單軸延伸聚亞醯胺薄膜光學異向性之研究

Abstract

本論文主要研究分為二部分：(1)以單軸拉伸方式來探討PI薄膜的光學異向性﹔(2)探討PI薄膜在拉伸時結構對其雙折射性的影響。首先，我們以動態機械分析儀(DMA)的應力緩和(Stress Relaxation)模式對芳香族FDA-BAPP PI及脂肪族D2192-BAPP PI薄膜進行拉伸實驗,並將拉伸後的PI薄膜置於一組相互交差的偏光片中觀察其光學異向性,發現拉伸後的PI薄膜試片可以降低在±45o視角的漏光的效果，展現出如同A-Plate的光學補償特性，所顯現的光學補償效果相當的均勻而且沒有色偏的情形發生，證實以拉伸方法可以造成PI薄膜很均勻的雙折射效應。PI薄膜在拉伸時之拉伸力(Pre-load Force)是影響PI雙折射率的重要控制因素，過高的拉伸力( >0.3N)會造成PI薄膜試片的Necking現象，將導致其雙折射率的不均勻。Prism Couping方法被用來量測PI薄膜的折射率與厚度，藉由雷射光耦合之方式可以在小範圍區域內精確量到PI薄膜的折射率，將經此方式量得的PI薄膜折射率與厚度計算其光學補償值，其結果與直接量測法所得的光學補償值相當，說明以此拉伸方法可以獲得很好的PI薄膜光學異向性。 本論文的第二部分在研究PI分子結構與其雙折性的相互關連性，我們合成7種不同結構之PI並將其薄膜以單軸拉伸之方式作延伸實驗後，量測其In-Plane的雙折射性。在此是以DMA 的另一種試片拉伸方式-Creep模式，來取代之前的Stress Relaxation模式對PI薄膜試片進行拉伸實驗，Creep模式是以最低的拉伸力量及最低的拉伸溫度的前提下，選擇Pre-load Force(N)與Isothermal Temperature(oC)這二項為拉伸控制因素，在此狀態下的PI薄膜拉伸比是取決於其拉伸的時間，此種拉伸模式將可得到較高的拉伸比，而高的拉伸比是獲取較高薄膜雙折射率的必要條件。經拉伸試驗後發現PI薄膜的雙折射率與其分子結構有相當的依存性，其中PI分子的排列程度將影響其拉伸雙折射性的大小。為了釐清PI結構對其拉伸雙折性之影響，我們引用ANDO’s團隊所提出的模擬理論，以計算推估出之Dno為指標來觀察PI結構對其拉伸雙折射性的影響，發現此法與實際量測PI薄膜的雙折射率間，在某一些PI結構上會有預測上的差異，無法準確的預測PI薄膜的拉伸雙折射率。我們考慮到PI薄膜在拉伸時所產生的分子交互作用的影響，以其分子鏈排列程度作一修飾參數來對ANDO’s的預估模式作修正，發現經修正過的預估模式可以較準確的預測PI薄膜結構與拉伸雙折射性之間的關係，其預測值與實際量測得到的雙折射性呈現較好的一致性，此一致性的結果也經由X-ray繞射實驗結果得到證實。

A new biaxial-retardation film prepared from the uniaxial-stretched polyimide (PI) film was developed for compensating the viewing angle of liquid-crystal displays. In the first part of this thesis, Stress relaxation method set up in the software of DMA was used to perform the aromatic (FDA-BAPP) and aliphalic (D2192-BAPP) PI film stretching process. To study the optical anisotropies property to the light leakage from the crossed polarizers, we put the stretched samples into one set of crossed polarizers and as expected, the stretched PI films could reduce the light leakage at view angle of ±45 deg, and exhibited a visually observed compensating property of an A-plate. No color shift was observed, and the birefringence was very uniform. These results prove a uniform birefringence can be obtained by a stretched process for PI films. Preload force is a key factor to control the birefringence in stretched process. The non-uniformity of birefringence induced by necking phenomenon was observed in the sample with high preload force of 0.3.To analyze the birefringence change in stretched PI films, the prism coupling method was used to measure the nx, ny, nz, and thickness of the stretched PI films. The prism coupler method can sense local deviation of birefringence better than direct retardation measurement. It may be due to the smaller beam size of Hi-Ni laser used in prism coupling. In conclusion, the biaxial retardation of uniaxial-stretched PI films was confirmed by both methods of RI measurement and direct retardation measurement and obtained a good optical anisotropies property. The second part of this thesis is to study the relationship between PI film structural and birefringence property. Seven polyimides (PI) were synthesized and their films were uniaxially stretched to observe the variation in their in-plane birefringence. The creep method of DMA was used instead in stretching the PI films. The preload force (N) and isothermal temperature (oC) were the two controlled factors that were chosen on the basis of the lowest stretchable force and lowest temperature for each PI structure. The elongation ratio of a PI film was then decided by the stretching time. Creep method can be reduce the necking phenomenon was observed and obtained high elongation ratio in the stretched process for PI film specimens. After the stretched process, the birefringence of PI have a strong relationship with it’s molecular structural. To clarify the structural effect on the stretch-induced Dn in PI films, we followed ANDO’s method to estimate the Dno of each PI structure. We found the birefringence from ANDO’s estimate method has some different comparing with measured by prism coupler. For modified the ANDO’s model, we used a molecular orientation degree factor in PI film stretched process to revise this model. We can obtain an accurate estimated stretched birefringence of PI film after revised model. Prism coupler measured also has same results proved by X-ray diffraction testing.