向列性混合物於磁場作用中與不同分子結構之鐵電性液晶的分子動力學研究

Study on the Molecular Dynamics of a Nematic Mixture under Magnetic Fields and Ferroelectric Liquid Crystals with Various Molecular Structures

黃蕾靜(Lei-Ching Huang)

指導教授 ： 傅昭銘

國立臺灣大學/理學院/物理研究所/博士(2016年)

https://doi.org/10.6342/NTU201610300

摘要

液晶在光學設備上已被廣泛應用了數十年，對於任何光學裝置來說，液晶的性能由其介電強度、弛豫頻率及反應時間等等由介電與光電參數所決定；因此，液晶的基礎性質已引起許多研究者的興趣；在這些研究之實驗方法當中，透過介電測量得以讓研究者得知液晶的分子動力學；所以，液晶分子的介電頻譜在其光電裝置上的應用扮演著很重要的角色。而本研究透過分析介電頻譜，來探討向列相液晶混合物E7在磁場效應下與不同分子結構的鐵電性液晶兩者的分子動力學。
在液晶顯示器的實際應用中，電場對液晶的效應已廣受研究；另一方面，以磁場控制液晶定向是一項有趣的課題。本研究中，首先探討磁場作用下，水平排列的向列相液晶混合物E7之分子動力學；透過此部分的介電頻譜量測結果，顯示約1000 Oe的低磁場即使得低頻(~kHz)( δ-弛豫)與高頻(~MHz) (α-弛豫)的介電弛豫頻譜產生變化。更進一步將實驗所得之介電頻譜轉換成介電模數(electric modulus)，並對介電模數進行Cole-Cole模型理論分析，發現由適配結果所得的弛豫時間與介電強度等適配參數，會隨著所施加的磁場強度產生系統性變化。本研究提出微觀分子動力學模型，用以描述E7分子隨磁場呈現之兩段式變化；希望透過此研究結果，得以提供磁場作用下之液晶分子動力學的磁控應用理論。
此外，因為鐵電性液晶展現快速轉換特性，且相較於其他液晶具有較大的各向異性變化(Δn~0.1)；因此，鐵電性液晶亦受到諸多學術研究的關注；再者，鐵電性液晶的物理特性深受其分子結構影響。本研究探究具有三種不同核心環與兩種對掌性結構之六種鐵電性液晶分子的自發極化值與分子動力學；由實驗結果發現，自發極化值與核心結構極為相關；經由比較六種實驗樣品所測得之自發極化值，發現具聯苯核心結構的鐵電性液晶之自發極化值為最大，而具苯環核心結構的鐵電性液晶之自發極化值卻為最小；另外，觀察實驗所得之介電頻譜，發現所有鐵電性液晶在鐵電性相(SmC*)溫區下的複數介電頻譜皆呈現Goldstone模式；然而，兩個以苯環為核心結構的樣品部分溫區之介電頻譜，則呈現似陶鐵電性液晶相(SmCγ*-like)的特性。對六種鐵電性液晶的複數介電頻譜進行理論適配，結果呈現，Debye模型與Cole ̶ Cole模型之理論可與實驗結果適切適配。此外，對於具有(S, R)-對掌性末端基的鐵電性液晶分子來說，在低直流電場作用下，Goldstone模式會有上升變化；然而，對於具有(S, S)-對掌性末端基的鐵電性液晶分子來說，在直流電場作用下，此模式卻只呈現受壓抑的變化；本研究提出微觀分子動力學模型以描述此Goldstone模式特殊的上升變化之潛在機制；另外，亦探討鐵電性液晶分子結構與介電頻譜及自發極化值的實驗結果之相關性。

關鍵字

液晶 ； 鐵電性液晶 ； 介電弛豫 ； 介電模數 ； 自發極化值

並列摘要

The application of liquid crystals to optical devices has been widespread for a few decades. For any optical device, the performance of liquid crystals is determined by their dielectric and electro-optical parameters such as dielectric strength, relaxation frequency, and response time. Thus, many studies have been carried out as a result of interest in the fundamental properties of liquid crystals. In the experimental methods of these studies, dielectric spectroscopy of the liquid crystal molecules plays an important role in their application for electric-optical devices, since measurement of the dielectric response enables the researchers to depict the molecular dynamics of liquid crystals. In this study, the dielectric spectra have been analyzed to investigate molecular dynamics of the nematic mixture E7 under magnetic fields and the ferroelectric liquid crystals with various molecular structures.
The effect of electric fields on liquid crystals has been studied extensively for practical applications of liquid crystal displays. Moreover, controlling liquid crystal orientation through the use of magnetic fields is a subject of interest. In this study, the molecular dynamics of the homogeneously aligned nematic liquid crystal mixture E7 subject to a magnetic field were studied. The dielectric spectra study revealed a low bias magnetic field (H ∼ 1000 Oe) effect on the evolution of dielectric relaxation spectra occurred at lower (~kHz) (δ-relaxation) and higher (~MHz) (α-relaxation) frequency regions. The complex electric modulus, which was converted from experimental dielectric spectra, was analyzed with the theoretical model of Cole-Cole relaxation. The obtained fitting parameters, such as relaxation time and dielectric strength, varied systematically with the strength of the applied magnetic field. A microscopic molecular dynamic model was proposed to describe the two-step variation of E7 molecules under the bias magnetic field. The results imply magneto-modulation of liquid crystal molecular dynamics under the bias magnetic field.
Moreover, since the ferroelectric liquid crystals exhibit faster switching compared to what had earlier been seen in liquid crystals, considerable attention has been given to ferroelectric liquid crystals. The physical properties of the ferroelectric phase are largely affected by the structure of the ferroelectric liquid crystal compounds. The spontaneous polarization and molecular dynamics of six ferroelectric liquid crystals with three different kinds of core rings and two types of diastereomeric structures were investigated in this study. The experimental results revealed that the spontaneous polarization was dependent on the core structure. Among the experimental samples, the ferroelectric liquid crystals with a biphenyl ring core structure showed the highest spontaneous polarization, and the ferroelectric liquid crystals with a phenyl ring core structure showed the lowest spontaneous polarization. The complex dielectric spectra exhibited the Goldstone mode in the ferroelectric (SmC*) phase for all ferroelectric liquid crystals, whereas a novel ferrielectric-like (SmCγ*-like) phase was observed in samples with phenyl ring core structures. The complex dielectric spectra of the six ferroelectric liquid crystals can be optimally fitted by the Debye model and the Cole ̶ Cole model. Moreover, the Goldstone mode was enhanced under low DC bias fields for the ferroelectric liquid crystals with the (S, R)- diastereomeric structure, whereas the mode was suppressed under DC bias fields for the ferroelectric liquid crystals with the (S, S)- diastereomeric structure. A microscopic molecular dynamic model was proposed to describe the underlying mechanism of the particular enhancement of the Goldstone mode. The experimental results of dielectric spectra and spontaneous polarization are explained in the discussion of mesomorphic properties and their relation to ferroelectric liquid crystals molecular structure.

並列關鍵字

Liquid crystals ； Ferroelectric liquid crystals ； Dielectric relaxation ； Electric modulus ； Spontaneous polarization

參考文獻

References for Chapter 1

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