超音波操控向列型液晶排列之研究(III)

Abstract

剪應變與壓力梯度都具有改變長鏈奈米材料的排列秩序，後者通常需要許多週期才 能完成整個程序，例如超音波誘發排列。本計畫以超音波原理及液晶的光學性質為基 礎，探討超音波改變向列型液晶配向的機制及其可能的應用。在過去兩年的研究中，已 分別根據沒水式超音波及界面聲波的實驗數據，證實超音波可以有效率的影響向列型液 晶的分子排列。前者實驗顯示穿透液晶的光強度極值發生於洩漏聲導波的模態頻率，惟 為了維持液晶層的聲壓梯度，大部分的聲波能量消耗於玻璃層。後者實驗採用指插換能 器激發界面聲波，大幅改進聲光效應的效能，超音波的能量侷限於液晶層，僅消耗極少 能量於玻璃層。並建立適當的理論模型，探討超音波重排直立及水平配向之向列型液晶 的最佳操作條件、反應時間及恢復時間。基礎於前兩年的研究成果，提出後續的第三年 計畫。 液晶配向層可以有效地節省開關向列型液晶穿透光所需之電場或超音波聲場能 量。第三年研究計畫將以界面聲波誘發棒狀分子的重排，取代傳統的絨毛摩擦刮痕或微 成型之凹槽，改變向列型液晶配向層(聚亞醯胺)的分子排列及液晶分子的預傾角。並開 發操控高分子奈米材料排列的實用技術，提供創新技術給國內以液晶光學為基礎的平面 顯示器產業。

Shear strain and pressure gradient can alter the orientational order of long chained nanoscale materials. The latter method, such as ultrasound induced alignment, usually needs a large number of cycles to complete the process. A research program based on the principles of ultrasound and optical properties of liquid crystals is proposed to explore the mechanism of acoustic re-alignment of nematic liquid crystals and feasible application in polymer alignment. In the past two-year research project, experimental evidence indicates that re-alignment of nematic liquid crystals was effectively caused by guided acoustic waves propagating over the liquid crystal cell immersed in water and interface acoustic waves, respectively. The former results reveal that the extreme intensity of optical transmission through the liquid crystal cell occurs at the modal frequencies of leaky guided waves. But most energy carried by guided waves is consumed in glass plates to sustain sound pressure gradient in the liquid crystal layer. The latter method utilized interdigitated transducer to generate interface acoustic waves. Acoustic energy is confined in liquid crystal layer instead of dissipation into glass plates. A theoretical model was developed to determine the optimal operating conditions, response time, and relaxation time for acoustic re-alignment of both homeotropically and homogeneously aligned nematic liquid crystals. Based on the preliminary study in the past two years, a further one-year research program is proposed. Polymer alignment layer can cost-effectively reduce energy consumption in switching light transmission through nematic liquid crystal layer by either electric field or ultrasound. In the third year, a novel method is presented for molecular reorientation of rod-like polymers using ultrasound. Instead of mechanical scratching by fibrous materials or grooving by micromachining technique, standing interface acoustic waves is used as a buffing process to orient molecular chains of polyimide alignment layer and pretilt angles of nematic liquid crystals. This research can benefit the progress in manipulation of long chained nanoscale materials and provide innovative technology to domestic industry of liquid crystal based flat panel display.