In situ observation of smectic layer reorientation during the switching of a ferroelectric liquid crystal

A thin plastic cell containing the ferroelectric liquid crystal ZLI3654 (4 μm) was placed edge-on to a pin-hole collimated horizontal X-ray beam. In this way, the smectic layers were brought into register. Subsequently, triangular voltage waves with various peak (V_p) values were applied across the cell and diffraction photos were obtained during the application of the alternating voltage. Up to V_p = ± 30 V, no significant movements in the initial tilted orientation of the smectic layers with respect to the surfaces (chevrons) could be observed. During the application of V_p = ± 32 V an increasing fraction of the smectic layers changed their initial tilt angle with respect to the cell surfaces to make larger tilt angles. At a slightly higher voltage, the layers became upright (bookshelf structure). Upon removing the voltage and short circuiting the cell, the quasi-bookshelf structure was sustained. The new method described here can be used in combination with a fast detector for time resolved experiments.     

Evaluation of image sticking property on liquid crystal displays with polymer layers produced from phenanthrene-carrying monomers

As a model of polymer-sustained-alignment liquid crystal display (PSA-LCD), the liquid crystal (LC) cells having the polymer layers produced from monomers of phenanthrene (Phen)-carrying monomers, 2,7-, 3,8-, 1,8- and 3,6-dimethacryloyl-oxy-phenanthrene (DMAPhen), were prepared and investigated image sticking phenomenon with evaluations of residual direct current voltage (V_rDC) and difference in pretilt angle before and after application of alternate current voltage (Δtilt). The V_rDC values were small for the LC cells with the polymer layers produced from the Phen-carrying monomers used in this study. On the other hand, Δtilt depends on the molecular structure of the Phen-carrying monomer. The Δtilt values became small with increasing the linearity of the monomer structure for the Phen-carrying monomers. The findings would be useful for obtaining the small level of image sticking in the PSA-LCDs produced from the Phen-carrying monomers.     

Tilt and temperature dependence of the pitch in the chiral smectic C phase

Abstract

The chirality of the constituent molecules in the chiral smectic phase induces a helical structure with a pitch, p ₀. Because of the tilt and chirality there is a spontaneous polarization and a bend deformation which act upon the induced helix. The resulting pitch is described as a function of p ₀ using the phenomenological theory of a chiral smectic C phase. The pitch, p ₀, is then calculated using a molecular theory of the cholesteric phase. The results obtained explain the experimental observations, at least qualitatively.     

Deformed helix ferroelectric liquid crystal display: A new electrooptic mode in ferroelectric chiral smectic C liquid crystals

Abstract

A new electrooptic mode of operation of ferroelectric chiral smectic C liquid crystal displays (LCDs) is proposed and demonstrated. The effect, which is called the deformed helical ferroelectric (DHF) effect, is based on the deformation of the helical structure by weak electric fields. In the unbiased device the smectic layers are arranged in the bookshelf geometry with the helix axis parallel to the electrodes [1]. Systems with a very small pitch (<1 μm) and a large tilt angle are especially well suited for this mode. The key characteristics of DHF-LCDs are: (a) low driving fields (1 V_p-pμm^?1 for maximum contrast); (b) grey scale which is approximately linear with the applied electric field; (c) easy alignment even for thick cells using standard wall-aligning methods; and (d) response times at room temperature of 300 μs.     

Computer modelling of the director configuration in a supertwist nematic liquid crystal cell

Abstract

The director distribution in a supertwist nematic cell, containing La-Roche liquid crystal mixture 3010, has been studied extensively using Berreman's computer simulation approach. It is seen that the director distribution in the cell depends critically on the total twist angle θ_t, the surface tilt angle θ_o and the ratio of the cell thickness to the pitch d/p. The values of θ_o and φ_t have been optimized to yield a small bistability (ΔV = 0.06 V) and a relatively large change in the midplane tilt angle (Δθ_m = 51°) in an unstrained cell with ?_t = (d/p) × 360°. The optimum values of θ_o and Ø_t were found to be 15° and 240°, respectively. The effect of varying d/p on the director distribution has also been studied in great detail in supertwist cells with θ_o = 30° and Ø_t = 270°. Some interesting features in understrained and overstrained cells have been observed.     

Improvement of image sticking on liquid crystal displays with polymer layers produced from mixed monomers

As a model of polymer-sustained-alignment liquid crystal display (PSA-LCD), the LC cells having the polymer layers produced from the homopolymers and copolymers of 4,4?-dimethacryloyl-oxy-biphenyl (4,4?-DMABiph) and 2,6-dimethacryloyl-oxy-naphthalene (2,6-DMANaph) were prepared and investigated image sticking phenomenon with evaluations of residual direct current voltage (V_rDC) and difference in pre-tilt angle before and after application of alternate current (AC) voltage (Δtilt). The V_rDC was effectively improved by adding a small amount of 2,6-DMANaph to 4,4?-DMABiph because the concentrations of radicals and ions in the LC layer were decreased due to increase in the rate constant of the polymerisation with the addition of 2,6-DMANaph under UV light exposure. The Δtilt was proportionally increased with increasing the weight ratio of 2,6-DMANaph in the mixed monomers of 4,4?-DMABiph and 2,6-DMANaph. We confirmed that the range of the weight ratio for 2,6-DMANaph in the mixed monomers of 4,4?-DMABiph and 2,6-DMANaph from 10 to 25 wt% was useful for obtaining the small level of image sticking in the PSA-LCD.     

Antiferroelectric alignment and mechanical director rotation in a hydrogen-bonded chiral SmC* A elastomer

A new type of chiral smectic elastomer based on poly[4-(6-acryloyloxyhexyloxy)benzoic acid] is discussed. The layer structure and the molecular tilt stabilized by hydrogen bonding between side groups are identified by X-ray measurements. Well aligned and optically clear monodomain samples with smectic layers in the film plane are obtained by uniaxial stretching and then frozen-in by additional gamma-radiation crosslinking. In this monodomain state, two opposite orientations of director tilt are distributed through the sample thickness and alternate between neighbouring layers in a zigzag fashion. This structure of the stress-aligned chiral smectic C elastomer is similar to that of antiferroelectric liquid crystals of the smectic C^* _A type. Further mechanical stretching in the layer plane induces a gradual c-director reorientation along the new stress axis, when a threshold deformation ~ 20% is exceeded. The (reversible) transition proceeds as a director azimuthal rotation around the smectic C cone, with the layers essentially undistorted and the tilt angle of the side mesogenic groups preserved.     

X-ray diffraction study of the smectic mesophase of some azobenzene-containing polyacrylates

Abstract

An X-ray study is performed on powder specimens and on stretched oriented fibres of two liquid crystalline polyacrylates containing the azobenzene mesogenic unit with either a 4′-n-pentyloxy (sample 1–4) or 4′-n-hexyloxy (sample 1–5) substituent. The X-ray diffraction patterns of both samples showed the presence of a bilayer smectic C mesophase with the mesogneic groups tilted by an angle β ～ 45° (1–4) or β ～ 38° (1–5) with respect to the layer normal. The electron density profile p(z) along the direction normal to the smectic layers was calculated by Fourier inversion and possible structural models of the smectic mesophase are discussed. A partial interdigitation of the terminal alkyloxy substituents appears to occur.     

X-ray study of substrate-induced alignment of a smectic A liquid crystal

Abstract

We have performed a structural study of the liquid crystal (LC) octylcyanobiphenyl (8CB), deposited on gratings and flat surfaces, using high resolution X-ray scattering as a function of film thickness. 8CB is a room temperature smectic A₂, with a layer spacing of 31·6 Å. Glass was used as substrate and treated with either one of the organic surfactants MAP or DMOAP. Surface tension forces cause the liquid crystal molecules to align perpendicularly with respect to the plane of the substrate at the air interface. Competing with the LC-air interface, which is a strong aligner, a grating at the LC-substrate interface produces distortions in the smectic layering with an excess of elastic energy, which favours alignment parallel to the substrate and the grooves. Our purpose was to detect the onset and evolution of parallel alignment as a function of film thickness. The studies used 9 keV (1·403 Å) X-rays focused to a spot size of 2 mm² at the sample position. In-plane scans, which detect the smectic layers perpendicular to the plane of the substrate, were done at angles φ = 0° and 90° with respect to the gratings to ascertain the molecular orientation, at a nominal X-ray incidence angle of α = 0°. In order to observe regions of varying smectic layer orientation within the film, we performed a series of scans where the out-of-plane tilt angle χ changed from 0°, corresponding to scattering in the plane of the film, to 90°, which corresponds to scattering normal to the surface of the film. The results from these scans were fitted to a multilayer model where the orientation of the smectic layers varies as a function of film depth. The analysis confirmed our earlier observations that surface tension at the air interface plays a dominant role in the alignment of the LC molecules.     

Field dependence of the optical activity of a chiral liquid crystal near the isotropic–smectic A transition

Abstract

Chiral liquid crystals exhibit molecular optical activity in the isotropic phase. We have studied the evolution of the optical activity as a function of an applied electric field on a 76·2 μm film of the chiral liquid crystal W7, which exhibits an isotropic–smectic A transition at approximately 40°C. We measured the optical activity by recording the rotation of the plane of polarization of an incident linearly polarized ray of light, provided by a He–Ne laser. The applied biasing electric field is parallel to the direction of the incident beam. We find that at 41·0°C, the plane of polarization shifts from ?1·1° for an applied voltage of 30 V to a maximum of ?4·0° at 70 V. The absolute value of the signal decreases beyond this voltage. These shifts are in the direction of the smectic A phase and are in general larger than those observed as a function of temperature. Close to the isotropic–smectic A phase transition, molecules inside the liquid coalesce to form dynamic coherent groups, which have smectic nature. These groups are randomly oriented with respect to each other in the absence of an electric field. The application of an electric field causes the molecules within these groups to align along the direction of the field and to contribute coherently to the optical activity of the system. The way the molecules align with the field depends on the relative values of the polarizability α, which contributes to the alignment of the long axis of the molecule, and the dipole moment p, which contributes to the alignment of the short axis of the molecule. Our preliminary results and calculations suggest that for small fields, the electric field couples with the dipole moment p, whereas for fields in excess of 70 V, the field couples with the polarizability of the long axis of the molecule, causing a rotational reorientation of the molecules in the isotropic phase. The value of the field at which this reorientation occurs may be controlled by temperature.     

The influences of surface polarization on NLC cells

The influence of the surface polarization, P_s , on a nematic liquid crystal (NLC) cell is investigated analytically. Flexoelectric polarization is considered, but selective ion absorption is ignored. The differential equations are derived for tilt angle, θ, of director n and the corresponding boundary conditions based on Gibbs free energy, and their solutions discussed. Equations for the reduced threshold voltage, u_th , and the reduced saturation voltage, u_sat , are deduced and the relationships between u_th , u_sat and reduced strength of surface polarization, p, derived.     

Bend-stripes in hybrid aligned nematic layers

Abstract

The occurrence of bend-stripes also involving twist distortion is investigated in hybrid aligned nematic layers, with stronger tilt-anchoring at the homeotropic substrate than at the planar one. The modulated structure is found to exist above the thickness threshold d _a for the aperiodic bend-splay deformation. The periodicities of the director azimuth and polar angle are incommensurate along the normal to the cell plates. In principle, two independent periodic modes are possible, the first one stable, with threshold d _p = d _a, the second one metastable, with threshold d _p > d _a, according to the values of the torsional extrapolation lengths. Both transitions are continuous with respect to the in-plane wavenumber. The second mode can appear only if the difference between the torsional and the tilt extrapolation lengths is positive at the planar substrate. Moreover, both thresholds are independent of the saddle-splay elastic constant.     

Spontaneous Enantiomeric Resolution in a Fluid Smectic Phase of a Racemate

Right- and left-handed homochiral domains segregate in a fluid smectic phase of the racemic (R*,S*)-β-Me-TFMHPOBC analogue shown. This CF₃-containing liquid crystalline compound exhibits an electrooptic response in a homogeneous cell, although no macroscopic dipole is expected to exist. Moreover, the homeotropic cells of this material exhibit striped domains, which are associated with fine stripes having two opposite tilt senses with respect to the predominant stripes.     

A Photoelectrochemical Solar Cell Consisting of a Cadmium Sulfide Photoanode and a Ruthenium–2,2′‐Bipyridine Redox Shuttle in a Non‐aqueous Electrolyte

A photoelectrochemical (PEC) cell consisting of an n‐type CdS single‐crystal electrode and a Pt counter electrode with the ruthenium–2,2′‐bipyridine complex [Ru(bpy)₃]^2+/3+ as the redox shuttle in a non‐aqueous electrolyte was studied to obtain a higher open‐circuit voltage (V_OC) than the onset voltage for water splitting. A V_OC of 1.48 V and a short‐circuit current (I_SC) of 3.88 mA cm^?2 were obtained under irradiation by a 300 W Xe lamp with 420–800 nm visible light. This relatively high voltage was presumably due to the difference between the Fermi level of photo‐irradiated n‐type CdS and the redox potential of the Ru complex at the Pt electrode. The smooth redox reaction of the Ru complex with one‐electron transfer was thought to have contributed to the high V_OC and I_SC. The obtained V_OC was more than the onset voltage of water electrolysis for hydrogen and oxygen generation, suggesting prospects for application in water electrolysis.     

Influence of the smectic A-nematic transitional order on the formation of the smectic phases of 4-n-hexyloxybenzylidene-4′-n-butyIaniline and 4-n-butyloxybenzylidene-4′-n-octyIaniline from an electrically deformed nematic phase

Abstract

The crucial role of the smectic A-nematic transitional order for the formation of the smectic A, B and G phases from an electrically deformed nematic phase of the liquid crystal 4-n-hexyloxy-benzylidene-4′-n-butylaniline (6O.4) with a typical smectic A-nematic first order transition and the formation of the smectic A and B phases from an electrically deformed nematic phase of the liquid crystal (4-n-butyloxy-benzylidene-4′-n-octylaniline (40.8) with a smectic A-nematic second order transition has been demonstrated. The nematic phase was deformed by an AC voltage of 2U,_th 5U _th and 10U _th, where U _th is the threshold voltage which causes the appearance of the Fréedericksz transition in the homeotropic nematic layer. The smectic textures have been observed on free cooling of the nematic phase or after the use of an oven. The smectic A phase of the liquid crystal 60.4 was observed with the formation of a clear smectic A-nematic phase boundary while the smectic A phase of the liquid crystal 40.8 has been formed from intermediate pretransitional stripes, observed by Cladis and Torza [1]. The homeotropic anchoring of the direction was crucial for the formation of the smectic phases of the liquid crystal 40.8 but not significant for the liquid crystal 60.4.     

Application of CdSe–PVK Nanocomposite as a Hole Transport Layer for OLEDs

An organic light‐emitting diode was fabricated using cadmium selenide (CdSe)/poly(N‐vinylcarbazole) nanocomposite as the hole transport layer (HTL). The CdSe nanoparticles (NPs) with a mean crystallite size of 6.2 nm were prepared by high‐energy ball milling. Based on the current–voltage curves, the threshold voltage (V _th) of the composite diode was found to be ~1.3 ± 0.1 V lower than that of the diode without CdSe, with a significant increase in the current density for the composite diode. Moreover, the electroluminescence (EL) properties (luminous flux, emittance, and intensity) of the diode were found to be enhanced by ~16% with respect to those of the diode without CdSe. The decrease of the threshold voltage and the increase of the current density and the EL were due to the CdSe NPs that operate as hole trap centers in the HTL.     

An innovative technique based on voltammetric profiling and current–voltage characteristics for determination of residual direct-current voltage of liquid crystal cells

ABSTRACT

Generation of residual direct-current (DC) voltage (V_rDC) induces a serious image sticking of liquid crystal displays (LCDs). In this study, an innovative technique to determine the V_rDC of LC cells was proposed from the current-voltage (I-V) curves obtained by application of triangular voltage. The I-V curve shows the maximum and minimum current peaks due to the rotation of the LC director, and the V_rDC is determined from the average value of the two peaks. The V_rDC values determined from the proposed technique corresponded to the values determined from the flicker minimisation, which is the well-known method to determine the V_rDC. The results indicate that the proposed technique is useful for evaluation of the image sticking of the LCDs.     

Creation of nematic and reentrant nematic phases in bi- and multi-component smectic mixtures

Abstract

Experimental results referring to the transformation of smectic phases, mainly smectic A, into nematic and reentrant nematic phases are reviewed. A new explanation of some experimental results is proposed. Factors which are responsible for the depression of smectic phases in mixtures of polar mesogens are discussed and the possibility of forming mixtures with a broad temperature range of nematic phase from smectic compounds, which can be useful for liquid crystal displays (LCDs), is shown. A nematic gap observed in some cases between monolayer (S_A1) or monolayer and partially bilayer (S_Ad) smectics results from the differences in the organization of the molecules in the smectic layers. It is concluded that polar phase from smectic A₁ phases can be divided into two groups: (a) the first one is characteristic for compounds with the -NCS, -F, -CI, -I or ?COC _m H_{2m + 1} terminal group. The spacing of the smectic layer slowly expands with the increase in alkyl chain length and the structure of the smectic A₁ phase slowly changes to be more like the smectic A_d phase (d/1 > 1). It is proposed that such a smectic is called an enhanced monolayer smectic (S_A1e (b) the second one is typical for compounds with the -CN terminal group. This kind of smectic A₁ phase is rapidly transformed into the smectic A_d phase with increasing alkyl chain length. These latter monolayer mesogens easily form the reentrant nematic phase when they are mixed with other polar smectic mesogens.     

Structural study of the smectic phases of several 4-cyanoalkoxybenzylidene-4'-alkylanilines

Abstract

A series of 4-cyanoalkoxybenzylidene-4'-alkylanilines (CN-nO. m; n = 3, 4, 6 and m = 4, 5, 8) have been synthesized. Their thermotropic liquid-crystalline polymorphism has been investigated using optical microscopy, differential scanning calorimetry and dilatometry. Nematic, smectic A and smectic B mesophases have been identified. The structure of the smectic phases has been studied with X-ray diffraction. Depending upon the compound and the temperature, the smectic layers have been found to be either single (A₁) or double (A₂) layers of molecules. Unexpected from a simple comparison with the well-known behaviour of the very similar nO. m smectogens, which belong to the class of the so-called symmetric and therefore exhibit single-layered smectics only, this structural behaviour has been interpreted to mean the importance of the dipole interactions of the terminal cyano groups. For CN-60.8, a transition has been detected at 64°C between the single and double layered structure.     

Solubility of thioindigo dopants in a smectic liquid crystal host evaluated by 2H NMR spectroscopy

The solubility of two partially deuterated thioindigo dopants in a smectic liquid crystal host was evaluated by variable temperature ²H NMR spectroscopy and polarized microscopy. 2H NMR spectra showed that the dopant (±)-6,6'-bis(2-octyloxy)-5,5-dinitrothioindigo-d ₆ forms a homogeneous solution with the smectic phases of the liquid crystal host (±)-4-(4-methylhexyloxy)phenyl 4-decyloxybenzoate (PhB) up to its saturation point of 3 mol %. These results are consistent with polarized microscopy observations of the dopant crystallizing out of solution upon reaching a concentration of 3 mol %. On the other hand, ²H NMR spectra of (±)-5,5'-dichloro-6,6'-bis(2-octyloxy)thioindigo-d ₆ dissolved in PhB showed evidence of a partitioning of the solution between smectic and isotropic microdomains, which increases with increasing dopant concentration—from 1.2 to 9.1 mol %. To a large extent, this smectic/isotropic microphase separation could not be detected by polarized microscopy. These results suggest that ²H NMR spectroscopy can provide a more accurate determination of the occurrence and extent of microphase separation in doped liquid crystal samples.