旋光性液晶分子結構對藍相液晶生成的影響

The Effect of Molecular Structures on the Formation of Blue Phases in Chiral Liquid Crystal Materials

王力(Li Wang)

指導教授 ： 吳勛隆

大同大學/工程學院/化學工程學系所/碩士(2011年)

摘要

本研究的目的主要在探討非旋光末端烷鏈的長度、硬核結構與搭配不同的旋光基團對形成液晶相及其性質的影響，並且建立分子結構與藍相液晶材料的關係。五系列旋光液晶材料其分子結構如下所示：
目標化合物之液晶相及其相轉移溫度分別利用偏光顯微鏡與熱微差掃描卡計來做鑑定，而其誘電性液晶相則利用電流轉換行為與介電常數來做進一步確認。
實驗結果顯示：第一系列化合物 I-b(m=6, 7) 只具有BP、N*液晶相，而化合物 I-b(m=8) 則有Iso.-BP-N*-TGBA*- TGBC*-SmC*-Cr.的相變化順序。與化合物 I-b(m=8) 相比，化合物 I-b(m=9) 多了SmA*相而少了TGBC*相。第二系列化合物I-c(m=6, 7) 同樣擁有BP、N*液晶相而化合物 I-c(m=8) 則是多了TGBA*與SmA*相。在這兩系列當中，隨著非旋光末端烷鏈的增長皆有規則度較高的液晶相產生。
第三系列 II-b(m) 與第四系列 II-c(m=7) 化合物皆具有BP、N*液晶相。化合物 II-b(m=6-9)，藍相液晶相的溫度範圍隨著非旋光末端烷鏈的增長有遞減的現象；相反地，膽固醇型液晶相的溫度範圍則有增加的趨勢。在這兩系列當中，化合物 II-c(m=7) 具有最寬廣的藍相溫度範圍。而第五系列化合物 III-b(m=7) 只具有膽固醇型液晶相。
物理性質量測方面：化合物 I-b(m=8, 9) 其最大自發性極化值為68.9和97.2 nC/cm2；而光學傾斜角的最大值為17°和23°。與結構類似的化合物相比，結果顯示，當導入雙氟取代基於單苯環時，對其自發性極化值與光學傾斜角沒有顯著的影響。
總而言之，由五系列化合物中可觀察到BP、 N*、 TGBA*、TGBC*、SmA* 與SmC*相；挫敗相的產生顯示出化合物具有高旋光性。當導入雙氟取代基於單苯環時澄清點只有些微的下降，然而，當非旋光末端烷鏈與單苯環間無連接基時澄清點有明顯的降低；一般而言，單苯環上取代基與連接基的改變對藍相液晶相的溫度範圍無顯著的規則性。由 (R)-2-methyl-3-(pentyloxy)propanoic acid所衍生出的化合物，容易形成穩定的藍相液晶相；於所有的化合物中，以化合物I-c(m=7)具有最寬廣的藍相液晶相生成(約為15.8℃)。

關鍵字

藍相 ； 旋光性液晶材料 ； 雙氟取代基 ； 挫敗相 ； 光電性質

並列摘要

The aim of the research work was an attempt to investigate structure-property relationships of the formation of blue phases in terms of various achiral alkyl chain length (m), rigid core structures and chiral tails in the chiral liquid crystal compounds.
Thus, optically active chiral acids, (2S,3S)-2-chloro-3- methylvaleric acid and (R)-2-methyl-3-(pentyloxy)propanoic acid, and an alcohol, (S)-1-butyloxy-2- propanol were used as the chiral moieties for the preparation of chiral compounds. Five series of chiral compounds containing two kinds of core structures, I-b(m=6-9), I-c(m=6-8), II-b(m=6-9), II-c(m=7) and III-b(m=7), were then successfully synthesized by using these chiral groups for investigation. The corresponding molecular structures of the target compounds are depicted below:
The mesophases and their corresponding phase transition temperatures were determined by the microscopic textures and DSC thermograms. Additional, the
ferroelectric phase were identified by measurements of switching current behavior and dielectric constant ε'.
The results show that compounds I-b(m=6, 7) only exhibit BPII and N* phases. Compounds I-b(m=8) shows the phase sequences of Iso.-BP-N*-TGBA*- TGBC*-SmC*-Cr.. Compare with the compound I-b(m=8), the TGBC* phase were disappeared in compound I-b(m=9) but an additional SmA* formed, resulting in the formation of BPII, N*, TGBA*, SmA* and SmC* phases. Compounds I-c(m=6, 7) also possess BPII and N* phases, however, compound I-c(m=8) displays BPII, N*, TGBA* and SmA* phases. In the series, the results demonstrate that compounds with higher achiral aliphatic chain possess additional phases.
In the second part, the results show that both compounds II-b(m) and II-c(m=7) exhibit BP and N* phases. In the series of compounds II-b(m=6-9), the temperature range of BP phase is decreased and the temperature range of N* is increased when the alkyl chain length (m) is ascended. In addition, the compound II-c(m=7) exhibits the widest temperature range of BP phase among them. Compound III-b(m=7) as the last series, only possesses N* phase and the blue phase is suppressed.
The physical properties of the chiral materials in ferroelectric SmC* phases were measured. The maximum magnitudes of spontaneous polarization measured for the materials are in the range of 68.9 to 97.2 (nC/cm2). The maximum optical tilt angles of compounds I-b(m=8, 9) are 17° and 23°. When comparing with structurally similar compounds that the spontaneous polarization and optical tilt angle are nearly the same, so that core structure has no significant effect on the physical properties.
In conclusion, the results from the study of these five series of chrial compounds show that, depending on the molecular structure, various mesomorphic phases: BP, N*, TGBA*, TGBC*, SmA* and SmC* phases can be found. The appearance of frustrated derived from (2S,3S)-2-chloro-3-methylvaleric acid.

並列關鍵字

electro-optical property ； difluoro substituent ； chiral liquid crystal material ； frustrated phases ； blue phase

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