以交流電場促成均勻橫向螺紋態之關鍵頻率

Abstract

本論文透過介電頻譜術研究膽固醇液晶中的離子行為，依據電流體效應（electrohydrodynamic, EHD）在膽固醇液晶中產生橫向螺紋態（uniformly lying helix, ULH）的機制，本文著重於形成均勻橫向螺紋態的關鍵頻率範圍。本研究使用向列型液晶E7分別摻雜右旋手性分子形成右旋膽固醇液晶，藉由電容電感阻抗分析儀量測樣品的介電實部（ε’）與虛部值（ε”）並計算出正切損耗值（tan δ），透過介電頻譜上這三條曲線可以訂出三個頻率點，分別是fL、fR與fH並將介電頻譜劃分成四個頻率區間，透過偏光顯微鏡觀察不同頻率區間下膽固醇液晶的相態切換並分析其離子行為，並提出能夠產生均勻的橫向螺紋態之關鍵頻率範圍落於fL與fR之間 （fL < f < fR），在此操作頻率範圍內，液晶盒中的離子能夠在液晶盒上下基板間做完整遷移，因而能使得膽固醇液晶形成均勻橫向螺紋態。另外我們同時也利用氦氖雷射光與光學紋理圖分析了電場強度與溫度條件。 在探討完此款右旋膽固醇液晶的離子行為與相態切換後，我們使用另一款左旋手性分子摻雜入同樣的液晶本體E7中形成左旋膽固醇液晶，但並無法透過外加電場的方式形成ULH態，因此透過介電頻譜術量測介電值並代入公式擬合得出離子濃度與擴散常數，並對樣品照射UV光的方式增強離子效應，在曝光後電流體效應的強度便可使用上述所提出的方法找出關鍵頻率範圍並形成均勻橫向螺紋態，為了探討CLC中離子濃度與擴散常數對於ULH形成的影響，使用溫寬較寬的液晶E44摻雜S5011對這兩個參數做詳細探討。 透過介電頻譜術，本文提出利用電流體效應形成均勻橫向螺紋態的關鍵頻率範圍，並且同時分析操作電場強度、環境溫度與離子效應強度的影響。

Based on the mechanism of voltage-induced electrohydrodynamic (EHD) effect in a cholesteric liquid crystal (CLC) with positive dielectric anisotropy, this study is aimed to investigate the optimized frequency regime for the formation of the well-aligned uniformly lying helix (ULH) state. Firstly, we focus on the CLC mixture (R-CLC) made of the nematic liquid crystal host E7 doped with the right-handed chiral agent R5011. By means of complex dielectric and the tan δ spectra, three frequencies designated fL, fR, and fH can be identified at a given temperature. These frequencies are meaningful to characterize the transport behavior of mobile ions under the application of a probe ac electric field and to divide the dielectric spectra into four frequency regimes. According to the textural changes of CLC cell in each frequency regime, it is found that the frequency regime of fL < f < fR where the mobile ions can effectively oscillate in the bulk of the cell is the most appropriate one for the well-structured EHD-induced ULH state. Furthermore, the effect of other parameters, such as voltage and temperature, on the formation of the ULH state and its uniformity is discussed by measurements of voltage- and frequency-dependent transmission spectra with optical texture. To confirm the frequency regime (i.e., fL < f < fR) is nonspecific for the generation of the EHD-induced ULH texture, we further investigated a counterpart containing S5011 whose helical twisting power has the same strength as that of but opposite handedness to R5011. Our results indicate that the ion effect in the S-CLC cell is too weak to induce the EHD effect. After performing ultraviolet (UV) light exposure to the S-CLC to promoting the ionic effect, it is worth mentioning that the EHD-induced ULH state can be formed by external voltages whose frequency is set in the between fL and fR（fL < f < fR）.Furthermore, we employ nematic LC E44 instead of E7 as the host material in the S-CLC system to enlarge the temperature range so as to discuss the influence of ion density and diffusivity on the formation of the ULH texture in a CLC. As a result, this study provides a pathway to determine the optimal frequency regime for generation of the EHD-induced ULH state by means of dielectric spectroscopy.