手性添加物在膽固醇液晶中的離子貢獻

Abstract

本論文以介電頻譜術為研究手段，主要探討手性材料的添加對於膽固醇液晶中離子行為的影響。實驗中，我們選用兩款具有相同螺旋扭轉力之右旋手性材料R5011與左旋手性材料S5011，並分別摻雜至向列型液晶E44中形成右旋與左旋膽固醇液晶。此外，我們亦將相同濃度之右旋與左旋膽固醇液晶以1:1的比例混合，製作具消旋結構之手性分子/液晶混合物。根據介電量測結果，我們分析各樣品在低頻段(頻率小於 1 kHz）的介電頻譜，透過數據擬合得到直流導電率、離子濃度與擴散係數，藉此闡述手性添加物在膽固醇液晶盒中的離子貢獻。 首先固定手性材料濃度，我們透過光譜與介電結果比較得知，改變表面配向結構；如水平與90度扭轉配向，並不會影響膽固醇液晶的光學與介電特性。另一方面，針對單一手性材料摻雜，實驗結果顯示，含R5011之膽固醇液晶中的離子效應比E44大且會隨著R5011濃度的上升而增強。相對之下，含S5011之膽固醇液晶的離子特性與E44相同且不具濃度依存性。特別的是，我們發現，具消旋結構之手性分子/液晶混合物，其離子特性取決於R5011的濃度。由上述結果可推論，以摻雜手性材料於向列型液晶中所形成之膽固醇液晶，其離子效應的強度主要取決於材料的離子特性，與旋性以及螺旋結構無關。

This thesis focuses on inveatigating the effect of chiral additives on the ionic behaviors of a cholesteric liquid crystal (CLC) by means of dielectric spectroscopy. In this study, two kinds of chiral additives—R5011 with right-hand chirality and S5011 with left-hand chirality were used and doped individually into the nematic LC host E44 to form right-hand and left-hand CLCs, respectively. In addition, chiral/LC mixtures with racemic molecular structure were also prepared by mixing right-hand and left-hand CLCs with identical chiral concentration in a weight ratio of 1:1. Based on measured dielectric results, the dielectric spectra of CLC cells in the low-frequency regime (f ≤ 1 kHz）characterized by space-charge polarization, are discussed to characterize the impurity-ion properties of CLC cells. By further fitting the dielectric results to deduce the dc conductivity, ion density and ion duffusivity, the ionic contribution of the chiral additives in CLCs is clarified. In cases of CLC cells with fixed chiral concentration, results from transmission and dielectric spectra suggest that the optical profiles and dielectric properties of a CLC do not be affected by the cell configuration (i.e., planar and 90° twisted alignments). On the other hand, For CLCs containing single type of chiral additives, experimental results indicate that the ionic effect of the CLCs doped with R5011 is higher than that of pure host LC E44 and it is enhanced with increasing dopant concentration of R5011. In contrast, the ionic properties of CLCs with S5011 is identical to that of pure E44 and it is nearly invariance to the S5011 concentration. Specifically, it is found that the ionic behavior of the chiral/LC mixtures with racemic structure depends on the content of R5011. For the CLC consisting of a nematic LC host doped with chiral additives, It can be concluded from aforementioned results that the strength of ionic effect is determined by the ionic properties of the host LC and chiral additives rather than the chirality of the dopant and the helical structure of the CLC.