Photo-orientation at liquid crystal—polymer interfaces

It is suggested that liquid crystal—polymer interfaces are coupled systems, in which the components mutually influence the orientational state of each other. The photo-orientation process at liquid crystal-polymer interfaces provides a striking example of such a coupling. Experiments show that the anisotropic structure generated by polarised light at a polymer surface is strongly affected by the phase of the liquid crystal covering the polymer. Photo-orientation is significantly more efficient when the liquid crystal is in the isotropic phase than when it exhibits orientational order. The observations are interpreted by assuming that in the smectic and nematic phases the liquid crystal stabilises to a large extent polymer chain-segments aligned parallel to the director, while it blocks the photo-induced formation of chain-segments in the perpendicular direction. Other situations, in which the coupling between the liquid crystal and the polymer can be important, are also discussed briefly.     

The Phase Transitions from Chiral Nematic Toward Smectic Liquid Crystals

A Chen-Lubensky energy is used to investigate phase transitions from chiral nematic to smectic C* and smectic A* liquid crystal phases. We consider a liquid crystalline material Ω confined between two parallel plates, where the dimensions of Ω are assumed to be large relative both to the width of a smectic layer and the material’s chiral pitch. We take boundary conditions so that the smectic phase melts at the plates’ surfaces and prove the existence of energy minimizers in an admissible set consisting of order parameters and molecular directors . Then under the physically observed assumption that the Frank elasticity constants become large near a phase transition, we establish estimates for the transition region separating phases. In particular we derive analytic estimates proving that chirality lowers the transition temperature regime above which minimizers are nematic and below which minimizers are in a smectic phase.Research supported by NSF grants DMS-0306516 and DMS-0456286.     

Studies of thin smectic C films by X-ray reflectivity

The smectic order in thin and ultra thin films (150–600Å) of the chiral ferroelectric liquid crystal mixture ZLI-3654 is studied using the X-ray reflectivity technique. The spin cast films on various substrates (float glass, Si wafer, polymer coated glass, etc.) order spontaneously with smectic layering parallel to the substrate surface. A simple model which assumes a sinusoidal density modulation can describe well the experimental reflectivity profiles. The X-ray reflectivity provides a method to evaluate the phases of the structure factor. We demonstrate, for the first time, that is possible to extract the molecular tilt angle, , in ferroelectric liquid crystals from X-ray reflectivity measurements of ultra thin films. The temperature dependence of the tilt angle in the smectic C^* phase are almost independent of the film thickness (down to 200 Å) and are similar to those in the bulk.     

Unusual thickness-dependent thermal behavior and anticlinic coupling in chiral smectic free-standing liquid-crystal films

We observe, in free-standing films of a chiral smectic liquid crystal, a series of discrete transitions in the relative orientation of the tilt of the interior and surface layers. These transitions include a remarkable reentrant synclinic-anticlinic-synclinic ordering sequence of the film surfaces in the presence of an electric field upon cooling. The profiles of the associated heat-capacity anomalies are found to be strongly thickness dependent and exhibit a novel crossover behavior in reduced dimensions. We measure the anticlinic coupling between tilted surface layers in the smectic- A phase.     

Optical properties of the locally anisotropic liquid L-phase in a chiral anticlinic liquid crystal series

Optical Rotatory Power (ORP) measurements and Raman scattering experiments have been performed in various compounds of the nCTBB9 chiral series, which displays a locally anisotropic liquid phase referred to as “L-phase”. The ORP results are very reminiscent of the behaviour theoretically predicted for the Blue Phase III-to-Isotropic transition, close to and above the critical point in the temperature-chirality phase diagram. It is suggested however, considering both the Raman scattering results and the high stability of smectic phases in this series, that the supercritical state should be characterized by both chiral and smectic order with short correlation lengths. Received 20 December 1999 and Received in final form 6 March 2000     

Banana-shaped 1,2,4-oxadiazoles

3,5-Disubstituted 1,2,4-oxadiazoles are a new type of liquid crystalline (LC) compounds with asymmetrical five-membered heterocycle as a central unit. They have a bent shape and are very convenient model-compounds for studying the dependence of the LC properties on the molecular design. We have also synthesized and investigated ‘banana-shaped’ 1,2,4-oxadiazoles using the ester groups as the linkage units. The new compounds exhibit spontaneous polarization in the smectic phase, even if there is no chiral group in the molecules. Preliminary experimental data suggest the presence of spontaneous polarization in the nematic phase as well. In order to study the structural properties of the LC phases, X-ray diffraction (XRD) measurements on powder samples have been carried out. Based on the XRD data, a model of the structural arrangement of the bent molecules in the smectic phase is provided, which accounts for the macroscopic spontaneous polarization as well as the ferroelectric switching behavior.     

Pressure variation of reentrant transition temperature in liquid crystals

High pressure experimental studies show that in certain mesogenic materials, the nematic-smectic A (N-Sm A) transition temperature T_AN exhibits nonlinear pressure dependence. As a consequence, the material shows reentrant phenomena that is a phase sequence nematic — smectic A — reentrant nematic appears. The characteristic features of this phenomenon have been addressed here within the framework of Landau-de-Gennes theory, where the coupling between nematic and smectic A order parameters (γ, λ_eff) plays an important role. The cubic coupling γ is chosen to be negative in order to form Sm A phase whereas the biquadratic coupling λ_eff is made large and positive to obtain reentrant behaviour. In the present work, we incorporate the pressure dependence in the theory through γ and λ_eff which justifies the experimental pressure dependence in the reentrant transition temperature . The pressure dependence of γ and λ_eff are employed in the calculation of excess specific heat capacity near the reentrant transition. The computed heat capacity shows strong pressure dependence near the reentrant transition which can be confirmed from high pressure measurement.     

Spectral investigations of surface ordering in ultrathin molecular films

Surface molecular ordering in ultrathin molecular films is investigated. The optical transmission spectra of molecular films ranging in thickness from 2 to 13 smectic layers (6.7–43 nm) in the region of the electronic absorption bands in the smectic A phase of cyanobiphenyl CB9 are measured. The thickness and temperature dependences of the permittivity are determined. It is found that the orientational ordering of the molecules depends on the film thickness. The penetration depth of the surface molecular orientational order does not exceed two smectic layers (<7 nm). Zh. éksp. Teor. Fiz. 115, 1833–1842 (May 1999)     

Surface effects in thin films of antiferroelectric smectic liquid crystals in terms of the short-pitch long-pitch competition model

A theoretical model based on the competition between short-pitch and long-pitch types of helical order is developed for thin films of antiferroelectric smectic liquid crystals. In the case of the “bookshelf” structure of the film and non-polar surfaces, subsurface perturbations of the ordering are found out analytically. Corresponding contributions to the free energies of the different phases are analysed. The possibility of sufficient influence of the boundaries on phase sequences is predicted even in the case of weak surface anchoring. A consistent explanation of the controversial experimental information is given; further ways of experimental justification of the model assumptions are discussed. Received 23 May 2002 RID="a" ID="a"e-mail: mgorkoun@uos.de     

“Elastic” fluctuation-induced effects in smectic wetting films

The Li-Kardar field theory approach is generalized to wetting smectic films and the “elastic” fluctuation-induced interaction is obtained between the external flat bounding surface and distorted IA (isotropic liquid-smectic A) interface acting as an “internal” (bulk) boundary of the wetting smectic film under the assumption that the IA interface is essentially “softer” than the surface smectic layer. This field theory approach allows calculating the fluctuation-induced corrections in Hamiltonians of the so-called “correlated” liquids confined by two surfaces, in the case where one of the bounding surfaces is “rough” and with different types of surface smectic layer anchoring. We obtain that in practice, the account of thermal displacements of the smectic layers in a wetting smectic film reduces to the addition of two contributions to the IA interface Hamiltonian. The first, so-called local contribution describes the long-range thermal “elastic” repulsion of the fluctuating IA interface from the flat bounding surface. The second, so-called nonlocal contribution is connected with the occurrence of an “elastic” fluctuation-induced correction to the stiffness of the IA interface. An analytic expression for this correction is obtained.     

Quantitative analysis of lyotropic lamellar phases SANS patterns in powder oriented samples

We have developed a detailed numerical method based on the Caillé model to fit Small Angle Neutron Scattering profiles of powder-oriented lyotropic lamellar phases. We thus obtain quantitative values for the Caillé parameter and the smectic penetration length from which we can derive the smectic compression modulus and the membrane mean bending modulus. Our method, applied to a surfactant lamellar phase system decorated by amphiphilic copolymers, provides excellent fits for any intermembrane spacing or membrane concentration over the entire q-range of the SANS experiments. We compare our fits with those obtained from the model of Nallet et al. (J. Phys. II 3, 487 (1993)), which is reviewed. Good fits are obtained with both methods for samples exhibiting “hard” smectic order (sharp Bragg peak, moderate small angle scattering). Only our procedure, however, gives good fits in the case of “soft” smectic order (smooth Bragg peak, strong small angle scattering). A quantitative criterion to discriminate between these “soft” and “hard” samples is also proposed, based on a simple analogy with smectic-A liquid crystal in contact with an undulating solid surface. This allows us to anticipate the type of thermodynamic information that can be derived from the fits.     

Discontinuity walls terminated by twist disclinations in smectic a liquid crystals

A discontinuity wall in smectic A liquid crystals was discussed in M. Kléman:Points, Lines and Walls in Liquid Crystals, Magnetic Systems and Various Ordered Media (J. Wiley, Chichester, 1983). Possible solutions describing a singularity — twist disclination which terminates the discontinuity wall in an infinite model smectic A liquid crystal are discussed. One of solutions proposed can be used to describe the so-called zigzag defect line in chiral smectic C liquid crystals on which opposite oriented chevrons are terminated. This work was supported by Grant No.: 202/96/1687 of the Grant Agency of the Czech Republic.     

Hexagonal order in crystalline and columnar phases of hard rods

We report a study of colloidal suspensions of highly monodisperse semiflexible chiral rodlike viruses, denoted fd, in the range of high concentrations. Small angle x-ray scattering experiments reveal the existence of two hexagonal phases: the first one is crystalline and the second one is hexatic columnar, as shown by its short-range positional order. The suspension of rodlike viruses is the first experimental system showing the whole phase sequence with increasing particle concentration theoretically predicted for systems of hard rods, ranging from the chiral nematic via the smectic to columnar and crystalline phases.     

Short-pitch helicoidal modes in antiferroelectric liquid crystals and scattering of resonant X rays

The structure of antiferro-and ferroelectric phases in chiral smectic liquid crystals is studied on the basis of the theory of modes with a short pitch. The existence of ferroelectric phases is explained both by the characteristic incommensurability of short-pitch modes with respect to the thickness of the smectic layer and by distortions of the short-pitch antiferroelectric structure of a long-wavelength mode. It is pointed out that the details of the short-pitch order can be observed in resonant x-ray scattering. The fundamental features of the pattern of such scattering in the phases considered are predicted. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 3, 226–231 (10 February 1999)     

Self-assembled chiral superstructures composed of rigid achiral molecules and molecular scale chiral induction by dopants

We explore the phase behavior of a rigid achiral bent-core model system. Nematic and smectic phases form at higher densities, whereas micelles and columns composed of chiral clusters of these molecules self-assemble at lower densities. No nucleation mechanism requiring transient chirality is possible in the formation of these chiral superstructures due to the rigid achiral nature of the substituents. We show the chiral micelles are minima of the potential energy surface using energy minimization and parallel tempering simulations. Chiral dopants were found to induce the system to adopt a consistent chiral twist direction, the first molecular scale computer simulation of this effect.     

Twisted-grain-boundary (TGB) phases: nanostructured liquid-crystal analogue of Abrikosov vortex lattices

Twisted-grain-boundary (TGB) phases shown by some liquid-crystalline materials have properties common to those of both smectic and cholesteric phases. Following analogy between liquid crystals and super conductors proposed by de Gennes [Solid State Commun., 10, 753 (1972)], Renn and Lubensky [Phys. Rev. A, 38, 2132 (1988)] theoretically predicted a chiral analog of the smectic A (SmA*) phase, which is now known as TGBA phase. The TGBA phase was experimentally observed for the first time by Goodby et al. [Nature, 337, 449 (1989)] in the chiral homologous series of ferroelectric liquid-crystal material R- and S-1-methylheptyl 4′-[(4′′-n-alkoxyphenyl)propionoyloxy]-biphenyl-4-carboxylates (nP₁M₇), with n?=?13,14,15. Since then, more than a hundred pure and mixed systems showing TGBA phase have been found. Later, Renn derived a mean-field phase diagram based on the chiral Chen-Lubensky model, and predicted two more TGB phases, namely TGBC and TGBC?. These two phases have also been experimentally observed in many systems, and in a few, the phase diagram is similar to that predicted by Renn. Unlike the TGBA phase, several theoretical models have been proposed for TGBC and TGBC? phases, and it remains to be ascertained whether all the types of proposed TGBC/TGBC? structures experimentally exist. A review of the theoretically predicted and experimentally observed TGB phases is given in the present article. Some recently observed novel optical textures of the TGB phases are also reported.     

Quark potential in a quark–meson plasma

We investigate the effect of the restoration of chiral symmetry on the quark potential in a quark–meson plasma by considering meson exchanges in the two flavor Nambu–Jona-Lasinio model at finite temperature and density. There are two possible oscillations in the chiral restoration phase; one is the Friedel oscillation due to the sharp quark Fermi surface at high density, and the other is the Yukawa oscillation driven by the complex meson poles at high temperature. The quark–meson plasma is strongly coupled in the temperature region 1≤T/T _c≤3, with T _c being the critical temperature of the chiral phase transition. The maximum coupling in this region is located at the phase transition point.     

Molecular structure and physical parameters of some ferroelectric liquid crystal systems

We investigate thermodynamic properties and spontaneous polarization of ferroelectric liquid crystal (SmC^*LC) systems: smectic C matrix + chiral adding (ChA). The fact of existence of an optimum concentration of ChA, for which the studied SmC^*LC systems have the maximum value of spontaneous polarization, and a certain concentration, at which the phase transition of the chiral tilted smectic C phase to the smectic A phase occurs, is established. Temperature dependence curves for the free energy, configuration energy, specific heat, and the orientational order parameter are calculated in the temperature range of existence of liquid crystal mesophases. The curves are in good agreement with the experimentally observed trends of the SmC^*LC properties.     

On some liquid crystalline phases exhibited by compounds made of bent-core molecules and their mixtures with rod-like molecules

In most homologous series of compounds made of bent-core (BC) molecules, the B₂ B₁ and B₆ phases occur as the chain length decreases. We have studied binary mixtures of the compound 1,3-phenylene bis[4-(3-methylbenzoyloxy)]4-n-dodecylbiphenyl 4’-carboxylate (BC12) which exhibits the B₂ phase with the compound 4-biphenylyl 4’-n-undecyloxybenzoate (BO11) made of rod-like (R) molecules. We find the above sequence of occurrence of the B phases with increasing concentration of BOH. In this paper we describe the physical origin for the formation of these phases in both pure compounds and in the mixtures. We have also found the occurrence of the biaxial smectic A phase when the BO11 concentration is increased to 87–95.5 mol%. We also report on another binary system composed of BC12 and 4-n-octyloxy 4’- cyanobiphenyl (8OCB) made of R molecules. This system exhibits the biaxial smectic A phase down to 30°C. Using polarized infrared spectroscopy we find that the mutual orientation of the R and BC molecules in the SmA_db liquid crystal is such that the arrow axes of the BC molecules are along the layer normal of the partial bilayer smectic structure formed by the rods. We also describe unusual growth patterns obtained when the nematic phase transforms to the SmA_db phase in a mixture with 24 mol% of BC12.     

Influence of pressure on the electric-field-induced phase transitions in liquid crystals

Within the framework of Landau-de-Gennes formulation, we analyse the effect of pressure on electric-field-induced phase transitions in a liquid crystal which shows spontaneously an isotropic-smectic A transition. Inferring from the experimental pressure dependences on the layer spacing in smectic A phase, as well as the nematic-smectic A metastable temperature T^*_AN, we incorporated the pressure dependence in the free energy through (the surface energy term) and the coupling between the quadrupolar nematic ordering Q_ij and the smectic order parameter ψ. From the S-T phase diagram, we found that the stability of field-induced nematic phase increases with pressure, whereas the discontinuity of the transition decreases. Also, the region where paranematic phase transits directly to smectic A phase increases with pressure.