Optical microscopy study for director patterns around disclinations in side-chain liquid crystalline polymer films

Polarizing optical microscopy is employed to study director fields around disclinations in side-chain liquid crystalline polymer films. Optical black brushes together with stripes around disclinations are observed. The stripes run parallel to the local director and thus decorate overall patterns of nematic director around disclinations. Three director patterns involving radial, spiral, and circular microstructures of a positive integer disclination with s = +1 and one hyperbolic pattern of a negative integer disclination with s = -1 are observed in the thin film. It is found that the specific configurations of a pair of (+1, -1) disclinations form during the late stage of annihilation. Increasing the film thickness leads to disclination instability. We observe that black four-brushes of disclinations with s = +/-1 split into black two-brushes, where two types of director patterns of disclinations with half-integer strengths of s = +/- 1/2 produce. Theoretical analysis is presented to explain this instability.     

Flexoelectric response at defect sites in nematic inversion walls

Electric field experiments have been carried out on +1 and ?1 defects formed in alignment inversion walls, in a planarly aligned nematic phenyl benzoate. The results show that the defects are non‐singular in the core and exhibit a flexoelectric response to an applied d.c. or low frequency a.c. field. When the c‐director flux lines are circular, as in a +1 defect in a wall parallel to the easy axis, flexoelectro‐optic switching characterized by an azimuthal angle variation is observed. When the c‐director flux is radial, the response is seemingly through polar angle variations involving no rotation of the extinction brushes due to crossed polarizers. This conclusion follows from the field‐induced structural distortions observed at a ?1 defect having a combination of radial and tangential c‐director fields.     

The identification of the sign and strength of disclinations in the schlieren (nucleated domain) texture of the nematic phase,by optical microscopy

ABSTRACT

The optical texture of the nematic phase, variously known as the schlieren, structure à noyuax or nucleated domain texture, was identified over a century ago as being an array of point singularities. When viewed between crossed polars, patterns of dark brushes radiate from each point nucleus. The sign and strength of each nucleus can be uniquely determined from the changes in the orientation of these brushes when either the sample or the crossed polars are rotated, from two formulae given by Chadrasekhar in 1977. However, these were given with little exemplification and have been largely overlooked. Consequently, the majority of the discussions given in current literature are either incomplete and confusing or, in some cases, incorrect. Here, we provide a detailed explanation of the textures and their behaviour as viewed with the most commonly used experimental geometry (i.e. with a rotating sample and stationary polars).     

Ferritin immobilization on patterned poly(2-hydroxyethyl methacrylate) brushes on silicon surfaces from colloid system

In this paper, we describe a graft polymerization/solvent immersion method for generating poly(2-hydroxyethyl methacrylate) (PHEMA) brushes in various patterns. We used a novel fabrication process, involving very-large-scale integration and oxygen plasma treatment, to generate well-defined patterns of polymerized PHEMA on patterned Si(100) surfaces. We observed brush- and mushroom-like regions for the PHEMA brushes, with various pattern resolutions, after immersing wafers presenting lines of these polymers in MeOH and n-hexane, respectively. The interaction between PHEMA and ferritin protein sheaths in MeOH and n-hexane (good and poor solvent for PHEMA, respectively) was used to capture and release ferritins from fluidic system. The “tentacles” behaver for PHEMA brushes was found through various solvents in fluidic system. Using high-resolution scanning electron microscopy, we observed patterned ferritin Fe cores on the Si surface after pyrolysis of the patterned PHEMA brushes and ferritin protein sheaths, which verify the “tentacles” behaver for PHEMA brushes.     

Polar electro-optic switching in droplets of an achiral nematic liquid crystal

A polar electro-optic response is observed in droplets of an achiral nematic liquid crystal in coexistence with the isotropic phase. Between crossed polarizers each pancake-shaped droplet shows extinction brushes in the form of a centred cross aligned with the polarizer axes. An applied electric field E induces a rotation of the crosses about the field direction, with about half the droplets switching clockwise and the other half anticlockwise. The sense of rotation in each droplet changes when E is reversed. We propose that a twisted bipolar director structure is stabilized in the droplets by a relatively large splay elastic constant and tangential boundary conditions. The molecules twist along the diameter of the droplets, perpendicular to the applied field, which results in a linear rotation of the director by the inverse flexoelectric effect. Since the molecules are achiral, the handedness of the twist, and hence the sense of the switching, in any droplet is arbitrary.     

Polar electro-optic switching in droplets of an achiral nematic liquid crystal

A polar electro-optic response is observed in droplets of an achiral nematic liquid crystal in coexistence with the isotropic phase. Between crossed polarizers each pancake-shaped droplet shows extinction brushes in the form of a centred cross aligned with the polarizer axes. An applied electric field E induces a rotation of the crosses about the field direction, with about half the droplets switching clockwise and the other half anticlockwise. The sense of rotation in each droplet changes when E is reversed. We propose that a twisted bipolar director structure is stabilized in the droplets by a relatively large splay elastic constant and tangential boundary conditions. The molecules twist along the diameter of the droplets, perpendicular to the applied field, which results in a linear rotation of the director by the inverse flexoelectric effect. Since the molecules are achiral, the handedness of the twist, and hence the sense of the switching, in any droplet is arbitrary.     

Characterization of patterned poly(methyl methacrylate) brushes under various structures upon solvent immersion

In this paper we describe a graft polymerization/solvent immersion method for generating various patterns of polymer brushes. We used a very-large-scale integration (VLSI) process and oxygen plasma system to generate well-defined patterns of polymerized methyl methacrylate (MMA) on patterned Si(1 0 0) surfaces through atom transfer radical polymerization (ATRP). After immersion of wafers presenting lines of these PMMA brushes in water and tetrahydrofuran, we observed mushroom- and brush-like regimes through grafting densities and surface coverages, respectively, for the PMMA brushes with various pattern resolutions. In the mushroom-like regime, the distance between lines of PMMA brushes was smaller than that of the lines patterned lithographically on the wafer; in the brush-like regime, this distance was approximately the same. This new strategy allows polymer brushes to be prepared through graft polymerization and then have their patterns varied through solvent immersion.     

Atomic force microscopy study for supermolecular microstructures in side-chain liquid crystalline polymer films

Atomic force microscopy (AFM) is employed to study the supermolecular microstructures of disclinations and inversion walls in thin films of a smectic side-chain liquid crystalline polymer. Two-dimensional nanostripes are formed in thin films when the material enters the smectic phase. A possible mechanism for their formation is also suggested. The stripes run parallel to the local director and thus decorate the overall patterns of nematic director around the disclinations and inversion walls. Three patterns involving radial, spiral, and circular supermolecular microstructures of a positive disclination with s = +1 and one hyperbolic pattern of a negative disclination with s = -1 are observed. The cores of all these microstructures exhibit circular dark centers in AFM height images. It is found that the specific configurations of a pair of (+1, -1) disclinations form during the late stage of annihilation and inversion walls always separate a pair of (+1, +1) disclinations. The analysis on the director fields around disclinations and inversion walls shows that the bend and splay elastic constants are of the same order of magnitude in the side-chain liquid crystalline polymer.     

Flexoelectric response at defect sites in nematic inversion walls

Electric field experiments have been carried out on +1 and -1 defects formed in alignment inversion walls, in a planarly aligned nematic phenyl benzoate. The results show that the defects are non-singular in the core and exhibit a flexoelectric response to an applied d.c. or low frequency a.c. field. When the c-director flux lines are circular, as in a +1 defect in a wall parallel to the easy axis, flexoelectro-optic switching characterized by an azimuthal angle variation is observed. When the c-director flux is radial, the response is seemingly through polar angle variations involving no rotation of the extinction brushes due to crossed polarizers. This conclusion follows from the field-induced structural distortions observed at a -1 defect having a combination of radial and tangential c-director fields.     

On the nematic-nematic phase transition in mixtures composed of sheet-shaped palladium organyls and apolar organic solvents

Abstract

The three sheet-shaped palladium organyls 1a-c exhibit lyotropic nematic properties in apolar organic solvents. The occurrence of two nematic lyomesophases was verified by the investigation of the orientational properties of these liquid crystalline systems. In cells with obliquely deposited SiO _x layers, the director of the high temperature phase N₂ is aligned in a homogeneous, planar manner. In the low temperature phase N₁, a different orientational pattern is observed. The specific pattern type depends on the deposition conditions of the SiO _x layers. In samples homeotropically aligned in the N₂ phase, the director is tilted away from the cell normal after the transition into the N₁ phase. These changes of orientation are reversible. The experimental observations presented here support our proposed model for the structural change at the nematic-nematic phase transition.     

Drying dissipative structures of Chinese black ink on a cover glass and in a dish

Macroscopic and microscopic dissipative structural patterns are formed in the course of drying a suspension of Chinese black ink on a cover glass and in a dish. The time for the drying and the pattern area increased as the particle concentration increased. The broad ring patterns of the hills accumulated with the particles formed around the outside edges on a macroscopic scale. The height and the width of the broad ring increased as the particle concentration increased. The spokelike patterns of the rims accumulated with particles were also formed on a macroscopic scale. Microscopic patterns of colloidal accumulation were observed over the whole region of the pattern area. Various types of convection cells were observed on a cover glass and in a dish at 25–80 °C. A time-resolved observation of the drying process was also made. The convections of water and the colloidal particles at different rates under gravity and the translational and rotational Brownian movement of the particles were important for the macroscopic pattern formation. Microscopic patterns were determined by the translational Brownian diffusion of the particles and the electrostatic and the hydrophobic interactions between the particles and/or between the particles and the cell wall in the course of the solidification of the particles.     

The alignment of nematic liquid crystals on photolithographic micro-groove patterns

Abstract

The alignment of nematic liquid crystals on micro-groove patterns was studied. It has been found that the order of the alignments is determined by the edge shape, spacing, and line pitch of the micro-groove patterns. On coarse patterns, with pitches greater than 2 μm, striped patterns of the liquid crystal alignment were observed using polarized light which manifested different orientations of the liquid crystals on the top, edge, and bottom of the grooves, respectively. On fine patterns, with pitches less than 2 μm, a uniform device-quality alignment has been realized, with which twisted nematic cells were constructed in combination with the rubbed alignment layer on the opposite substrate. Their viewing angle characteristics and tilt orientations of the director were also investigated.     

Microscopic observations of axial and radial nematic droplets for a dual-frequency addressable liquid crystal

Abstract

We report observations on nematic droplets of a dual-frequency addressable nematic mixture using polarization microscopy. When low frequency (f = 100 Hz) electric fields are applied, the dielectric anisotropy is positive and the droplets show the usual behaviour, i.e. dark polarization rings which move outward with increasing field strength. When high frequency (f = 50 kHz) fields are applied, however, the dielectric anisotropy is negative and the dark polarization rings move inward with increasing field strength. The appearance of the droplets in transmission is compared to computer-simulated pictures using a simple model which describes the change of the director field within a weak field limit.     

String defects connecting pairs of half-integer disclinations and tilting transition of a langmuir monolayer

The static and dynamic string defect textures connecting pairs of half-integer disclinations have been observed by Brewster angle microscopy in the solid phase of pentacosadiynoic acid Langmuir monolayers. The static string defect structures have appeared coexisting with two kinds of point disclinations that have four and two black brushes. The use of local laser heating has allowed one to observe kinetics of creation and annihilation of string defects connecting the two-half-integer disclinations in the splitting process of an s = 1 point disclination into fractional disclinations. These kinetics have been analyzed by studying the competition between the orientational elasticity of the molecules and the line tension of the string and the drag force of the disclinations.     

高分子液晶向列相的向错结构

高分子液晶态向错结构在正交偏振片下呈现出具有不同数目黑刷子的纹影织构，是由于分子指向矢取向排列上的不连续性所引起的一种光学效应。近年来，高分子液晶态向错结构的研究已取得了较大进展，发展和应用片晶装饰、条带织构装饰和表面裂纹装饰等技术可以在电镜和偏光显微镜下直接观察各种向错结构。本文简要介绍高分子液晶态向列相的向错和反转壁结构的几何学、高强度向错以及近年在实验上观察各类型向错形态的研究进展。     

Relationship between the Frank elastic constant anisotropy measured from the lamellar director and that of the molecular director around disclinations in main chain liquid crystalline polymers

Abstract

The transmission electron microscope technique enables direct visualization of disclination core structure of main chain liquid crystalline polymers. Generally, the lamellar structure, that is a function of type of polymer and molecular length, and of which the thickness is 10–200 nm, can be observed. In terms of distortions in the director field produced by a disclination, the elastic anisotropy, ?, can easily be calculated. However, the ? does not indicate the molecular distortion produced by the disclination. Continuum mechanics is used to describe the relationship between the elastic anisotropy, ?_a, measured from the lamellar director and that, ?_b, of the molecular director in main chain liquid crystalline polymers. The relationship is ?_a = ??_b, indicating that lamellar distortion is completely different from the molecular distortion.     

High field carbon 13 relaxation of 5CB. Chemical shift anisotropy as a probe of nematic dynamics

Abstract

The carbon 13 spin-lattice relaxation times of the ring carbons at 22·6 MHz in the nematic phase and at 20·0 MHz in the isotropic phase of 5CB have been published previously. An analysis of the results in the nematic phase based on diffusion constants obtained from deuteron relaxation was presented. In this paper measurements of the carbon 13 spin-lattice relaxation times of the ring carbons of 5CB at 125·6 MHz are given along with an analysis which makes use of the wide variation in the field to interpret the relatively simple relaxation of the protonated aromatic carbons. An effort to interpret the more complicated relaxation behaviour of the unprotonated carbons using the simplification in relaxation behaviour at high fields where chemical shift anisotropy dominates the relaxation is less successful. However, the conclusion that order director fluctuations account for almost 50 per cent of the relaxation rate in the nematic phase at high fields suggests that study of high field rotating frame relaxation for these carbons might be a rich source of information on critical fluctuations.     

Director patterns and inversion walls in 2D inhomogeneously deformed nematic LC layers

Abstract

With progressing resolution of dot matrix displays, effects of inhomogeneous electric fields at the edges of structured electrodes become more and more important in the optical performance. We present a new method of polarizing microscopy for the study of the director field in two-dimensionally inhomogeneous nematic LC layers. Planar cells with electrode strips are investigated. The novel effects observed in these systems are two types of inversion walls, depending upon the preferred director orientation at the glass plates with respect to the electrodes.     

Orientational ordering of 4-substituted phenylcyclohexanes studied by carbon-13 two-dimensional N.M.R.

Abstract

An N.M.R. method combining the techniques of separated local field spectroscopy (SLF) and variable angle spinning (VAS) is valuable in the investigtion of nematic liquid crystals. Rapid sample spinning causes the nematic director to align along the spinning axis, resulting in narrow peaks in the C-13 N.M.R. spectrum. SLF is a two-dimensional N.M.R. method which produces a first order splitting pattern for each carbon signal from which C–H dipolar coupling constants can be determined. The order parameters for all segments of the liquid crystal molecule can then be calculated. Results for three 4′-cyanophenylcyclohexanes are considered here. These compounds are trans-substituted at the 4 position of cyclohexane ring with n-pentane (PCH5), 1-pentene (3d ₁CP) and 3-pentene (1d ₃CP), respectively.     

Light-induced LC director freezing on a chalcogenide surface

First observations of permanent light-induced director patterning in a transparent liquid crystal (LC) are reported. The patterning was observed in an LC cell with one of the substrates covered with a chalcogenide film. Concentric director ring pattern appears following the irradiation of the chalcogenide surface through the LC layer by a Gaussian beam. The experimental results are explained in terms of the heat transfer from the chalcogenide film to the LC after the light absorption by the chalcogenide film. Heating the LC changes its birefringence according to the spatial intensity distribution of the Gaussian beam. This in turn leads to the spatially oscillating changes of the polarisation state of light in the plane of the chalcogenide film. The oscillating changes of the light polarisation result in producing the light-induced easy orientation axis and the oscillating changes of the anchoring energy. It causes the spatial modulation of the director on the chalcogenide surface and the formation of the concentric ring twist structures in the cell.