Surface Of Alignment Layer Research Articles

63.3: Fast Switching Surface‐Polymer‐Assisted IPS Liquid Crystal Displays

AbstractWe demonstrate fast switching surface‐polymer‐assisted in‐plane switching (SPA‐IPS) liquid crystal displays (LCDs). The formation of polymer nano islands at the substrate surface as the alignment layers is achieved by liquid crystal (LC) directed polymerization of a small amount of a reactive monomer in homogenously aligned liquid crystal. At appropriate concentration, the phase‐separated nano‐sized polymer islands, with a preferential orientation direction following the rubbing direction, act as internal surface alignment layers and enable the fast switching of field‐induced reorientation of liquid crystal molecules. Furthermore, significant improvement in switch‐off time of a SPA‐IPS will be due to the increased surface anchoring strength by the low‐density surface‐localized polymer islands.

The effect of surface alignment on analog control of director rotation in polarization stiffened SmC* devices

The effect of surface alignment on the achievement of analog (“V”-shaped) electric field control of director rotation in SmC* liquid crystal devices is investigated experimentally and through numerical modeling. Ferroelectric SmC* liquid crystals are intrinsically analog and thresholdless, i.e. the director can be rotated freely around the tilt cone. Whether or not a SmC* liquid crystal cell exhibits thresholdless switching depends strongly on the influence of the cell’s alignment layers, on the magnitude of the liquid crystal’s spontaneous polarization, and on whether smectic layers adopt a bookshelf or chevron configuration. To study the effect of the surface alignment layers, we have exploited a technique for the vertical (bookshelf) alignment of the smectic layers that does not depend on surface anisotropy. The alignment technique allows an experimental study of the influence of surfaces spanning a wide range of pretilt angles, azimuthal and zenithal anchoring energies. This technique is used to study the effect of surfaces on the threshold behavior of director rotation in SmC* materials under the influence of an electric field. The alignment technique also allowed us to use a high-PS liquid crystal material having an I-A-C phase sequence and reduced layer shrinkage thought to be well suited to thresholdless switching. We show that the alignment layer has a strong effect, and that excellent analog response can be achieved for the case of alignment layers which promote homeotropic director orientation. We further model and discuss the potential effect of a thin layer of nematic at the surface and the possibility of gliding of the easy axis during switching.

TFT-LCD取向层表面的针孔缺陷分析

The pinholes on the surface of alignment layer was studied, which formed during the TFT-LCD manufacturing process and caused defects of the products. Herein, a vitally typical defect, black spot, was analyzed via FIB, SEM and OMS. The analysis results of statistical software BO (Business Objects) show that the pinhole is the main cause of the blackspot. Based on theoretical analysis and further experimental results, it is found that the hydrodynamic in stability of the solution may lead to the pinholes on the alignment layer surface, and the precure time of the alignment film solution is significantly responsible for hydrodynamic instability. It is noteworthy that the instability needs times to grow full enoughand then to influence the film structure. Therefore, the pinholes on the alignment layer surface will be reduced or eliminated if the precure time is less than or equal to that time.

62.1: Reduction of the Threshold Voltage and Enhancement of Contrast Ratio in Liquid Crystal Devices with BaTiO3 Nanoparticle Embedded Surface Alignment Layers

AbstractHerein, we report the effect of a 40% weakening of the surface polar anchoring energy of liquid crystal devices when the polyimide layers were doped with the nanoparticles of BaTiO3. This effect leads to the reduction of the threshold voltage by 6% and also the enhancement of the contrast ratio of twisted‐nematic liquid crystal displays (TN‐LCDs) at the saturation voltage by 130%.

P‐139: Investigation of the Behaviors of Liquid Crystal at the Surface under Applied Electrical Field by Reflective Measurements

AbstractVariation of tilt angle on surface of alignment layer under electric field was investigated by reflection measurement in thin (2–5 μm) twisted LC cell. Dependence of tilt angle from voltage was used for determination of polar anchoring properties of two alignment materials with different polar anchoring energy. It introduces the new method for investigation of polar anchoring properties of nematic liquid crystal cell, which has twisted structure and small cell gap.

Selective Crosslinking of Polyimide for Photonic Devices

Vertically aligned nematic (VAN) liquid crystals (LC) are interesting for many photonics applications. The performance of VAN cells depends on different factors; the boundary interactions between the LC monolayer and the alignment layer surface are the most important ones. These interactions are determined by anchoring energy. A set of procedures have been developed to modify by crosslinking the alignment properties of a specific polyimide (PI) for VAN cells; this PI produces vertical alignment with a small pretilt. We used two diepoxides as crosslinking agent of PI. The cells show different switching voltages for crosslinked and not crosslinked areas in the device. A small variation of the voltage, such as 1–2 V, can be adequate to generate phase grating with vertically aligned cell.

Anchoring of Liquid Crystals on Self-Organized Microwrinkles

The self-organized microwrinkles can serve as a surface alignment layer to align nematic liquid crystals, which is primarily based on the groove mechanism. The azimuthal anchoring energy is discussed and estimated from the groove topography and the actual twist angle in the twisted nematic cell.

P‐114: Effect of the Surface Alignment Layer on Analog Switching of Ferroelectric Liquid Crystals

AbstractWe investigated the effect of the surface alignment layer on the analog switching properties of ferroelectric liquid crystals. Using a liquid crystal that does not exhibit a nematic phase allowed us to obtain alignment of the smectic layers independent of the surface alignment layer used. This allowed to test various alignment layers that have not been tested before, and to demonstrate improved performance.

Photostability of liquid crystals and alignment layers

— Photostability of liquid-crystal (LC) materials and surface alignment layers was evaluated using a UV lamp and a blue laser beam. Both organic polyimide (PI) and inorganic silicon-dioxide (SiO2) alignment layers were studied under nitrogen environment. Two commercial TFT-grade LC mixtures (low-birefringence MLC-9200-000 and high-birefringence TL-216) were used for comparisons. Results indicate that SiO2 alignment layers are much more robust than PI layers, and low birefringence LCs are more stable than the high-birefringence ones. At the He-Cd laser wavelength (λ = 442 nm), both LC mixtures and SiO2 alignment layers are hardly damaged. To lengthen the lifetime of an LCD projector, inorganic SiO2 alignment layers, high-optical-density UV filter, long cutoff-wavelength blue filter, and short-conjugation (low birefringence) LC materials should be considered.

Orientation of pentacene films using surface alignment layers and its influence on thin-film transistor characteristics

We have investigated the effect of surface order on the orientation and mobility of pentacene. The surface order was created using monolayers and polymers that are normally used to align liquid crystals. Rubbed polyvinylalcohol layers were found to align approximately 27% of the pentacene grains within a 30° range. When introduced in a thin-film transistor, they were found to enhance the saturation current by a factor of 2.5. A mechanism for this enhancement is proposed.

Alignment of polymer network-stabilized ferroelectric liquid crystal using a magnetic field

Polymer network-stabilized ferroelectric liquid crystals with homogeneous alignment have been produced in cells without a surface alignment layer. In this technique, a crosslinkable monomer is mixed into a ferroelectric liquid crystal and polymerized in a magnetic field to form a polymer network that will stabilize the alignment of the ferroelectric liquid crystal. The concentration of the monomer is an important factor in achieving alignment of the ferroelectric liquid crystal. Both the morphology of the final composite layer and the molecular alignment of the host FLC are affected by the curing temperature at which the UV curing of the sample is started.

Effect of surface alignment layers on the configurational transitions in cholesteric liquid crystals

To understand the mechanism behind the pitch change during the transient planar to the planar state transition of a cholesteric liquid crystal (while it is relaxing from the homeotropic state to the planar state), we studied a more controlled situation of pitch change in a thermochromic liquid crystal. We studied the pitch change in the liquid crystal, as a function of temperature, under various surface alignment conditions such as homogeneous and homeotropic surface alignments. In a cell with homogeneous surface treatment, the liquid crystal showed a discontinuous pitch change, while the pitch change was rather continuous in a cell with homeotropic treatment. The numerical simulations also showed an existence of continuous pitch change in a homeotropic cell, but not in a homogeneous one. This led to a prediction that when a cholesteric material is switched from the homeotropic state to the planar state, the transition may be much faster for the homeotropically treated cell as compared to the homogeneously treated cell. We performed experiments to verify this prediction with a chiral nematic mixture. Indeed, the homeotropic to planar state transition time of a homeotropic cell was 5 msec, much faster compared to about 200 msec for corresponding transition in a homogeneous cell.

A mechanistic picture of the effects of rubbing on polyimide surfaces and liquid crystal pretilt angles

A mechanistic picture of the effects of rubbing on polyimide alignment layer (AL) surfaces and nematic liquid crystal (LC) pretilt angles (ΘP), leading to establishing a mechanism of determination of ΘP’s, was proposed on the basis of the study of the change in surface polarity of a polyimide AL with the rubbing force and density, and the subsequent changes in the ΘP and the azimuthal anchoring energy. In the regime of relatively weak rubbing where inhomogeneous, patchy surface modification (the microscopic reorientation of polymer functional groups and the orientation of polymer backbones) occurs, the surface polarity, the anchoring energy, and the ΘP all increase monotonically with rubbing strength. These increases correlate to an increase in the area fraction of the reoriented/oriented AL surface which generates in-plane orientation of the first LC monolayer at the surface and subsequently bulk alignment. Beyond this region in which the AL surface gets modified sufficiently to induce fully developed in-plane orientation of the LC monolayer, the surface chemistry and roughness have dominant effects on the ΘP and the anchoring energy. The increase in surface polarity and the generation of a larger reoriented surface area by high-force rubbing, which enhances the attractive interaction between the LC monolayer and the rubbed surface, contribute to increases in both the anchoring energy and the thermal stability of ΘP and to a decrease of ΘP. This decrease of ΘP with increasing AL surface polarity was verified by studying the relationship between ΘP and surface polarity under constant rubbing conditions by manipulating the surface polarity via the degree of imidization of a poly(amic acid) AL.

Relationship between alignment layer polymer surface structures and liquid crystal display parameters

AbstractThe polar alignment layer (AL) surface provided relatively small liquid crystal (LC) pretilt angles while polyimides with long alkyl side chains gave relatively large LC pretilt angles. The results suggest that LC pretilt angles, in addition to an anchoring effect, are greatly affected by both electronic and steric interactions between LC molecules and a polyimide alignment layer surface. Rubbing with a cotton cloth induces functional groups, side chains, and repeat units at the surface of a liquid crystal polyimide AL to re‐orient. It was discovered that rubbing induced polar functional groups and repeat units to re‐orient out‐of‐the‐plane of the surface, and it made non‐polar aliphatic side chains partially re‐orient inwards, toward the bulk of the film.

Surface Properties of Rubbed Polyimide for Alignment of Liquid Crystal

Abstract The surface tensions with the structural changes in the alignment layer surface due to rubbing was compared with the curing temperature and the rubbing strength. At low curing temperature, the polar component of surface tensions had a high contribution, but at high curing temperature, the dispersion component was greater than the polar components. These indicated that the surface cured at high temperature were less polar than those at low temperature, while, rubbing resulted in the larger polar component and induced the ordering of polar groups on the surface in the X-ray measurement. Then, the ordering of polar groups in the rubbed surface could be conjectured to play an important role in the alignment of liquid crystals from the fact that the layer with a high dispersion component also aligned liquid crystals along the rubbing direction.

Microscopic Molecular Reorientation of Alignment Layer Polymer Surfaces Induced by Rubbing and Its Effects on LC Pretilt Angles

The orientation of functional groups, side chains, and repeat units at the surface of a liquid crystal (LC) alignment layer (AL) polyimide was changed by rubbing with a cotton velvet cloth. It was discovered that rubbing induced polar functional groups and repeat units to reorient out-of-the-plane of the surface, and it made nonpolar aliphatic side chains partially reorient inward, toward the bulk of the film. The polar AL surface provided relatively small LC pretilt angles while polyimides with long alkyl side chains gave relatively large LC pretilt angles. The results suggest that LC pretilt angles are greatly affected by both electronic interaction and steric repulsion between LC molecules and an alignment layer polyimide surface.

Polyimide, Polyamide-Imide and Polyamide Alignment Layers for Antiferroelectric Liquid Crystal Display Cells and Their Structural Effect on Hysteresis Behavior

Abstract Polyimide, polyamide-imide and polyamide alignment layers for antiferroelectric liquid crystal cell devices were developed. Good quality alignment with improved electrooptical properties of the cell was observed by using structurally different fluorine and non-fluorine containing polymers as surface alignment layers. The double hysteresis loops characteristic of the antiferroelectricity arising from the chiral smectic phase were found to be different depending on the different molecular structure of polymer alignment layers. The switching between antiferroelectric (AF) and ferroelectric (F) states significantly depended on the molecular structure of the alignment layer and liquid crystals used. The liquid crystal blend (AFLCB-1) used in the experiment exhibited a wide temperature range of the smectic C*A phase. In the present paper, we report the broader hysteresis with higher driving margin for the non-fluoro polymer alignment layers as compared to those of the fluoro polymer aligning layers under identical conditions. The driving margin (M) varied from the 0.21 to 2.2 as the alignment layers were changed in the cells. All the alignment layers developed in our laboratory showed superior properties to that of the commercially available alignment layer material (LQ-1800).

The Cone Angle Between Relaxed States in SSFLC's—A Pure Surface Effect?

Abstract It is found that the two ferroelectric liquid crystal mixtures ZLI-3774 and FELIX-004 give identical cone angles between the relaxed states in the SSFLC geometry on all tested surface alignment layers, but the cone angle will change with the nature of the surface. On two of the layers a number of fluids are tested, and within experimental errors all give the same cone angle irrespective of their smectic C tilt or composition.