向列型液晶4-n-pentyl-4'-cyanobiphenyl於具微溝槽結構之高分子基材上之配向行為研究

Abstract

液晶顯示器基本組成為兩片透明的導電玻璃基板中夾入一層液晶物質，而在兩片基板內面與液晶接觸的表面上必須塗佈一層配向膜以提供液晶分子方向性。由於平面顯示器隨著應用領域的不斷開發，顯示器朝向軟性材料的應用，本研究主要以高分子作為配向膜的基材，希望未來可朝向可撓式基材做配向膜的研究。 配向膜的配向能力與表面化學性質及表面結構有關，預傾角的調控為控制顯示效果的關鍵技術所在。有別於定向摩擦法製備配向膜，本研究根據Berreman (1972、1973)提出了溝槽理論，利用軟性模板壓印法提供了有別於摩擦法所製備之溝槽，可避免摩擦法所產生之靜電與碎屑汙染問題。第一部分是針對結晶性高分子聚對苯二甲酸乙二酯 (PET)、與非結晶性高分子聚甲基丙烯酸甲酯 (PMMA)高分子進行電漿改質處理，以末端碳鏈長度不同的有機矽烷來做表面改質形成自聚性分子膜進而調整表面能量。第二部分是合成化學性質不同的聚胺酯丙烯酸樹酯，以不同比例混摻聚合來調整不同的表面能量，並可藉由測量對水的前進接觸角來了解材料的表面能量。本研究以固定溝槽寬度 (~450 nm)及液晶分子4-n-pentyl-4’-cyanobiphenyl (5CB)的條件下，探討液晶分子在微溝槽表面上的排列行為。 本研究發現表面較親水，由其製備的配向膜會使液晶分子呈水平配向；而表面較疏水，由其製備的配向膜則會使液晶分子呈垂直配向。更發現材料表面的親疏水性質與向列型液晶之預傾角有高度相關性。任何本研究所使用之高分子 (PET、PMMA與urethane-acrylate resins)，若固定液晶種類及表面起伏的條件下，水的前進角與預傾角均落在同一條關聯曲線。當水的前進角為70o ~ 100o時，液晶的預傾角亦隨之自0o ~ 90o變動。

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.