V. K. Dolganov

Phase Transformations And Dynamics Of 4-Cyano-4′-Pentylbiphenyl (5cb) By Nuclear Magnetic Resonance, Analysis Differential Scanning Calorimetry, And Wideangle X-Ray Diffraction Analysis

The polymorphism of 4-cyano-4′-pentylbiphenyl (5CB) was investigated by means of differential scanning calorimetry (DSC), X-ray diffraction, and proton nuclear magnetic resonance (NMR) experiments. Three new phases has been identified and characterized structurally and dynamically. By a slow cooling at a rate ≤ 0.2 K/min, the nematic phase crystallizes into a phase noted C1. Upon a fastest cooling (≥5 K/min) a glassy liquid crystal (GLC) phase with a glass transition temperature T g =208 K, is formed. Upon heating up both the solid state C1 and the GLC phase, a second crystalline phase C2 (ΔH=49 J/g.K) which is more stable than C1 (ΔH=36 J/g.K) can be obtained. The proposed phase diagram has been investigated by proton NMR lineshape analysis. The motional correlation time of end-chain methyl group motion was determined by means of spin-lattice relaxation time ( T 1 ) analysis.

Two-dimensional ordering of inclusions in smectic C phase

The organization of inclusions nucleated in free standing films of smectic C (SmC) liquid crystal is investigated using polarized light microscopy. Anchoring on the inclusion boundaries induces distortions of the in-plane orientational order of the SmC phase, which drive the elastic interactions between inclusions. Such interactions show a quadrupolar character. At low concentration, the inclusions self-organize in linear or branched chain structures, while at high concentrations two-dimensional patterns appear in the film.

Free-standing films above the bulk smectic-nematic-isotropic transitions

Abstract Optical reflectivity of free-standing films in the temperature interval of the smectic phases and above the bulk smectic-nematic transition temperature has been studied. Melting phenomena have been found to be dependent on the film thickness. For thick films, nematic-like droplets and an unstable state over the entire film have been observed above the bulk smectic-nematic transition temperature. These droplets and the unstable state of the film look like the “quasi-smectic” phase predicted by Mirantsev.

Droplets Nucleation in Smectic-C* Free-Standing Films

Abstract The behavior of freely suspended smectic-C* (SmC*) films at the bulk Smectic-C*- cholesteric (N*) phase transition has been investigated. According to our experimental observations, the heating of the SmC*free-standing film above the bulk SmC*-N* phase transition can give rise to conventional layer thinning transitions or to nucleation of N* droplets trapped inside the film. The optical texture observations show that a N* droplets confined in the SmC* host phase are accompanied by topological defects. Due to the boundary condition fixed by the anisotropic host phase elastic interactions between droplet-defect pairs appear. These interactions drive the self-organization of the droplet-defect pairs in chain-like structures. In this paper, we present the experimental conditions acting on the droplet nucleation process and on the droplet chaining.

Shape of nematic droplets in smectic membranes

We found that the nematic droplets in smectic membranes and the configuration of the c-director field near droplets may rearrange drastically with droplet size and temperature. Investigations were made in the membranes with quadrupolar droplets. Large droplets are nearly circular, whereas small droplets become elongated. The elongation of the droplets increases with decreasing droplet size. At high temperature the droplets are found to be spindlelike with two cusps on their boundary. The observed transformations are explained by the competition between the elastic energy of the director field and the surface tension of the interface between the droplets and the smectic membrane.

Formation of two-dimensional crystal-like structures from inclusions in smectic C films

The structures formed by inclusions in smectic C (SmC) free-standing films are investigated using polarized light microscopy. The domains confined in these two-dimensional (2D) systems induce distortion of the inplane orientational order, which governs the elastic interaction between the inclusions. The balance between long-range quadrupolar attraction and short-range repulsion gives rise to a nontrivial collective behavior of domains. Various 2D structures are created as a function of the concentration and size of inclusions. We observe the formation of chains and then a 2D square lattice when the concentration of domains increases. Further increase in the domain size leads to the transition from square to hexagonal close-packed structure.

Free-standing smectic films at high temperature

Optical reflectivity studies on free-standing liquid crystal films above the bulk smectic temperature range have revealed different melting phenomena. Our measurements are performed on tilted smectic phases (smectic C*, smectic C) using optical microscopy in polarized light in order to visualize the changes of the film structure. We observe the formation of twodimensional defect structures from string-like lines in very thick (about 1000 layers) as well as in thin (about 20 layers) films. In thick films these structures nucleate around the temperature of the bulk smectic-cholesteric phase transition, while in thin films the formation of the defects occurs well above this temperature and just before the thinning transitions. In thick and intermediate thickness films, cholesteric or nematic droplets and a ‘quasi-smectic’ structure are observed. The films exhibiting the ‘quasi-smectic’ structure definitely exist at higher temperatures than the smectic films with the same thickness.

Ferrielectric smectic phase with a layer-by-layer change of the two-component order parameter

One of the most remarkable properties of smectics is the wide variety of possible equilibrium structures. In this paper, based on the Landau theory of the phase transitions, the transitions between ferroelectric and antiferroelectric phases and the structure formed by smectic layers with different azimuthal and polar orientations of the molecules were calculated. This unique structure has been predicted [P.V. Dolganov et al., JETP Lett. 76, 498 (2002)] using the minimization of the free energy with respect to the phase and modulus of the two-component order parameter, but never before detected. Recently, a nonresonant Bragg reflection, consistent with the predictions of the model, was found [P. Fernandes et al., Eur. Phys. J. E 20, 81 (2006)] in the ferrielectric smectic C*FI1(SmC*FI1) phase. In the three-layer ferrielectric structure with a macroscopic helical pitch, the modulus of the order parameter is larger in anticlinic-like layers and smaller in layers with mixed ordering. The values of the interlayer interactions were determined for smectic liquid-crystalline materials forming different polar structures.

Different mechanisms of nucleation and self-organization of droplets in ferroelectric smectic membranes

Abstract.New mechanisms of droplet nucleation and self-organization in ferroelectric membranes are described. The droplets may be accompanied by different number of topological defects (zero, one, two) whose location may be on the droplet boundary or in the membrane. Nucleation and self-organization of droplets with total topological charge S = 0 , S = + 1 and S = - 1 were investigated. We found that an S = - 1 topological defect may be the center of both droplet nucleation and chain formation. This mechanism of chaining drastically differs from the droplet self-organization described earlier which is realized by attraction of droplet-defect pairs. Our observations demonstrate new possibilities for manipulating the inclusions and their self-organization in smectic membranes.

Anomalous orientation of ferroelectric liquid crystal films in an electric field

An anomalous orientation of ferroelectric liquid crystals in an electric field is observed. The tilt planes of the molecules are oriented in a direction parallel to the electric field. The anomalous orientation is observed in freely suspended films above the smectic-C*-smectic-A bulk phase transition temperature. The effect is explained by the appearance of giant flexoelectric polarization, 104 times greater than that in bulk liquid-crystal samples, in the film.