Nematic-smectic Transition Research Articles

A density functional study of nematic-smectic A-solid transitions of parallel hard spherocylinders

A density functional theory is formulated to study the nematic, smectic A and solid phases and phase transitions of a system of parallel hard spherocylinders (PHSC). For the direct correlation function the analytic expression suggested by Pynn is employed and an effective liquid approach is proposed to calculate the excess free energy of the inhomogeneous (smectic and solid) phases. The nematic-smectic A transition is found to be second order. The first-order nematic-solid and smectic A-solid transitions are calculated. As a function of the elongation of the spherocylinders, the phase behaviour and coexistence data obtained from the theory are in reasonable agreement with the available computer simulation results.

Chemical reactions in liquid crystalline solvents: anti-Arrhenius behavior as a consequence of nematic-smectic A-reentrant nematic phase transitions

Chemical reactions in liquid crystalline solvents: anti-Arrhenius behavior as a consequence of nematic-smectic A-reentrant nematic phase transitions

On temperature calibration of power compensation DSC in cooling mode

Differential scanning calorimeters (DSC) are used to determine transition temperatures on cooling. Often temperature scale corrections are linear extrapolated from temperature calibration on heating. For power compensated DSC this extrapolation is not possible in an easy manner due to the influence of the power compensation control circuits. Therefore it is necessary to calibrate the calorimeter on cooling. A smectic-nematic phase transition can be used for this calibration. For four different power compensated DSC a deviation between temperature scales up to 0.7 K in heating and cooling mode is observed.

Micellar structures in lyotropic liquid crystals and phase transitions

The formation of micellar nematics is discussed with emphasis on the transitions between nematic phases and nematic-smectic transitions. Phase diagrams for MTAB/l-decanol/D,O systems show a direct transition between uniaxial nematics. Electrical conductivity and birefringence measurements on a mixture of sodium decylsulfate. 1-decanol, D,O demonstrate, on the other hand, the existence of a biaxial nemantic range that separates the Uniaxial nematics. On a mixture of cesium perflouroctanoate and H 2O the electrical conductivity and rotational viscosity are used to discuss the relevant features of nematic-lamellar-smectic transitions. The formation of elongated ribbon-like micelles at the nematic-smectic transition is suggested. Transitions between different nematic phases in the MTAB system may be connected with a structural change from long micelles with a fairly circular cross section to similar micelles with a more elliptical cross section.

Relaxation near Smectic-Nematic Transition Temperature in Smectic A Liquid Crystals

Abstract Slightly below the smectic A to nematic phase transition temperature (TSN), relaxation from an electric-field-induced texture to a preferred texture was investigated under various boundary conditions. The initial texture of a sample was formed by applying an ac voltage during the cooling process from the nematic phase. For a compound having positive dielectric anisotropy, relaxation from a homeotropic to planar texture was studied. The observed degree of relaxational texture change is found to depend strongly on cell temperature (T). The capacitance of the cell, which reflects the layer deformation, was also measured during the relaxation process. Several types of focal conics appeared as the layer was deformed. Also the relaxation from a fan to homeotropic texture was studied for a compound having negative dielectric anisotropy. In addition, the time required for the relaxation of the texture was measured and found to increase exponentially with increasing TSN - T.

The observation of a novel biphase behavior in thermotropic side chain polymers

A novel biphasie behavior is detected by thermal analysis and polarizing microscopy in the polyacrylates [CH 2 −(CH(CO 2 R)] n R=(CH 2 ) 6 −O−Ph−N=N−Ph−OR 1 Ri=C 5 H 11 , C 2 H 5 ). Upon annealing these polymers at different temperatures inside the biphasis gap associated to their nematic-isotropic transition, the two coexisting phases separate and afterwards undergo distinct thermal transitions including the isotropic-nematic and nematic-smectic transitions

Monte Carlo Simulation Study of a Smectic-Nematic-like Transition in a Two-dimensional Lattice Gas Model of Cylindrical Particles

Monte Carlo simulation of a two-dimensional lattice gas model of orientable particles is used to study the coupling between the smectic-nematic phase transition and the orientational order parameter in Liquid Crystals. The phase diagram of the model is obtained, and the critical behaviour of the smectic-nematic-like transition studied. Results can be qualitatively compared with experimental data and suggest a microscopical explanation of the continuous variation of the effective critical exponents in some Liquid Crystal mixtures. The existence of a Tricritical Point is also reproduced. Finally, results are analyzed in terms of a general Landau free energy functional (Anisimov et al 1990 Phys. Rev. A 41 6749) that suggests that the Tricritical Point is only apparent.

Nematic Smectic-A Tricritical Points for Nonseparable Interactions

Abstract Relistic interactions in liquid crystals depend not only on the relative orientation of the molecules but also on the orientation of the radius vector connecting molecular centers. The last feature of the interactins is commonly referred to as nonseparability. In this prsentation we address the question how nonseparability influences the nematicsmectic A tricritical behaviour in classical and strongly polar liquid crystals. We study the influence of this effect on the classical nematic-smectic A and the nematic-smectic A2 phase transitions. Detailed calculations are presented for some mean-field free energies.

Nuclear magnetic relaxation study of the nematic-smectic (G) transition

Abstract Members of the homologous series, alkyloxy benzylidene alkylanilines (nO. m series), show rich polymesomorphism and 60.2 has the phase sequence: crystal-smectic (G)-nematic-liquid. Molecular dynamics in this system are investigated using proton spin-lattice relaxation (T 1) at different Larmor frequencies (10 to 30 MHz), and proton dipolar relaxation (T 1D ), as a function of temperature. These results are supplemented by cw measurements at 40 MHz and an angular dependence of T 1 in the nematic phase. The results indicate that diffusion is too fast for the spin probe and order director fluctuations essentially mediate the T 1 process in the nematic phase. The N-S G transition shows dramatic effects on T 1 and T 1D . The change in T 1 near this transition temperature is due to the modifications in order director fluctuations brought about by the formation of layers, whereas the abrupt drop in T 1D reflects the sudden translational order within the layer. The T 1 process does not show any frequency...

Curvature elasticity and rotational viscosity of the nematic decylammonium chloride system

Experimental data are reported on curvature elasticity and rotational viscosity of micellar nematics formed in the decylammonium chloride/NH4Cl/H2O system. Two mixtures of different salt concentrations were studied (DACl:NH4Cl=10:1 and 20:1 by weight). The constants for the bend elasticity and the rotational viscosity were found to vary over nearly three orders of magnitude from 0.2×10−6 to 150×10−6 dyn and from 1 to about 1000 P, respectively. The large range is due to the divergence at the nematic–smectic transition. Both constants diverge with the same exponent. We obtained an exponent of 1.07±0.05 for a weight ratio DACl/NH4Cl of 20 and 0.87±0.02 for a ratio of 10, but an unexpected thermal hysteresis interfered with a reliable determination of the critical properties. Overall, the elastic properties and the divergence is this micellar nematic are similar to those observed in thermotropic nematics.

Synthesis and Mesomorphic Properties of N-n-Alkyl-N-[4-(2-Cyanoethenylphenyl)]-Piperazines

Abstract We have synthesized the N-n-alkyl-N′-[4-(2-cyanoethenylphenyl)]-piperazines and studied their physical properties using calorimetric, optical and X-ray methods. Depending on alkyl chain length there are one or two liquid crystal phases. For the lower homologs there is a nematic and a smectic phase with the smectic being identified as probably smectic A and in the higher homologs only the smectic is present. The smectic phase has an interdigitated bilayer and the transition heat of the nematic-smectic transition becomes very small with decreasing alkyl chain length.

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...

Reentrant transitions in liquid crystals: Theoretical predictions of new phenomena

A new mechanism is proposed for the reentrant nematic-smectic A-nematic transitions seen in several liquid crystals. The mechanism applies in particular to systems with strong dipolar head-group interactions, and involves a subtle interplay between these forces and the Lennard-Jones interactions of the tails. A simple model which embodies the mechanism is developed, and solved exactly. In addition to providing an understanding of the experiments to date, our model makes definite predictions for new types of reentrant phase diagrams.

The electric conductivity of the lamellar smectic, the micellar nematic, and the isotropic micellar solution of ammonium perfluorononanoate in water

We present electric conductivity measurements on a mixture of 25 wt. % ammonium perfluorononanoate in H2O. The system shows a lamellar smectic phase up to 26.3 °C, followed by a nematic phase consisting of disk-like aggregates, which at 34.2 °C transforms to an isotropic micellar solution. The relative anisotropy of the conductivity increases from ∼0.07 at 34 °C to ∼0.29 at 26.3 °C. There is no discontinuity, but a small change in slope occurs at the nematic–smectic transition. The experimental results, interpreted in terms of structural models for the two anisotropic phases, shows that the aggregates in the nematic phase have diameter to thickness ratios, decreasing from about 8.3 at the lamellar to nematic transition, to about 4.0 at the transition to the isotropic phase. We estimate that about 30% of the total area are disrupted by holes or channel-like defects. The results are in agreement with previous studies on a mixture of decylammonium chloride/NH4Cl/H2O. It is also indicated that increasing the size of the aggregates increases the binding of the counterions to the aggregates.

Magnetic Anisotropy and Latent Heat of Some Smectic Liquid Crystals

The magnetic anisotropy of 4'-Methoxybenzylidene-4-aminocinnamic Acid Ethyl Ester, 4'-Methoxybenzylidene-4-aminocinnamic Acid n-Butyl Ester and 4'-n-Butoxybenzylidene-4-aminoacetophenone was measured in the smectic, nematic and isotropic phases. The magnetic anisotropy of their liquid crystals depends on the strength of magnetic field in the smectic phase. It suggests that the domains form in the smectic phase. The order parameter in the smectic and nematic phases was determined by the measurement of magnetic susceptibility. The transition entropy of their liquid crystals at the phase transitions was obtained from the thermal analysis. Their nematic-smectic transitions correspond to the behaviour of the first order transition predicted from the McMillan theory.

On the Even Odd Effect in Smectic-Nematic Transitions

Abstract An extension of an earlier work by the present authors on the even-odd effect in smectic A-nematic (S-N) transition is presented. The earlier model showed too large an alternation in TSN. In the present work the mean field is modified by including the contribution from the orientation independent term of the Kobayashi pair potential. Numerical calculations on three homologous series with alkyl chains, namely 4-(4′-n-alkoxybenzylideneamino) biphenyls, (p'-n-alkoxybenzoyloxy) acetophenones and PAA show a marked diminution in alternation of TSN. The present mean field, though a definite improvement over the earlier one, is still found to be inadequate quantitatively, especially for samples with very small or virtually nonexistent alternation in TSN. It seems that a purely attractive pair potential may not be adequate in explaining this aspect of S-N transition. A possible role of repulsive interaction in suppressing this ‘even-odd’ amplitude is discussed.

A Molecular Statistical Theory of Smectic A Liquid Crystalline Mixtures: Thermal Stability of Smectic A Phase in Binary Mixtures

A molecular statistical theory of smectic A liquid crystalline mixtures is presented. Both the intermolecular repulsions and the attractions are consistently introduced in similar to the previous work on smectic A liquid crystals. 1) The repulsive interaction is approximated in terms of the hard-core potential and is taken into account within the second-virial approximation. The temperature dependence of the order parameters over the smectic-nematic (or isotropic) and nematic-isotropic phase transitions is numerically calculated for several binary mixtures. In addition the phase diagrams of them are constructed. The smectic-nematic (or isotropic) transition temperatures in binary mixtures tend to be increased in comparison with the predicted values in terms of the linear approximation, being consistent with some experimental findings. It is also found that the symmetry-breaking component of one-body pseudo-potential contributes to the thermal stability of smectic A phase in smectic A binary mixtures.

Density and Susceptibility Measurements on a Micellar Nematic System

Abstract The densities and diamagnetic susceptibilities in the nematic and isotropic ranges of two mixtures of decylammonium chloride, ammonium chloride, and water are reported. The mixtures have a lamellar smectic phase, a nematic phase, and an isotropic micellar solution. The smectic-nematic transition is second order, the nematic-isotropic transition first order. The density does not show a discontinuous change at the nematicisotropic transition, and the expansion coefficients of the nematic and isotropic regions are the same within error limits. The diamagnetic susceptibility in the isotropic range and the susceptibility perpendicular to the optical axis in the nematic range were determined using the Gouy method. The diamagnetic anisotropy of the nematic range was derived. It varies from 2 X10−9 near the nematic-isotropic transition to 5 × 10−9 cgs near the nematic-smectic transition.

Smectic-nematic transition in a lyotropic liquid crystal

Etude sur le systeme ternaire chlorure de decylammonium, eau, chlorure d'ammonium des variations de structure et d'ordre apparaissant quand on passe de la phase Nd a la phase Lα en fonction de la temperature. Utilisation de la diffraction RX et de la spectrometrie RMN

Emergence of Smectic Mesophase in Binary Mixtures of Pure Nematogens

Abstract Binary mixtures both components of which are liquid crystals are known to form continuous mixed liquid crystals.1 Most of them do not undergo any change in their mesomorphic textures, though a few have been found to give rise to different textures.2 We report here a binary system consisting of (A) p-nitrophenyl-p'-n-amyloxycinnamate (90.5–120.5°) and (B) p-methoxyphenyl-p'-n-heptyloxycinnamate (91.0–120.5°) which transforms the original nematic texture into a mixed smectic A texture over a range of temperature and concentration. Both components individually are nematic liquid crystals. In their binary mixtures, a two dimensional more ordered mixed Smectic A mesophase emerges between 5.0–92.5 mol% of p-methoxyphenyl-p'-n-heptyloxycinnamate over a temperature range of about 50–105° at its maximum. The mixed smectic-nematic transition curve shows a marked rounded concavity, extrapolation of which on either side yields values of latent transition temperatures 77.0° and 66.5° for the components A and ...