Smectic H-smectic VI pretransitional effect by orientational ordering in TBBA

New neutron quasi-elastic scattering data on TBBA in its supercooled smectic H phase are presented and analyzed in terms of the elastic incoherent structure factor. Contrary to what was found in the normal H phase, the results support the idea of partial orientational ordering of the molecules. It is suggested that this ordering (of dynamical nature) is a precursory effect to the smectic H-smectic VI phase transition.     

Induced anticlinic ordering and nanophase segregation of bow-shaped molecules in a smectic solvent

Recent experiments indicate that doping low concentrations of bent-core molecules into calamitic smectic solvents can induce anticlinic and biaxial smectic phases. We have carried out Monte Carlo simulations of mixtures of rodlike molecules (hard spherocylinders with length/breadth ratio L(rod)/D = 5) and bow-shaped molecules (hard spherocylinder dimers with length/breadth ratio L(ban)/D = 5 or 2.5 and opening angle psi). We find that a low concentration ( 3%) of L(ban)/D = 5 dimers induces anticlinic ( SmC(A)) ordering in an untilted smectic ( SmA) phase for 100 < or = psi < 150. For L(ban)/D = 2.5, no tilted phases are induced. However, with decreasing psi we observe a sharp transition from intralamellar nanophase segregation (bow-shaped molecules segregated within smectic layers) to interlamellar nanophase segregation (bow-shaped molecules concentrated between smectic layers) near psi = 130.     

On the emergence of mysterious liquid crystal phases: chirality and non-chirality issues

The structure–property correlation has been systematically investigated by the modification of the central core part of the dichiral materials so that the chiral and/or non-chiral issue having an effect on the formation of the layered or inter-connected structure can be studied. The central phenyl core of the dichiral compound showing the inter-connected ordered smectic Q phase has been modified in the following three ways: introduction of the substituent in the central phenyl core, introduction of the alkyl spacers between the central phenyl core and the peripheral mesogenic groups, and replacement of the central phenyl ring by the alkyl spacer. It has been found that (1) a symmetric structure stabilizes the tilted molecular ordering and the inter-connected structure, whereas the non-symmetric structure stabilizes the smectic A ordering; (2) a flexible molecular structure stabilizes the layered phase rather than the inter-connected; (3) increase in the degree of the segregation in the molecular assemblies stabilizes the anticlinic layered ordering; (4) increase in the local liquid-crystallinity stabilizes the layered structure; and (5) the ordered inter-connected structure is more sensitive to the chirality of the system than the disordered inter-connected structure.     

Design,synthesis and characterization of a linear hydrogen bonded homologous series exhibiting reentrant smectic C ordering

Design, synthesis and characterization of seven linear hydrogen bonded liquid crystal complexes derived from mesogenic p–n-decyloxy benzoic acid and p–n-alkyl benzoic acids designated as 10OØn (where n varied from ethyl to octyl) are reported. FTIR studies confirm the hydrogen bond formation in all these complexes. The phase transition temperatures and their corresponding enthalpy values are experimentally deduced from Differential Scanning Calorimetry (DSC) studies. POM and DSC data are further utilized for the construction of 10OØn phase diagram. Two Odd–even effects have been evinced, one in enthalpy values and the other in corresponding transition temperatures across the isotropic to nematic phase transition. An interesting result is the observation of re-entrant smectic ordering, designated as smectic C_R in three higher ordered mesogens. A new smectic ordering, smectic X, has been observed which is sandwiched between traditional smectic C and re-entrant smectic C_R. Magnitudes of optical tilt angle in smectic C, smectic X and smectic C_R are experimentally found to attain saturation with decrement of temperature in the corresponding phase. The occurrence of smectic X and smectic C_R are discussed with relation to the molecular chemical structure. The optical filtering action in smectic C and re-entrant smectic C_R phases have been analyzed.     

Main-chain smectic liquid-crystalline polymers as randomly disordered systems

We report a high-resolution X-ray lineshape study of main-chain smectic polymers. The results indicate that the layer ordering differs fundamentally from the algebraic decay typical for other smectic liquid-crystalline systems. The lineshapes are best described by broad squared Lorentzians indicating some form of short-range correlations. However, several higher harmonics are observed, which excludes simple liquid-like short-range order. This behaviour is tentatively attributed to a random field of defects associated with entangled hairpins in the main-chain polymer structure.     

Phase transitions in 8CB liquid crystal confined to a controlled-pore glass: Deuteron NMR and small angle X-ray scattering studies

We present results of the deuteron nuclear magnetic resonance (NMR) and small-angle X-ray scattering (SAXS) study of ordering and phase transition behavior of octylcyanobiphenyl (8CB) liquid crystal confined to a controlled-pore glass (CPG) with nontreated and silanes-treated pore surfaces. The deuteron NMR spectra allowed to determine the degree of nematic liquid crystal ordering and also provided an indirect information on the confined 8CB smectic ordering via its influence on the nematic ordering. For the smectic phase these data are supplemented with measurements of the temperature dependence of the first-order SAXS diffraction pattern. The NMR results indicate that the average nematic and smectic order parameters of 8CB in the nontreated CPG are only weakly perturbed by the confinement. The SAXS data further suggest that in confined 8CB for both nontreated and silane-treated CPG a domainlike pattern appears in accordance with the Imry-Ma theorem.     

Structure Formation from the Oriented Glassy States of Poly(Ethylene Terephthalate)

The ordering process of poly(ethylene terephthalate) (PET) from the oriented glassy states during the isothermal annealing process was investigated using real-time X-ray scattering experiments with synchrotron radiation sources. The X-ray scattering intensity of the 001' reflection, which is a measure of the smectic order with respect to chain alignment, increases with time and then begins to decrease after reaching a maximum. The intensity of the four-point pattern in the small-angle X-ray scattering region, which is a measure of the fraction of a crystal-like structure with a tilted lamellar structure, increases monotonically with time. Hence, the fraction of PET chains with smectic-like order has a maximum during the isothermal process, while that of the crystal-like structure increases monotonically. The kinetics of structure formation from the nematic-like structure to the crystal-like structure by way of the smectic structure will be discussed.     

Capillary smectization and layering in a confined liquid crystal

Using density-functional theory, we have analyzed the phase behavior of a model liquid crystal confined between two parallel, planar surfaces (i.e., the so-called slit pore). As a result of confinement, a rich phase behavior arises. The complete liquid-crystal phase diagram of the confined fluid is mapped out as a function of wall separation and chemical potential. Strong commensuration effects in the film with respect to wall separation lead to enhanced smectic ordering, which gives capillary smectization (i.e., formation of a smectic phase in the pore), or frustrated smectic ordering, which suppresses capillary smectization. These effects also produce layering transitions. Our nonlocal density-functional-based analysis provides a unified picture of all the above phenomena.     

Universal non-monotonic smectic fluctuations of liquid crystal films in a magnetic field

Free-standing liquid crystal films with positive diamagnetic susceptibility can have the smectic ordering enhanced by an external field applied perpendicular to the plane layers. Within a quadratic functional integral approach, we investigate the interplay between the smectic order induced by an external field H and that due to the surface tension γ between the film and the surrounding gas. We find that the average smectic fluctuation is a non-monotonic function of film thickness, with a characteristic thickness scale ξ _H delimiting the predominance of surface tension and magnetic field effects. This characteristic thickness obeys simple asymptotic power-law relations as a function of the ordering terms which allows us to represent the average smectic fluctuations in a universal scaling form. Received 7 January 2003 Published online 1st April 2003 RID="a" ID="a"e-mail: marcelo@ising.fis.ufal.br     

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)     

Abnormal switching behavior of nanoparticle composite systems

We have investigated the properties of liquid crystals (LCs) doped with ZnO (8% Cu doped) nanoparticles. The electro-optic properties of LCs have changed with varying concentration of ZnO nanoparticles. The dielectric anisotropy obtained from the values of dielectric permittivity at 5 kHz in the nematic and smectic phases was found to increase with increasing concentration of nanoparticles in LCs. It has been established that the effect of nanoparticles on the dielectric anisotropy depends on the physical properties of LCs; the nanoparticle disturbs the orientation ordering of LC molecules. The nanoparticle also influences the switching behavior, splay elastic constant, rotational viscosity and threshold voltage of pure LCs. A small quantity of nanoparticles causes slight reduction of the splay elastic constant and rotational viscosity of LC cells.     

Self-diffusion of rodlike viruses through smectic layers

We report the direct visualization at the scale of single particles of mass transport between smectic layers, also called permeation, in a suspension of rodlike viruses. Self-diffusion takes place preferentially in the direction normal to the smectic layers, and occurs by quasiquantized steps of one rod length. The diffusion rate corresponds with the rate calculated from the diffusion in the nematic state with a lamellar periodic ordering potential that is obtained experimentally.     

Photoinduced Ordering Transition in Microdroplets of Liquid Crystals with Azo‐Dendrimer

Ordering transitions are observed in azo‐dendrimer‐dissolved nematic liquid crystal (NLC) droplets dispersed in a glycerol matrix. The dendrimer molecules are spontaneously attached at the interface between NLC and glycerol, so that the nematic directors orient perpendicular to the interface, in the radial configuration. Photoisomerization makes the directors be tangential to the interface, in a photoinduced ordering transition from radial to bipolar structure. Similar experiments are conducted both in cholesteric (Ch) and smectic‐A (SmA) LC droplets. Complicated photoinduced ordering transitions are also observed in Ch and SmA droplets, and the associated molecular orientation changes are discussed. Photoisomerizable azo‐dendrimer molecules provide a possible way to control orientation, even in microdroplet systems in which the interface cannot be treated by conventional surface agents.     

Kossel line analysis on crystallization in colloidal suspensions

The processes of crystallization in salt-free aqueous suspensions of highly-charged latex particles were observed by laser diffraction experiments for semi-dilute specimens of concentration 0.1–10 vol%. The Kossel diffraction patterns provide accurate three-dimensional information on the suspensions enabling the time evolution of the crystal structure to be recorded in detail. The results showed that the ordering formation evolved by way of the following intermediate processes: two-dimensional hep structure ← random layer structure ← layer structure with one sliding degree of freedom ← stacking disorder structure ← stacking structure with multivariant periodicity ← fcc structure with (111) twin ← normal fcc structure. For less concentrated suspensions (< 2 vol %), the phase transition proceeded further from the normal fcc structure to the normal bcc structure via the bcc twin structure with twin plane (112) or (112).     

Shear-induced smectic ordering in the melt of isotactic polypropylene

We report an unusual shear-induced smectic phase of high-molecular-mass isotactic polypropylene at temperatures well above its melting point. The necessary stiffness in the polymer chain is provided by shear-induced coil-helix transitions. Coupling between the conformational ordering and the density generates the smectic layering. The periodicity of about 4 nm is built up from a rigid helical sequence of at least 11 monomers and a random-coil part of about 1.4 nm. These results also provide new clues to the question of preordering before polymer crystallization.     

Electronic structure,pairwise interactions and ordering energies in binary f.c.c. transition metal alloys

Effective pair interactions for binary f.c.c. transition metal alloys have been computed using a generalized perturbation method starting from the completely disordered state. Their properties, as a function of interatomic distance, band filling, disorder and concentration are discussed. The ordering energy computed from the pair interactions is succesfully compared, for the Cu₃Autype structure, to the energy derived from the band structure determined by the recursion method.     

X-ray scattering from freely suspended smectic-A films

This work is devoted to a theoretical analysis of specular reflection and diffuse scattering of x-rays from a freely suspended smectic-A film, taking into account its spatial inhomogeneity and deviations of orientational and position ordering in smectic layers from ideal ordering. The results of the calculations are compared with the experimental data on small-angle scattering of x-rays from freely suspended films of 7 AB liquid crystals. It is found that the theoretical results agree with the experimental data.     

磁通密度对第Ⅱ类超导体磁通动力学的影响

计算了二维无序钉扎系统中磁通运动的平均速度、微分电阻、纵向电压噪声和静态结构因子.通过在不同磁通密度下的磁通运动形式,给出了磁通运动的动力学相图.研究表明,磁通晶格存在钉扎相、塑性流相、近晶流相,和运动玻璃相.在运动玻璃相中,随着驱动力的进一步增加,横向玻璃态和运动Bragg玻璃态相继出现.磁通密度增大有利于有序相的出现.当磁通密度增大到一定程度时,近晶流动相会消失.磁通运动随着外加驱动电流增大发生从塑性流动相到运动玻璃相的转变 关键词： Ⅱ类超导体 磁通动力学 运动玻璃     

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. The text was submitted by the authors in English.