以溫變介電頻譜技術測量藍相材料之相轉變溫度

Abstract

本論文研究藍相液晶材料之相態結構。手段上係利用溫變介電頻譜技術來定義藍相液晶材料之相轉變溫度，並探討不同相態下的離子行為。我們測量水平配向藍相液晶盒之實部介電常數隨溫度變化的曲線圖，並透過將實部介電值對溫度做一階與二階微分的方式，說明HDE/R5011、E7/R5011以及E7/S811等三組藍相液晶材料的相轉變溫度與順序。更進一步地，基於本實驗室針對HDE/R5011之液晶盒中離子行為的相關研究成果，測量待測液晶盒在低頻段（頻率小於5 kHz）的介電頻譜，藉數據擬合得到離子濃度與擴散係數，闡述在不同相態時液晶盒中的離子行為。 實驗結果顯示，將溫變介電頻譜術應用於將溫過程可以精準地擬定出藍相液晶材料的相轉變順序。此方法的量測誤差非常小，在一階相變溫度的誤差小於0.13%，而二階相變溫度的誤差小於0.42%。溫變介電頻譜術相對於其它測量相轉變溫度的方式，不會受到樣品的量測面積影響，也不會受到儀器量測範圍的限制。針對液晶盒中的離子行為，可發現對掌性分子之濃度多寡並非影響離子濃度的主要因素。離子擴散係數之變化主要是受樣品盒的環境溫度影響，並非液晶層的相態變化。

This thesis focuses on phase behaviors of blue-phase-liquid-crystal (BPLC) materials. The phase-transition temperatures and ionic behaviors of BPLC materials are investigated by means of temperature-dependent dielectric spectroscopy. By introducing BPLCs into planar-aligned cells, the phase sequence and transition temperatures between two adjacent mesophases of three distinct BPLC mixtures (R5011-doped HDE, R5011-doped E7, and S811-doped E7) are clarified according to the data of the first and the second derivatives of the real-part dielectric permittivity at a specific frequency with respect to the temperature. Furthermore, based on our previous studies for the ionic effect in BPLC (HDE/R5011) cells, the dielectric spectra in the low-frequency regime (f ≤ 5 kHz) characterized by space-charge polarization allow one to investigate the impurity-ion properties of BPLC cells in comparison with those of their neat counterparts. Experimental results indicate that the phase sequence of a BPLC can clearly be defined through the temperature-dependent dielectric spectroscopy on the cooling process. The percent error of measurement of this dielectric method is very small. The maximum percent errors of the first-order and the second-order phase transition are 0.20% and 0.46%, respectively. Compared to other phase identification methods, the area of a BPLC sample and the measurement-range restriction of instrument cannot affect the data of the temperature-dependent dielectric spectroscopy. In regard to the ionic behavior in the BPLC cells, experimental results show that the ion- concentration value of BPLC cells changes owing to different phases. Moreover, the environmental temperature of BPLC cells can affect the ionic diffusivity in cells rather than phase transition.