由第四章的實驗結果我們知道摩擦會引起配向層表面自由能及 濕潤特性的改變，且會影響液晶器件之光電特性。在本研究中利用接 觸角分析的辦法，得到了摩擦與表面自由能及濕潤特性的關係。且在 表面自由能與液晶器件之光電特性的探討中，發現了初步的關聯性。
以下條列整個研究所得之結果，做為最後的結論。
(1) 接觸角量測液體對環境之穩定性，會影響表面自由能運算的準確 度。
(2) 對於摩擦配向所引起的表面濕潤特性改變，水平配向的材料會隨 著摩擦強度增強而促使表面拒水性增強；而垂直配向材料之親水 特性變化並無明顯改變。並且在水平配向材料各方位角親水特性 的量測中發現，配向層親水性在逆於摩擦方向的位置較順於摩擦 方向位置來的高。
(3) 摩擦配向所引起之動態接觸角變化，在水平配向材料 BISAF-25 (3%)的流動穩定性較 JASL-9800-R1(5%)來的差。且在順向及逆向 摩擦方向所量測到之動態接觸角延遲量有些微的差異，並在摩擦次 數位於 3~4 次時，順逆向動態接觸角延遲量最為相近。
(4) 對於摩擦配向所引起的表面自由能變化，在垂直配向材料中並無 規律性。而水平配向材料的表面自由能隨摩擦強度增強而下降，
且逆摩擦方向所量得之表面自由能皆比順摩擦方向高。另外在三 種配向材料的摩擦方位角對應表面自由能量測中發現，能量皆呈 現對稱性分佈，且最大值皆發生於摩擦方位角 90 度的位置上。
(5) 最佳摩擦參數可利用表面自由能及濕潤特性的分析而獲得。且在 分析中發現，入毛深度在 0.3mm 及摩擦次數在 3 次時，可以使配 向層表面自由能及濕潤特性趨於均勻且穩定。
(6) 利用最佳摩擦參數的選擇，我們改善鐵電液晶配向時所會面臨的 ZigZag 缺陷問題，而獲得單一區域無 ZigZag 缺陷之鐵電型液晶器 件。
(7) 配向層表面自由能變化會對液晶器件之光電特性產生影響。當表 面自由能下降時，液晶分子預傾角則隨之增大。而當摩擦深度在 0.2mm 時，響應時間則是在摩擦次數 4 次之前呈現上升狀態，在 4 次之後則呈現下降狀態。另外在受電場影響的開關時間皆比受表 面能量影響的弛逸時間來的短。且隨著摩擦次數的增加，開關時 間呈現下降趨勢，弛逸時間則呈現上升的趨勢。
而在配向層表面自由能對液晶分子影響性的相關研究中指出，影 響液晶分子最主要的能量為極性表面自由能。由於本實驗室所用來測 試極性表面自由能的液體，對環境穩定性相當的差，目前也接續的在 做這方面的改善，並且也找到了替代方法來量測配向層之極性表面自 由能。配向層表面的研究為本實驗室一大研究方向。在未來我們會繼 續探討表面自由能對液晶器件光電特性的影響性，還會繼續深入了解 極性表面能與液晶配向之關聯性。並且會對鐵電型液晶器件的配向做 更深入的探討。
參考文獻
[1] F.Reinitzer and Monatsh. Chem.9. 421(1888) [2] O.Lehmann and Z. physic. Chem.4. 462(1889)
[3] G.Vertogen,W.H.de Jeu,“Thermotropic Liquid Crystals,Fundamentals
”,Volume45(1940).
[4] D. Demus,and L. Richter,“Textures of liquid crystals”,Verlag Chemie, New York(1978).
[5] G. W. Gray,and J. W. Goodby,“Smectic Liquid Crystals:Textures and Structures”,Blackie Press,Glasgow London(1984).
[6] 松本正一,角田良市, “液晶之基礎與應用”,劉瑞祥 譯,國立編譯館, 臺北市,(1996).
[7] H.S. Nalwa,“Liquid Crystals, Display and Laser Materials”,Volume7 (2000).
[8] Pochi Yeh, and Claire Gu,“Optics of Liquid Crystal Displays”,A Wiley Interscience Publication, New York,(1999).
[9] CB. Bahr：”Chirality in Liquid Crystal”, Wiley (2000).
[10] D. Demus, J. W. Goodby, G. W. Gray, H. W. Spiess, V. Vill, Handbook of Liquid crystal, 2(A) (1998)
[11] R. B. Meyer, L. Liebert, L. Strzelecki, P. Keller, J. Phys. Lett,36 (1975), L69
[12] N. A. Clark, S. T. Lagerwall, “Submicrosecond bistable electro-optic switching in liquid crystals”,Appl. Phys. Lett., 36, 899 (1980).
[13] 王其仁,謝玉英, “配向膜材料及應用技術”,工業材料.166,125
(2001).
[14] 郭惠隆, “可溶性polyimide配向膜材料”,工業材料.144,96(1998).
[15] Y. Ouchi, J. Lee, H. Takezoe, A. Fukuda, K. Kondo, T. Kitamura, A.
Mukoh, “Smectic Layer Structure of Thin Ferroelectric Liquid Crystal Cells Aligned by SiO Oblique Evaporation Technique”,Jpn.
J. Appl.Phys., 27, L1993 (1988).
[16] Chenhui Wang,Ryuji Kurihara,Philip J. Bosa) , Shunsuke Kobayashi,
“How to achieve zigzag free Sm-C* state with a low pretilt angle:A numerical calculation”, J. Appl. Phys., Vol. 90, No. 9(2001).
[17] Shalel-Levanon S, Marmur A, “Validity and accuracy in evaluating surface tension of solids by additive approaches”[J]. Journal of Colloid and Interface Science,2003,262(2):489-499.
[18] 王曉東,彭曉峰,陸建峰,王補宣,“接觸角測試技術及粗糙表面上 接觸角的滯後性Ⅰ:接觸角測試技術”,中國應用科技基礎與科學 學報.Vol 11,No 2(2003).
[19] Morra M , Occhiello E , Garbassi F. Knowledge′s about polymer surfaces from contact angle measurements [ J ] .Advances in Colloid and Interface Science , 1990 ,32 :79-116.
[20] Johnson R E J r. Conflicts between Gibbsian thermodynamics and recent treatments of interfacial energies in solid2liquid2vapor system [J ] . Journal of Physical Chemistry , 1959 ,63 :1655-1658.
[21]L. T. Creagh and A. R. Kmetz: Mol. Cryst. & Liq. Cryst. 24 (1973) 24.
[22] G. Porte, J. de Physique, 37, 1245 (1976).
[23] D.K. Owens, R.C. Wendt, “Estimation of the surface energy of polymer”, J. Appl. Polym. Sci., 13, 1741-1747 (1969).
[24] D.-S. Seo, and S. Kobayashi, “Study of the pretilt angle for 5CB on rubbed polyimide films containing trifluoromethyl moiety and analysis of the surface atomic concentration of F/C (%) with an electron spectroscope for chemical analysis”, Appl. Phys. Lett., 61, 2392-2394 (1992).
[25] Shin-Tson Wu, Uzi Efron, and LaVerne D. Hes, “APPLIED OPTICS”, 23, 3911 (1984).
[26] 李政道, “彩色液晶顯示器的配向技術與材料進展”,工業材 料.150,115(1999).
[27] B. S. BAN, Y. B. KIM, “Surface Free Energy and Pretilt Angle on Rubbed Polyimide Surfaces”, Journal of Applied Polymer Science, Vol. 74, 267–271 (1999).
[28] Hidehiro Seki a; Yoichiro Masuda a; Tatsuo Uchida b, “Rubbing Effect on Geometrical Anisotropy of Alignment Surface of Liquid Crystals”,
Mol. Crysr. Liq. Crysr., Vol. 282, pp. 323-329(1996).
[29] T. Krupenkin,a) S. Yang, and P. Mach, “Tunable liquid microlens”, Appl. Phys. Lett.,Volume 82, Number 3(2003).
[30]A. RUDAWSKA*, J. KUCZMASZEWSKI, “Surface free energy of zinc coating after finishing treatment”,Materials Science-Poland, Vol. 24, No. 4(2006).
附錄
碩士班期間所發表之會議論文:
[1]W. Zheng*, C. Lu, Y. Yeh, and Y. Tien, “ surface free energy of rubbed polyimide thin films ” ,in Roc.OPT 2006.
[2]W. Zheng*, C. Lu, Y. Yeh, and Y. Tien, “Anisotropy in Surface Free Energy of Rubbed Polyimide Thin Films”, in Proc. SID Asia Display 07,volume2,1733~1737.
[3] 王君旗,于紹偉,盧志宏,鄭文軍*,“液晶配向層表面自由能對鐵電液 晶配向之影響”,OPT 2007.
[4] 王君旗,于紹偉,鄭文軍*,王信評,柯仲隆 ,盧志宏,“摩擦聚乙醯胺薄 膜表面形態與浸潤特性對鐵電液晶配向的影響”,中國液態晶體學 會 2007 年會既研討會.
SURFACE FREE ENERGY OF RUBBED POLYIMIDE THIN FILMS
W. Zheng*, C. Lu, Y. Yeh, and Y. Tien
Institute OF Electro-Optical Engineering, National Sun Yat-Sen University, 70, Lien-hai Road Kaohsiung 804, Taiwan, R.O.C.
Tel:+886-07-5252000 ext 4445, Email: [email protected]
Abstract Surface free energy of rubbed polyamide thin films has been determined. The influence of rubbing strength on the surface free energy is investigated.
Key words:
surface free energy, rubbed polyamide, rubbing strength
INTRODUCTION
The alignment mechanism of liquid crystal (LC) molecules in contact with a rubbed polyimide surface is a subject of both scientific interest and importance for practical applications. Although it is widely used in LCD industry, the alignment mechanism of rubbed PI has not yet been well understood. It is accepted that LC alignment results from the orientation of polymer chains in the outmost layer of the PI film due to rubbing [1].
More and more studies have shown that LC alignment on a rubbed PI surface is dominated by chemicophysical characteristics of the surface. Surface energy of substrates plays a very important role in LC alignment [2]. For a solid substrate, the surface energy can be evaluated using Owens-Wendt equation [3]
(1 cos )
where θ is the contact angle, σl is liquid surface tension and σs is the solid surface tension (c.f. figure 1), or free energy, D and P denote dispersed part and polar part, respectively.
Figure 1. The contact angle and surface tensions of a liquid droplet on a clean solid surface.
In this work, the surface free energy of rubbed PI thin films is studied. The influence of rubbing conditions on the surface energy is presented.
EXPERIMENTAL
In the present studies, polyimide (Chriskev Bisaf-25) was diluted to make a 3 wt% solution, and spin-coated onto ultrasonically cleaned ITO glass plates. The samples were then thermal cured to form a thin
layer of PI on the substrates. The PI films were unidirectionally rubbed using an in house made rubbing machine. The rubbing strength (RS) can be calculated using the following equation [4],
(2 R 1),
RS N π ω
= ⋅ Λ ν − (2)
where N is the cumulative number of rubs, Λ is the pile impression of the velvet fibers, ω is the rotation speed of the roller, R is the radius of the roller, and v the lateral speed of the substrate. In the present studies, Λ was fixed at 0.3 mm, ω was fixed at 135 rpm, and v at 30 mm/min. The measurement of surface free energy of the PI thin films was performed using a surface tension meter (KRÜSS DSA100).
RESULT AND DISCUSSION
Firstly, we observed the wettability of the rubbed PI. Wettability of a solid is governed by its surface free energy. However, it is difficult to measure wettability directly for rubbed PI surface. In this study, the wettability of the PI surface was evaluated by contact angle (CA) of water in contact with the PI surfaces. For this purpose, we rubbed the PI surface with a rubbing strength of 762.51mm, and measured the CA of a drop of water on the surface as a function of azimuthal angle against the rubbing direction. The CA along the rubbing direction, as illustrated in Figure 2, is found to be greater than that along the anti-rubbing direction indicating that the rubbed PI is less wettable in the rubbing direction than in the anti-rubbing direction. In the direction deviated from the rubbing direction, initially the CA increased with azimuthal angle, and then decreased.
0 20 40 60 80 100 120 140 160 180 200
Contact Angle / o
Azimuthal Angle / o
Figure 2. The contact angle of water on the rubbed PI as a function of azimuthal angle against the rubbing direction.
The model used for the evaluation of the surface free energy is based on the assumption that the surface free energy of a solid consists of two independent or partially independent components, the dispersion part and the polar part, with each of them represents a distinctly different type of intermolecular interaction. We here focused on investigating general variation in the total surface free energy without distinguishing the two parts.
In order to determine the surface energy, four wetting liquids, water, diiodomethane, glyceroi, and ethylene glycol, were used as probes. Figure 3(a) shows the response of surface energy of rubbed PI to RS. As can be seen, mechanical rubbing to PI surface can reduce free energy of the polymer surface.
Very little work has been done on rubbing causing anisotropy in surface energy of PI films. In the present studies, we evaluated the anisotropic surface free energy of rubbed PI film based on the values of CA. We assume that the CA of the wetting liquids on rubbed PI is symmetric about the rubbing direction and so does the surface free energy. Figure 3(b) shows the azimuthal surface free energy against rubbing direction for different RS. As can be seen, surface free energy of the PI films is anisotropic. The surface free energy initially decreases with the increase of azimuthal angle, and than increases when the azimuthal angle increases. The surface free energy in the direction parallel to the rubbing direction is smaller than that in the direction anti-parallel to the rubbing direction.
0 200 400 600 800 1000 1200 1400
20
Surface Free Energy / mN/mm
Rubbing Strength / mm
0 20 40 60 80 100 120 140 160 180 200
Surface Energy / mN/mm
Azimuthal Angle / o
(a) (b)
Figure 3. (a) Response of surface free energy of rubbed PI to rubbing strength. (b) Variation of the surface free energy of the rubbed PI with azimuthal angle against rubbing direction.
CONCLUSION
Mechanical rubbing to PI will cause reduction in surface free energy of the PI film. The surface free energy of rubbed PI films is anisotropic, with a smaller value in the direction parallel to the rubbing direction.
It is also observed that the rubbed PI is less wettable in the rubbing direction than in the anti-rubbing direction.
This work is supported by the Grant for “Aim for the Top University Plan” from National Education Committee of ROC.
REFERENCES
[1] J.M. Geary, J.W. Goodby, A.R. Kemtz, and J.S. Patel, The mechanism of polymer alignment of liquid crystal materials, J. Appl. Phys., 62, 4100-4108 (1987).
[2] T. Uchida, and H. Seki, Surface Alignment of Liquid Crystals, in Liquid Crystals: Applications and Uses, Ed by B. Bahadur, World Scientific, Singapore, 1-63 (1992).
[3] D.K. Owens, R.C. Wendt, Estimation of the surface energy of polymer, J. Appl. Polym. Sci., 13, 1741-1747 (1969).
[4] D.-S. Seo, and S. Kobayashi, Study of the pretilt angle for 5CB on rubbed polyimide films containing trifluoromethyl moiety and analysis of the surface atomic concentration of F/C (%) with an electron
Anisotropy in Surface Free Energy of Rubbed Polyimide Thin Films
W.Zheng*, C. Lu, Y. Yeh, and Y. Tien
Institute of Electro-optical Engineering, National Sun Yat-Sen University, 70 Lien-hai Road, Kaohsiung 804, Taiwan ROC Tel : +88675252000 ext 4445, Fax : +88675254499, Email : [email protected]
Abstract: The application of mechanical rubbing to polyimide breaks the symmetry on the polymer surface and promotes a template for liquid crystal alignment. Macroscopic observation of the surface symmetry of the rubbed polyimide is made by means of the measurement of the free energy in the surface region. It is found that the free energy of the rubbed polyimide thin films varied with the azimuthal angle against the rubbing direction. The increase of rubbing strength results in the decrease in the surface free energy of the polymer thin films. Mechanical rubbing also modifies the wettability of the polyimide surface.
Key Words: Rubbed polyimide, Surface free energy, Surface Anisotropy, Surface Wettability.
* Corresponding author.
1 Introduction
The uniform orientation of liquid crystal (LC) molecules is a prerequisite for the operation of LC in electrooptical devices.
Over the years, much effort has been made to achieve a desired molecular alignment of LCs in a cell. The most commonly used technique, up to date, to achieve homogeneous LC alignment is rubbed polymer technique, in which a thin layer of polymimide (PI) is spin-coated onto the inner surfaces of the substrates and subsequently rubbed the polymer surface unidirectly [1]. Although this technique is widely used in LCD industry for achieving desired LC alignment, the alignment mechanism of rubbed PI has not yet been clearly elucidated. It is accepted that the alignment of LC molecules is dominated by chemicophyical characteristics of the surface, and the LC alignment originates from the breaking of the symmetry of the surface of the substrates. Many experimental evidences have shown that rubbing causes polymer chains to orient unidirectly along the rubbing direction [2-6]. Such anisotropy in the distribution of the polymer chains is thought to be responsible for the liquid crystal alignment [7].
In the current literature, the LC alignment is explained as follows. The first layer of LC molecules that are in contact with the surface of the alignment layer is aligned to have a certain orientational order, which is determined by the chemicophysical characteristics of the surface and the LC molecules, and the other molecules that stack on will tend to align themselves to be parallel to the molecules in the first layer through intermolecular interactions. This orientational self-organization of LC molecules grows expitaxially to form an alignment state in the bulk. The molecular alignment of LC is governed by many factors such as intra- and intermolecular interactions, molecule-surface interactions, etc. Surface free energy of the substrates plays a very important role in control of interfacial behavior of LC molecules and LC alignment [8].
As the surface isotropy of a polymer must be broken to become an alignment layer, it is expected that the surface free energy of the alignment layer is also asymmetric.
Recently, we have studied the effects of mechanical rubbing applied to PI thin films on the surface free energy of the polymer films. The model used for the evaluation of the
surface free energy is based on the assumption that there are
surface free energy is based on the assumption that there are