Binary Liquid Crystal Mixtures Research Articles

X-ray diffraction study of the smectic-A-tilde fluid antiphase and its transitions to smectic-A1 and smectic-A2 phases.

A high-resolution x-ray diffraction study has been carried out on a binary liquid crystal mixture of pentylphenylcyanobenzoyloxy benzoate (${\mathrm{DB}}_{5}$CN) and cyanobenzoyloxypentyl stilbene (${\mathrm{C}}_{5}$ stilbene) that is 49.5 mole % ${\mathrm{C}}_{5}$ stilbene. This mixture exhibits on cooling the phase sequence monolayer smectic-A (${\mathrm{SmA}}_{1}$)--smectic-A fluid antiphase (Sm\~A)--bilayer smectic-A (${\mathrm{SmA}}_{2}$). We observe the presence of a broad coexistence region between the Sm\~A phase and the ${\mathrm{SmA}}_{2}$ phase, contrary to previous work which reported a smectic-A crenelated phase. The behavior of the smectic-A fluid antiphase ordering and the evolution of the system to Sm${\mathit{A}}_{2}$ order via a two-phase coexistence region are described in detail. Our results are compatible with previous high-resolution heat capacity measurements on the same sample.

Rotational Viscosity (γ1) Study of Nematic Phases in Binary Liquid Crystal Mixtures of 60CB and 8OCB

Abstract Rotational viscosity coefficient is measured in different nematic phases of a binary liquid crystal system 6OCB-8OCB, as a function of relative concentration of the constituents, and of temperature at each concentration, using a transient NMR technique. Experiments are performed on four such mixtures with relative concentrations chosen such that for two of them there is a re-entrant nematic phase with an intervening smectic A phase (high temperature (HN) and re-entrant (RN) nematic phases), whereas for the other two there is only a single nematic phase (SN). Variation of γ1 with concentration and temperature is distinct in the HN and RN phases, while the corresponding behaviour in the SN phase is qualitatively the same as that in the RN phase. The variation of activation energies in all the phases as a function of the relative concentration further confirms this observation. It is suggested that the dynamics in HN phase is influenced by the smectic order fluctuations remnant in the system due to ...

Scattering from mixtures of flexible and stiff polymers

The random phase approximation (RPA) is used to calculate the scattering function for multicomponent mixtures of flexible and stiff polymers. These calculations will be useful for the qualitative interpretation of small angle neutron scattering (SANS) data from binary mixtures of polymer liquid crystals and flexible polymers in the isotropic phase region. They can predict the spinodal line as well as the isotropic-to-nematic phase transition line. Depending on the volume fraction of the rigid polymer and on the relative values of the Flory–Huggins and Maier–Saupe interaction parameters, either the spinodal line or the isotropic-to-nematic phase transition is reached first.

Nematic-smectic a tricritical phase transition in binary mixture of liquid crystals

The nematic-smectic A tricritical phase transition has been studied in binary mixtures of liquid crystals. By making the analogy between the “quasi-binary” fluid mixture and binary mixture of liquid crystals, we adapt the thermodynamic model for fluid mixtures due to Griffiths to the case of mixtures of liquid crystals. In this study we obtained the temperature dependences of the smectic order parameter, nematic susceptibility and the specific beat discontinuity near the tricritical phase transition region of the binary mixture of liquid crystals. We then predicted the tricritical exponents for these quantities, which give the predictions of the non symmetric systems. Our prediction compare well with some experimental data for the binary mixtures of liquid crystals. This shows that the binary mixture of liquid crystals exhibiting NA phase transition can be described satisfactorily by the mean field model.

Microscopic theory of anchoring transitions at the surfaces of pure liquid crystals and their mixtures. I. The Fowler approximation

We have generalized earlier work on anchoring of nematic liquid crystals by Sullivan, and Sluckin and Poniewierski, in order to study transitions which may occur in binary mixtures of nematic liquid crystals as a function of composition. Microscopic expressions have been obtained for the anchoring energy of (i) a liquid crystal in contact with a solid aligning surface; (ii) a liquid crystal in contact with an immiscible isotropic medium; (iii) a liquid crystal mixture in contact with a solid aligning surface. For (iii), possible phase diagrams of anchoring angle versus dopant concentration have been calculated using a simple liquid crystal model. These exhibit some interesting features including re-entrant conical anchoring, for what are believed to be realistic values of the molecular parameters. A way of relaxing the most drastic approximation implicit in the above approach is also briefly discussed.

Deuterium spin relaxation and molecular motion in a binary liquid-crystal mixture

Deuterium spin relaxation has been studied as a function of temperature, concentration, and site of deuteration for both components of a binary nematic liquid-crystal mixture of 4-methyl-4′-cyanobiphenyl- d 11 (1CB) and 4- n-pentyl-4′-cyanobiphenyl- d 6 (5CB). Relaxation rates of Zeeman order R 1Z and quadrupolar order R 1Q were determined by two different techniques. It was found that the standard method, based on sum and difference (SAD) magnetization generated by a Jeener-Broekaert sequence, suffers from serious systematic error due to frequency-selective excitation, large linear phase corrections, and unavoidable baseline distortion. Broadband Jeener-Broekaert (BBJB) sequences can be used to achieve uniform excitation of quadrupole order, while echo techniques can be used both with inversion-recovery (IR) and BBJB sequences to eliminate baseline artifacts. This new approach of performing two separate experiments (IR/BBJB) yields spectral densities J 1( ω 0) and J 2(2 ω 0) with precision and accuracy significantly greater than those determined by the SAD method. For 1CB, the SAD results were not accurate enough for detailed analysis but a satisfactory interpretation of the temperature-dependent IR/BBJB data for 1 CB methyl and aromatic deuterons was achieved in terms of a model with three correlation times. The same model was successfully used to account for relaxation of the alkyl chain C, deuterons in the liquid-crystal molecule 5CB. Measurements on 10 and 25 mol% mixtures showed that the correlation times and activation energies for rotational motion of both 1CB and 5CB are concentration independent.

Enthalpies of Mixing in Binary Systems of Nematic Liquid Crystals

Abstract The mixing enthalpies of binary mixtures of nematic liquid crystals are studied using a Calvet calorimeter. The systems studied are: 4-ethoxybenzylidene-4′-n-butylaniline-4-n-pentylphenyl-4′-methoxybenzoate and 4-n-butyl-4′-methoxyazoxybenzene-4-n-pentyl-4′-cyanobiphenyl. In the second system the induced smectic A phase occurs in the medium concentration range. The results are discussed in terms of molecular properties and interactions.

Weak Crystalization in Liquid Crystals: Experimental Evidence

Abstract The heat capacity in the isotropic phase and the latent heat on the IN transition line have been measured in binary liquid crystal mixtures by high-resolution adiabatic calorimetry. It shown that an anomalous behaviour of the entropy jump on the IN phase transition line in the vicinity of the INA triple point is defined only by an particular behaviour of the crystallization fluctuations in the isotropic phase. The contribution of the crystallization fluctuations to the heat capacity is comparable with the pretransitional heat capacity of the nematic fluctuations in the isotropic phase.

Mean-field theory of binary mixtures of nematic liquid crystals consisting of biaxial molecules.

Binary mixtures of liquid crystals whose constituent molecules are not cylindrically symmetric are considered. For a mixture consisting of biaxial molecules with ${\mathit{D}}_{2\mathit{h}}$ symmetry, two tensor order parameters are required to specify the orientational order of each component. Using mean-field theory, expressions for the free energy of the mixture and the orientational order parameters are obtained as functions of temperature. In the geometric-mean approximation, the parameters of the theory are ratios of anisotropic interaction strengths, which can be determined from measurements on the pure components. The theoretical predictions are compared with recent experimental results of $^{2}\mathrm{NMR}$ measurements on the binary mixture of the liquid-crystal 2-fluorenyl-4'-tetradecyloxy-benzoate-${\mathit{d}}_{9}$ (${\mathrm{FLOC}}_{14}$) and p-xylene-${\mathit{d}}_{10}$ (p-Xy).

Dielectric relaxation of a nematic eutectic mixture with a high glass temperature

Abstract Dielectric relaxation measurements in the frequency range from 2 Hz to 100 kHz have been carried out on an oriented binary nematic mixture of low molecular liquid crystals with a glass temperature of 307 K. For both relaxation frequencies, f R‖ and f R⊥, a glass behaviour according to the Vogel-Fulcher law could be detected. The Cole-Cole distribution parameter for reorientation around the long molecular axis increases strongly with decreasing temperature. This effect is discussed with regard to the optical biaxiality of the mixture.

129Xe NMR in Liquid Crystals: Detection of Induced Smectic Phases in Binary Mixtures of Nematic Liquid Crystals by Use of Xenon Gas as a Probe

The deuteron quadrupole splitting of d 2 -EBBA and the nuclear shielding of the 129 Xe isotope of xenon gas dissolved in various binary mixtures of the nematic liquid crystals ZLI1132 and d 2 -EBBA and in the mixture of S1114 (16.8 wt percent) and d 2 -EBBA (83.2) were studied over the temperature range from 360 K to 280 K. The 129 Xe shielding is found particularly sensitive to phase transitions, and consequently, the 129 Xe NMR spectroscopy can be utilized in determining phase diagrams of liquid crystals. In the cases studied, I-N, N-S A (induced) and S A (induced) - S B (induced phase transitions have been detected, and the phase diagram of the ZLI1132/d 2 -EBBA system is reported

Molecular rotational potentials in the SC* phase of binary ferroelectric liquid crystal mixtures

Abstract The spontaneous polarization and tilt angle have been measured as a function of temperature and concentration in the SC* phase of four binary ferroelectric liquid crystal mixtures. The substances have been chosen in a way that different situations could be studied, for example mixtures of a chiral substance (P s non-zero) with a non-chiral one (P s = 0) and mixtures of chiral compounds with the same sign of P s as well as with opposite signs. The results have been interpreted by means of a molecular rotational model and potentials associated with the tilt as well as with the chirality have been determined as a function of concentration.

Twist viscosity and splay elastic constant near the nematic-smectic-A-smectic-C multicritical point.

The twist viscosity has been measured in a binary liquid-crystal mixture above the nematic--smectic-C transition near the nematic--smectic-A--smectic-C multicritical point using the dynamic Fr\'eedericksz deformation and found to exhibit a monotonic divergence. This study also shows that the splay elastic constant has a relatively mild temperature dependence compared with that of the bend elastic constant.

THE NEMATIC-SMECTIC-A-SMECTIC-C MULTICRITICAL POINT IN BINARY LIQUID-CRYSTAL MIXTURES

Properties near the nematic-smectic-A-smectic-C multicritical point present in binary liquid-crystal mixtures are reviewed. Recent dynamic Fréedericksz deformation and quasielastic light-scattering results are described and their relationship with data from X-ray scattering and static light scattering is discussed.

High pressure study on induced SAphases in binary liquid crystal mixtures

Abstract Binary mixtures of terminal polar and non-polar liquid crystals exhibiting induced smectic phases are studied under high pressure. For terminal polar compounds with smectic phases, there are two types of T, x phase diagrams known up to now. Diagrams with a nematic gap between the induced phase and the smectic phase of the terminal polar compound and diagrams with an uninterrupted miscibility of the smectic phases. We find a continuous transformation between these phase diagrams with pressure. At a certain pressure, the phase transition lines form a cross separating two nematic and two smectic phases.

Electro-Optical Studies on Monotropic Nematic-Smectic Phase Transitions

Abstract The formation of nematic phases which are monotropic either with respect to a smectic A or to a smectic B phase was shown to be possible both for binary mixtures of low molar mass liquid crystals as well as for unmixed systems. The existence of these monotropic states was demonstrated experimentally by means of electro-optical Kerr effect studies. The phase diagrams of such systems are characterized by the fact that the transition temperatures vary continuously in mixtures and in homologeous series as a function of the composition or of the chemical structure, respectively. This holds even if the nematic phase becomes monotropic with respect to a smectic phase.

On the nature of the biphasic region in binary nematic liquid crystal mixtures

The biphasic region, in which the nematic and isotropic phases can coexist, in binary mixtures of p-azoxyanisole (PAA) and 4,4′-di- n-hexyloxyazoxybenzene (PHAB) is studied by dilatometric measurements. It is seen that in 0.1 PAA/0.9 PHAB and in 0.7 PAA/0.3 PHAB mixtures that the densities in the biphasic regions follow a linear temperature dependence. This behavior is interpretated as being evidence that the relative fraction of coexisting isotropic phase decreases linear with decreasing temperature.

Binary Liquid Crystal Systems with Two Eutectics

Abstract Some binary liquid crystal mixtures based on phenyl-pyrimidine derivatives are investigated. Some of these derivatives are new and were found to have SmC or SmC* phases. They exhibit unusual binary phase diagrams like the existence of two eutectics and induced smectic A and smectic C phases. In some cases the melting point at intermediate concentrations is considerably higher than that of the two single substances. This indicates that a new compound is formed at about equal concentrations of the two substances. A possible explanation, the formation of an EDA complex, is discussed.

Microscopic Description of Nematic-Smectic A2 and Nematic-Smectic A1-Smectic A2 Phase Transitions in Binary Mixture of Polar and Nonpolar Liquid Crystals

Abstract In the present paper, in a mean field approximation, we considered binary mixtures of polar and nonpolar liquid crystals (LC) with ideal orientational order. It is assumed that nonpolar molecules can form only monolayer smectic A-structure whereas polar molecules can form both monolayer smectic A-structure with short-range antiferroelectric order and bilayer smectic A-structure with long-range antiferroelectric order. By analogy with McMillan's theory the self-consistent equations determining order parameters for these smectic A structures are introduced. It is shown that smectic A1 phase with monolayer smectic A structure and smectic A2 phase with coexisting monolayer and bilayer smectic-A structures can ocur in the mixture. Phase diagram for mixture and entropy discontinuities at the nematic-smectic A1 and smectic A1-smectic A2 phase transitions obtained by numerical solution of self-consistent equations are similar to experimental ones. Also the temperature and concentration dependences of int...

ChemInform Abstract: Simple Model of Induced Smectic Phases in Binary Mixtures of Liquid Crystals

ChemInform Abstract: Simple Model of Induced Smectic Phases in Binary Mixtures of Liquid Crystals