Study On Photoalignment Film Of Liquid Crystals Based On Self-Assembled Film

Compared to conventional CRT displays, Liquid crystal displays (LCD) have many advantages, such as environmental protection, low power, flatness and portability. The statistic result of Display Search showed that the sell of LCD was 2.07 million sets in 2003 in the mainland of China, and the number was larger than the quantity of CRT displays. LCD manufacture is a high-tech region, technology must be developed and refreshed, so that it can keep up with the requirement of LCD market. In the region of LCD research and manufacture, most of LCD need alignment layers that can give liquid crystals a preference alignment direction. In the industry of LCD, rubbing alignment method has been widely used because its process can be easy to carry out. Photoalignment technique is another important alignment method, its mechanism was: a photosensitive solid film becomes anisotropy when it was irradiated by linear polarized ultraviolet (LPUV), the anisotropic film would have the ability to induce liquid crystal to alignment. Photoalignment is an un-contacted alignment method, so it can avoid the shortcomings of rubbing method, such as particle contamination and static charge buildup. Moreover, photoalignment is particularly useful in multi-domain applications; by this technique the LCD will have large view-angle. In 1992, Schdat M. had already given photoalignment method, but the method with superiority has not been applied in the LCD industry now. The reason may be as follow: the stability of photoalignment layer is not good, and alignment result is worse than that of rubbing treatment. The photosensitive materials are always polymer, when the photochemical reaction carries out, the backbone of polymer probably can obstruct the photosensitive group, which make the reactivity ratio of the film to be low. To resolve these problems, we give some new schemes: 1) A photosensitive monomer was used as film-formed material, a high reaction ratio would be obtained; 2) A photosensitive self-assembled film was introduced to photoalignment systems, because of self-assembled film is an ordered system, the photosensitive group was easy to react, this means the reaction would be complete and efficient. The content of the thesis mainly included three parts: 1) Three kinds of low molecular photosensitive monomers were synthesized, and they were used to fabricate photoalignment layer of nematic liquid crystals by photocycloaddition. The UV and IR spectroscopic studies of the monomer films showed that the photoreaction ratio of monomer (about 50%) was higher than the corresponding photosensitive polymer (about 10%, PVCi). The polarized IR spectra proved that obvious anisotropic absorption was displayed in the irradiated photosensitive film. With polarizing microscopy method, we found that these kinds of photoalignment films exhibited good alignment ability, and the stability of the photoalignment film could reach about 100℃. 2) A photosensitive self-assembled monolayer was fabricated that contained photo-cycloadditable groups, such as, coumarin and cinnamate. The reaction process were detected by contact angle method because the IR absorption of monolayer was very weak. The photosensitive self-assembled monolayer promote uniform alignment of nematic liquid crystals by LPUV, and it thermal stability could reach about 130℃. 3) A cationic bolaform amphiphiles was synthesized that had two photosensitive double bonds, and can be polymerized by ultraviolet. A layer-by-layer self-assembled deposition film was prepared in aqueous solution by using this material and poly (sodium 4-styrenesulfonate). The UV-Vis. spectra proved that the multilayers film was uniform and layer-by-layer. When LPUV irradiated the film, the photopolymerizable double bond underwent [2+2] cycloaddition, and the production of the reaction distributed along the polarized direction of incident LPUV. Polarized UV-Vis. spectra indicated that the irradiated self-assembled multiplayer film was anisotropic, and the dichromic ration is about 0.09, it is higher than the spin-coating method of photosensitive polymer or monomer. We used this anisotropic film as alignment layer of nematic liquid crystals, and obtained a uniform and stability alignment result, the static contrast ratio is about 100, which is very close to application of liquid crystal device. This thesis is an instance that the photochemical material was used in the physics region of liquid crystals, the route that was from photosensitive monomer, self-assembled monolayer film to self-assembled multiplayer film was undergone, by this logical research method photoailgnment of liquid crystal was extended and consummated.