Liquid Crystal Molecular Alignment Research Articles

The effects of side-chain flexibility on the photosensitivity of polyimide alignment film

ABSTRACT To examine the impact of side-chain flexibility on the photosensitivity of polyimide liquid crystal (LC) alignment films, two polyimides (OD-HAB-CA and OD-HAB6-CA) were synthesised. OD-HAB6-CA contains flexible hexamethylene side chains. This study demonstrates that the OD-HAB-CA alignment film can induce LC molecular alignment after 80 min of non-polarised ultraviolet light (NPUVL) irradiation. In contrast, the OD-HAB6-CA alignment film can achieve the same result after only 35 min of irradiation. This suggests that the flexibility of the side-chain can significantly improve the photosensitivity of the alignment film and shorten the required irradiation time. In addition, OD-HAB-CA and OD-HAB6-CA exhibit good thermal stability and light transmittance properties. The resulting LC cell have pretilt angles of 0.2° and 0.5°, respectively, which are suitable for in-plane switching mode LCDs.

Electrically tunable dual-layer twisted nematic liquid crystal THz phase shifters with intermediate composite polymer thin film

We demonstrate electrically tunable twisted nematic (TN) aligned liquid crystal (LC) terahertz phase shifters with a novel structure, in which a single LC composite polymer thin film is inserted in the middle of the LC cell. For designing such types of phase shifters, we have applied the Mauguin formalism used in LC-based switches for applications in the visible. Experimental results are in good agreement with those predicted by the theoretical model. Using this design, the phase shift has been increased by 14.3° at 1.2 THz in comparison to the conventional design (98.1°). The threshold voltage has been lowered from 0.81 VRMS to 0.48 VRMS while the driving voltage for quarter-wave (π/2) phase shift operation has been reduced from 8.4 VRMS to 6.3 VRMS at 1.2 THz. The dynamic response of devices also enhances significantly. Such design has also successfully been adopted to demonstrate a 2π phase shifter with similar improvement. Full-wave operation is achieved at a bias of 25 VRMS at 1.2 THz using a highly birefringent LC material. Order parameter calculations show that the polymer film improves significantly LC molecular alignment in the thick LC cell for THz applications. We have also compared the figure of merits (FOMs) of the present devices with previously reported THz π/2 and 2π phase shifters.

Transformation between elastic dipoles, quadrupoles, octupoles, and hexadecapoles driven by surfactant self-assembly in nematic emulsion.

Emulsions comprising isotropic fluid drops within a nematic host are of interest for applications ranging from biodetection to smart windows, which rely on changes of molecular alignment structures around the drops in response to chemical, thermal, electric, and other stimuli. We show that absorption or desorption of trace amounts of common surfactants can drive continuous transformations of elastic multipoles induced by the droplets within the uniformly aligned nematic host. Out-of-equilibrium dynamics of director structures emerge from a controlled self-assembly or desorption of different surfactants at the drop-nematic interfaces, with ensuing forward and reverse transformations between elastic dipoles, quadrupoles, octupoles, and hexadecapoles. We characterize intertransformations of droplet-induced surface and bulk defects, probe elastic pair interactions, and discuss emergent prospects for fundamental science and applications of the reconfigurable nematic emulsions.

A coupled Ericksen/Allen–Cahn model for liquid crystal droplets

We present a model and discretization that couples the Ericksen model of liquid crystals with variable degree of orientation to the Allen–Cahn equations with a mass constraint. The coupled system models liquid crystal droplets with anisotropic surface tension effects due to the liquid crystal molecular alignment. The total energy consists of the Ericksen energy, phase-field (Allen–Cahn) energy, and a weak anchoring energy that couples the liquid crystal to the diffuse interface. We describe our discretization of the total energy along with a method to compute minimizers via a discrete gradient flow algorithm which has a strictly monotone energy decreasing property. Numerical experiments are given in three dimensions that illustrate a wide variety of droplet shapes that result from their interaction with defects.

Effect of crosslinking polymer networks on the molecular reorientation and electro‐optical performance of in‐plane switching vertically aligned liquid crystal devices

ABSTRACTThe effects of crosslinking polymer networks (PNs) on the molecular reorientation and electro‐optical properties of vertically aligned (VA) liquid crystal (LC) devices are investigated by applying an in‐plane switching (IPS) electric field. Through the polymerization process, crosslinking PNs are developed on the substrate surface, effectively increasing the anchoring energy and governing the LC molecular reorientation. With its stronger anchoring effect, the PNs cell shows good light transmittance and excellent vertical alignment quality, as compared to the pure LC cell. Furthermore, the alignment transformation and transmittance bounce resulting from the transient process of LC molecular reorientation are eliminated when the cell is operated at high voltages. The rising‐time (tr) and falling‐time (tf) responses of the PNs cell are significantly improved, and around 36% improvement in the optical switching response is obtained. In addition, the dynamic gray‐level tr and tf responses of the PNs cell are enhanced by around 55% and 42%, respectively, at a low driving voltage (∼12 V). This developed VA‐IPS LC/PNs cell benefits not only the LC molecular alignment but also the electro‐optical performance. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015, 53, 1123–1130

Performance enhancement using a non-uniform vertical electric field and polymer networks for in-plane switching of multi-pretilt, vertically aligned liquid crystal devices.

A simple and reproducible alignment method for fabricating vertically aligned (VA) liquid crystal (LC) cells with a multi-pretilt structure is developed. A non-uniform vertical electric field is employed in the LC/monomer mixed cells during the photocuring process, and two pretilt domains with a functional small pretilt angle (∼1.6°) in the stabilized VA LC/polymer cells are achieved. The enhanced electro-optical performance of the cell driven by an in-plane switching field is demonstrated. Compared to the pure cell, the 2 wt.% pretilt angle cell shows 36%, 64%, and 76% improvement in the optical switch, the gray-level rise time, and the gray-level fall time responses, respectively, which are obtained at a low driving voltage (≤12 V). When applied to LC devices, the proposed method not only effectively benefits the LC molecular alignment, but it also significantly boosts the electro-optical performance.

Synthesis of Chiral Azobenzene and their Effect on Cholesteric Liquid Crystal

A kind of chiral azobenzene was synthesized. The formation of chiral azobenzene was confirmed by IR and1H NMR spectroscopy, and mesomorphic properties were investigated by DSC and POM. The reversible optically induced switching effect betweentransandcisisomer of chiral azobenzene was demonstrated by a UV/Vis/NIR spectrophotometer. It was found that cholesteric liquid crystal molecular alignment could be adjusted by these chiral azobenzene.

Relationship between rubbing-induced anisotropy and surface azimuthal anchoring energy of nematic liquid crystals

The relationship between the rubbing-induced anisotropy of alignment films and the surface azimuthal anchoring energy of nematic liquid crystals was investigated using three kinds of rubbing cloths. Reflection ellipsometry revealed that the optical surface anisotropy of polyimide alignment films increases monotonically with increasing RS. The surface azimuthal anchoring energy measured by the torque balance method depends on the rubbing-induced anisotropy of alignment films. This indicates that liquid crystal molecular alignment can be controlled by monitoring the rubbing-induced optical anisotropy of alignment films when a suitable rubbing cloth is used.

Fluorescence enhancement of dye-doped liquid crystal by dye-induced alignment effect

We investigated fluorescence from hemicyanine dye molecules in a liquid crystal (4,4′-n-pentylcyanobiphenyl) (5CB) medium at different temperatures. The fluorescence decay lifetime decreased monotonically irrespective of the thermodynamic phases of the host medium as the temperature was increased. This behavior is due to an intramolecular motion of the dye promoted with the decrease in the viscosity of the medium facilitating a nonradiative decay of the excited dye molecules. By contrast, fluorescence intensity from the dyes in the nematic phase was about 3 times stronger than that in the crystalline or isotropic phase. This fluorescence enhancement in the nematic phase was found to be due to an anisotropic alignment of the dye molecules following the anisotropic alignment of the host liquid crystal medium along the pump-beam polarization direction. This light-induced liquid crystal molecular alignment was markedly enhanced by the guest dyes preferentially excited along the pump-beam polarization direction. The orientational order parameter of the dyes in the liquid-crystalline phase deduced from fluorescence anisotropy measurement was similar to the known order parameter of the liquid crystalline 5CB.

全息方法制备嵌段式液晶弹性体

The preparation method of triblock Liquid Crystal(LC) elastomer films with thermal induced contraction was researched and their polarized characteristics and thermal induced contrasction rates were analyzed.By using the holographic exposure technology,a liquid crystal elastomer with a grid-like structure was prepared.Based on a polarization microscope and a He-Ne laser,grid-like structure and polarization were detected,respectively.The relationship between the LC molecular alignment and the polarization of LC elastomer was analyzed.In addition,the thermal induced contraction rate for LC elastomer was tested.The experimental results indicate that LC molecules are arranged perpendicular to grid-like structure.The deformation rate of thermal induced contraction for the liquid crystal elastomer can be achieved 12% in the LC phase transition temperature(65-80℃).

An efficiently tunable microring resonator using a liquid crystal-cladded polymer waveguide

An electrically tunable polymer microring resonator of large tunability and low applied voltage is demonstrated using active liquid crystal (LC) cladding. A large tuning range of 0.73 nm is achieved due to more homogenous LC molecular alignment and enhanced interaction of the light with the LC cladding in the simplified polymer waveguide structure. The operating voltage decreases to 10 V with a threshold of only 3 V by the utilization of interdigital electrodes.

Improvement of Electro-Optical Characteristics of Liquid Crystal Display by Nanoparticle-Embedded Alignment Layers

The effect of various metal-oxide nanoparticles embedded in the alignment layer of a liquid crystal (LC) display is experimentally investigated. The study mainly focuses on the electro-optical properties of a twisted-nematic LC display. It is confirmed that a certain type of nanoparticles contribute to the enhancement of contrast ratio. In addition, it is observed that for almost all types of nanoparticles used in the present study, the statistical error of electro-optical properties of LC test cells reduces as compared to that of cells without nanoparticles, indicating the improvement of LC molecular alignment. For a further clarification, AFM study and other related measurements are performed.

Polymer planar waveguide device using inverted channel structure with upper liquid crystal cladding

We propose a composite waveguide configuration based on an inverted polymer channel structure with upper nematic liquid crystal cladding. This configuration can achieve a more homogenous liquid crystal molecular alignment between the core and the liquid crystal material by minimizing the rubbing damage during preparation of the alignment layer. We demonstrated our idea with a variable optical attenuator which exhibited a 24 dB of attenuation range over a tuning peak voltage of 10 V at 1550 nm.

Polarization-Independent Liquid-Crystal Grating with Microscale Alignment Pattern

A polarization-independent liquid crystal (LC) grating with a microscale LC molecular alignment pattern is proposed. The microscale LC alignment pattern is achieved using a microrubbing technique. The optical properties of the proposed LC grating are theoretically discussed in detail using Jones matrix method. Optimum condition for obtaining maximum +1st or -1st diffraction efficiencies is derived. The diffraction efficiency of the LC grating is measured and the diffraction efficiency characteristics are compared with the theoretical result. Furthermore, the dependence of the polarization direction of the incident light on the diffraction efficiency is discussed.

Surface alignment of liquid crystal multilayers evaporated on photoaligned polyimide film observed by surface profiler

The investigation of the surface alignment of liquid crystal (LC) multilayers evaporated on photoaligned polyimide vertical alignment (PI-VA) film was carried out by means of the novel three-dimensional (3D) surface profiler. We report the first use of the surface profiler to visualize a microscopic image of the monolayer arrangement of LC molecules in contact with the surface of photo-treated PI-VA film. The photoinduced anisotropy of partially UV-exposed PI-VA film can be visualized as a topological image of LC multilayers. It seems that the topology of LC multilayers is indicating the orientational distribution of LC molecules on the treated film. It was found that the periodically photoaligned PI-VA film surface can align an adsorbed LC monolayer and the LC molecular alignment can be extended to the bulk via the epitaxylike LC–LC interaction, i.e. a short-range molecular interaction. With regard to the unexposed PI-VA film surface, noticeable anisotropy in the monolayer alignment was not observed, indicating that the long-range elastic interaction may be responsible for the bulk alignment. The appearance of small droplets in the masked region may be presumably related to the dewetting phenomena.

Liquid-Crystal Fresnel Zone Plate Fabricated by Microrubbing

A novel liquid-crystal (LC) Fresnel zone plate (FZP) is fabricated only by the LC molecular alignment process without the need for surface relief structures and patterned electrodes. It is theoretically described that the Fresnel-lens-like phase profile is achieved under optimum LC molecular orientation conditions and the polarization state of incident light, that is, the retardation of the LC cell is (n+1/2)λ (n is an integer) and the incident light is circularly polarized. The optical properties of the fabricated LC FZP are experimentally investigated. Good focusing properties are confirmed (the full width at half maximum of the spot is approximately 8% larger than that of the diffraction limit) and the light intensity of the focusing spot is found to be controllable by adjusting applied voltage.

Alignment mechanism of liquid crystal in a stretched porous polymer film

This article discusses the mechanism of nematic liquid crystal alignment in stretched porous polymer films. The polymer films were formed by extreme stretching of an isotropic porous polyolefin, such that the draw ratio was 12:1. A 6-μm-thick porous film with a high porosity coefficient of 92% revealed fine string-shaped areas that exhibited optical anisotropy due to their possessing a high degree of molecular alignment. The porous film was filled with nematic liquid crystal and then the composite film was sandwiched between transparent electrodes coated onto glass substrates, without the use of conventional alignment layers. From polarizing microscopy observations it was found that the string-like polymer areas induce liquid crystal molecular alignment. The liquid crystal cells can exhibit an electrically controlled birefringence effect. This alignment technique enables us to realize three-dimensional control of liquid crystal alignment.

Morphological Control of Polymer Network in Ferroelectric Liquid Crystal.

The morphology of polymer fiber network formed in ferroelectric liquid crystal devices was closely observed by optical and scanning electron microscopes, and the formation mechanism of the polymer was investigated for obtaining a rigid polymer structure supporting substrates. It was found that aligned polymer fibers are formed with fibril aggregation, which is induced by photopolymerization of monofunctional acrylate monomer aligned in parallel with the liquid crystal molecular alignment. As the monomer concentration increases, the polymer fibers are transformed into thicker and harder structures. This self-assembling phenomenon is thought to be induced by the interfacial tension of the liquid-state polymer with comparatively low molecular weight during the polymerization, and the transient unstable polymer morphology that appears in the phase separation process can be fixed by rapid curing of the polymer material.

Polymer Surface-Induced Order of Liquid Crystalline Molecular Alignment Based on Nematic–Isotropic Phase Transition Behavior

Alignment order of liquid crystal (LC) molecules at the (polymer/LC) interface was evaluated by comparing nematic-isotropic phase transition temperatures (TNI) at the interface with TNI in the bulk. The thickness dependence of TNI was measured using the LC cell in which an LC thin layer was sandwiched between polymer surfaces without surface treatment. The order of LC molecular alignment at the (polymer/LC) interface increased with surface free energy of the polymer substrate. The surface of crystalline polymer provided higher order of LC molecular alignment. Hydrophobic intermolecular interactions at the (polymer/LC) interface may be more responsible for increase in the interfacial molecular alignment order of LC in comparison with the hydrophilic one.

1C10 shearingによる液晶分子配向処理 (II)

1C10 shearingによる液晶分子配向処理 (II)