Cotton-Mouton Effect in Monodisperse Suspensions of Liquid Crystals

Abstract

The collective Cotton-Mouton (C-M) effect in the system of micron-size liquid-crystal droplets freely suspended in the isotropic liquid is studied experimentally and theoretically. The collective C-M constant in liquid-crystal suspensions is found to be from 10⁴ to 10⁸ times higher than the respective “molecular” C-M constant in the isotropic phase of the same substance. These large values cause the saturation in the C-M effect at relatively low magnetic fields. Using the generalized nth order Langevin's functions L, _n the statistical therory is presented for the collective C-M effect in the monodisperse liquid-crystal suspensions. As a result, the induced birefringence is obtained in the form Δn = 1/2(Δn)∞ (3L ₂ – 1), where (Δn)∞ is the induced birefringence at saturation. The collective C-M constant is C _c = CN _s S _g where N ₈ is the number of molecules in the droplet, S ₈ is the orientational order parameter of the long molecular axes in the droplet, and C is the “molecular” C-M constant. The resultant (measured) C-M constant is C ₈ = C(1 + N ₈ S ₈). The relaxation time τ depends on the rotational (Brownian) diffusion constant D and after removing the field B the orientational order parameter of the optical axes of droplets in the suspension S _s = 1/2(3L ₂ – 1) vanishes exponentially as S _s = exp(?6Dt). Hence, the relaxation time τ = (6D)^?1.

The collective C-M effect and its saturation can be used as methods for the analysis of the presence and change of the orientational order within colloidal grains and the derivation of several parameters characterizing the aggregation, self-organization, and morphogenesis processes in multicomponent complex systems.     

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.     

Simulation of metastable zone width and induction time for a seeded aqueous solution of potassium sulfate

The metastable zone width (MSZW, ΔT_m) and induction time (t_ind) were determined with computer simulation for seeded batch crystallization of potassium sulfate from aqueous solution. The MSZW and induction time determined with simulation showed the same behavior as experimental values reported in the literature; log (ΔT_m) increased linearly with an increase in log R (R: cooling rate) and t_ind decreases in proportion to (ΔT)⁻ⁿ (ΔT: supercooling, n: nucleation order in the secondary rate expression of B=k_n(ΔT)ⁿ). The secondary nucleation parameters (k_n and n) were deduced both from the simulated MSZW and induction times by using the previously proposed model [J. Cryst. Growth, 2010, 312, 548–554]. The secondary nucleation rate calculated with the deduced parameters was in agreement with that calculated with the parameters input for simulation.     

Investigation on birefringence property in uniaxial crystal (Ca0.28Ba0.72)0.75(Sr0.60Ba0.40)Nb2O6

The birefringence of (Ca_0.28Ba_0.72)_0.75(Sr_0.60Ba_0.40)Nb₂O₆ crystal is measured basing on polarization‐ extinction method and conoscopic interference method. Experimental results show that the birefringence in this crystal is up to 10^‐5, the natural birefringence in this crystal is (n_o ‐n_e) = 5.89 x 10^‐5 with the He‐Ne laser wavelength at 632.8nm and the birefringence grads with wavelength at 532nm is 5.68 x 10^‐5, which indicate the good optical homogeneity of this crystal. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

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.     

Differential Thermal Analysis Studies on the Crystallization of Strontium Tungstate from Sodium Tungstate Melts Thermodynamic and Kinetic Parameters

From the estimated diffusion rate-constants (k_D1) of strontium tungstate crystal growth from sodium tungstate melts in platinum crucibles, energy (E), enthalpy (ΔH_a), entropy (ΔS_a) and free-energy (ΔG_a) of activation and the pre-exponential factor (k₀) for the process were estimated using an ordinary Arrhenius equation k_Dl = k₀e^−E/RT. These thermodynamic parameters were virtually unaffected by the changes in crystallization temperatures (T₀) and cooling rates (R)_T. The distance (r₁₂), between a diffusing species and its host necesary for a successful diffusion, increased with T₀ and T_T but there was no direct correlation.     

Nematic-Isotropic Transition in MBBA and its Mixtures with 4-Methoxybenzaldehyde: Applications to the Purity Control of Liquid Crystals

The nematic-isotropic transition temperatures of MBBA(1) + 4-methoxybenzaldehyde(2) mixtures (where MBBA=N(4-methoxybenylidene)-4-n-butylaniline) for 0  x₂  0.06 and additionally the transition enthalpy of pure MBBA were measured by a calorimetric method. Straight lines were found for the T¹(x₂) and Tⁿ(x₂) curves respectively where T¹ and Tⁿ are the upper and the lower limiting temperatures of the transition range for a given x₂. The results are compared with literature data on similar systems and confirm the wellknown rule that the decrease of the nematic-isotropic transition temperature with increasing x₂ depends more strongly on the size and the shape than on the polarity of component 2. From a plot of δT [tbnd] T¹ - Tⁿ against x₂ the mole fraction of impurity in the original sample can be extrapolated.     

Estimation of activation parameters for diffusion-controlled crystallization of barium tungstate from sodium tungstate melts by differential thermal analysis

The proximity (d₁₂) between a diffusing species and its host crystal necessary for a successful diffusion for diffusion-controlled crystallization of barium tungstate from sodium tungstate melts in platinum crucibles was estimated. These distances increased with increased cooling rates (R_T) and crystallization temperatures (T₀). Energy (E), enthalpy (ΔH_a), entropy (ΔS_a) and free-energy (ΔG_a) of activation and the pre-exponential factor (k₀) were evaluated using an ordinary Arrhenius equation k_D1 = k₀e^−E/RT, where k_D1 was the diffusion rate-constant. These parameters were virtually unaffected by the changes in T₀ and R_T.     

Optical and Electrooptical Properties of “Guest-Host” Effect in Twisted Liquid Crystal Layers

Abstract

Optical and electro-optical characteristics of a positive dichroic dye-doped Twisted Nematic Liquid Crystal Layer (TNLCL) for two different orientations of the input light polarization to the NLC director α (α = 0°, α = 90°) and various NLC optical path lengths A = Δnd/λ (Δn = real part of birefringence, d = cell thickness, λ = wavelength) are investigated. In the case of one polarizer, the maximum contrast ratio is obtained for A ? 1.2 (α = 0°) and A ～ 0 (α = 90°), regardless of whether Δn or d is changed. The viewing characteristics of a dye doped TNLCL with one polarizer are shown to be more uniform than those for a “pure” TNLCL between parallel polarizers.     

Crystal structure of complexes of bivalent Co,Ni, and Cd with anions of benzoic and 2-(acetylamino)-5-nitrobenzoic acids

The structure of three complexes of bivalent metals (cobalt, nickel, and cadmium) with anions of benzoic (HL ¹) and 2-(acetylamino)-5-nitrobenzoic (HL ²) acids, namely, [Co₂¹ (H₂O)₂(μ-C₄H₄N₂)] _n (I), [NiL²(H₂O)₅]L² · 2H₂O (II), and [Cd(μ-L ²)₂(H₂O)₂] _n · 2nH₂O (III), is determined. In chainlike structure I, cobalt atoms are connected by bridging pyrazine molecules; structure II contains isolated complexes. In structure III, centrosymmetric (CdOCO)₂ cycles and polymeric ribbons are formed due to the coordination of the carboxylate group of the L ² ligand to two cadmium atoms.     

Structure and magnetic properties of heterometallic coordination carboxylate polymers with cobalt and lithium atoms

Reactions between lithium pivalate and cobalt coordination polymers [Co₅(OH₂)(OH)(Piv)₉)L ¹)₄] _n (I) and [Co₂(OH₂)(Piv)₄(L ²)₂] _n (II), where Piv—is the pivalate anion, L ¹ is pyrazine, and L ² is pyrimidine, result in new heterometallic polymers {[Li₂Co₂(Piv)₆(L ¹)₂]₂ MeCN} _n (III), {[Li₂Co₂(Piv)₆(L ²)]_0.5 MeCN} _n (IV), and [Li₂Co₂(Piv)₆(L ²)₂] _n (V). The resulting compounds contain tetra-nuclear {Li₂Co₂(Piv)₆} fragments connected by neutral bridging ligands (pyrazine or pyrimidine) into layer structures. Crystal structures III–V are determined, and the magnetic properties of III and IV are studied.     

Synthesis,structure, and mesomorphism of novel liquid crystalline acrylate monomers and polymers

A series of novel liquid crystalline monomers (M₁?M₈) and side chain polymers base polyacrylate backbone were synthesized. The chemical structures were characterized by FT-IR and ¹H-NMR spectra. The mesomorphism and thermal behavior was investigated by polarizing optical microscopy, differential scanning calorimetry, and thermogravimetric analysis. The relationships of structure and mesomorphism are discussed in detail. The eight monomers and their corresponding polymers all show enantiotropic nematic phase. With increasing the spacer length or flexibility of the terminal group, the melting temperature (T_m) and isotropic temperature (T_i) of the corresponding monomers and polymers all decreased. However, with increasing the rigidity of the mesogenic core, T_m and T_i of the corresponding monomers and polymers all increased. TGA showed that all the polymers obtained in this study had excellent thermal stability.     

Solidification behaviour of eutectic and monotectic alloys: Succinonitrile - acenaphthene system

The phase diagram of succinonitrile-acenaphthene system, determined by the thaw-melt method, shows the formation of a eutectic at 0.99 mole fraction of succinonitrile and a monotectic at 0.35 mole fraction of succinonitrile with a large miscibility gap. The linear velocity of crystallization (v) of the pure components, eutectic and monotectic, studied at different undercooling (ΔT) by measuring the rate of movement of the growth front in a capillary, suggests that crystallization data obey the equation: v = u(ΔT)ⁿ where u and n are constants depending on the nature of solidification of the compounds. While the microstructural investigation of eutectic shows broken lamellar and eutectic dendrite-type morphologies that of monotectic exhibits fibrous and broken fibrous-type microstructures depending on the rate of solidification.     

Synthesis,Crystal Structure and Hirshfeld Surfaces of the N-(bis(benzylamino)phosphoryl)-4-Methylbenzenesulfonamide Polymeric tetrakis-Complex {Na[Yb(L)4(iPrOH)]}n

Lanthanide containing polymer chain of title compound {Na[Yb(L)₄(ⁱPrOH)]}_n is formed by alternating complex anions and sodium cations. The anion is composed by four deprotonated bidentate chelate ligands of sulfonylamidophosphate type N-(bis(benzylamino)phosphoryl)-4-methylbenzenesulfonamide (L^?) coordinated to the Yb(III) ion via O atoms belonging to the sulfonyl and phosphoryl groups. The coordination polyhedron of the ytterbium has a slightly distorted triangular dodecahedron conformation, TDD-8, D_2d. The Na⁺ cation is coordinated by four sulfonyl O atoms, the NaO₄ polyhedra shape is intermediate between axially vacant trigonal bipyramid C_3v and tetrahedron T_d. Single-crystal X-ray diffraction studies show that one 2-propanol molecule is hydrogen-bonded to the sulphonyl oxygen of one of the chelate ligands. Also the extensive network of hydrogen bonding interactions between the aromatic rings both in the structural unit and between the polymer chains were detected. They were investigated by the analysis of Hirshfeld surfaces and fingerprint plots. The correlations between structure parameters obtained and the physico-chemical properties of the {Na[Yb(L)₄(ⁱPrOH)]}_n were investigated.

Graphic Abstract

New tetrakis-complex {Na[Yb(L)₄(ⁱPrOH)]}_n was synthesized and characterized by X-ray crystallography. Structural packaging characteristics were investigated by the analysis of Hirshfeld surfaces.

     

Elasticity and Orientational Order in Some trans-p-n-Alkoxy-α-methyl Cyanophenyl Cinnamates

The splay (k ₁₁), twist (k ₂₂) and bend (k ₃₃) elastic constants determined by the Freedericksz method and the orientational order parameters (s) derived from optical measurements in the nematic phase of six homologues of trans-p-n-alkoxy-α-methyl cyanophenyl cinnamates (n OMCPC) are reported. The data close to the nematic-isotropic transition point (T _NI) are compared with T _NI and the heats of transition (ΔH). The temperature-variation of elastic constants is discussed in terms of existing theories. The pretransitional increase in the twist and bend constants near the nematic-smectic A transition point (T _NA ) of 10 OMCPC has also been analysed.     

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)     

Induction periods and mechanism and kinetics of nucleation of barium tungstate crystal growth from sodium tungstate melts in platinum crucibles

Induction periods (t) and mechanism and kinetics of nucleation in barium tungstate crystallization from sodium tungstate melts in platinum crucibles were studied. A theoretical relation has been developed to express the dependence of t on the cooling rates (R_T) and the rate (R_c) of development of excess solute concentration. At any crystallization temperature the average rate-constant (k_n) for heterogeneous nucleation was related to R_c by (k_n/p)^1/(p+1) = 1/(tR_c^γ), where γ is a constant and p is the average number of particles in the critical nuclei. The critical temperature (T), critical supersaturation (S) and γ values were estimated.     

Structural Characterization of Heterodinuclear ZnII-LnIII Complexes (Ln = Pr,Nd) with a Ring-Contracted H2valdien-Derived Schiff Base Ligand

Synthesis and structural characterization of two heterodinuclear Zn^II-Ln^III complexes with the formula [ZnLn(HL)(µ-OAc)(NO₃)₂(H₂O)_x(MeOH)_1-x]NO₃ · n H₂O · n MeOH [Ln?=?Pr (1), Nd (2)] and the crystal and molecular structure of [ZnNd(HL)(µ-OAc)(NO₃)₂(H₂O)] [ZnNd(HL)(OAc)(NO₃)₂(H₂O)](NO₃)₂ · n H₂O · n MeOH (3) are reported. The asymmetrical compartmental ligand (E)-2-(1-(2-((2-hydroxy-3-methoxybenzylidene)amino)-ethyl)imidazolidin-2-yl)-6-methoxyphenol (H₂L) is formed from N¹,N³-bis(3-methoxysalicylidene)diethylenetriamine (H₂valdien) through intramolecular aminal formation, resulting in a peripheral imidazoline ring. The structures of 1–3 were revealed by X-ray crystallography. The smaller Zn^II ion occupies the inner N₂O₂ compartment of the ligand, whereas the larger and more oxophilic Ln^III ions are found in the outer O₂O₂’ site.

Graphic Abstract

Synthesis and structural characterization of two heterodinuclear Zn^II-Ln^III complexes (Ln?=?Pr, Nd) bearing an asymmetrical compartmental ligand formed in situ from N¹,N³-bis(3-methoxysalicylidene)diethylenetriamine (H₂valdien) through intramolecular aminal formation are reported.

     

Effect of buffer layers on the dislocation structure of heteroepitaxial systems

Transmission electron microscopy (TEM) as well as X-ray topography (XRT) and X-ray diffractometry have been used for investigation of the structure of the LPE heteroepitaxial system In_0.05Ga_0.95As-In_yGa_1−yAs_1−xP_x-GaAs(111) A. A critical value of the lattice misfit has been shown to exist at the metallurgical boundary ((Δa/a)* ≈ 10⁻³) which results in the change of the film nucleation and growth mechanism as well as the change of misfit dislocations (MDs) generation mechanism. With (Δa/a)₀ > (Δa/a)* the nucleation and growth mechanism is mixed: island growth at the first stages of growth and layer-by-layer growth at large thicknesses. MDs are created in an “island film” developing a non-ordered dislocation network. The density of threading dislocations (N_d) is ∼ 10⁸ cm⁻². With (Δa/a)₀ < (Δa/a)* there is layer-by-layer mechanism of film's nucleation and growth from the very first stages of crystallization. MDs are injected into continuous layer along the inclined slip planes {111}, thus forming a regular three-dimensional grid of MDs. N_d is less than 10⁶ cm⁻² in the case. A model of dislocation structure formation in heterolayers has been proposed. Within the frame of this model the two critical values of phosphorus concentration in the quaternary melt have been quantitatively determined. These are corresponding to the change of MD generation mechanism. The expected values of N_d for (Δa/a)₀ > (Δa/a)* and (Δa/a)₀ < (Δa/a)* have been theoretically determined.     

Two Novel Three-dimensional Networks Constructed by Mn or Cd Ion and Tartrate: Syntheses,Structures and Properties

Abstract  

Two coordination polymers with the same ligand but different metal ion [Cd(C₄H₄O₆)] _n (1) and [Mn₂(C₄H₄O₆)₂(H₂O)] _n (2) (C₄H₄O₆ = tartrate dianion) have been synthesized. Compound 1 exhibits novel (4, 4)-connected 3-D network with (4.6.8⁴)₄(4.6².8².10)(4.6².8³)(4.6³.8²)₄(4.8⁵)₂ topology, Cd centers are connected by η⁶, μ⁴-mode tartrate ligands. For compound 2, Mn centers are link by η⁶, μ⁴-mode and η⁵, μ³-mode tartrate ligands to form (4, 3)-connected 3-D network with (4.8²)(4.8⁴.10) topology. Additionally, 1 displays an intense luminescent emission and 2 displays weak antiferromagnetic coupling.