Nematic-smectic Phase Research Articles

Influence of structure of two ring ester on the induction of smectic Ad Phase in bicomponent mixtures containing a compound with virtual Ad phase

Mixtures of 4-cyano-4′-hydroxyazobenzene 4-n-octyloxybenzoate (8OBCAB) - constant compound and 4′-X-phenyl 4-n-decylbenzoate or 4′-X-phenyl 4-n-decyloxybenzoate (10.X, 10.O.X) - the variable compound in which X refers to the terminal group CN, NO2, NCS, COCH3, Cl or F, were studied by thermomicroscopic and X-ray methods. The influence of the terminal groups on the kind of phases and their miscibility was observed. The enhancement of the smectic Ad phase together with the appearance of the nematic gap between Ad and A1 smectic phases and re-entrant nematic phase were observed in mixtures with high polar compounds (-CN, -NO2). Complete miscibility together with an increase of smectic A phase persistence were observed in mixtures with medium polar compounds (-NCS, -COCH3). Additive miscibility was observed in mixtures with low polar compounds (-Cl, F).

Thermotropic Liquid Crystalline Glucose Derivatives with Mesogenic side Chains

Abstract Many glucose derivatives have been studied so far for their liquid crystalline behavior. Penta-O-n-alkanoylglucopyranoses have been reported to form discotic columnar mesophases. Thermotropic liquid crystals of glucose with mesogenic side chains, consisting of a mesogen, an alkyl spacer and a tail, form the subject of this study. The glucose derivatives were prepared by complete esterification of the hydroxyl groups of glucose with the mesogenic side chains. Their characterization was based on the analysis from cross polarizing microscopy, differential thermal calorimetery, wide-angle X-ray diffraction pattern and nuclear magnetic resonance (both 13C and 1H) spectra. Smectic and chiral nematic (cholesteric) phases have been observed in these macromolecules. The effect on the phase behavior of varying the tail length in the side chains of the molecule was studied. The structural arrangement of such molecules has been proposed based on the observations. These are reported here as the first such obs...

Evidence for a discotic smectic-nematic phase induced in Langmuir-Blodgett films.

Evidence for a discotic smectic-nematic phase induced in Langmuir-Blodgett films.

Dislocation loops in finite systems.

The distortions induced by dislocation loops are studied in finite systems. The influence of finite size, D, of the system, surface tension, and surface-bending elastic constant on the nematic-smectic (NA) phase transition is discussed. In thin freely suspended smectic liquid-crystal films the nematic-smectic transition temperature, ${\mathit{T}}_{\mathrm{NA}}$(D), is proportional to 1/ \ensuremath{\surd}D , providing the transition is initiated by the unbinding of the dislocation loops. The smectic phase is stabilized in films, i.e., ${\mathit{T}}_{\mathrm{NA}}$(D)g${\mathit{T}}_{\mathrm{NA}}$(\ensuremath{\infty}). The loop-unbinding mechanism for the NA transition is completely suppressed in films sandwiched between solid boundaries, because the distortion energy in this case is proportional to the area of the loop, while the loop configurational entropy is proportional to its length. For temperatures T\ensuremath{\ge}${\mathit{T}}_{\mathrm{NA}}$(\ensuremath{\infty}) the equilibrium size of the loop, ${\mathit{R}}_{\mathrm{eq}}$, is proportional to the distance between solid boundaries, D. It could grow to infinity only for D\ensuremath{\rightarrow}\ensuremath{\infty} and T\ensuremath{\ge}${\mathit{T}}_{\mathrm{NA}}$(\ensuremath{\infty}).

The influence of the liquid crystalline core geometry on the mesogenicity of novel chiral 2-(4-substituted-phenoxy)propanonitriles

Abstract The synthesis and characterization of seven novel (R)-2-(4-substituted-phenoxy)propanonitriles are described. The propanonitriles were prepared to evaluate their potential use as thermochromics and ferroelectric dopants, as well as to determine their twist sense properties. The materials exhibit smectic and chiral nematic phases of high thermal stability; the mesogenic behaviour of the nitriles is directly related to the type of two-ring core unit employed. The effects of the different molecular geometries and polarizabilities of the liquid crystalline cores on mesophase stability are discussed, particularly in relation to other members of this series. The chiral nematic phase of the propanonitriles is assigned as having a left-handed twist sense from contact preparation studies, and this is in agreement with rules relating absolute configuration and molecular structure to helical twist sense.

Dielectric absorption in polymer-liquid crystal systems

Abstract Dielectric studies of multiphase systems of polymer-ceramic type1 were extended by us to polymer-liquid crystal composites. The results of the investigation include: —separation of dielectric dispersion whose origin is the polymer matrix —demonstration that the temperature of dielectric anomaly in the smectic-nematic phase increases with frequency, as in the case of clustering —proving that the properties of the composite polymer-LC are not additive with respect to the properties of its components.

Orientational Order Parameters of Binary Mixture 80CB + 60CB Showing Re-entrant Nematic Phase

Abstract Orientational order parameters have been determined for the mixture of hexyloxy cyanobiphenyl (60CB) and octyloxy cyanobiphenyl (80CB) for three different compositions containing molar concentration 0.289, 0.337 and 0.401 of 60CB over the complete mesomorphic ranges. Both x-ray diffraction and refractive index measurements on aligned samples have been used for order parameter calculations. The experimental order parameter values show a small enhancement with decreasing temperature in the smectic phase, but decrease on entering the re-entrant nematic phase. Smectic fluctuations were observed even in the nematic phase of the mixtures not having any smectic or re-entrant nematic phase. Previous order parameter values obtained from NMR studies are much smaller than ours. However, since this NMR measurement used a rather small molecule (p-xylene) as a probe, the ordering of these probe molecules may be less than that of the liquid crystalline mesophase studied.

Is the smectic-nematic transition in 8CB first or second order?

We present here measurements on the smectic-nematic phase transition in 8CB using the proton nuclear-magnetic-resonance technique. Our results show that this transition is first order rather than second order as some observations have suggested.

Order in thin films of a liquid crystalline polymer

Ordering effects of a side group liquid crystalline polymer PMA64 have been observed by the x-ray reflectivity technique. Thin films on float glass substrate are investigated as a function of temperature and film thickness. At 100 °C, a phase transition from a smectic-A to a nematic phase is observed. The layers of the smectic-A phase are oriented parallel to the surface giving rise to a Bragg peak which is coherently superimposed on Kiessig fringes. The Kiessig fringes are caused by interference of beams reflected at the polymer surface and the polymer–glass interface, while the Bragg peak originates from the electron density modulation perpendicular to the surface. As a function of film thickness, we observe a significant deviation of the integrated intensity of the Bragg peak as compared to the integrated intensity of a Bragg peak due to perfect periodic one-dimensional density modulations. A model which includes the influence of the interfaces as well as distortions of the first and second kind is used to describe this thickness dependence of smectic order in thin films of liquid crystalline polymers. The Bragg peak is lost at the smectic–nematic phase transition where we observe a significant roughening of the free surface. This effect is discussed with respect to molecular order.

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.

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.

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.

Thermal and structural properties of tri‐O‐substituted cellulose ethers

AbstractTri‐O‐substituted cellulose ethers prepared by the use of SO2–diethylamine (DEA)–dimethylsulfoxide (DMSO) and powdered sodium hydroxide were characterized by measurement of melting points and degradation points and by differential scanning calorimetric (DSC) and X‐ray analyses. Their thermal and structural characteristics were ascertained to be dependent on the kinds of substituents. Among the ethers examined, four kinds of arylmethylcelluloses showed the characteristics of thermotropic liquid crystals (smectic). Tri‐O‐α‐naphthylmethyl‐cellulose showed the three transformations due to a solid–smectic phase, a smectic–nematic phase, and a nematic–isotropic phase transformation. This thermotropic liquid crystal was enantiotropic below 200°C.

Mesomorphic Homologous Series 4-Formylphenyl 6-n-Alkoxy-2-Naphthalenecarboxylates (I) and 2-Acetylnaphthalen-6-yl 4-n-Alkoxybenzoates (II)

Abstract Two homologous series of naphthalene derivatives have been synthesized and the mesomorphic behaviour of their members has been studied. In series I the first nine members are enantioropic nematic, the decyl derivative exhibits a reentrant nematic phase and enantiotropic smectic and nematic phases while the remaining members are enantiotropic smectic. In series II the first seven members exhibit nemati, the octyl derivative exhibits monotropic smectic and enantiotropic nematic phases and the remaining members are enantiotropic smectic and nematic or only smectic. The thermal stability of these series is compared with the corresponding homologous series of coumarin derivatives.

ChemInform Abstract: ELLIPTIC PHASE BOUNDARIES BETWEEN SMECTIC AND NEMATIC PHASES

Smectic–nematic phase boundaries that resemble arcs of ellipses can be analyzed by a quadratic expansion in temperature and pressure of the change ΔG of Gibbs function. If the change of entropy or volume is known, ΔG and its first two derivatives can be determined. Data on 8OCB and mixtures of 8OCB and 6OCB and of CB9A and CB7A fit an ellipse well, which suggests that the boundary is an ellipse. The smectic phase is therefore enclosed, which appears to be unique in single‐component systems and to disagree with simple mean‐field theory.

Elliptic phase boundaries between smectic and nematic phases

Smectic–nematic phase boundaries that resemble arcs of ellipses can be analyzed by a quadratic expansion in temperature and pressure of the change ΔG of Gibbs function. If the change of entropy or volume is known, ΔG and its first two derivatives can be determined. Data on 8OCB and mixtures of 8OCB and 6OCB and of CB9A and CB7A fit an ellipse well, which suggests that the boundary is an ellipse. The smectic phase is therefore enclosed, which appears to be unique in single-component systems and to disagree with simple mean-field theory.

Fluctuation of Smectic Phase Order Parameter in Nematic Phase

X-ray critical scattering from a nematic above the smectic-nematic phase transition temperature has been investigated in order to clarify the nature of the phase transition, and to test the Landau theory proposed by de Gennes.

Pretransitional behavior of conductivity and dielectric properties above a smectic C–nematic transition in 4-4′-di(n-alkyloxy)azoxybenzenes

Electrical conductivity and dielectric relaxation measurements have been made in the nematic phases of the 4-4′-di(n-alkoxy)azoxybenzenes. The higher members of this series have a smectic C phase at lower temperatures. In that case, we have observed that (1) the ratio of the conductivities parallel and perpendicular to the local optical axis is abnormally low (<1) above the smectic–nematic phase transition and increases with temperature, and (2) the activation energy corresponding to the molecular rotational diffusion in the frequency range 0.1–10 MHz is significantly larger than for the lower homologs. We tentatively suggest that our results are due to fluctuations of the smectic order in the nematic phase as first pointed out by Chystiakov and Chaikowsky [Mol. Cryst. Liq. Cryst. 7, 269 (1969)] through observation of x-ray diagrams.

Surface waves in liquid crystals

The propagating fluctuations on the free surfaces of nematic, cholesteric, and smectic liquid crystals are evaluated in the absence of dissipation from the hydrodynamical equations. In the nematic there is full translational symmetry and this implies that the dispersion relation for propagating waves is the same as in an ordinary fluid. Surface fluctuations in a cholesteric distort the helical structure and the first-order elastic constant ${k}_{22}{q}_{0}^{2}$ associated with the twist gives rise to an additional restoring force besides the usual surface tension $\ensuremath{\gamma}$. When $\ensuremath{\gamma}q\ensuremath{\gg}{k}_{22}{q}_{0}^{2}$ ($q$ is the wave vector), the surface tension dominates and the dispersion relation $\ensuremath{\omega}(q)$ approaches that in an ordinary fluid: $\ensuremath{\omega}\ensuremath{\propto}{q}^{\frac{3}{2}}$. When $\ensuremath{\gamma}q\ensuremath{\ll}{k}_{22}{q}_{0}^{2}$, the twist elastic forces dominate and $\ensuremath{\omega}\ensuremath{\propto}q$. In both cases the waves penetrate less into the bulk than in an ordinary fluid. Similar results apply in the smectic-$A$ case provided that the elastic constant associated with layer displacement is small compared to the compressibility. This may be the case near the smectic-nematic phase transition.