Vladimir M. Kozenkov

Photoanisotropic Effects in Poly(vinyl-cinnamate) Derivatives and Their Applications

The effect of LC photo-aligning by poly(vinyl-cinnamate) (PVCi) derivatives is consequent to the phenomena of the photo-induced optical anisotropy, found in PVCi films during their illumination by UV-light within the self-absorption band of the PVCi film. We proved in our experiment, that the phenomena of the photo-induced birefringence in thin films of PVC derivatives takes place due to the two main factors: described earlier irreversible photochemical process (2 + 2) cycloaddition of cinnamoyl groups and reversible photophysical reorientation of the chromophore groups, that accumulates photochemical non-active part of the UV-light energy. We reviewed various applications of the effect of photo-induced optical anisotropy in PVCi films including: writing the polarization holograms and images, optical data processing, development of different polarization optical elements, non-destructive testing of defects, LC photo-aligning. Most of the early papers, devoted to the above mentioned applications were published in 1977–1987 in USSR only and are not well known worldwide.

Liquid-Crystal Photoalignment by Super Thin Azo Dye Layer

A novel liquid crystal (LC) photoalignment method, based on a super thin azo dye molecular layer is proposed. The basic idea of this method is to form a very neat textile knitwear and uniform alignment by azo dye layer without spin coating and rubbing processes. The thickness of the alignment layer is smaller than 3 nm, which is much thinner than traditional PI alignment film. In addition to the advantages of a conventional photoalignment method, the use of super thin layer simplifies the alignment procedure, making possible a high electrooptical performance, good photo-tolerance and thermal stability, better adhesion on indium tin oxide (ITO) surface and compatibility with roll-to-roll process.

P‐104: Aligning Layers Using Azo Dye Derivatives for Liquid Crystal Devices

The photo-induced alignment of an azo dye film and liquid crystal (LC) alignment on it are discussed for the liquid crystal display applications. Considerable dichroism is induced in the azo dye SD1 film not only by polarized UV light but also by obliquely incident non-polarized UV light irradiation. It was found that sulfo groups in the azo dye molecule play an important role in the LC alignment on the film. Large pretilt of LC on the photo-aligned film was achieved simultaneously, because the azo dye molecules can be easily reoriented out of the film plane by obliquely incident non-polarized light irradiation. In order to improve stability of the photo-aligned film, a monomer derived from the azo dye was synthesized. After thermal polymerization, the film of it also exhibit good photo-alignment properties. Improvement of durability against light exposure and moisture was observed in the LC cell using it.

New results on liquid crystal alignment by photopolymerization

A review of LC alignment properties on UV-cured photoanisotropic films is given. Special attention is attracted to photopolymers (like PVC), as aligning agents. Such aspects as orientation quality, tilt angle, etc. are taken into account. The new aligning methods seem to be very attractive in LCD technology as they allow the user to avoid electrostatic charges and impurities, which usually exist in conventional rubbing techniques. Such applications of photoanisotropic aligning methods as the improvement of viewing angles of TN LCDs or the restoration of bistability in FLCs are discussed.

Bistable Switching in FLC Cells Aligned by Photoanisotropic Films

Abstract Long-term perfect bistable switching of 1,5μm thick Ferroelectric Liquid Crystal (FLC) cell oriented by photoanisotropic (PA) films was demonstrated even in2 case of a large spontaneous polarization Ps=100nC/cm. The condition of a good SmC* orientation quality was shown to be the existence of a nematic phase N*. The bistability properties of PA-films aligned FLC cell, kept for a while in a short-curcuiting state, are easily restored, which is not observed in FLC cells oriented by usual rubbing technique.

New photo-aligning and photo-patterning technology: superthin internal polarizers, retarders, and aligning layers

The photo-aligning materials based on azodye layers are proposed. The azodye aligning layers enable (i) high order parameter; (ii) excellent alignment quality of LCD with a high contrast ratio; (iii) temperature stability, suitable for LCD manufacturing; (iv) perfect adhesion and high voltage holding ratio due to the specific molecular groups (v) pretilt angle generation. The azodye layers can be used to fabricate thin internal patterned (pixelated) polarizers with different local orientations of the absorption axis and/or absorption colors. Our new methods allow to produce defect-free highly uniform alignment of lyotropic LC or iodine-doped azodye layers themselves with a fine resolution of the polarization pattern. The photo-aligned internal polarizers are cost-effective and enable new LCDs with excellent electro-optical response, including good viewing angles and high brightness. We prepared an internal phase retarder using UV-cured photo-polymerized material. 4-(6- acryloyloxyhexyloxy) benzoic acid had been synthesized and the synthesis procedure was modified for a better yield. We had shown that by applying an electric or magnetic field, the director deformation of the liquid crystalline monomer could be in-situ UV-cured for the optimal phase compensation generation.

Azo-dye aligning layers for liquid-crystal cells

The photo-induced alignment of liquid crystal onto a photochemical stable azo-dye film was studied for liquid-crystal display (LCD) applications. The photo-aligning of azo dye takes place due to the pure reorientation of the molecular absorption oscillators perpendicular to the UV-light polarization. The order parameters S = -0.4 (80% of the maximum absolute value S m = -0.5) was measured at a wavelength of 372 nm from the polarized absorption spectra. The temperature-stable pretilt angle of 5.3° was obtained by a two-step exposure of azo-dye film using normally incident polarized light followed by oblique non-polarized light. The azimuthal anchoring energy of the photo-aligned substrate was A φ > 10 -4 J/m 2 , which is of the same magnitude as the anchoring of the rubbed PI layer. The VHR value of the photo-aligned LC cell was also found to be very high (98-99%) at room temperature and more than 95% at T= 80°C. The thermal stability of the photo-aligned azo-dye layers is sufficiently high, while UV stability has to be improved, e.g., by polymerization. The new LCD aligning technology based on photochemical stable azo-dye layers is envisaged.

38.2: Azodye Aligning Layers for Liquid Crystal Cells

The photo-induced alignment of liquid crystal onto a photochemical stable azo dye film is studied for the liquid crystal display (LCD) applications. The photo-aligning of azo dye takes place due to the pure reorientation of the molecular absorption oscillators perpendicular to the UV-light polarization. The order parameters S= −0.4 (80% of the maximum absolute value Sm= −0.5) was measured at the wavelength of 372 nm from the polarized absorption spectra. The temperature stable pretilt angle of 5.3° was obtained by a two-step exposure of azo- dye film using normally incident polarized light followed by oblique non-polarized light. The azimuthal anchoring energy of the photo-aligned substrate was Aɛ> 10−4 J/m2, which is of the same magnitude as the anchoring of the rubbed PI layer. VHR value of the photo-aligned LC cell was also found to be very high (98–99%) at room temperature and more than 95% at T=80°C. The thermal stability of the photo-aligned azo-dye layers is sufficiently high, while UV-stability has to be improved e.g. by polymerization. The bright perspectives for the new LCD aligning technology are envisaged.

Novel photoaligned twisted nematic liquid crystal cell

A novel photoaligned twisted nematic liquid crystal display (TN-LCD) cell is fabricated by one-step illumination with oblique nonpolarized UV light of an empty cell with azo-dye layers coated on indium tin oxide (ITO) substrates. If the incident angle is sufficiently large (>75°), the p-polarization of the light is more pronounced for the bottom azo-dye layer in comparison with that for the upper one. Thus, the photo-alignment of the azo-dye layer on the bottom substrate is perpendicular to that of the upper one, and LC directors on the top and bottom substrates also become perpendicular. The method provides a new simple way of manufacturing photoaligned TN-LCDs.

45.3: Azo Dye Materials for the Alignment of Liquid Crystal

In this paper, the photo-induced alignment of liquid crystal on an azo dye film is discussed for the liquid crystal display (LCD) applications. This is a non-contact technique to align the liquid crystals so that the dust particles and static charges generated by the rubbing process can be eliminated. To prepare for the photo-alignment layer, the azo dye film is exposed to a linearly polarized or non-polarized light. As a result of the dipole absorption, the photo-induced molecular distribution is non-uniform and has an angular dependence. Consequently, since the dispersion forces between the dye and the liquid crystal molecules dominate, the induced orientation of the liquid crystal molecules is possible. The oblique incidence of the light exposure will also favour a non-zero pretilt angle. We find that the order parameter associated with the aligned liquid crystal medium is high, and very good contrast has been measured in the Twist Nematic (TN) LCD.