Rubbed Polyimide Layers Research Articles

Application of Photoalignment Technology to Liquid-Crystal-on-Silicon Microdisplays

We applied photoalignment technology to liquid-crystal-on-silicon microdisplays and characterized their performance. A three-step photoexposure method was developed to solve problems with alignment disturbances on reflective silicon panels. The azimuthal and polar anchoring energies of photoalignment layers produced by this method were high and comparable with that of rubbed polyimide layers. We assembled both the rubbed and photoaligned microdisplays on a MTN 90° configuration for characterization and comparison. These two kinds of silicon microdisplays had good and comparable characteristics in terms of contrast, response time, voltage holding ratio and residual DC charge. The rubbed microdisplays were more uniform, which we attributed to a more mature fabrication technology, while the photoaligned microdisplays had far fewer defects due to the noncontact nature of the photoalignment technology.

Pinning effect on the phase separation dynamics of thin polymer-dispersed liquid crystals

The surface pining effects on phase separation dynamics of polymer-dispersed liquid crystals (PDLCs) with thin cell gaps are demonstrated. Comparing various boundary conditions, the inner surfaces of the substrates with or without polyimide layers [but no rubbing] cannot provide enough anchoring force, so in either case the liquid crystal (LC) droplets flow and coalesce to form larger and less uniform droplets. However, if the inner surfaces of the substrates are coated with rubbed polyimide layers with anchoring energy >1x10-4 J/m2, almost all the nucleated LC droplets grow at a fixed position during phase separation. The appearance of the coalescence is not obvious and the formed LC droplets are relatively uniform. The surface anchoring has a significant effect on the morphology of PDLCs.

Influence of the Casting Solvent on the Thermotropic Alignment of Thin Liquid Crystalline Polyfluorene Copolymer Films

The morphology of a conjugated polymer thin film determines how it will behave as an active layer in optoelectronic devices. The liquid crystalline phase present in some conjugated polymers offers a unique way of controlling the film morphology. For example, films in which the chains are aligned in a single direction can be fabricated by processing films on a templating layer and annealing them above the liquid crystalline melting point of the polymer. However, the ability to align to a templating layer will be influenced by the character of the as-cast film. This effect was examined by casting poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) films from a range of both aromatic and chlorinated solvents on top of rubbed polyimide layers and aligning via the thermotropic melt. Measurements of the dichroic ratio as a function of annealing time revealed that films cast from high boiling point aromatic solvents like isodurene (ISO) and o-xylene (OXY) aligned well with a maximum dichroic ratio over 10. Film...

Glass Forming Nematic-Discotic Oligomesogens

New star-like oligomers are presented characterized by the chemical linkage of a defined number of disc-shaped multialkynylbenzene moieties via a central linking unit. The compounds exhibit a nematic mesophase which can be quenched into a glassy state. The molecular orientation of the discotic oligomesogens on rubbed polyimide layers has been investigated.

Voltage Holding Ratio and Residual DC Property of the IPS-LCD on Rubbed Polymer Layers by Voltage-Transmittance Hysteresis Method

Voltage holding ratio (VHR) and residual DC phenomena for the in-plane switching (IPS)-liquid crystal display (LCD) on a rubbed polyimide (PI) surface were investigated. It was found that the VHR of the IPS-LCDs on the rubbed PI layer increased with increasing concentration of cyano LC and specific resistivity of fluorine LCs. Also, the residual DC voltage in the IPS-LCD by voltage-transmittance (V-T) hysteresis method decreased with increasing cyano concentration of LCs. The residual DC voltage of the IPS-LCD can be improved by high polarity of LCs. Finally, the new residual DC voltage measurement of the IPS-LCDs by light minimum/maximum method is proposed.

Planar and twisted lyotropic chromonic liquid crystal cells as optical compensators for twisted nematic displays

We have explored the use of lyotropic chromonic liquid crystals (LCLCs) as promising materials for optical compensators for liquid crystal displays. We used aqueous solutions of disodium cromoglycate in the range of concentrations that yield a chromonic N phase. The N phase is aligned by rubbed polyimide layers giving planar orientation. Two different types of LCLC-based elements are described: (a) a uniformly aligned planar N phase that forms an optically uniaxial plate with a negative birefringence; (b) twisted cells of LCLC obtained by doping the N phase with chiral amino acids. Both planar and twisted N* phase cells were used for compensation of the positive birefringence of twisted nematic (TN) displays; the TN devices were appreciably improved. We achieved achromatic dark state and contrast ratio up to 50:1 at all directions within a 40° cone of viewing angle.

Alignment Control of a Nematic Liquid Crystal on a Doubly Treated Substrate

We have investigated the molecular orientation of a nematic liquid crystal (LC) on a doubly treated substrate. One of the substrate surfaces was first treated with the photo-exposure or rubbing process and then subsequently treated with the second photo-exposure process as a function of the irradiation time while the other was prepared with a rubbed polyimide layer for strong anchoring. The LC orientation on the doubly treated substrate was well controlled either by the irradiation time or by the rubbing strength. The observed molecular orientation was consistent with the simulation results based on a torque balance equation. The anchoring energies of the treated substrates were determined from the least-squares fits of the experimental data to the torque balance equation.

Synthesis and properties of azo dye aligning layers for liquid crystal cells

The synthesis and properties of azo dyes that can be used for photoaligning liquid crystals (LCs) have been investigated. The structures and the synthetic procedure for the azo dyes are presented. The photoaligning of azo dyes takes place purely due to the reorientation of the molecular absorption oscillators perpendicular to the UV light polarization. The qualitative model for the phenomenon in terms of the rotational diffusion of the azo dye molecules in the field of the polarized light is discussed. The order parameters S = -0.4 (80% of the maximum absolute value S m = -0.5) were measured from the polarized absorption spectra at the wavelength 372 nm. A temperature stable pretilt angle of 5.3° was obtained by a two-step exposure of the azo dye film using normally incident polarized light followed by oblique non-polarized light. The azimuthal anchoring energy of the photoaligned substrate was Aϕ , 10−4 J m−2, which is the same as the anchoring of the rubbed polyimide (PI) layer. The voltage holding ratio value of a photoaligned LC cell was found to be even higher than for a rubbed PI layer, which enables the applications of azo dyes as aligning layers in active matrix liquid crystal displays. The thermal stability of the photoaligned azo dye layers is sufficiently high, but UV stability has to be improved, e.g. by polymerization. A new LCD aligning technology based on polymerized azo dye layers is envisaged.

Technique for characterizing azimuthal anchoring of twisted nematic liquid crystals using half-leaky guided modes

The half-leaky guided-mode technique is used to explore the director profile through a thin, twisted, homogeneously aligned nematic liquid-crystal cell (Merck-E7). Twisted alignment is realized by rubbed polyimide layers on both inner walls of the cell. By monitoring first a particular angle of incidence range of the p-to-s-conversion reflectivity for the twisted-nematic cell under high voltage, the direction of the liquid-crystal director on the top interface of the cell, the easy axis, is accurately determined. Then by removing the external voltage and modeling the complete director profile in the cell to fit the half-leaky guided-mode reflectivity data recorded, the real twist of the director from the top to the bottom of the cell is accurately determined. By comparing the surface director orientation at zero volts with the easy axis of the top interface, the azimuthal (in-plane twist) anchoring coefficient, WT, is quantified through knowledge of the twist elastic constant K22. At 26.5 °C we find that for E7, WT=(4.4±0.3)×10-5 J m-2.

Rubbing-induced molecular reorientation on an alignment surface of an aromatic polyimide containing cyanobiphenyl side chains.

Surface lamellar decoration (SLD), surface enhanced Raman scattering (SERS) and optical second harmonic generation (SHG) experiments have been utilized to study the molecular orientation and conformation changes at a rubbed polyimide alignment-layer surface. This aromatic polyimide containing pendent cyanobiphenyl mesogens was synthesized via a polycondensation of 2,2'-bis(3,4-dicarboxy-phenyl)hexafluoropropane dianhydride (6FDA) with bis[omega-[4-(4-cyanophenyl)phenoxy]hexyl] 4,4'-diamino-2,2'-biphenyldicarboxylate (nCBBP, n = 6), abbreviated as 6FDA--6CBBP. Uniform alignment layers, possessing high pretilt angles ranging from 39 degrees to 43 degrees, have been achieved after mechanical rubbing of the polyimide thin film surface at room temperature and subsequent annealing. This is the first time that high pretilt angles have been detected to possess a negative angle (-theta(c)) with respect to the rubbing direction (i.e., opposite to the rubbing direction), considerably different from the conventional pretilt angle (theta(c)) observed along the rubbing direction. This observation is confirmed using magnetic null and SHG methods. Combined polyethylene (PE) SLD and atomic force microscopy experiments reveal that the azimuthal orientation distribution of the long axis of the edge-on PE lamellar crystals is oriented normal to the rubbing direction, indicating that the PE chains are aligned parallel to the rubbing direction. This SLD technique probes the anisotropic surface orientation of the outermost molecules of the rubbed polyimide layer. The SERS results show that prior to rubbing the surface, both the pendent cyanobiphenyls in the side chains and backbones possess nearly planar chain conformations at the polyimide surface. Mechanical rubbing causes not only tilting of the backbone moieties, such as imide-phenylene structure, but also significant conformational rearrangements of the pendent side chains at the surfaces. The molecular mechanism of this unusual alignment is due to the fact that the pendent cyanobiphenyls forms a uniformly tilted conformation on the rubbed surface, and the polar cyano groups point down toward the layer surface deduced from SHG phase measurements. This conformational rearrangement of the side chains results in the formation of fold-like bent structures on the surface, which directly leads to the long axis of cyanobiphenyls having the -theta(c) pretilt angle with respect to the rubbing direction.

Photoisomerization and photo-induced optical anisotropy of polymethacrylate containing aminonitroazobenzene chromophores in the side chain

Photoresponsive side chain copolymer and homopolymer containing an aminonitroazobenzene were synthesized for studying photoisomerization behavior and photo-induced anisotropy.Trans-to-cis photoisomerization was observed under the exposure of a circularly polarized visible light with UV-Vis absorption spectroscopy. Reorientation of polar azobenzene molecules induced optical anisotropy under a linearly polarized light at 532 nm. Polarized absorption spectroscopy was employed to investigate the anisotropy of the polymer film during irradiation of the excitation light. Layers of two photosensitive polymers were used for aligning liquid crystal (LC) molecules instead of one of the rubbed polyimide layers in the conventional twisted nematic cell. For producing homogeneous alignment of a nematic LC molecule, a linearly polarized light was exposed to the films of two polymers. The stability of the LC alignment upon the linearly polarized light exposure was also studied.

Determination of the torsional anchoring of a twisted nematic liquid crystal using the half-leaky guided mode technique

Optical excitation of half-leaky guided modes has been used to explore the director profile of a thin twisted homogeneously aligned nematic liquid crystal cell (Merck, E7). Twisted alignment is realized by the use of rubbed polyimide layers on both inner walls of the cell. By modelling the optical response of particular director profiles in the cell to fit the halfleaky guided mode reflectivity data, both the thickness and the twist of the director from the top to the bottom of the cell are accurately determined. Then comparing the rubbing directions with the twist of the easy axes, the torsional anchoring coefficient, W T is quantified through a knowledge of the twist elastic constant K 22 of E7. At 24.5°C we find that W T = (5.8 ± 0.4) X 10-5 J m-2.

Highly Aligned α-Type Copper Phthalocyanine Formed on Rubbed Polyimide Alignment Layer

We have fabricated oriented copper-phthalocyanine (CuPc) films by vacuum deposition on rubbed polyimide (PI) alignment layers. It was found that the b-axis of α-type CuPc microcrystals changes its direction depending on the thickness of PI layers, namely, the b-axis is perpendicular to the rubbing direction on a thin rubbed PI layer but is almost perfectly aligned to the rubbing direction on a thick rubbed PI layer. The oriented structure on a thick PI layer was confirmed by a high dichroic ratio of about 4, which is expected in the optical absorption of α-type CuPc, and by highly oriented needle-shaped microcrystals observed by atomic force microscopy.

Liquid crystalline polyfluorenes for blue polarized electroluminescence

A series of 9,9-dialkyl-poly(fluorene-2,7-diyl)s containing linear and branched alkyl substituents with a Mn of up to 200000 g/mol has been synthesized. Moreover, some of the polymers were end capped with a suitable hole transport functionality, such as a triphenylamine derivative, to improve their charge transport properties and to control the molecular weight. The thermal alignment of these novel polymers on a rubbed polyimide layer led to highly anisotropic film formation with dichroic ratios (absorption parallel and perpendicular to the rubbing direction) of up to 26 in absorption and 21 in emission.

Control of the Anchoring Energy of Rubbed Polyimide Layers by Irradiation with Depolarized UV-Light

Exposure of rubbing polyimide (PI) film to depolarized ultraviolet (UV) light suppressed the effective anchoring energy of liquid crystal (LC) with aligning surface. Polarized light changed the orientational distribution of PI molecules obtained by rubbing by changing both the anchoring energy and easy axis direction. These results show that ultraviolet exposure can be effectively used to control anchoring parameters.

Infrared Spectroscopic Studies of the Thermal Stability of Alignment Layers of Polarized UV-Exposed Polyimide Films for Liquid Crystal Alignment

Polarized Fourier transform infrared (FTIR) and optical microscopy with crossed polarizer were used to investigate the thermal stability of the alignment layers formed by rubbing and polarized UV (PUV) irradiation on the polyimide (PI) films. The thermal stability of the PUV irradiated PI alignment layer is lower than that of the rubbed PI layer due to the fragmentation reaction of the PI by PUV. We also investigated the alignment of the liquid crystal by rubbing or PUV irradiation. Liquid crystal (LC) molecules align parallel to the predominant orientation of the PI and perpendicular to the PUV irradiation direction.

Alignment of cholesteric liquid crystals using periodic anchoring

Previous work has shown that when short-pitch cholesteric liquid crystals are aligned in cells with the helical axis in the plane of the substrate in a well-defined direction, then application of an electric field across the cells may result in microsecond in-plane switching due to the flexoelectric effect. Moreover, the response to the electric field is linear and temperature independent. Hence this device geometry has great potential since fast, temperature-independent switching with analogue gray scale may be realized. Up to now, however, the alignment of the helix in the plane of the substrate achieved, for example, by rubbed polyimide layers has been rather unstable, since it relaxes to a Grandjean texture in a time scale determined by the cell thickness. This time is generally of the order of minutes. Here we report a method for the attainment of stable anchoring of the unidirectional lying helix texture using periodic anchoring conditions.

Mechanically and Light Induced Anchoring of Liquid Crystal on Polyimide Film

The interplay between the effects of mechanical rubbing and UV light induced deimidization on polyimide films has been studied. Exposure to depolarized UV light of rubbing PI film was found to suppress effective anchoring energy. Polarized light led to the alteration of the orientational distribution of PI molecules obtained by rubbing i.e to the change of both anchoring energy and easy axis direction. A simple model based on the selective UV light absorption with consequent deimidization of PI molecules describes the UV-initiated processes on the rubbed polyimide layers but contradicts the experimental data obtained for non-rubbed layers. The results obtained show that UV exposure can be effectively used to control anchoring parameters.

Polarized electroluminescence of light emitting liquid crystalline polymers

Abstract Smectic liquid crystalline arylenevinylene main chain polymers were used in light emitting diodes (LEDs) to produce linearly polarized electroluminescence. The degree of orientational order induced by a rubbed polyimide layer amounted to S = 0.64, as extracted from the electroluminescence spectra. The great advantages of these LC polymers are their good thermal stability, processability and broad mesophase range, and the possibility of applying established orientation techniques in order to achieve a macroscopic orientation.

Creation of submicron orientational structures in thin liquid crystal polymer layers

Submicron orientational patterns were created in thin liquid crystal polymer films. A rubbed polyimide layer was locally modified by hatching it with the small tip of a simplified atomic force microscope. A nematic liquid crystal mixture, that can be spin-coated and polymerized at room temperature was used to transfer the lithography from the polyimide into the liquid crystal polymer. Gratings as small as 240 nm periods were fabricated. The domain boundaries, which are nothing else than twist disclination lines attached to the substrate surface, influence the topography on a distance much larger than the film thickness. This topographic effect, which consists of a groove and a wall parallel to the boundary, was investigated as a function of the scratching angle, the film thickness and the size of the structure.