Ym Koo

In situ Photopolymerization of Polymerizable Liquid Crystal at the Air-Water Interface

The π-A isotherms and transfer characteristics of synthesized, polymerizable liquid crystals spread at the air-water interface were investigated. The liquid crystals mixed with a poly(amic acid) salt were spread, and the mixed monolayers were polymerized by UV-irradiation. The in situ polymerization was monitored by a multi-channel photo detector.

Stability and transferability of monolayers of polyamic acid salts

Abstract The stabilities and transferabilities of monolayers for the two kinds of polyamic acid alkylamine salts (PAAS) were investigated. The stability of monolayer increases as the chain length of salts increases. At well-balanced condition, the Langmuir film is transferred as a Y-type on various substrates, and the thickness of one layer of PAAS was found to be about 16.7 A,.

Optical retardation and FT-IR characteristics of rubbed polyimide langmuir-blodgett alignment layers of liquid crystals

Optical retardation and FT-IR characteristics of polyimide alignment layer of liquid crystals were investigated with Langmuir-Blodgett (LB) films rubbed perpendicularly to the dipping direction. While the retardations of LB films remained unchanged as the number of transfers increased, those of rubbed films increased up to 7 layers, but after a maximum retardation, they decreased steadily as the number of transfers increased. The retardations of films thicker than about 7–9 layers shifted negatively, indicating that the optical axis completely switched to the direction of rubbing. Further, difference of retardation between unrubbed and rubbed films indicates that there existed a limit in the number of layers, i.e., about 30 layers, to be influenced by the surface forces. It is an interesting observation that the substrate surface freezes the polymer chains up to 7–9 layers, persistent to the rubbing strength, while the surface rubbing force penetrates into about 30 layers. The IR dichroic ratio (A¦/A⊥) of 1,500 cm-1 and of 1,240 cm-1 decreases by rubbing, while little change at 1,720 cm-1, indicating that benzene groups are reoriented to the direction of rubbing, while imide groups are not reoriented.

Liquid crystal alignment film with mixture of polyimide and side chain LC by Langmuir-Blodgett technique

Abstract The mixed films of poly(amic acid) and a side-chain liquid crystal, 11-[(4-cyano-4′-biphenyl) oxy] undecanyl vinyl ether(CN-11-M) were investigated as a LC alignment film to control the pretilt angle of a nematic LC. The mixed films were fabricated by the Langmuir-Blodgett technique. A stable monolayer of poly (amic acid) alkylamine salt(PAAS) and CN-11-M was formed at the air-water interface. A Y-type film on 1TO glass was established at 25 mN/m and room temperature and was confirmed by FTIR spectroscopy. The number of layer was 4, and the ratio of PAAS and CN-11-M was varied. Surface tensions of these films were observed by measuring the contact angle of test liquid. Antiparallel cells were made to measure the pretilt angle of LC by crystal rotation method. Polar force in the mixed film increased with the amount of CN-11-M, but showed a maximum. The pretilt angles of nematic liquid crystal also showed the maximum at a fixed ratio of PAA and CN-11-M.

Electro-Optical Properties of the Antiparallel Liquid Crystal Cell

Abstract The normal incident light to the cell in which liquid crystal has an angle with the surface of cell, have a phase retardation by the refractive anisotropy of liquid crystal and the other cell parameters. At a field off-state, the transmission is easily calculated from the various methods, but at a field on-state the variation of tilt angles from the surface anchoring angle to maximum angle at z=d/2 across thickness makes the calculation difficult. Considering the distribution of director in the liquid crystal cell under the field with the folded anisotropic layers, to calculate the light propagating through these layers the Jones matrix was expanded sequentially and the transmitted intensity was accurately evaluated consistent with the experiment.

The Preparation and Transfer Characteristics of Polyimide Langmuir-Blodgett Film for Liquid Crystal Alignment

Abstract The monolayer stability of polyamic acid salts (PAAS) increases as the chain length of salts increases and the hysteresis of the π-A isotherms is found when compressed and expanded. The transferabilities of the monolayers were sharply influenced by the stability at the interface, but not affected by the dipping conditions. In a well-balanced condition, the Langmuir film is transferred as a Y-type on various substrates. The thickness of PAAS was found to be 16.7 A. After imidization, it was reduced to 5.6 A per layer, but some ordered structures sustained.

Relationship between Pretilt Angle of Nematic Liquid Crystal and Surface Structure of Alignment Layer

Abstract Fabrication method of alignment layer was investigated to verify which mechanism is dominant in generating the pretilt angle of LC. The surface energy and the geometrical surface structure were systematically controlled by combining the rubbing and Langmuir-Blodgett(LB) methods. Two layers of LB film were deposited on the top of rubbed polyimide films to remove the surface energy generated by rubbing. In a rubbed polyimide film, polymer chains array in a certain angle which increase with the rubbing strength. The rubbed film with two LB layer showed the contact angle to be the same as the LB film, while the pretilt angle of LB layer on rubbed film was reduced. Therefore it is concluded that the pretilt is more affected by the surface structure of the polyimide alignment layer than the surface energy.

Monolayers of polymerizable liquid crystal and in situ interfacial polymerization at the air-water interface

Abstract The π-A isotherm and transfer characteristics of polymerizable liquid crystals spread at the air-water interface were investigated. The limiting areas of n-[(4-cyano-4′-biphenyl)oxy] alkyl vinyl ether (CN-n-M) homologues series of n = 8, 10, and 11 were about 10, 13, and 17 A2/molecule respectively, and smaller than the area of hydrophilic cyanobiphenyl group. All the CN-n-M formed apparently stable monolayers and the transferred type of pure CN-11-M monolayer on the hydrophilic surface was Z-type for the first several layers. The monolayer of a mixture with poly(amic acid) salts at 25 mN/m was polymerized by UV-irradiation.