Liquid crystal binary mixtures of 8OCB + 10OCB. Evidence for a smectic A-to-nematic tricritical point

The two-component system octyloxycyanobiphenyl (8OCB) + decyloxycyanobiphenil (10OCB) has been studied by means of modulated differential scanning calorimetry as well as optical microscopy. The general trends of the phase diagram are similar to the two-component system octylcyanobiphenyl (8CB) + decylcyanobiphenil (10CB), previously published. Evidence for the existence of a TCP have been reported, the molar composition being about 0.33 of 10OCB. Additionally, the smectic mesophase of the 8OCB + 10OCB mixtures has been unmistakably characterized through optical measurements as smectic A for the whole composition range.     

Surface anchoring of nematic liquid crystal 8OCB on a DMOAP-silanated glass surface

Dynamic light scattering spectroscopy has been used to determine the temperature dependence of the anchoring strength of the nematic liquid crystal 8OCB on DMOAP-silanated glass surfaces inducing homeotropic alignment. Wedge-type glass cells with known thickness profile starting from 150 nm to several microns have been used in the experiments. The relaxation rates of the nematic fluctuations with the wave vector perpendicular to the confining surfaces have been measured as a function of the cell thickness. Fitting of the thickness dependence of the relaxation rate allows for straightforward determination of the surface extrapolation length and therefore also the strength of the surface anchoring, which is 1×10^-4 J m^-2. The overall experimental accuracy of the experiments is discussed.     

Phase diagram of a two-frequency chiral nematic mixture in an electric field

The behaviour of a chiral nematic mixture with low frequency dielectric relaxtion was studied between two electrodes, with homeotropic anchoring, in the presence of an AC electric field E. The phase diagram in the parameter plane (d/p, E) was determined for this mixture which could be dielectrically negative or positive in different frequency ranges. The determination was carried out by changing the sample thickness d and both the amplitude and frequency of the electric field at a constant cholesteric pitch p.     

Phase diagram investigation and thermodynamic study of Os-B system

The constitution of the Os-B system was investigated in this work. The phase regions were determined by metallographic investigations. The existence of the following borides was confirmed: OsB_1.1 (hex.), Os₂B₃ (hex.) and OsB₂ (orthorh.), while the new high temperature Os₂B₃ compound with a not yet identified structure was observed. OsB_1.1 melts congruently at 1820±15°C and OsB₂ at 1870±15°C, while the solid solubility of B in Os was found to be less than 0.5 at%B. Considering thermodynamic study of this system, the activities and activity coefficients for osmium and boron were determined in temperature interval 3300-3500 K, based on the known liquidus and solidus lines from phase diagram, according to calculation procedure given by Rao and Belton. This revised version was published online in July 2006 with corrections to the Cover Date.     

Phase diagram of a two-dimensional system with anomalous liquid properties

Using Monte Carlo simulation techniques, we calculate the phase diagram for a square-shoulder square-well potential in two dimensions that has been previously shown to exhibit liquid anomalies consistent with a metastable liquid-liquid critical point. We consider the liquid, gas, and five crystal phases, and find that all the melting lines are first order, despite a small range of metastability. One melting line exhibits a temperature maximum, as well as a pressure maximum that implies inverse melting over a small range in pressure.     

Phase diagram and electro-optical Kerr effect of nematic mixtures containingtolane and biphenyl cores

An experimental phase diagram for two types of nematic liquid crystals and their binary mixtures was established by polarised optical microscopy and differential scanning calorimetry methods. The mixtures comprised a tolane-base liquid crystal, 4-heptyl-3-fluoro-4-isothiocyanatotolane (7TOLF), and a biphenyl-base nematogen, 4-heptyl-3-fluoro-4-isothiocyanatobiphenyl (7BF). The static Kerr effect and third-order non-linearity were investigated for 7TOLF and 7BF and their nematic mixtures within the isotropic phase. Both the compounds have a positive and large Kerr constant. The second-order phase-transition temperatures, T?, were determined for these mixtures. The linear dependence of (T - T?)^??1 on the Kerr constant is found to be in good agreement with the predications of the Landau–de Gennes model. The third-order non-linear susceptibility,χ⁽³⁾, values were determined for these mixtures.     

Determination of elastic constants of a binary system (7CPB+9.CN) showing nematic,induced smectic Ad and re-entrant nematic phases

The temperature variation of the splay and bend elastic constants of a binary system exhibiting nematic-induced smectic A_d and re-entrant nematic phases measured by electric field-induced Freedericksz transition method has been reported. As bend deformation is not permitted in the smectic A phase, bend elastic constant (K₃₃) could only be determined in the nematic and re-entrant nematic phases. In both the nematic phases, the splay elastic constant has the same order of magnitude and does not show any pretransitional effect. However, in the induced smectic A_d phase, the value of K₁₁ is about one to two orders higher than that in the nematic phases. The bend elastic constant shows a strong pretransitional effect near the nematic–smectic and smectic–re-entrant nematic phase transitions. The influence of the presence of the induced smectic phase is observed even in those mixtures which have no induced smectic phases. As the smectic phase is approached, the ratio K₃₃/K₁₁ increases rapidly and diverges to infinity.     

Discrimination in isotropic, nematic, and smectic phases of chiral calamitic molecules: a computer simulation study

Racemic fluids of chiral calamitic molecules are investigated with molecular dynamics simulations. In particular, the phase behavior as a function of density is examined for eight racemates. The relationship between chiral discrimination and orientational order in the phase is explored. We find that the transition from the isotropic phase to a liquid crystal phase is accompanied by an increase in chiral discrimination, as measured by differences in radial distributions. Among ordered phases, discrimination is largest for smectic phases with a significant preference for heterochiral contact within the layers.     

Stability of nematic and smectic phases in rod-like mesogens with orientation-dependent attractive interactions

The stability of isotropic (I), nematic (N), smectic A (Sm A), and hexatic (Hex) liquid crystalline phases is studied for a fluid of molecules with a rod-like shape and dispersive interactions dependent on orientation. The fluid is modeled with the spherocylindrical Gay-Berne-Kihara interaction potential proposed in a recent work, with parameters favoring parallel pair orientations. The liquid crystal phase diagram is characterized for different molecular aspect ratios by means of Monte Carlo simulations in the isobaric-isothermal ensemble. Three types of triple points are observed, namely, I-Sm A-Hex, I-N-Sm A, and N-Sm A-Hex, leading to island-shape domains for the smectic A phase. The resulting phase diagrams are compared with those derived previously for prolate fluids of ellipsoidal and spherocylindrical symmetry. It is concluded that the stability of the layered phases with respect to the nematic phase is enhanced in the spherocylindrical fluids due to geometrical constraints. Furthermore, the anisotropy of the dispersive interactions induces a stronger dependence of the overall phase diagram on temperature and aids in the energetic stabilization of the hexatic crystalline phase with respect to the fluid smectic A phase.     

Phase diagram and universality of the Lennard-Jones gas-liquid system

The gas-liquid phase transition of the three-dimensional Lennard-Jones particles system is studied by molecular dynamics simulations. The gas and liquid densities in the coexisting state are determined with high accuracy. The critical point is determined by the block density analysis of the Binder parameter with the aid of the law of rectilinear diameter. From the critical behavior of the gas-liquid coexisting density, the critical exponent of the order parameter is estimated to be β = 0.3285(7). Surface tension is estimated from interface broadening behavior due to capillary waves. From the critical behavior of the surface tension, the critical exponent of the correlation length is estimated to be ν = 0.63(4). The obtained values of β and ν are consistent with those of the Ising universality class.     

Phase diagram of a tetrapropylammonium bromide-water system according to DTA data

The phase diagram of a tetrapropylammonium bromide-water binary system is studied by means of differential thermal analysis. Three tetrapropylammonium bromide hydrates are identified that include one stable compound of 1: 4 composition (bromide: water; mp, ?32.6°C) and two metastable hydrates of 1: 6 (mp, ?70.4°C) and 1: 24 compositions (mp, ?57.0°C).     

Phase diagram of a system of adipic,glutaric, and sebacic acids

Adipic acid–glutaric acid, glutaric acid–sebacic acid, and adipic acid–sebacic acid binary systems are studied, along with an adipic acid–glutaric acid–sebacic acid ternary system. It is shown all of these systems are eutectic. Phase equilibria for the diagram elements of the binary systems and the ternary system are described. It is concluded that the above low-melting compounds can be recommended for use as working bodies in heat accumulators, and for preparing electrolytes used in the thin-layer anodic oxidation of aluminum alloys.     

Investigation of orientational behaviour of nematic and smectic a compounds using the Raman microprobe

The Raman microprobe technique, has been used to probe orientational order in oriented samples of nematic CB₅ and nematic and smectic CB₈ versus spatial localization of the analysed microvolume. In the nematic state, the influence of long-wavelength director fluctuations is pointed out and an elastic constant derived.     

Chiral nematic phase in hydrogen-bonded blends of a side-chain smectic polymer with low molar mass dopants

The possibility of chiral nematic mesophase induction in blends of a smectic A side-chain liquid-crystalline copolymer with low molar mass dopants was studied. The chirality of the initial copolymer was determined by the cholesterol optical active groups; however, in the individual state it was not able to form any chiral liquid-crystalline phases. We have shown that the induction of the chiral nematic phase becomes possible in blends of such a copolymer with low molar mass dopants that are stabilized by hydrogen bonding. Phase behavior and optical properties of the blends were studied with X-ray scattering, differential scanning calorimetry and polarizing microscopy. Owing to hydrogen bonding the blends are stable over a wide range of contents and temperatures. The nature of the end group in the dopant molecules is shown to have an important influence on the chiral mesophase induction concentration and the clearing temperatures of the blends. Temperature and concentration dependences of the selective reflection maximum wavelength in the chiral nematic phase were also studied.

The thermally sensitive characteristics of a (smectic LCP/ nematic LC/chiral dopant) ternary composite system

A transition between the transparent smectic A (SmA) phase and the light scattering chiral nematic (N*) phase was realized based on the thermally induced SmA N* phase transition for the homeotropically aligned \[liquid crystalline polymer (LCP)/liquid crystal (LC)/chiral dopant] ternary composite system. The LCP played an important role in increasing the intensity of the light scattering of the heat-induced N* phase. Meanwhile the effects of the composition of the ternary composite system on the thermo-optical characteristics were also investigated.     

Magnetic effects and viscosity in the nematic phase of a polymer series

Magnetic measurements were made on a series of polyesters in their nematic state and compared to their apparent viscosity at the same temperature. At similar molecular weights, the length of a flexible segment in the main-chain polymer plays an important role in both the magnetic orientation effects and the apparent viscosity behavior. Moreover, on cooling from a perfectly oriented nematic sample, the alignment remains down to room temperature. In contrast with nematic monomers, these polymers behave like solids, even at the lowest velocity of a rotating magnetic field.     

Phase diagram and electrical conductivity of TbBr3-NaBr binary system

Several experimental techniques were used to characterise the physicochemical properties of the TbBr₃-NaBr system. The phase diagram determined by DSC, exhibits an eutectic and a Na₃TbBr₆ stoichiometric compound that decomposes peritectically (759 K) shortly after a solid-solid phase transition (745 K). The eutectic composition, x(TbBr₃)=39.5 mol%, was obtained from the Tamman method. This mixture melts at 699 K. With the corresponding enthalpy of about 16.1 kJ mol^-1. Diffuse reflectance spectra of the pure components and their solid mixtures (after homogenisation in the liquid state) confirmed the existence of new phase exhibiting its own spectral characteristics, which may be possibly related to the formation of Na₃TbBr₆ in this system. Additionally, the electrical conductivity of TbBr₃-NaBr liquid mixtures was measured down to temperatures below solidification over the whole composition range. This revised version was published online in July 2006 with corrections to the Cover Date.     

A model p-T diagram for a ternary system with two semi-volatile and one highly volatile components

A model p-T diagram for a three-component system composed of two semi-volatile and one highly volatile components is developed. The phase equilibria and phase transitions are analyzed over a wide range of variable parameters. Isobaric T-x-y diagrams typical of real metal and salt systems with isothermal and polythermal sections are considered.     

Thermodynamics of dimerization in solution as a rational tool for inducing nematic vs. smectic organizations in lanthanidomesogens

The judicious tuning of simple thermodynamic parameters controlling dimerization processes in solution induces a rational switch between smectic and nematic organizations in thermotropic lanthanide-containing liquid crystals.