Liquid Crystal Pretilt Research Articles

The Relationship between Pretilt Angle of Liquid Crystal and Optical Anisotropy of Alignment Film

We investigated the relationship between the pretilt of liquid crystal and the optical anisotropy of three kinds of polyimide films (PI-A, PI-B and PI-C) rubbed under various conditions. The optical anisotropy of the rubbed films was detected by reflection ellipsometry, and was characterized according to anisotropic dielectric constants, the tilt of the principal dielectric axis and the thickness of the molecular oriented layer. The optical anisotropy of the rubbed PI-A and PI-C films was largely dependent on the rubbing condition. A monotonic decrease in pretilt was observed as the difference between the ordinary and extraordinary dielectric constants of the molecular oriented layers. In the case of PI-B, the optical anisotropy is changed very little when rubbing conditions were changed. The correlation between the optical anisotropy in PI-B and pretilt was unclear. It was concluded that the influence of molecular orientation on pretilt is strongly dependent on the polyimide type.

Liquid-crystal alignment on rubbed polyimide films with various side chains

A series of polyimides containing various side chains was synthesized in order to investigate the effect of side chains on the alignment of liquid crystals on the rubbed surface. Here, the side chains include short flexible alkyl spacers and isomeric biphenyl mesogens. The pretilting of liquid-crystal (LC) molecules was found to be very sensitive to the isomeric structure of biphenyl mesogen end groups as well as the conformation and length of flexible spacers, in addition to the rubbing process. The pretilt angle of LC molecules in the LC cell was achieved in a wide angle range of 8-27°, depending upon the rubbing density as well as the side chains. The high performance in the pretilt and alignment of LCs might be attributed mainly to a strong interaction between the biphenyl mesogen end group in the side chain and the LC molecule in addition to the microgrooves generated in the rubbing direction.

Ultraviolet-light Induced Liquid-Crystal Alignment on Polyimide Films

Bond-breaking and oxidation occur during broadband ultraviolet light (UV light) illumination of the polyimide film in the air according to spectroscopic analysis. Surface tension and polarity are increased based on the measurements of contact angles. The liquid-crystal (LC) pretilt angles of the rubbed polyimide films are reduced by both unpolarized and polarized UV light illumination. Both the LC alignment and pretilt angle are generated on unrubbed polyimide films through the photoalignment process. The mechanism of photoalignment of LC is discussed.

The process to make a ridge- and fringe-field multidomain structure using a photoimageable polymer for a wide viewing angle liquid crystal display application

Liquid crystal display (LCD) is very important in the computer industry and many other areas. However, the application of LCD is limited by its narrow viewing angle, compared with a cathode ray tube (CRT) display. In this article a new type of LCD [ridge- and fringe field multidomain homeotropic (RFFMH) LCD], is introduced. In RFFMH LCD, a transparent photoimageable polymer resist is applied to build ridges on each pixel to control the liquid crystal pretilt direction. This photolithographic step imparts a wider viewing angle and simpler LCD production process. In this article the detailed process of building the ridge structure is described, and the thermal stability and chemical resistance of this polymer are studied. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 1197–1203, 1999

Investigation of Liquid Crystal Alignment and Pretilt Angle Generation in the Cell with Linearly Polarized UV Light Irradiation on Polymer Surface

We have investigated the generation of pretilt angle in nematic liquid crystal (NLC) and electro-optical (EO) characteristics of photo-aligned TN-LCD with polarized ultraviolet (UV) light irradiated on polyimide (PI) surface. It was found that the LC alignment capability increases with increasing the UV light irradiation time. In case of the fact that the polarized UV light of oblique angle of 60° irradiated on PI surface rotated by 90° at 10 min. after being normally irradiated on PI surface at 30 min., the generated pretilt angle of NLC is about 1.5°. Also, we obtained that the pretilt angle in NLC strongly depends on oblique angle and UV light irradiation time. We considered that the pretilt angle of NLC is attributed to the interaction between the LC molecules and the polymer surface due to photo-depolymerization of polymer. Finally, we measured that the EO characteristics of photo-aligned TN-LCD is almost same in comparison to rubbing-aligned TN-LCD.

Photoinduced Alignment of Liquid Crystals using Polyimide Irradiated with a Polarised Excimer Laser

A linearly polarised excimer laser is used to photoinduce an anisotropy in polyimide liquid crystal alignment layers. Exposure with the laser using 10 shots of fluence 13 mJ cm−2 results in saturated alignment with an azimuthal anchoring energy greater than 10−5J m−2. The liquid crystal alignment direction is perpendicular to the polarisation direction of the incident beam. A model is presented to explain the observation of a small liquid crystal pretilt angle on oblique exposure of the excimer laser. This assumes an initial predominantly planar and symmetric polyimide chain distribution which is tilted by oblique irradiation.

Liquid Crystal Alignment on Polymer Films

The rubbed polyimide (PI) and PI LB films were fabricated and treated to align the nematic liquid crystal (LC) 5CB. The rubbed polyimide (PI) films aligned LC cell shows a higher LC pretilt angle than the cell aligned by PI LB films. Atomic force microscope (AFM) observations indicate that the rubbed PI films have a needle-like structure, while PI LB films have a groovy structure. We think that the needle-like structure plays a major role in producing the higher LC pretilt angle.

Pretilt angle generation on photoreactive polymer films

The mechanism of liquid crystal pretilt angle generation on photoreactive polymer films doubly exposed to linearly polarized ultraviolet (uv) light has been investigated. The first exposure for time ${\ensuremath{\tau}}_{1}$ is normal to the surface, and the film is subsequently irradiated for time ${\ensuremath{\tau}}_{2}$ with obliquely incident uv light with a plane of polarization rotated by $90\ifmmode^\circ\else\textdegree\fi{}$ with respect to that of the first uv light. The state of the polymer film after two exposures was characterized with a tensor order parameter, which then served as a boundary condition for the surface state of the nematic liquid crystal. The bulk liquid crystal behavior was investigated using Landau--de Gennes theory extended to include the possibility for weak surface interactions linear in the tensor order parameter. The pretilt angle was calculated as a function of both ${\ensuremath{\tau}}_{1}$ and ${\ensuremath{\tau}}_{2},$ and as a function of the second uv light angle of incidence. The calculated behavior of the pretilt angle was found to be in qualitative agreement with experimental observations. Conclusions about the strength of the anchoring have also been made.

Microscopic Origin of Liquid Crystal Alignment on Rubbed Polymer Surfaces

Surface-sensitive and polarization-dependent near-edge X-ray absorption fine structure (NEXAFS) measurements clearly reveal a preferred in-plane and out-of-plane orientation of phenyl and CO groups at the surface of rubbed polyimide films. The unidirectional molecular alignment at the surface is argued to provide the template for liquid crystal (LC) alignment of the films. Both the LC orientation along the rubbing direction as well as the direction of the out-of-plane LC pretilt are explained in a simple model. In this model the LC direction follows the preferential orientation of the phenyl rings at the surface. The preferred phenyl orientation is explained in terms of preferential chain segment alignment through a pulling action of the rubbing cloth fibers. The proposed LC alignment model is based on the existence of a statistically significant unidirectional bond asymmetry at the polymer surface, and it does not require the existence of crystalline order.

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.

Effects of Ultraviolet-Light on Polyimide Films for Liquid Crystal Alignment

AbstractUV-light effects on the polyimide films have been studied by characterizing the film property changes and by measuring the liquid crystal pretilt angle. Infrared, UV-visible and X-ray photoelectron spectroscopy measurements indicate that bond-breaking and oxidation occur during broadband UV-illumination of the film in the air. Free radicals are formed during UV-illumination as evidenced by the analysis of electron spin resonance spectra. Surface tension and polarity are increased after UV-illumination based on the contact angle measurements. While the pretilt angle on the rubbed polyimide film is decreased after nonpolarized UV-illumination, polarized UV-light generates a relatively small pretilt angle on a nonrubbed surface.

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.

Analysis of Rubbed Polyimide Films by Polarized Infrared Spectroscopy: Effects of Immersion in Organic Solvents

We applied polarized infrared (IR) spectroscopy to the analysis of rubbed polyimide (PI) films and evaluated the PI orientation induced by rubbing by measuring the dichroic difference. The IR dichroic difference and liquid crystal (LC) alignment were analyzed for rubbed PI films that were immersed in various organic solvents. The solvents which reduced the dichroic difference and pretilt angle of LC molecules could be characterized in terms of their solubility parameters. When the dichroic difference of the PI films was greatly reduced by the immersion in the solvents, a correlation between the LC pretilt angle and the dichroic difference (i.e. the in-plane orientation of the PI molecules) was observed.

NEXAFS studies on the rubbing effects of the surface structure of polyimides

Surface sensitive NEXAFS (Near Edge X-ray Absorption Fine Structure) spectroscopy technique has been applied to the rubbed polyimide in order to examine the correlation between liquid crystal (LC) pretilt angle and surface structure of polyimides, BPDA-Cn and PMDA-Cn. These results indicate that the surface structure of the polyimides strongly affects the LC pretilt angle. An improved insight on the reported odd-even effect of the LC pretilt angle has also been provided.

Surface Orientation of Cyanobiphenyl Liquid Crystal Monolayer and Pretilt Angle under Various Rubbing Strengths

We have determined the orientational distribution function (ODF) of a liquid crystal (LC) monolayer on rubbed polyimide films by optical second-harmonic generation (SHG) using the maximum entropy method without assuming the independence of the polar and azimuthal angular distributions. The LC samples used were a homologous series of 4'-n-alkyl-4-cyanobiphenyl (n CB; n=5–8). The rubbing strength dependence of the pretilt angle was also measured and was found to be classified into two types according to even or odd number of carbon atoms in the alkyl chain. However, we could not observe a distinct difference in ODFs of 7CB and 8CB monolayers to explain the two kinds of rubbing strength dependence of the pretilt angle.

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.

高分子配向法を用いた液晶セルの熱処理による液晶分子配向への影響

Liquid crystal molecules on a rubbed polymer alignment layer are oriented at some polar angle away from the surface. This so-called pretilt angle is an important parameter determining the electro-optical properties of liquid crystal devices. The generation and stability of the pretilt angle should therefore be studied. Several researchers have been studying this problem but its physical mechanism is not thoroughly understood. We investigated the polymer alignment and polymer inclination angle of surface polymer, and the change in the Liquid Crystal (LC) pretilt angle caused by heat treatment. Then we studied the change in pretilt by heat treatment of various LC materials. We concluded that the generation of LC pretilt mainly depends on the average inclination angle of the surface polymer of the alignment layer. We also consider that the LC pretilt changes after heat treatment depend on solvent effect of LC materials because the stronger the polarity of the LC is, the larger the change in LC pretilt becomes. It is suggested that temperature of heat treatment of LC cell must be carefully controlled considering the change of uniform alignment as well as LC pretilt.

Relation between the orientations of polymer alignment layer and liquid crystal molecules.

Effects of surface topology and polymer molecule orientation on the liquid crystal(LC) orientation have been examined using photolithography and Langmuir-Blodgett techniques. It has been clarified that the polymer orientation in the alignment layer affects more effectively than surface topology. UV/Visible spectroscopy has shown that the elevation angle of the polymer main chain in the alignment layer from the substrate plane does not correspond to the LC pretilt angle, which suggests that long alkyl side chains produce high pretilt angle for some polyimides.

Polyamide-Type High Pretilt Surface: Effect of Side-Chain Structure and Type of Liquid Crystals on the Generation of Pretilt Angle

Abstract Highly N-aralkylated poly(p-phenylene terephthalamide) (PPTA) with various substituents having benzyl units in their structures were synthesized in a high yield, and their properties as a liquid crystal alignment film were studied. Good liquid crystal alignment and pretilt angles of 0.5 to 8.1 deg were observed on the spin-coated polymer films after rubbing when a nematic liquid crystal, ZLI-2293 was used. The induced pretilt angle showed no correlation with the surface-free energy of the polymer film, but was strongly dependent on the structure of the side chain. The effects of the structural difference of the side chain and type of liquid crystals are discussed in more detail.

Hysteretic Behavior of a Reversely-Pretilted Weakly-Anchored Large-Pitch Cholesteric and a Finger-Print Embedded Into a Homeotropic Matrix Under Different Sweeping Rates of an AC Voltages

Abstract The structural transformations of an initially planar Grandjean-like texture with a weak θ-polar anchoring and a reverse pretilt of the liquid crystal at the boundaries are studied under the influence of an AC voltage with a sweeping rate of 0,02 V/sec, 0,2 V/sec and 2 V/sec. At a sweeping rate larger than 1 V/sec, the liquid crystal oscillate with a large dynamic hysteresis between the Grandjean-like and homeotropic orientations passing through a conical deformation. The increase of the voltage at a sweeping rate smaller than 1 V/sec led to a textural hysteresis resulted in Helfrich's instability, a 90° rotation of the helix and a total unwinding. The decrease of the voltage however, caused after a relatively fast dielectric reorientation the formation of domains of Rault-Cladis with a zig-zag form which after relaxation in usual fingers disappeared into the initial Grandjean-like orientation. Dynamic hysteresis was also observed in electricallyexcitated finger-print embedded into a homeotropic ...