Sooman Lee

THERMAL AND OPTICAL STABILITIES OF PHOTOISOMERIZABLE POLYIMIDE LAYERS FOR NEMATIC LIQUID CRYSTAL ALIGNMENTS

We have investigated the thermal and optical stabilities of the photoalignment layers for nematic liquid crystals (LCs). For the photoalignment layers, three kinds of photoisomerizable polymer systems were studied; a polyamic acid doped with azobenzene molecules, a side-chain-substituted polyamic acid with azobenzene units, and a main-chain-substituted polyamic acid with azobenzene units. Photoinduced anisotropy was produced by illumination with linearly polarized near-UV light from a mercury lamp, and evaluated by measuring polarized UV absorption spectra and optical birefringence. The nematic LC-alignment properties were investigated using the photoalignment layers processed under various thermal and optical conditions. It was observed that the main-chain polyimide system exhibited good unidirectional LC alignment and excellent thermal (300°C for 1 h) and optical stabilities. Moreover, we suggest a new procedure, in which the photoillumination process is carried out before the thermal imidization process, to achieve a stable photoalignment layer.

DIRECTOR TILTING OF LIQUID CRYSTALS ON PHOTOISOMERIZABLE POLYIMIDE ALIGNMENT LAYERS DOPED WITH HOMEOTROPIC SURFACTANT

Director tilting of liquid crystal (LC) molecules on photosensitive alignment layers has been investigated. The layers, polyimides with photoisomerizable groups in their main chains, were irradiated with unpolarized near-UV (UPUV) light at oblique incidence. The irradiated film was found to align LC molecules homogeneously with a negligible pretilt angle. It was found that the pretilt angle can be generated successfully by introducing surfactant lecithin and decreasing the incidence angle of the UPUV light. The LC alignment and electro-optical properties of the LC cells with the photoalignment layers were demonstrated to be suitable for twisted nematic LC, vertical alignment LC, and surface-stabilized ferroelectric LC devices.

Alignment Mechanism of Nematic Liquid Crystal on Rubbed Polymer Surface Studied by Subsequent Processes of Rubbing and Photoalignment

A novel method for understanding the alignment mechanism was motivated by the texture observation of a nematic liquid crystal (LC) contacted with a photoaligned layer after rubbing. Reorientation of director occurs by subsequent photoalignment to different direction from that forced by rubbing. Moreover, it was found by polarized absorption spectra that the preferential average main chain axis over whole the alignment layer does not change, indicating that the orientation change by photoalignment occurs only at very top surfaces. This experiment without changing surface morphology indicates that the alignment priority for the nematic LC is mainly governed by the anisotropic short-range intermolecular interaction between alignment films and LC molecules and the effect of microgrooves plays a minor role.

Comparative Study of Anchoring Strengths for Photo- and Rubbing-Aligned Liquid Crystals on Photoisomerizable Polyimide Alignment Films

The high-electric-field technique was used to measure the out-of-plane (polar) anchoring strength at the interface between a nematic liquid crystal, pentylcyanobiphenyl (5CB), and a photosensitive polyimide (PI) alignment layer possessing an azo group in the main chain. The anchoring energy of the photoaligned state was found to be rather strong, on the order of 0.1 mJ/m2, being at least half of the anchoring energy on the rubbed surface of the same azo PI. The anchoring energy exhibited a conspicuous peak near the clearing temperature of 5CB, particularly in the photoaligned system, indicating a significant role of molecular motion in anchoring behavior.

Pyrooptic converter based on a ferroelectric crystal and a liquid crystal

Abstract The thermal image signals can be directly converted into optical signals in the visible region by a pyroelectric (PE) converter. We examine the potential use of a pyrooptic converter for signal processing which utilizes a PE, KDP doped TGS crystal and an electrooptic (EO), 854E liquid crystal. When TGSK is exposed to a pulsed infrared radiation produced from a near-infrared light-emitting diode at the wavelength of 0.95 μm, an alternating voltage is generated by the PE effect, and it subsequently induces the electrooptic effect in the LC layer. In the frequency region of 10 to 100 Hz, the EO modulation of the LC becomes saturated at about 0.4 V, and the response time is fairly constant.