表面聲波對於液晶配向層聚亞醯胺排列的影響

Abstract

本研究提出一種以表面聲波誘發液晶配向層分子排列的方法。配向層聚亞醯胺為長鏈狀分子，在固化過程中，以無數週期之表面聲波所形成的穩定壓力梯度，達成有秩序的分子排列。配向層之硬烤溫度為攝氏200度，表面聲波元件鈮酸鋰基材與金屬電極的熱膨脹係數差異大，爲避免環境溫度過高而損壞，本研究設計一玻璃楔型波導將表面聲波傳遞至配向層內，不僅可增加彈性波的強度，還可降低表面聲波元件之操作溫度。在不同操作條件下驅動超音波，可造成配向層分子不同的配向效果。實驗先以偏光顯微鏡定性觀察分子排列的聲光效應，再以穿透雷射光強量測及傅立葉轉換紅外線光譜儀定量量測配向效果。實驗結果顯示表面聲波可讓聚亞醯胺分子之秩序參數達到0.23，介於磨刷配向與無配向處理之間。目前採用的超音波配向實驗裝置尚無法均勻地改變聚亞醯胺分子的排列，未來仍有改善的空間。

This thesis presents a method of surface acoustic wave (SAW) induced molecular reorientation for polyimide (PI) alignment layers of liquid crystals. After being insonified by a large number of wave cycles, the long chain PI molecules could be aligned parallel by acoustic pressure gradient stably formed in the alignment layer during curing process. The coefficients of thermal expansion for aluminum electrode and lithium niobate substrate of SAW devices are quite different so the device could be broken down at the curing temperature up to 200 degrees Celsius. A wedge-shaped glass waveguide is used to transmit SAWs generated by the interdigital transducer (IDT) adhered on top surface of the glass to PI alignment layer. The wedge tip of the waveguide is used to increase the acoustic intensity and to reduce working temperature of the transducer. The qualitative results were observed by use of a polarized optical microscope. On the other hand, the order parameter of molecular orientation was determined quantitatively by measuring the intensity of laser light transmitted through liquid crystal cell and using Fourier transform infrared spectrometer (FTIR). The experimental results indicate that value of order parameter for molecules insonified by SAW achieves 0.23, which is less than average of rubbed PI layers but much greater than those of unrubbed films. Even though PI molecules cannot be uniformly aligned by SAW in current experimental setup, it is worth further investigation to improve the homogeneity.