated motion of tetrahedral groups. It is established that the deformation of tetrahedral groups increases the temperature range of existence of these regions.     

Measurement of the Refractive Index and Estimate of the Order Parameter in Oriented Liquid Crystals

The refractive indices of a cholesteric liquid crystal, cholesteryl dodecyl carbonate (CDC), and a smectic liquid crystal, p-n-octoxybenzilidene-p-n-butylaniline (OOBBA), have been measured as a function of temperature in the transition temperature regions. The measurements were made on oriented liquid crystals with an Abbe refractometer. By combining the data on ordinary and extraordinary refractive indices, the relative order parameter, α_aS/α, is plotted as a function of temperature. The behavior of the order parameter near the cholesteric to isotropic transition of CDC is similar to a nematic liquid crystal, whereas, the order parameter in OOBBA shows only a slight temperature variation like many solids.     

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.     

Thermal conductivity and refractive indices of Nd:GdVO4 crystals

The thermal conductivities of Nd:YAG, M(Y,Gd)VO₄ crystals were measured at 298 K. The value of Nd:GdVO₄ crystal along <001> direction was 11.4 W/mK, which was higher than that of YAG crystal measured to be 10.7 W/mK. The principal refractive indices of Nd:GdVO₄ crystal in the temperature range from 20 °C to 170 °C were determined by auto‐collimation method. Based on the measured values of refractive indices, the Sellmeier equation and expression of temperature dependence of refractive indices have been obtained. The measured results show that the birefringence Δn is 0.22007 at 20 °C and temperature coefficient of birefringence is 4.33 × 10⁻⁶/°C for 1.064 μm. These results prove that the GdVO₄ crystal is a laser crystal with excellent thermal and birefringence properties. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Physical Studies of Homologous 4-Cyanophenyl Esters of 4′- n-Alkylcinnamic Acids

The mean polarizabilities of homologous series of nematic liquid crystals of 4-cyanophenyl esters of 4′- n-alkylcinnamic acids are evaluated by using the MLδP method. From these polarizabilities the mean diamagnetic susceptibilities of the liquid crystals are also calculated. The orientational order parameters of these compounds are also estimated using the Vuks and Neugebauer methods. The variation of the order parameter with temperature is discussed.     

Physical Property of New Liquid Crystal Materials and Mixture Design for Active Matrix LCD

Abstract

We studied the physical properties of three series of new fluorinated liquid crystal components and prepared some mixtures by using these components. Decahydronaphthalenes have low Δn with relatively high T _NI. Naphthalenes and Tetrahydronaphthalenes have large Δ? and variety range of Δn (0.08–0.21). Moreover, we have revealed that the fluoro- substituent at C-1 position for the naphthalene and the tetrahydronaphthalene ring has effects to increase T _NI and to reduce γ₁ with good solubility. Then, we have designed some LC mixtures for AM-LCD having good performance with 4V-driving, quick response, high birefringence, low birefringence, wide temperature range and low driving voltage.     

Observation of strong virtual scattering under three-beam (220, 371) X-ray diffraction in TeO<Subscript>2</Subscript> single crystal

A strong effect of virtual scattering has been experimentally observed when studying the nearly coplanar three-beam (220, 371) X-ray diffraction in a paratellurite single crystal under high-resolution double-crystal X-ray diffraction using MoK _α1 radiation. One characteristic feature of this effect is that the angular dependence of the first (strong) reflection intensity and its shape barely change in the three-beam range of parameters, whereas very strong changes are observed for the second (weak) reflection not only in the three-beam range but also far beyond it, which is related to the variation in the two-beam diffraction parameter due to virtual scattering. The changes observed are asymmetric and make it possible to determine the triplet combination of structure-factor phases.     

Computer Simulation Studies of Anisotropic Systems,III. Two-Dimensional Nematic Liquid Crystals

We have studied a two-dimensional ensemble of cylindrically symmetric particles interacting via a weak anisotropic potential using the Monte Carlo technique of computer simulation. The calculation is simplified by confining the particles to the sites of a triangular lattice. The internal energy, specific heat, second rank orientational order parameter and second rank orientational pair correlation function were calculated at various temperatures. The variation of the order parameter and pair correlation function shows that the system exhibits a transition from an orientationally disordered to a partially ordered phase. The temperature dependence of the specific heat suggests that the transition is second order or higher.

The possibility of the existence of order-disorder transitions in two dimensions is discussed. The results of the simulation are then compared with the predictions of a molecular field theory of orientational phase transitions. As expected the theory is found to be in poor agreement with the calculations.     

Influence of TiO2 and ZnO nanoparticles on orientational order parameter of LC compounds – An optical study

In the present paper ZnO and TiO₂ nano particles are dispersed in Cyano-biphenyl liquid crystalline compounds (n?=?7, 12). The thermal polarizing microscopy and differential scanning calorimetry techniques are employed to measure the transition temperatures. The Nematic transition temperatures are decreased by 2.34?°C and 1.53?°C in Heptylbiphenyl, 1.07?°C and 1.12?°C in dodecylbiphenyl compounds due to the dispersion of nanoparticles. The refractive indices and Newton’s rings methods are exploited to measure birefringence at different temperature in nematic phase. The orientational order parameters are estimated by finding the birefringence in perfect order. The orientational order parameter is increased in nanoparticles dispersed liquid crystals when compare to pure liquid crystalline compounds.     

Refractive Indices,Density and Order Parameter of Some Technologically Important Liquid Crystalline Mixtures

Temperature variation of the refractive indices, birefringence, density and order parameter of liquid crystalline mixtures E₇, E₈, N10 and PCH-1132 are reported. The birefringence of PCH-1132 is found to be abnormally low compared to other mixtures.     

Orientational Distribution Function and Order Parameters of Two 4′-Alkoxy-4-Cyanobiphenyls in Mesomorphic Phase

X-ray diffraction studies on samples of 4′-Pentyloxy-4-Cyanobiphenyl and 4′-Octyloxy-4-Cyanobiphenyl over the entire mesomorphic range are reported in this paper. A magnetic field was used to align samples. The orientational distribution functions f(β) have been calculated from the angular distribution of X-ray intensities. Orientational order parameters ( P ₂) and ( P ₄) have been calculated and compared with Maier-Saupe theoretical values. Expressions for the angular part of the pseudopotentials have been obtained from the temperature variation of f(β). The intermolecular distances D have been calculated from the experimental data and it is found that D is significantly smaller for well aligned samples. The apparent molecular length in the nematic phase and the layer thickness in the smectic phase have also been determined and are found to be about 1.4 times larger than the true molecular length in the fully extended form. This is in accordance with the findings from other cyano compounds.     

Temperature variation of effective geometry parameter, αg,refractive indices and the determination of orientational orders in three room temperature nematic liquid crystal mixtures

Orientational order parameter, S and effective geometry, α_g are calculated from the refractive index in three nematic room temperature liquid crystal mixtures, blOO3, blOO6, and blO38. The method due to Kuczynski et al. used to determine S from birefringence. The effective geometry parameter is α_g = n_o/n_e. (n_o and n_e are the ordinary and extraordinary refractive indices of the liquid crystal material.) The behavior of α_g as a function of temperature and order parameter has also been reported. The S value calculated from α_g is identically equal to the value obtained from Kuczynski et al method, i.e., from birefringence, δn. Finally, dn_e/dT and dn_o/dT are obtained and the crossover temperatures, T_CO the characteristic temperature, which is in fact the temperature of minimum in the ordinary index of refraction in three mixtures for different wavelengths are calculated from dn_o/dT.     

Molecular Polarizabilities,Polarizability Anisotropies and Order Parameters of Nematic Liquid Crystals

Mean Molecular polarizabilities, polarizability anisotropies and order parameters are evaluated in members of three homologous series, using two theoretical models. The variation of polarizabilities and their anisotropies with number of carbon atoms in the alkyl chain is studied. The temperature dependence of order parameter is also presented. The results are compared with the reported values and the merits of the different models are discussed.     

Structure Effect of Solute on Orientational Order in Binary Mixture by 2H-NMR and Molecular Simulations

Abstract

The orientational order parameters of six solutes with different shapes and sizes dissolved in two nematic solvents have been measured as a function of temperature. The parameter, S^xx-S^yy is correlated with the molecular geometrical structure, while S^zz is affected by not only the geometry but also the anisotropy in the polarizability. The molecular dynamics simulations are carried out using a realistic atom-atom potential for evaluating the ordering properties in the nematic phase. The results provide reasonable values of S^zz corresponding to experimental ones for solutes and solvents, and also a prediction of uniaxiality for the nematic phase.     

Order Parameters and Distribution Function of 4'-(N-(p-Pentyloxy Carbonyloxy-Benzylidene)Amino) Valerophenone in Smectic Phase from X-ray Diffraction Measurements

Abstract

X-ray diffraction studies of 4'-(N (p-pentyloxycarbonyloxy-benzylidene)amino)valerophenone (PCBAV in short) oriented by a magnetic field is reported here. The compound has a smectic A phase which is identified by X-ray studies and texture studies. Orienta-tional order parameters (p ₂) and (P ₄) have been determined from the calculation of f(β), the orientational distribution function. An approximate expression for the angular part of the potential of the mean field is obtained from the temperature variation of the orientational distribution function. The value of d, the layer thickness, and D, the intermolecular distance, have been calculated. An approximate value for the translational order parameter τ₁ and average value of the root mean square displacement ?z²?½ are given. We could not get an accurate estimate of the translational order parameter due to the inaccuracy in the measurement of the intensity of the inner ring.