| Summary: | 碩士 === 國立臺灣大學 === 化學工程學研究所 === 99 === The liquid crystal display is composed by two conducted ITO glass and a liquid crystal material. In the between of two ITO glass, we need alignment film on ITO glass to induce the liquid crystal molecules to align at certain pretilt angle. With the innovation of LCD, we hope we can use other polymers rather than polyimide (PI) as alignment film to fabricate flexible alignment film.
The alignment behavior of the liquid crystal molecules is related to the surface chemical properties and topology. It is believed that the groove theory proposed by Berreman in 1972 and 1973 explain that the liquid crystal molecules would align along the groove direction that lowers the elastic free energy of nematic liquid crystal molecules. Therefore, we use soft-embossing method to fabricate the micro-grooved polymer surface in order to prevent electrostatics and debris causing by rubbing method. The first part is to use crystalline polymer, polyethylene terephthalate (PET) and non-crystalline polymer poly(methyl methacrylate) (PMMA) as substrate to demonstrate the potentials to apply those plastics as alignment film. We use O2 plasma to bombard the substrate in order to generate hydroxy group which reacts with alkoxysilane as self-assembly monolayer. The second part is to synthesize novel urethane-acrylate resins, and we use different combinations of end groups to tune the surface energy. The advancing contact angle of water is applied as an index of the surface hydrophobicity, that is, the surface energy of the material. In this study, we want to figure out the alignment behavior of liquid crystal molecules in the condition of using 450 nm period micro-grooved polymer surface and 4-n-pentyl-4’-cyanobiphenyl (5CB) liquid crystal molecules.
Our study shows that if the surface is hydrophilic, the liquid crystal alignment tends to homogeneous; and if the surface is hydrophilic, the liquid crystal alignment tends to homeotropic. Furthermore, we find out that there is a highly relationship between advancing contact angle of water and pretilt angle. In every polymer substrate we use, that is, PET, PMMA, and urethane-acrylate resins, as we tune the surface hydrophobicity in the same condition of surface topology and liquid crystal molecules, the pretilt angle falls on the same correlation curve. As the advancing water contact angle increase from 70oC to 100 oC, the pretile angle increase from 0oC to 90oC.
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