The structural transitions in ferronematics and ferronematic droplets

The paper presents an overview of the observations of structural transitions in ferronematic based on thermotropic nematic 6CHBT (4-trans-4′-n-hexyl-cyclohexyl-isothiocyanato-benzene) and in ferronematic droplets. The type of anchoring was determined using the results of the capacitance measurements. The ferronematic droplets were observed in solutions of nematogenic 6CHBT with fine magnetic particles and phenyl isocyanate. The phase diagram of the transitions between isotropic and nematic phases was found. The magneto-dielectric measurements of various structural transitions in this new system enabled to estimate the type of anchoring of nematic molecules on magnetic particles surfaces in droplets.     

Light transmission at the uniaxial-biaxial lyotropic nematics phase transitions

Results are reported on light transmission versus the temperature near the phase transitions between lyotropic nematics. Strong pretransitional effects are observed when the phase transitions occur between biaxial nematic and uniaxial phases. The light is strongly depolarized both at the calamitic-biaxial and at biaxial-discotic phase transitions. The results are qualitatively explained.     

Transitions in lyotropic liquid crystals caused by concentration change and γ-radiation

A molecular-statistical theory of phase transitions in lyotropic liquid crystals, which describes the phase transitions between the isotropic (micellar), nematic and lamellar phases was developed. The equations describing the dependence of parameters of orientation and translational long-range order on the concentration were obtained. It was shown that depending on the values of the model microscopic constants, the nematic phase–lamellar phase transition can be both of the first and the second order. The influence of intensive and low intense γ-radiation on the phase transitions mentioned herein was considered. It was shown that the irradiation changes the model constants responsible for the phase transitions. On this basis, it can be assumed that the γ-radiation influences the course of the dependence of the long-range order parameters on concentration as well as it changes the values of the critical concentrations of the phase transitions and even the phase transition order.     

Phase transitions in lyotropic nematic gels

In this paper, we discuss the equilibrium phases and collapse transitions of a lyotropic nematic gel immersed in an isotropic solvent. A nematic gel consists of a cross-linked polymer network with rod-like molecules embedded in it. Upon decreasing the quality of the solvent, we find that a lyotropic nematic gel undergoes a discontinuous volume change accompanied by an isotropic-nematic transition. We also present phase diagrams that these systems may exhibit. In particular, we show that coexistence of two isotropic phases, of two nematic phases, or of an isotropic and a nematic phase can occur. Received 15 February 2002 and Received in final form 14 June 2002     

Quantum oscillations in dual-layered quasi-two-dimensional organic metal (ET)<Subscript>4</Subscript>HgBr<Subscript>4</Subscript>(C<Subscript>6</Subscript>H<Subscript>4</Subscript>Cl<Subscript>2</Subscript>)

We study the effect of interionic anisotropy on the phase states of a non-Heisenberg ferromagnet with magnetic ion spin S = 1. It is shown that depending on the relation between the interionic anisotropy constants, uniaxial and angular ferromagnetic and nonmagnetic phases exist in the system. We analyze the dynamic properties of the system in the vicinity of orientational phase transitions, as well as a phase transition in the magnetic moment magnitude. It is shown that orientational phase transitions in ferromagnetic and nematic phases can be first- as well as second-order.     

Effect of electric and magnetic fields on the orientation structure of a ferronematic liquid crystal

We analyze uniform orientation phases in soft ferronematics (suspensions of magnetic nanoparticles in nematic liquid crystals) induced by electric and magnetic fields. It is shown that the competition between the electric and magnetic fields can lead to various sequences of orientation transitions in a ferronematic depending on the energy of coupling between the director and magnetization. We obtain and analyze phase diagrams of these transitions. A sequence of re-entrant transitions in the orientation structure (angular phase-homeotropic phase-angular phase-planar phase) is predicted for a certain range of the coupling energies and electric field strengths.     

Temperature dependence of the specific heat of anisaldazine close to phase transitions

The experimental data for the specific heat C_p is analyzed at various temperatures close to the nematic–isotropic liquid (T_NI = 180.5 °C) and the solid–nematic (T_SN = 168.9 °C) transitions in anisaldazine.

Values of the critical exponent for the specific heat, which describe a λ-transition between nematic and the isotropic liquid, and a jump discontinuity between solid and nematic phases, are deduced for anisaldazine. They are compared with the model predictions.     

Phase diagram of transitions from an isotropic phase to nematic and smectic (uniaxial,biaxial) phases in liquid crystals with achiral molecules

The phase diagrams of transitions from an isotropic phase to nematic and smectic phases are investigated within a simple phenomenological model of the Landau thermodynamic potential. The conditions of the isomorphic phase transition between two uniaxial smectic phases and the direct transition from the isotropic phase to the uniaxial and biaxial smectic phases are determined. The behavior of the order parameters is described along different thermodynamic paths. The theoretical results are discussed using the example of liquid-crystal phases in compounds with banana-shaped achiral molecules.     

First order phase change during isotropic-nematic and nematic-smectic a transitions of NPOB

The temperature variation of the magnetic susceptibility (χ) of NPOB are reported in isotropic, nematic and smectic A phases, χ and Δχ undergo sudden changes at the phase transitions. The isotropic-nematic and nematic-smectic A transitions are assigned to be first order phase transition.     

Tricritical phenomena in ferronematic liquid crystals

Orientational transitions induced by an external magnetic field in a ferronematic (a suspension of single-domain magnetic particles in a nematic liquid crystal) are considered in the framework of the continuum theory. The surface potential of the interaction between the ferronematic and the bounding plates is used taking into account the fourth-order anisotropy (the modified Rapini potential). It is shown that the ferronematic can be in one of three phases that correspond to three types of orientational ordering: homogeneous ordering, inhomogeneous ordering, and saturation state. The influence of the segregation effect of magnetic particles on the nature of orientational transitions of the ferronematic in an external field is studied. It is established that the transitions between these phases can be of the first and second order depending on the values of the anchoring parameters and the segregation parameter. Tricritical values of the parameter of anchoring anisotropy and the segregation parameter are determined.     

Synthesis,characterization and phase transition studies in 4-hexyloxy benzylidene 4’-alkoxyanilines

Synthesis and the characterization through polarizing optical microscope (POM) textures in number of 4-hexyloxy benzylidene 4’-alkoxyanilines, 6O.Om with m = 4, 6 to 10 liquid crystalline materials are carried out. Furthermore, the density and thermal expansion coefficient results reveal that the phase transitions present, viz., isotropic–nematic, nematic–SmC and SmC–SmI show first-order nature as expected. The transition temperatures obtained thorough the differential scanning calorimetry is found to be in agreement with the literature data. The first two compounds exhibit only the nematic phase while the next four compounds show enantiotropic SmC phase as per the literature data. The compound with m = 10 exhibits monotropic SmI phase. However, we, the authors, are able to observe this phase in addition to SmC and nematic through POM only as per the literature. The parameters calculated across the phase transitions and in the phases are in agreement with the body of the data available.     

Orientational Ordering of a Liquid-Crystal Suspension of Carbon Nanotubes in a Magnetic Field

A statistical theory is proposed to describe a suspension of carbon nanotubes in a nematic liquid crystal. The mean-field approach is used, and dispersion attraction, the excluded volume effects, the diamagnetism of liquid crystal molecules, and the strong diamagnetism of nanotubes are taken into account. The influence of the volume fraction of impurity, temperature, and magnetic field on the orientational ordering of a liquid crystal matrix and carbon nanotubes is studied. The concentration and temperature phase transitions in the suspension are investigated for various magnetic fields. The concentration and field shifts of the point of the phase transition between nematic and isotropic or paranematic phases are studied.     

Comparative study of liquid-crystalline ordering in a monomer,linear polymer,and graft copolymer by the photon transmission technique

The photon transmission technique was used to study the phase transitions of a liquid crystalline acrylate monomer, 6-(4-cyanobiphenyl-4′oxy)hexyl acrylate (LC6), its homopolymer (PLC6) and its graft copolymer (GLC6) with polytetrahydrofuran grafts. The phase transitions were also confirmed by DSC and polarizing microscopy. We observed the phase transition sequence isotropic–nematic–smectic A–smectic C in the LC6 monomer. In PLC6 and GLC6 polymers, the nematic and smectic A phases appear dominant. The apparent nematic–smectic A transition is of first order in PLC6 and of second order in GLC6, with the transition temperature remaining the same. The effects of quenched random constraints introduced in GLC6 are consistent with the theory of quenched random interactions. The critical exponents were also evaluated.     

Texture transitions in the liquid crystalline alkyloxybenzoic acid 6OBAC

The 4,n-alkyloxybenzoic acid 6OBAC has a very rich variety of crystalline structures and two nematic sub-phases, characterised by different textures. It is a material belonging to a family of liquid crystals formed by hydrogen bonded molecules, the 4,n-alkyloxybenzoic acids (n indicates the homologue number). The homologues with 7?≤?n?≤?13 display both smectic C and N phases. In spite of the absence of a smectic phase, 6OBAC exhibits two sub-phases with different textures, as it happens in other materials of the homologue series which possess the smectic phase. This is the first material that exhibits a texture transition in a nematic phase directly originating from a crystal phase. Here we present the results of an image processing assisted optical investigation to characterise the textures and the transitions between textures. This processing is necessary to discriminate between crystal modifications and nematic sub-phases.     

The hard ellipsoid-of-revolution fluid I. Monte Carlo simulations

We present the results of Monte Carlo simulations on a system of hard ellipsoids of revolution with length-to-breadth ratios a/b = 3, 2.75, 2, 1.25 and b/a = 3, 2.75, 2, 1.25. We identify four distinct phases, viz. isotropic fluid, nematic fluid, ordered solid and plastic solid. The coexistence points of all first order phase transitions are located by performing absolute free energy computations for all coexisting phases. We find nematic phases only for a/b ≥ 2.75 and a/b ≤ 1/2.75. A plastic solid is only observed for 1.25 ≥ a/b ≥ 0.8. It is found that the phase diagram is surprisingly symmetric under interchange of the major and minor axes of the ellipsoids.     

Peculiarities of biphasic regions at phase transitions in oriented textures of 4-cyano-4′-n-pentylbiphenyl

Peculiarities of the biphasic regions at the direct and reverse phase transitions between homeotropic oriented nematic mesophase and isotropic liquid have been investigated in detail in this work. The shift of phase transition temperatures to the low temperatures by the reverse phase transitions have been observed for this mesophase. Nonlinearity of thermotropic and thermo-optical properties at the phase transitions between nematic mesophase and isotropic liquid have been found.     

Positron lifetimes in five phases of BEA

Positron lifetimes have been measured as a function of temperature in 4-butyloxybenzal-4′-ethylaniline (BEA). BEA has been previously reported to have two liquidcrystalline phases (smectic and nematic) with transition temperatures as follows: solid → smectic, 40.5° C; smectic → nematic, 51.0° C; and nematic → isotropic liquid, 65.5° C. Positron life time spectra were resolved into two components, with the shorter component τ₁ remaining approximately constant in all phases, and τ₂ exhibiting reversible changes at all of the above transitions. In addition, an irreversible discontinuity in the τ₂ lifetime was observed in the vicinity of 28° C, indicating the presence of a new phase (phase X) of BEA. The τ₂ andI ₂ values obtained for the various phases of BEA are: solid (1.25 nsec, 7.1%), phase X (1.97 nsec, 26.2%), smectic (2.36 nsec, 23.6%), nematic (2.72 nsec, 28.3%), and isotropic liquid (2.69 nsec, 29.8%).     

Uniaxial and biaxial spin nematic phases induced by quantum fluctuations

It is shown that zero point quantum fluctuations completely lift the accidental continuous degeneracy that is found in mean field analysis of quantum spin nematic phases of hyperfine spin-2 cold atoms. The result is two distinct ground states which have higher symmetries: a uniaxial spin nematic and a biaxial spin nematic with dihedral symmetry Dih4. There is a novel first-order quantum phase transition between the two phases as atomic scattering lengths are varied. We find that the ground state of 87Rb atoms should be a uniaxial spin nematic. We note that the energy barrier between the phases could be observable in dynamical experiments.     

Texture transitions in binary mixtures of 6OBAC with compounds of its homologous series

Recently the authors have observed in compounds of the 4,n-alkyloxybenzoic acid series, with the homologous index n ranging from 6 to 9, a texture transition in the nematic range which subdivides the nematic phase in two sub-phases displaying different textures in polarised light analysis. To investigate a persistence of texture transitions in nematic phases, we prepared binary mixtures of 4,6-alkyloxybenzoic acid (6OBAC) with other members (7-, 8-, 9-, 12-, 16OBAC) of its homologous series. Binary mixtures exhibit a broadening in the temperature ranges of both smectic and nematic phases. A nematic temperature range of 75°C is observed. In the nematic phase, in spite of the microscopic disorder introduced by mixing two components, the polarised light optics analysis of the liquid crystal cells reveals a texture transition. In the case of the binary mixture of 6OBAC with 12OBAC and with 16OBAC, that is of compounds with monomers of rather different lengths, the texture transition temperature is not homogeneous in the cell, probably due to a local variation in the relative concentrations of compounds.     

双轴性向列相液晶的相变理论

假设一双轴性向列相液晶分子简单相互作用模型，利用平均场理论，得到从各向同性相至单轴向列相、单轴向列相至双轴向列相的相变. 结果表明，在一定温度范围内，分子形状的非轴对称性可导致系统双轴向列相的产生. 在核磁共振实验中，导出双轴向列相序参数与谱线分裂频率间的一般关系，并讨论序参数测量性的可能性. 关键词： 向列相液晶 相变