Chevron Layer Structure Research Articles

Main-Chain Chiral Smectic Polymers Showing a Large Electroclinic Effect in the SmA* Phase

The synthesis and characterization of a main-chain smectic liquid-crystalline polymer system designed for development into electromechanical actuators is described. The chemical structure is chosen to provide a large electroclinic effect in the SmA* phase, with large concomitant layer shrinkage (a rare combination). The polymers are prepared by acyclic diene metathesis polymerization (ADMET) of liquid-crystalline ,-dienes. Oligomers with a degree of polymerization of {approx}10-30 are obtained using Grubbs first-generation catalyst, while oligomers with a degree of polymerization of {approx}200 are obtained using Grubbs second-generation catalyst. All polymer samples show the following phase sequence: I - SmA* - SmC* - Glass. X-ray analysis of polymer powder samples demonstrates the desired layer shrinkage at the SmA* - SmC* transition. The polymers form well-aligned fibers by pulling from the isotropic melt, and X-ray analysis of fibers in the SmA* phase shows that in the bulk of the fiber the layers are oriented perpendicular to the fiber axis, while at the surfaces there appears to be a thin sheath where the layers are parallel to the fiber/air interface. The desired layer shrinkage with tilt at the SmA* - SmC* transition in these fibers is seen as well, and in the SmC* phase the fibers exhibitmore » an interesting conical chevron layer structure. Electro-optic investigation of aligned thin films of the polymer, prepared from quenched fiber glasses using a novel technique, exhibit a large electroclinic effect, with substantial degradation of alignment quality upon field-induced tilt. This degradation in alignment quality, coupled with the layer shrinkage at the SmA* - SmC* transition demonstrated by X-ray scattering, strongly suggests the desired layer shrinkage with electroclinic tilt is in fact occurring in the polymer films.« less

Dielectric relaxation in chevron surface stabilized ferroelectric liquid crystals

The dielectric response of surface stabilized ferroelectric liquid crystals with chevron layer structure is studied within low and intermediate frequency ranges, characteristic for collective molecular excitations. By analytically solving the dynamic equation for collective molecular fluctuations under a weak alternating electric field, it is demonstrated that chevron cells stabilized by both nonpolar and polar surface interactions undergo at medium frequencies two Debye relaxation processes, connected with two chevron slabs, on opposite sides of the interface plane. This result is confirmed, experimentally, making use of the electro-optic technique. Based on qualitative arguments supported by microscopic observations of zigzag defects at different frequencies and amplitudes of the external electric field, it is shown that, at low frequencies, the electro-optic response of chevron samples is determined by three kinds of motions of zigzag walls. The first two dynamic categories are related to collective relaxation processes at weak fields, within smectic A layers forming zigzag walls, and drift or creep motions of thick walls occurring at stronger field amplitudes. Dynamic processes of the third kind correspond to sliding of zigzag walls, which appear at yet stronger field amplitudes, but below the switching threshold.

Influence of alignment layer thickness on ferroelectric liquid-crystal structure

We have investigated the influence of the thickness of polyvinyl alcohol alignment layer on azimuthal anchoring of ferroelectric liquid crystals by measuring the azimuthal angle of the chevron C2 structure. By varying not only the alignment but also the ferroelectric liquid-crystal layer thickness, we also studied the influence of the ions screening the polarization charge on the chevron layer structure. A simple model describing the free enthalpy of the system was used to calculate the anchoring coefficient from the azimuthal angle of the structure.

Time-resolved X-Ray Scattering Studies of Reversible Layer Flexing in a Surface Stabilised Ferroelectric Liquid Crystal Device

Recent time resolved x-ray scattering studies of a surface stabilised ferroelectric liquid crystal (SSFLC) device have demonstrated that the smectic layers move in two orthogonal planes during low field switching of the device. This paper extends our initial studies of the layer motion and presents a detailed examination of the influence of low field switching on the chevron layer structures in a 3 μm SSFLC device. A symmetric bipolar pulse was applied to the device at a temperature 5°C below the ferroelectric chiral smectic-C to smectic-A phase transition. X-ray scattering data, corresponding to the intensity and position of the Bragg peak, were collected with microsecond time resolution. Results are presented which show a rapid, reversible flexing of the smectic layers to much lower angles during switching with fields of magnitude ±3 Vμm−1 followed, in some cases, by a slow (millisecond) return to the original layer structure. The data are to some extent consistent with the layer flex model proposed by Gießelmann, followed by domain motion within the device.

X-Ray Analysis of the Layer Structure in the Chiral Smectic Phase Showing V-shaped Switching

The layer structure of the chiral smectic phase showing V-shaped switching was investigated using the X-ray diffraction method. In thin planar cells of a three-component mixture in which V-shaped switching was first reported, the chevron layer structure was confirmed. The layer structure almost kept chevron even in the application of an electric field, and additional small diffraction peaks suggesting the existence of a substructure during V-shaped switching were observed. Preliminary time-resolved results are also reported.

The effect of substrate surface smoothness on the chevron layer structure of surface-stabilized ferroelectric liquid crystals

For freedom from zigzag defects in SSFLCD, it is important to research in detail the effect of surface smoothness of substrate on the chevron layer structure. We have studied the effect of surface roughness of undercoating layer on the surface smoothness of alignment film and then the alignment structure of SSFLC. Furthermore, we suggest a model for explaining the freedom from defects in a SSFLC by investigating theoretically the free energy of Cl and C2 chevron structure.

Spontaneous layer reorientation in smectic C liquid crystals

A defect structure has been observed for certain ferroelectric liquid crystal mixtures in the surface stabilised device geometry when cooled towards ambient temperature. X-ray and electro-optic studies suggest that the sample, initially in the chevron layer structure, forms a uniformly tilted layer structure spontaneously. This seriously degrades device contrast and switching properties. The effect of both liquid crystal composition and alignment layer pretilt are reported and recommendations made to ensure that the defect does not occur, thereby allowing good device performance to be maintained.

Defects in ferroelectric and antiferroelectric liquid crystals

In ferroelectric (FE) and antiferroelectric (AF) phases of chiral smectic C (Sc *) liquid crystals typical line defects like dislocations and disclinations exist. In this review a few examples of these defects will be mentioned: –In thick samples the anchoring conditions impose the presence of two systems of (2π)-twist disclinations (called also, ‘'unwinding lines'') near both glass plates. –In Sc * samples with the chevron layer structure the chevron plane is acting as a barrier to the propagation of the director deformation. Overcoming this barrier leads to the creation of so called ‘‘inversion line'’ which is π-twist disclination in the chevron plane. –In thicker free-standing FE or AF films the observed stripe texture was interpreted as a periodic system of director discontinuity walls starting on both free film surfaces and terminated in the volume of the film. –Line defects called dispirations which are combinations of layer dislocations and director disclinations were observed in AF phases.

Smectic A and C materials with novel director tilt and layer thickness behaviour

We report a smectic liquid crystal in which a tilt of the molecular orientation away from the layer normal in the Smectic C phase of up to 24 occurs with minimal layer contraction. This characteristic, accompanied by an exceptionally large underlying negative thermal layer expansion coefficient, enables the formation of chiral smectic electro-optical cells having nearly the ideal 'bookshelf' layer geometry, free from the complexities of the chevron layer structure produced by contraction.

Molecular Motion in A Smectic Liquid Crystal Showing V‐shaped Switching As Studied by Optical Second‐harmonic Generation

AbstractThe molecular motion during the V‐shaped switching in a homogeneously aligned smectic C*‐like liquid crystal (LC) cell has been investigated by measuring second‐harmonic generation (SHG) interferometry. It was shown that observed SHG behaviors were successfully explained using the collective model and that azimuthal rotation of LC molecules is limited within half a cone and that LC molecules undergo counter azimuthal rotations between the top and bottom halves of the LC chevron layer structure. Possible molecular orientation within a layer was also discussed using the 4 × 4 matrix method.

The Effect of Polymer Stabilization on the Alignment Structure of Surface-Stabilized Ferroelectric Liquid Crystals

It has been shown that a polymer network, formed in the SmA phase by UV photocure of photocurable liquid crystalline monoacrylates that are doped in a surface-stabilized ferroelectric liquid crystal (SSFLC), eliminates zig-zag defects in the SmC* phase, Since a chevron layer structure is transformed into a quasi-bookshelf layer structure by the polymer main chain. In this study, we have observed the electrooptical (EO) effect of this polymer-stabilized SSFLC (PS-SSFLC) with the aim of exploring the effect of the liquid crystalline polymer network on the alignment structure of FLC molecules. As experimental results, it was found that liquid crystalline side chains bond strongly with a main chain and attract FLC molecules in the rubbing direction. Furthermore, we propose and demonstrate a new PS-SSFLC fabricated also by UV photocure of doped monoacrylates particularly at a temperature where the LC medium is in the SmC* phase under the simultaneous application of a monopolar electric field. This device exhibits monostable EO characteristics with a high contrast ratio owing to being defect free, having grayscale capability without threshold, and a fast response speed.

Polymorphism and chevron layer structure of an antiferroelectric liquid crystal studied by X-ray diffraction

We report X-ray diffraction experiments performed on an antiferroelectric compound exhibiting a very rich polymorphism ( $$({S_{I*A}} - {S_{C*A}} - {S_{C*FI1}} - {S_{C*FI2}} - {S_{C*}} - {S_{C*\alpha }} - {S_A})$$ ). The structural study of the unknown phases only allows us to exclude some phenomenological models. The use of oriented planar samples prepared between solid glass plates generate by cooling from the SA phase a chevron structure of tilted layers already well characterized for the SC* phase. The extensive analysis of the evolution of the chevron structure through the numerous smectic-smectic phase transitions provides some original information in three distinct areas: fundamental data on the important physical parameters in the chevron structure formation, detection of the smectic-smectic phases transition by small change of the chevron structure, and information on the local molecular order induced by the alignment layer (interaction with a rubbed polymer).

Liquid Crystalline Polymer Stabilized FLCDs with Conventional Rubbed Polyimide Films or with Photo Alignment Films of Poly(vinyl Cinnamate)

Polymer stabilized SSFLCDs have been successfully fabricated. A liquid crystalline polymer network in a device cell is formed by the photo-cure of UV photo-currable liquid crystalline monomers that are doped in an FLC, where both the rubbed polyimide (PI) and non-rubbed but linearly polarized UV irradiated poly(vinyl cinnamate) (PVCi) films are utilized as surface FLC molecular alignment. The actual monomers adopted is acrylate with a mesogenic side chain. The PVCi is one of the photo alignment polymer. The fabrication process of these SSFLC cells is as follows; first the photo-cure is done on a monomers and initiators doped LC medium at the temperature where this material takes SmA phase and then the cell is cooled down to room temperature, at which the medium takes SmC* phase. The layer structure of the liquid crystalline polymer stabilized (LC-PS) FLC cells has been investigated by X-ray diffraction measurement. As the experimental results, it is found that the formed polymer network suppresses the formation of a chevron layer structure, resulting in the formation of a quasi-bookshelf layer structure; the only C1-uniform state is spontaneously formed without giving a high surface pretilt angle; and a fairly good memory state with an excellent contrast ratio and a low threshold voltage are obtained for a low concentration of doped monomers (2 ∼ 3 wt%) in particular with photo-alignment layers of PVCi; and furthermore, in a cell with a higher concentration of doped monomers (4 wt%) an excellent electroclinic like behavior (called the quasi-electroclinic effect) is obtained.

Electric Field Effect on Layer Structure in Surface-Stabilized Ferroelectric Liquid Crystals

Electric field dependence of the layer structure in surface-stabilized ferroelectric liquid crystals (SSFLCs) was investigated by means of an X-ray diffraction experiment. With increasing amplitude of the applied electric field (square wave, 20 Hz), in the SmC* phase, the layer structure in our SSFLCs changed from the chevron layer structure to bookshelf layer structure. In the SmA phase (high temperature than the SmC* phase), the tilted bookshelf layer structure changed to a nontilted bookshelf one. The layer structure in the SmC* phase changed from an “asymmetric” chevron layer structure to “symmetric” chevron layer structure when the samples were cooled from the SmA phase. The application of electric field to SSFLCs in the SmA phase induced the well-oriented bookshelf layer structure, and it is also useful to obtain the symmetric chevron layer structure in the SmC* phase.

Inversion line in finite samples of ferroelectric liquid crystals with the chevron layer structure

Abstract In finite samples of ferroelectric liquid crystals with parallel smectic layers inclined with respect to the sample surfaces, the chevron structure usually occurs. For the same surface anchoring conditions, the twisted molecular organization may also appear, but in contrast with the bookshelf layer geometry, the twist is confined either to the upper part of the chevron, with the lower part untwisted or vice versa. Therefore two different director configurations with respect to the chevron interface can be recognized. These different configurations can coexist and they can be mediated by the so called inversion line. In this communication, we describe this inversion line within the simplest approximation of the chiral smectic C∗ elasticity. The self-energy of the inversion line, together with its core energy estimated on the basis of the Peierls-Nabarro model, gives the energetically more favourable solution from two that are possible. Finally the interaction of the inversion line with 2/pi-twist ...

Switching Behavior of Surface Stabilized Ferroelectric Liquid Crystal Devices with Chevron Layer Structure

Abstract The switching behavior in the C1-uniform (C1U), C1- twisted C1T) and C2-uniform (C2U) states, which were observed in surface-stabilized ferroelectric liquid crystal (SSFLC) devices with the chevron layer structure, were investigated. The voltage dependence of the extinction angle was investigated in the C1U and C2U states and was discussed in terms of the switching model of these states. The boat-shape disclination loops were observed in the C1U, C1T and C2U states, suggesting that they were attributed to the molecular switching at the chevron interface. In addition, this paper describes a gray scale with smooth voltage-transmission characteristic, which were induced by the voltage dependence of the extinction angle.

Characteristics of C1 and C2 States in Surface-Stabilized Ferroelectric Liquid Crystals with Chevron Layer Structure

Abstract Characteristics of the dynamic response of the surface- stabilized ferroelectric liquid crystals (SSFLC) in the C1 and C2 states to an electric field is investigated by means of simulations based on the continuum theory of SmC* phase. The effects of the surface pretilt on them are discussed. The effects of dielectric anisotropy are also studied.

Layer Structure Reformation of Surface-Stabilized Ferroelectric Liquid Crystal Treated with Electric Field

The smectic layer reformation caused by the electric field treatment (EFT) was studied in terms of X-ray diffraction profiles, electro optical (EO) response, polarization reversal current and transitional dielectric constant (TDC) measurement. From the X-ray diffraction study, it was found that the chevron layer structure was reformed to a quasi-bookshelf structure by applying a suitable amplitude of the EFT voltage. The structural reformation made the threshold characteristics of the EO steep. The shape of the polarization reversal current indicated that the surface molecular switching was delayed as the EFT voltage increased. Furthermore, from TDC measurement, it was found that the amplitude of the dielectric constant variation corresponding to FLC switching was decreased when the EFT voltage was high. These results were consistently examined on the basis of the consideration for the surface anchoring potential barrier.

Switching characteristics of surface-stabilized ferroelectric liquid crystals with chevron layer structure

Abstract The switching characteristics of SSFLC cells with chevron layer structure are investigated by means of simulations based on a model of the surface-anchoring energy at the substrates and at the chevron interface of SSFLC cells. The memory states which appear after a voltage pulse is turned off are studied. It is found that the switching process of SSFLC cells is deeply affected by the surface anchoring conditions.

X-ray and optical studies of orientational state in surface-stabilized ferroelectric liquid crystal

Abstract Surface-stabilized ferroelectric liquid crystals (liquid crystal was SCE-8) were investigated by means of the X-ray diffraction and optical measurements as a function of a temperature. An asymmetric chevron layer structure was observed. Near the transition temperature of SmC-SmA, X-ray profile of the coexistence of three peaks and the hysteresis of the transition temperature of about 2°C were observed. Phase transition was discussed in terms of the layer structure.