Mechanism Of Liquid Crystal Alignment Research Articles

Physicochemical Modification Effect on Homogeneously Aligned Liquid Crystals Based on the Nickel Oxide Thin Film.

We demonstrated homogeneous liquid crystal (LC) alignment on Nickel Oxide (NiO) films subjected to ion beam (IB) irradiation. Uniform LC alignment was achieved at high IB intensity values of 1200 and 1800 eV. To determine the mechanism of LC alignment following IB irradiation, physicochemical analysis was performed using atomic force microscopy and X-ray photoelectron spectroscopy. IB irradiation with high intensity increases uniformity of the surface, and IB irradiation induces the formation of oxygen vacancies and increases the NiO (Ni2+) phase components. Hence, both of the smooth surface and the strong van der Waals interactions between the NiO film and the LC molecules provides the LC molecules with a stable anchor on the surface, leading to uniform LC alignment on NiO films after IB irradiation. IB-irradiated NiO exhibited a high transparency of 85% in the visible light range as compared with the 83% average transmittance of conventional polyimide, which makes the IB-irradiated NiO alignment layer attractive in display devices.

Process Design and Fabrication of Sample for Non-contact Liquid Crystal Alignment with High Yield

In this study, we report the results of an experiment for aligning liquid crystals (LCs) by irradiating ultraviolet rays that are not polarized to an alignment layer, and a simulation result on the alignment principle. In the case of irradiating un-polarized ultraviolet rays to the substrate coated with the alignment film, there have been many experimental reports on the alignment mechanism of the LC due to the anisotropy generated in the substrate, but the theoretical approach is somewhat lacking. As a result of the experiment, the best orientation was observed at around 45°. As a result of the simulation, it was found that the maximum difference between the p-polarization and the s-polarization was within the range of similar angles there was. The theoretical approach to the alignment mechanism of LC was possible. And we have experimentally confirmed that the mechanism of liquid crystal alignment by conventional Groove theory is possible by irradiation of unpolarized UV light. The AFM analysis confirmed that the surface anisotropy that causes this orientation principle actually occurs in the unpolarized UV light. It is expected that this non-contact orientation method can realize higher production yield than the conventional contact type rubbing method, and the result of this study could contribute to the improvement of the yield.

Transition of vertically aligned liquid crystal driven by fan-shaped electric field

Interdigital electrodes are implemented in many commercial and novel liquid crystal devices to align molecules. Although many empirical principles and patents apply to electrode design, only a few numerical simulations of alignment have been conducted. Why and how the molecules align in an ordered manner has never been adequately explained. Hence, this investigation addresses the Fréedericksz transition of vertically aligned liquid crystal that is driven by fishbone electrodes, and thereafter identifies the mechanism of liquid crystal alignment. Theoretical calculations suggest that the periodic deformation that is caused by the fan-shaped fringe field minimizes the free energy in the liquid crystal cell, and the optimal alignment can be obtained when the cell parameters satisfy the relation p/2d=k11/k33, where p is the spatial period of the strips of the electrode; d denotes the cell gap; and k11 and k33 are the splay and bend elastic constants of the liquid crystal, respectively. Polymer-stabilized vertical alignment test cells with various p values and spacings between the electrodes were fabricated, and the process of liquid crystal alignment was observed under an optical microscope. The degree of alignment was evaluated by measuring the transmittance of the test cell. The experimental results were consistent with the theoretical predictions. The principle of design, p/2d=k11/k33, greatly improves the uniformity and stability of the aligned liquid crystal. The methods that are presented here can be further applied to cholesteric liquid crystal and other self-assembled soft materials.

Thermally stable poly(styrene-maleic anhydride) layer modified by ion-beam for liquid crystal orientation

We report uniform and homogeneous liquid crystal (LC) alignment using ion beam (IB)-irradiated poly(styrene-maleic anhydride) (SMA) films as an alignment layer when the IB incidence angle is controlled. The molecular polarity on the SMA surface increased with IB irradiation, especially at an incidence angle of 30°, resulting in strong van der Waals forces between the LC molecules and the surface. Atomic force microscopy and X-ray photoelectron spectroscopy were used to determine the LC alignment mechanism. Chemical structure analysis indicated that the orientation of LC molecules was due to van der Waals forces and selective breaking of C–C bonds by IB irradiation. The potential application of SMA substrates as alignment layers for display devices was further supported by their higher thermal stability (220 °C) compared with that of conventional rubbed SMA (160 °C) and rubbed polyimide (160 °C). Therefore, LC cells with IB-irradiated SMA are potential candidates for alternative alignment layers in LC applications.

Stable and fast switching of liquid crystals on solution-derived compound oxide films irradiated by ion beam

We fabricated stable, homogeneous, fast-switching liquid crystal cells on solution-processed zinc tin oxide films treated by ion-beam irradiation >1200 eV. The ion-beam-irradiated zinc tin oxide films were amorphous and demonstrated good transparency. X-ray photoelectron spectroscopy analysis was used to determine the liquid crystal alignment mechanism with respect to the ion-beam-irradiated zinc tin oxide film. The total response time of electrically controlled birefringence-liquid crystal cells on 2200-eV ion-beam-irradiated zinc tin oxide films was 5.04 ms; the threshold voltage was reduced by 15% compared with rubbed polyimide. Thus, ion-beam-irradiated zinc tin oxide films showed great potential as an alignment layer for high-performance electrically controlled birefringence-liquid crystal cells in industrial applications.

Liquid Crystal Elastomer Actuators from Anisotropic Porous Polymer Template.

Controlling self-assembly behaviors of liquid crystals is a fundamental issue for designing them as intelligent actuators. Here, anisotropic porous polyvinylidene fluoride film is utilized as a template to induce homogeneous alignment of liquid crystals. The mechanism of liquid crystal alignment induced by anisotropic porous polyvinylidene fluoride film is illustrated based on the relationship between the alignment behavior of liquid crystals and surface microstructure of anisotropic polyvinylidene fluoride film. Liquid crystal elastomer actuators with fast responsiveness, large strain change, and reversible actuation behaviors are achieved by the photopolymerization of liquid crystal monomer in liquid crystal cells coated with anisotropic porous films.

Effect of the ion-beam bombardment and annealing temperature on sol-gel derived yttrium aluminum oxide film as liquid crystal alignment layer

We demonstrated a homogeneous liquid-crystal (LC) alignment state on yttrium aluminum oxide (YAlO) films, where the alignment was induced by ion-beam (IB) irradiation. Topographical analysis was performed by atomic force microscopy as a function of annealing temperature. Higher annealing temperatures yielded a smoother surface, accompanied by reduced light scattering. Transparency in the visible region increased on the surface fabricated at higher annealing temperatures. LC alignment mechanism was determined by X-ray diffraction (XRD) analysis. Moreover, IB-irradiated YAlO films annealed at temperatures greater than 200 °C exhibited good thermal stability and low capacitance–voltage hysteresis. The IB-irradiated YAlO films are suitable as alternative alignment layers in advanced LC display applications.

Ion beam-induced topographical and chemical modification on the poly(styrene-co-allyl alcohol) and its effect on the molecular interaction between the modified surface and liquid crystals

We demonstrated uniform liquid crystal (LC) alignment on ion beam (IB)-irradiated poly(styrene-co-allyl alcohol) by modifying the chemical bonding on the surface. An IB-irradiated copolymer was used for the alignment layer. We used physico-chemical analysis to determine the IB-irradiated surface modification and LC alignment mechanism on the surface. During IB treatment on poly(styrene-co-allyl alcohol), IB irradiation induces breaking of chemical bonds on the surface to give rise to new bonds with oxygen atoms. This causes a strong Van der Waals interaction between LCs and the modified surface, thereby resulting in uniform LC alignment. The results of contact angle (CA) studies of the copolymer support the chemical bonding changes that were investigated by XPS. We achieved uniform homogeneous LC alignment and obtained stable electro-optical performance by controlling the IB energy. Therefore, the LC cells with IB-irradiated poly(styrene-co-allyl alcohol) exhibited a good potential for alternative alignment of layers in LC applications.

Superior electro-optical properties of electrically controlled birefringence mode using solution-derived La2O3 films

The authors demonstrate a high performance electrically controlled birefringence (ECB) mode with solution-derived La2O3 films at various molar concentrations. Uniform and homogeneous liquid crystal (LC) alignment was spontaneously achieved on the La2O3 films for lanthanum concentrations at ratios greater than and equal to 0.2. A preferred orientation of LC molecules appeared along the filling direction, and the LC alignment was maintained via van der Waals force by nanocrystals of the La2O3 films. The LC alignment mechanism was confirmed by x-ray photoelectron spectroscopy and high-resolution transmission electron microscopy analysis. Superior electro-optical characteristics of the ECB cells constructed with solution-derived La2O3 films were observed, which suggests that the proposed solution-derived La2O3 films have strong potential for use in the production of advanced LC displays.

Effect of Anisotropic Photo-crosslinking on Liquid Crystal Alignment with Duplicated Surface-relief-gratings†

Surface-relief-gratings with further photo-crosslinkable property were prepared by softlithography using photo-sensitive polyimide containing cinnamate groups as "ink ". The effect of anisotropic photocrosslinking on the thermos-stability of gratings and the alignment of liquid crystal( LC) on gratings was studied. The thermos-stability of the photo-cured gratings was significantly increased. No shape deformation was observed for the gratings after treated at a temperature 31 ℃ above the glass transition temperature of the uncured polyimide. Both the un-exposed and exposed gratings induced uniform LC alignment on their surfaces.By irradiation with linear polarized UV light perpendicular to the gratings grooves,the LC alignment was enhanced. By changing the polarized direction of the UV light and controlling the depths of gratings,the LC alignment direction on the gratings was adjusted. This study result shows that the photo-crosslinkable gratings can achieve better thermo-stability,better LC alignment and adjustable aligned direction to achieve multi-domain alignment. The above research can lead to further understanding of the LC alignment mechanism.

Effect of ion beam irradiation and rubbing on the directional behavior and alignment mechanism of liquid crystals on polyimide surfaces

We investigated the effects of ion beam (IB) irradiation and rubbing on the directional behavior and alignment mechanism of liquid crystals (LCs) on polyimide (PI) surfaces. We found that the LC direction follows the IB irradiation alignment direction on the PI surface regardless of whether the irradiation occurs before or after rubbing. We assumed that the LC direction depends strongly on the C–O bonds created from C=O bonds on the PI surface broken by IB irradiation and conducted an investigation of the chemical bonding state of the PI surface by x-ray photoelectron spectroscopy.

Mechanism of liquid crystal alignment on submicron patterned surfaces

The alignment mechanism of liquid crystals on polymeric surfaces that were patterned using an atomic force microscope (AFM) tip was studied by polarizing optical microscopy. Depending on the thickness of the polymer, polymer chain alignment or grooves appear to be responsible for the liquid crystal alignment. In thick polymer films (above 100 nm) the polymer chains will align in the direction of the scan due to the large lateral force exerted by the tip. In thin polymer films (5–20 nm) the polymer chains are strongly fixed to the surface and will not realign by the tip; instead grooves will be formed. The azimuthal surface anchoring the energy of thick polymer films does not depend on the scan line separation and scan force, is of the same order of magnitude as for conventionally rubbed polyimide surfaces, but increases by the number of scans. AFM patterned pixels do not show a pretilt angle.

Infrared Spectroscopic Studies of the Mechanism of Orientation of Polarized UV-exposed Polyimide Films for Liquid Crystal Alignment

Polarized Fourier transform infrared (FTIR) and ultraviolet-visible (UV-VIS) spectroscopy were used to investigate the mechanism of liquid crystal alignment on polarized UV (PUV) irradiated polyimide (PI) films. PI films strongly absorb below 320 nm resulting in photochemical reaction of the PI. PUV irradiation of the PI film caused decrease of all the peak intensities in the IR, except the newly formed 1396cm−1 peak, due to degradation of the PI molecules. The preferential degradation of PI molecules parallel to PUV irradiation direction results in the predominant orientation of the remaining PI molecules perpendicular to the PUV irradiation direction. But the rubbing of the PI films induced reorientation of the PI molecules parallel to the rubbing direction.

Alignment of a nematic liquid crystal induced by anisotropic photo-oxidation of photosensitive polyimide films

The alignment of a nematic liquid crystal (LC) could be induced by three polyimide (PI) films exposed to linearly polarized ultraviolet (UV) light of long wavelength (366 nm). The alignment could also be modified by changing the direction of polarization of the linearly polarized light (LPL). Infrared (IR) and UV-visible spectroscopy indicated that among the three PI films the PI with a benzophenone moiety and the PI with a diphenyl methane moiety were sensitive to UV radiation and susceptible to remarkable ablation after irradiation with LPL in air. Polarized UV-visible spectroscopy indicated that dichroism of the PI films was caused by LPL and altered by changing the polarization of the LPL. Furthermore, the PI films irradiated under vacuum showed little reduction in intensity of IR bands, which is in sharp contrast to the marked decreases in samples irradiated in atmospheres containing oxygen. This proves that oxygen is necessary to the ablative process of the PI films. Our results indicate that the mechanism of LC alignment on the photosensitive PI films is photoinduced anisotropic oxidative ablation of the PI films. In contrast, the PI film with a diphenyl ether moiety was quite stable under similar irradiation conditions even though the irradiated PI film could also induce alignment of the LC molecules.

Alignment of liquid crystals on polyimide films exposed to ultraviolet light

The influence of unpolarized and linearly polarized UV exposure on previously rubbed as well untreated thin polyimide (PI) alignment layers was studied. Optical retardation and surface morphology measurements were carried out to understand the nature of surface modification as a function of the polarization and the exposure time of the UV light under different surface conditions. The exposure of the UV light on the PI layer was found to change drastically the morphological anisotropy due to photochemical dissociation. The control of anisotropic surface forces by the linearly polarized UV (LPUV) exposure combined with the rubbing process is important to study the alignment mechanism of liquid crystals on various substrates. A simple model incorporating the effect of the LPUV exposure is presented together with the essential features of the experimental results.

LC Alignment Mechanism for the Polarized UV‐Exposed Polyimide Films

AbstractWe used IR dichroism measurements to investigate the mechanism of liquid crystal (LC) alignment in polyimide (PI) films exposed to polarized ultraviolet (PUV) light. After PUV irradiation, we found preferential photodegradation of the PI molecules which were parallel to the PUV polarization direction. The remaining molecular chains of the PI were perpendicular to the PUV polarization direction.

Photopolymerization induced orientation transition in a nematic liquid crystal cell

Abstract We report the observation of a liquid crystal tilt transition from homeotropic to planar orientation induced by photopolymerization of the alignment layer in the absence of liquid crystal. The alignment agent is a unique, polymerizable lecithin (DC23PC), which induces homeotropic alignment before UV exposure. After non-polarized UV exposure, a tilted orientation is obtained. Moreover, further buffing of the UV treated substrate yields a homogeneous alignment. We believe that the conformation change in the lecithin array caused by solid state polymerization is primarily responsible for the transition. These results help to explain the mechanism of liquid crystal alignment and will lead to several potential applications.

Studies on mechanism of liquid crystal alignment on pyrolyzed polyimide langmuir-blodgett films

The mechanism of liquid crystal alignment on pyrolyzed polyimide Langmuir-Blodgett (PPI LB) films was investigated. Structural change of polyimide LB films caused by the pyrolysis was examined by monitoring the in-plane anisotropy of characteristics four vibrational bands. Before the pyrolysis, the polymer chain aligned preferentially along the dipping direction. The pyrolysis at 400°C caused the polymer chain to align more along the dipping direction. The C=O group of the imide ring was suggested to take a more random orientation after the heat treatment, which may be caused by the fission of the C-N bond of the imide ring. These four peaks disappeared when the pyrolyzing temperature was above 600°C. Microcrystals, several nm in size, were observed for a 201-layered PPI LB film pyrolyzed at 1000°C by TEM. The distribution of the lattice spacing and the distortion of the layered structure indicated the incomplete graphitization in the film, which was supported by the observed lattice spacings larger than 0.34 nm expected for a graphite. The molecular arrangement was suggested to be responsible for the liquid crystal alignment on the PPI LB films

Correlation Between Bulk Orderings and Anchoring Structures of Liquid Crystals Studied by Scanning Tunneling Microscopy

The anchoring structures of a homologous series of liquid crystals, n-alkylcyanobiphenyls (mCBs: m=7-12), on molybdenum disulfide (MoS2) are directly observed by scanning tunneling microscopy (STM) in order to analyze the alignment mechanism of liquid crystals. Based on highly reproducible STM images, the anchoring structures of mCBs are clearly divided into two categories, a monolayer (single-row) type (7CB, 8CB, 9CB and 11CB) and a bilayer (double-row) type (10CB and 12CB). Here we propose the monolayer is the anchoring structure of a nematic phase, while the bilayer is the anchoring structure of a smectic phase. The correlation between bulk phase orderings and anchoring structures of liquid crystals is discussed from the viewpoint of surface-phase formation for the mCBs and different solid substrates.

Inte raction between Liquid Crystals and Treated Solid Surfaces; Single Crystalization

It is well known that liquid cystals are used for the display devices in watches, calculators, and TV receivers. To improve such devices, we understand more exactly the mechanism of liquid crystal alignment, the synthesis new liquid crystals, the substrate treatment method, and the electrical driving method. We only present here phenomenologically the alignment, the treatment method, and the liquid crystals now in use.