Electric force microscopy study of the surface electrostatic property of rubbed polyimide alignment layers

Two polyimides were synthesized for use as alignment layers. The pretilt angles of the liquid crystals, 4-cyano-4′-n-pentylbiphenyl, on the two polyimides were measured by the crystal rotation method. The relative surface atomic concentrations of F/C (%) were measured by X-ray photoelectron spectroscopy. Electric force microscopy was utilized to investigate the surface electrostatic property of the two thin polyimide alignment layers before and after rubbing. All results demonstrate that rubbing causes trifluoromethyl moieties to migrate towards the surface, absorb negative charges and orient along the rubbing direction. Thus, it is proposed that distributions of functional groups on the surface of the polyimide after rubbing are anisotropic and the van der Waals forces between the polar groups and liquid crystal molecules play an important role in the uniform orientation of the liquid crystal molecules.     

Polyimide photo-alignment layers for inclined homeotropic alignment of liquid crystal molecules

We have succeeded in realizing an inclined homeotropic alignment of liquid crystal (LC) molecules by using photo-aligned films of a polyimide containing azobenzene in the backbone structure. To induce such an LC alignment, a side chain structure was introduced into the backbone structure. The LC pretilt angle, measured from the surface normal, could be controlled up to 1.75° by varying the light exposure in oblique angle irradiation with unpolarized light. Its thermal stability was examined by annealing the LC cell at 100 °C. No change was observed in the pretilt angle even after annealing for 36 h, indicative of its excellent thermal stability. Since photo-alignment has patterning capability, the photo-aligned polyimide film is expected as a promising alignment film for multi-domain vertical alignment mode LC displays.     

基于1,2,4-三羧基-3-羧甲基环戊烷二酐的可溶性聚酰亚胺垂直取向剂的制备及性能

采用1,2,4-三羧基-3-羧甲基环戊烷二酐（TCA）、4,4’-二氨基二苯甲烷（MDA）及长侧链二胺4-十二烷氧基苯甲酸酯联苯酚-3’,5’-二胺基苯甲酸酯（DBPDA）共聚制备了一系列的聚酰亚胺。并对其溶解性能、透光性能及取向性能进行了测试。结果显示,聚酰亚胺在有机溶剂中可溶,成膜后紫外透光率较高,在波长400 n...     

Use of polydimethylsiloxane thin film as vertical liquid crystal alignment layer

The possibility of the use of polydimethylsiloxane to align liquid crystals is studied. The polydimethylsiloxane thin films coated on glass substrates exhibit a very low surface free energy, and are found to be capable of vertical liquid crystal alignment. Comparing to thin films made using a typical polyimide polydimethylsiloxane films possess a more stable surface energetic state. The pretilt angle of the calamitic liquid crystal molecules sandwiched between the silicon elastomer thin films was virtually 90°, and the molecular tilt can be maintained in a wide temperature range. Liquid crystal sandwiched between the silicon-based polymer thin films can operate in the in-plane-switching mode.     

Van der Waals force contribution to the vertical alignment of liquid crystal on Al2O3 films using ion-beam method

This study demonstrates liquid crystal (LC) alignment on Al₂O₃ films using the ion-beam (IB) method as well as the optimization of the IB irradiation condition. Uniform LC alignment was achieved at an IB incident energy of 1800 eV with an IB incident angle of 45°, while inferior LC alignments were observed in other tested conditions. The pretilt angles and transmittances of the LC cell were also shown as part of the same trend for the LC alignment states. This result was subject to van der Waals forces which were caused by topographical changes during the IB process.     

Effects of backbone conformation and surface texture of polyimide alignment film on the pretilt angle of liquid crystals

Polyimides (PIs) with different inclination angle of polymer backbones, together with polar hydroxyl group and/or nonpolar trifluoromethyl group at various sites of the backbone were synthesized and used as liquid crystal alignment layers. The molecular conformation, surface chemistry, surface energy, surface morphology, and pretilt angle of the PI film were investigated. The distributions of fluorinated group and hydroxyl group at different depths of the PI surfaces were analyzed by X-ray photoelectron spectroscopy. Effects of the conformation of the PI molecular backbone on the surface morphology of the rubbed PI layer, the pretilt angle and surface energy of the alignment film were studied. The PI which contains both nonpolar fluorinated groups sticking out of the surface and the polar hydroxyl groups on the surface exhibits high pretilt angle.     

Effect of the shape of imprinted alignment layer on the molecular orientation of liquid crystal

The molecular alignment of liquid crystals was studied with three different imprinted patterns. The three different shapes of molds (square-, V- and U-shaped) replicated the microstructures with electroplating process was fabricated. Surfaces with three different microgrooved patterns were used as the alignment layer of the liquid crystal cell. The effect of the shape of imprinted alignment layer on the molecular orientation of LC was investigated. Among these patterns, U-shaped groove pattern resulted in a high performance of liquid crystal orientation, pretilt angle and anchoring energy, which are quite comparable to the conventionally rubbed layer. It clearly shows that the molecular orientation of liquid crystals is influenced by the shape of microgrooved patterns.     

Near-field characterization of planar photonic-crystal-waveguide structures

Characterization of photonic-crystal-waveguide (PCW) structures at telecommunication wavelengths with a collection scanning near-field optical microscope (SNOM) is considered. The propagation of light in silicon-on-insulator PCWs, formed by removing a single row of holes in the triangular lattice and connecting to access ridge waveguides, is imaged with the SNOM. High-contrast and high-resolution SNOM images of PCW structures containing straight PCWs as well as 90 degrees gradual and 60 degrees sharp PCW bends are obtained in the wavelength range of 1520-1570 nm, allowing for the determination of PCW mode characteristics and bend loss. We relate optical-signal variations along straight PCWs to the interference between a quasi-homogeneous background field and Bloch harmonics of the PCW mode, and use spatial-frequency spectra of the intensity variations to determine the dispersion of the PCW mode propagation constant. The bend loss is evaluated using averaged signal cross-sections and taking into account the fact that the signal is proportional to the PCW mode amplitude. It is directly shown that the bend loss for 60 degrees bends is significantly decreased for smoothened bends having one or three holes moved from the inner side of the bend corner to the outer one. The possibilities and limitations of SNOM imaging for the characterization of PCW structures are discussed.     

Tunable liquid crystal microlens array using hole patterned electrode structure with ultrathin glass slab

A configuration of hole patterned electrode liquid crystal microlens array with an ultrathin glass slab was fabricated. To reduce the fringing electric field effect and avoid the occurrence of disclination lines, an ultrathin glass slab was introduced between the patterned electrode and liquid crystal layer. The glass slab thickness played an important role in effecting the optical performance of the liquid crystal microlens array. An optimum thickness of 30 μm was selected employing numerical simulation method. Using this method, we demonstrated a microlens array that greatly improved the phase profile and focus power. The dynamic focal range of the liquid crystal microlens array may extend from <1.2 mm to >8 mm and the minimum diameter of the focus spot could be as small as 15 μm.     

Homogeneously aligned liquid crystal display on silicon oxynitride thin film using ion beam bombardment

This paper introduces the homogeneously aligned liquid crystal (LC) display using ion beam (IB) bombardment of a new silicon oxynitride (SiON) alignment layer for the first time. The pretilt angle was shown to be a function of the IB incident energy, and possible mechanisms of LC alignment were investigated with physical and chemical methods. An X-ray photoelectron spectroscopy analysis showed that the LC alignment on the IB-bombarded SiON inorganic surface was due to the reformation of Si–O bonds as a major factor. The electro-optical characteristics were comparable to those of rubbed polyimide films.     

Screening effect of a-C:H alignment layer in nematic liquid crystal cell

The electrical and optical characteristics of nematic liquid crystal (NLC) cells and their dependence on the thickness of an amorphous hydrogenated carbon (a-C:H) alignment layer have been studied. An increase in the a-C:H layer thickness favors enhancement of the screening of a bias voltage applied to this layer, which is manifested by an increase in the threshold voltage of the electrooptical splay effect. This is accompanied by a decrease in the initial (pretilt) director angle, which is evidence for an increase in the anchoring energy due to the field of space charge localized at the interface.     

Polyazomethine/carbon nanotube composites

Composites were synthesized by “in-situ” polymerization of polyazomethine, a liquid crystal polymer (LCP), in presence of multi-walled carbon nanotubes (MWNTs) previously dispersed in one of the employed monomers. Fiber processing was carried out by extrusion from the composites containing 1 and 10 wt.% of MWNTs at the mesophase temperature. We have observed that the typical highly oriented internal fibrillar structure can be significantly disrupted by increasing the nanotube content in the composite fibers. Evidences of MWNT alignment were found in the studied LCP/MWNT composites.     

Long memory retention time and high contrast ratio in a tristate liquid crystal display device

We propose a method to obtain long memory retention time and high contrast ratio in a tristate liquid crystal display device. The proposed device has three unique liquid crystal (LC) states that are known as splay, π twist, and bend, with different operating schemes for the two modes of operation, which are the memory (M) mode or dynamic (D) mode. A form of selective vertical or horizontal switching changes the optically compensated π twist LC as a common black state in both the M and D modes into a bend LC state for the D mode or into the splay LC state for the M mode, respectively.     

Dynamic characteristics of dual-frequency nematic liquid crystal with quasi-homeotropic twist structure

Dynamic characteristics of a liquid crystal (LC) cell with a quasi-homeotropic twist structure formed in a dual-frequency nematic liquid crystal (DFNLC) layer with the director pretilt angle increased to 60° have been experimentally studied. The cell was switched from the off to on state using a 30-kHz electric field, while the reverse (off/on) switching was effected by a 1-kHz field. An increase in the director pretilt angle allowed the switch-on time of a 6.4-μm-thick DFNLC cell to be reduced to 1 ms and the relaxation (switch-off) time, to 0.5 ms.     

Chain flexibility versus molecular entanglement response to rubbing deformation in designing poly(oxadiazole-naphthylimide)s as liquid crystal orientation layers

Four poly(oxadiazole-imide)s containing naphthalene rings, with different flexibility and molecular weight, are investigated with respect to their rheological properties to establish the optimal processing conditions from solution phase to film state for liquid crystal orientation purposes. The film uniformity and strength are determined by monitoring the flow behavior and chain entanglements. The solution rheological data are in agreement with film tensile testing, revealing that higher molecular weight favors chain entanglements and implicitly the film mechanical resistance. In order to analyze the suitability of these films as alignment layers their surface is patterned by rubbing with two types of velvet. Liquid crystal alignment of 4′-pentyl-4-biphenylcarbonitrile nematic is tested by polarized light microscopy. The resulting behavior is correlated with the polyimide malleability and characteristics of the textile fibers, namely their polarity, size, and mechanical features. The competitive effects between chain flexibility and entanglements, together with the interactions occurring between the polymer and velvet are analyzed in order to explain the surface regularity, which influences the uniformity of the liquid crystal alignment. The contrast between dark and bright states recorded on the liquid crystal cell indicates that some of these polynaphthalimides are promising candidates for liquid crystal display devices.     

Photo-induced liquid crystal alignment of poly(vinyl cinnamate) and fluorinated polyimide blends

Liquid crystal(LC) alignment properties were mainly affected by surface properties of alignment layers. In our previous work, we prepared poly(vinyl cinnamate) (PVCi) and polyimide blend alignment layer for thermally stable LC alignment. In this work, we utilized fluorinated polyimide for blend alignment layers in order to modify surface properties of alignment layers. For this purpose, polyimides containing fluorine unit were synthesized and used for the blend alignment layers. Fluorine containing diamine, 4,4′-bis[2-(4-aminophenyl)hexafluoropropyl]-diphenyl ether(BDAF), is used for the polyimide synthesis. We prepared the fluorinated polyimide and PVCi blend alignment layers and investigated the effect of fluorine on the LC alignment properties and pretilt angle of LC.     

Distributed feedback leaky laser emission from dye doped gel-glass dispersed liquid crystal thin film patterned by soft lithography

Gel-glass dispersed liquid crystal (GDLC) thin films doped with organic laser dye Rhodamine 6G (R6G) were prepared via a sol-gel procedure of tetraethyl orthosilicate (TEOS) and ethyl triethoxisilane (ETES). As characterized by scanning electronic microscope (SEM), surface-relief structures were successfully patterned on lower refractive index GDLC thin films by soft lithographic technology, which support distributed feedback (DFB) laser emission based on leaky mode propagation. The performance of the DFB laser emission was investigated and the spectral narrowing of the emitted radiation and the fine structure pattern were found to be controlled by the doping concentration of liquid crystal (LC) 4-cyano-4′-pentylbiphenyl (5CB). We also showed the synchronous excitation of a DFB lasing with random lasing mediated by light scattering inside the same GDLC leaky waveguide.     

The driving force for homeotropic alignment of a triphenylene derivative in a hexagonal columnar mesophase on single substrates

An interesting discotic triphenylene derivative, 2,7-biscarbethoxy-3,6,10,11-tetrapentyloxytriphenylene, can be homeotropically aligned on a large scale as a film on single substrates, i.e. the film with one side on the substrate and the other side freestanding. The alignment is achieved in a controlled condition close to a thermodynamic equilibrium state in a liquid crystal phase when interactions at the substrate surface are limited. Phase changing properties of this material are also a key factor in the alignment processes. The aligned structures are observed with an optical microscope. Within a single domain a hexagonal lattice of a d-spacing 16.3 Å is confirmed by X-ray diffraction measurement.     

Cells cultured on microgrooves with or without surface coating: Correlation between cell alignment,spreading and local membrane deformation

The behaviors of cells cultured on patterned substrates vary with the material stiffness, the geometry and the biochemical properties of the pattern. By using a reversed cell imprinting (RCI) technique, together with phase contrast microscopy, scanning electron microscopy (SEM) and atomic force microscopy (AFM), we have exploited reversed side cellular morphology on patterned microgrooves of different geometries with or without surface coating of adhesion molecules. We have shown a close correlation between the effect of contact guidance and penetration of cellular membrane. Without surface coating, roughly 80% of HeLa cells were aligned along the groove direction regardless of the groove spacing. When the microgrooves were coated with fibronectin, the area of cell spreading was increased but the percentage of aligned cells was significantly decreased. In both cases, the deformation of cell membrane at the cell-pattern interfaces could be measured. We found that the local penetration of the cellular membrane into the grooves was correlated to the cellular alignment for both HeLa and NIH 3T3 cells, and that such a correlation was cell-type dependent.     

Light‐Patterned Crystallographic Direction of a Self‐Organized 3D Soft Photonic Crystal

Uniform and patterned orientation of a crystallographic direction of ordered materials is of fundamental significance and of great interest for electronic and photonic applications. However, such orientation control is generally complicated and challenging with regard to inorganic and organic crystalline materials due to the occurrence of uncontrollable dislocations or defects. Achieving uniform lattice orientation in frustrated liquid‐crystalline phases, like cubic blue phases, is a formidable task. Taming and tailoring the ordering of such soft, cubic lattices along predetermined or desired directions, and even imparting a prescribed pattern on lattice orientation, are more challenging, due to the entropy‐domination attribute of soft matter. Herein, we disclose a facile way to realize designed micropatterning of a crystallographic direction of a soft, cubic liquid‐crystal superstructure, exhibiting an alternate uniform and random orientation of the lattice crystallographic direction enabled by a photoalignment technique. Because of the rewritable trait of the photoalignment film, the pattern can be erased and rewritten on‐demand by light. Such an oriented soft lattice sensitively responds to various external stimuli such as temperature, electric field, and light irradiation. Furthermore, advanced reflective photonic applications are achieved based on the patterned crystallographic orientation of the cubic blue phase, soft lattice.