向列型液晶於具微溝槽結構表面上之排列行為研究

Abstract

配向膜為液晶顯示器中相當重要的組成，而工業上是利用定向摩擦法製備配向膜。由於該法的配向機制相當複雜，學者們也提出多種理論來解釋當中的機制。其中，Berreman(1972、1973)提出了溝槽理論，並以彈性位能以及自由能的觀點來解釋液晶分子為何會順著溝槽方向排列。在本研究中，我們將同時透過模擬以及實驗，探討液晶分子在微溝槽表面上的排列行為。此外，預傾角(pretilt angle)的調控為控制顯示效果的關鍵技術所在，也為本研究中的重點。 在模擬液晶分子在微溝槽上排列情形的部分，我們根據Oseen-Frank所提出的彈性自由能以及Rapini-Papoular針對表面能量的計算式，利用變分法計算出所需的主導方程式以及對應的邊界條件。其邊界的溝槽結構配合實驗所使用的軟性模板壓印法(soft embossing)，不侷限於以往的正弦結構，而改採方波形結構；藉由模擬計算，我們不僅可以改變不同的表面配向性質下各種溝槽結構，得到其預傾角以及方位角(azimuthal angle)隨之變化的趨勢，也可以觀察複合配向性質的平面上液晶的排列行為。研究成果顯示在表面為垂直配向的條件下，液晶分子於系統中的方位角皆為0°，而其預傾角將隨著溝槽深度增加而下降，但超過某一臨界深度後預傾角即不再繼續變化。進一步於相同溝槽深度下改變溝槽底部與頂部平台的寬度比例，發現預傾角隨其比例的增加會有先減後增的趨勢；但若深度條件皆大於臨界深度，則預傾角則隨其比例增加而持續下降。上述溝槽結構對預傾角影響在降低系統錨附能後，其趨勢不變，但變化幅度縮小。表面性質為水平配向時，液晶的預傾角為0°、方位角為90°，若增加表面最適角(preferred tilt angle)，預傾角將隨之增加而方位角下降。此外我們發現在複合配向性質的平面上，交錯的垂直配向及水平配向區塊也能誘導液晶分子均勻排列，所得方位角為0°，預傾角則依錨附能及兩相異配向性質之間的比例而變化。 在實驗的部分，我們利用軟性模板壓印法來製備次微米等級的微溝槽結構，所採高分子基材為常用於可撓式基材材料之一的PET(Polyethylene terephthalate)。將製備好的PET微溝槽表面合成液晶盒(liquid crystal cell)後發現其預傾角為0°，4’-n-pentyl-4-cyanobiphenyl(5CB)液晶分子沿溝槽方向水平排列，與模擬結果相符。當於PET微溝槽表面上覆蓋一層OTS(Octadecyltrichlorosilane)自聚性分子膜，其製備而成的液晶盒之預傾角為90°且偏光顯微鏡在各角度下皆為暗態，代表表面性質為垂直配向。各取一水平配向及一垂直配向的微溝槽表面合成液晶盒，發現在偏光顯微鏡下任何角度皆無暗態出現，間接印證了模擬中當表面性質為垂直配向時，液晶方位角為0°之結果。除此之外，我們進一步使用末端碳鏈長度不同的有機矽烷於PET微溝槽表面上形成自聚性分子膜(self-assembled monolayers, SAMs)，改變其表面之配向性質。透過調整不同的成膜條件，我們成功的在0°到90°的區間範圍內調控液晶之預傾角。除了微溝槽結構表面以外，複合配向性質表面也如同模擬所得結果，成功的誘導液晶分子於其表面上均勻配向。

Nowadays, rubbing is the most common method to fabricate alignment layers. Due to the complex mechanism of liquid crystal alignment through rubbing method, many models had been proposed. A well known mechanism proposed by Berreman (1972, 1973) indicated that the alignment was based on strain deformation that makes the free energy to be a minimum as the liquid crystals align along the groove direction. In this study, the alignment of liquid crystal on the micro-grooved surface was investigated through both the simulation and experiment. In the simulation part, modeling liquid crystals aligned on a specific groove system was realized. The Oseen-Frank Equation and Rapini-Papoular form were used respectively for the bulk energy and surface energy of liquid crystal molecules. Making use of calculus of variations, we first deduced the govern equation and its corresponding boundary conditions. Through finite difference method, we could visualize the alignment of the directors in the semi-finite medium and also investigate the pretilt angles and azimuthal angles of various surface geometries and chemical properties. The soft embossing method is proposed to fabricate grating structure on PET (Polyethylene terephthalate) surfaces. These grating PET surfaces are assembled to form liquid crystal cells. It is found that the liquid crystal 4’-n-pentyl-4-cyanobiphenyl(5CB) director uniformly aligns along the groove direction, and the phenomena was consistent with the simulation results. We also formed self–assembled monolayers(SAMs) on the micro-grooved PET surfaces for different surface polarities. By tuning the hydrophobicity of the surfaces, we successfully adjusted the liquid crystal pretilt angle over the whole range from 0° to 90°.