偶氮手性向列型液晶的光致相變與介電特性

Abstract

偶氮手性材料之螺旋扭轉力可經由外在光源調控。因此，常被用來作為製作膽固醇液晶的添加物，藉此達到光控反射光譜特性（如中心波長與頻寬）的效果。在本論文中，我們研究含偶氮手性材料之膽固醇液晶，主要利用即時量測之溫變介電頻譜以及偏光顯微鏡觀察，探討其相轉變行為在不同強度之紫外光曝照下之變化。在數據分析上，我們繪製在10 kHz之介電實部對溫度的變化圖並將此曲線對溫度作一階微分，目的在於判斷待測液晶盒之相轉變順序與溫度。更進一步地，我們透過光學顯微鏡觀察，探討紫外光曝照對於各液晶相態之紋理的影響。 實驗結果顯示，在未照光與照射綠光的條件下，含偶氮手性材料之膽固醇液晶在液態相與液晶相之間存在一個預轉相態。當照射紫外光時，增加曝照強度會抑制此預轉相態的形成。此外，當紫外光強度大於2 mW/cm2時，在液晶相溫度範圍內存在一個紋理轉換。此紋理轉換的溫度點會隨著照光強度的增加而降低。因含偶氮手性材料在曝照紫外光下會降低其螺旋扭轉力，我們將以膽固醇液晶螺距的改變來解釋所觀察到的現象。

The helical twisting power (HTP) of an azo-chiral material is known to be tunable by illuminating external lights. Therefore, it is commonly used as an additive in a cholesteric liquid crystal to show photo-tunable spectral properties, such as central wavelength and bandwidth. In this thesis, we focus on investigating the effect of ultraviolet（UV）exposure on the phase behaviors of an azo-chiral-doped liquid crystal（ACD-LC）by measurements of temperature-dependent dielectric spectra and observations of optical images. The phase sequence and transition temperatures of the ACDLC cell were determined by the temperature-dependent real-part dielectric permittivity at 10 kHz and its first-order derivative with respect to the temperature. Furthermore, the textural changes as a function of the UV intensity in LC phases were discussed by using the polarizing optical microscope. When the cell is illuminated with green light, a pretransitional phase generated between the isotropic and the LC phases is obtained which is the same as that of the cell without light exposure. In contrast, by exposing UV light to the ACDLC cell, the pretransitional phase gets suppressed with increasing UV intensity. Moreover, when the UV intensity is higher than 2 mW/cm2 a textural change is occurred within a temperature range where the cell exhibits the LC phase. The transiting temperature of such a textural change gets decreased with increasing UV intensity. Because the HTP of the azo chiral is decreased by the UV exposure, the observed results are explained in terms of the change of the helical pitch.