探討具醚基連結基之誘電性及反誘電性液晶材料之液晶相及其光電性質

Study on the Mesophases and Electro-optical Properties of Ferroelectric and Antiferroelectric Liquid Crystals Containing Methyleneoxy Linking Group

趙珮君(Pei-Chun Chao)

指導教授 ： 吳勛隆

大同大學/工程學院/化學工程學系所/碩士(2008年)

摘要

本研究目的為以(S)-propylene oxide 為起始物並與半氟化與未氟化醇類在鹼性的條件下反應合成三系列新型旋光性液晶材料以探討(1) 硬核連結基由酯基改為醚基觀察是否有其他液晶相的生成，(2) 改變液晶分子旋光末端烷鏈氟化的程度觀察其液晶相的變化 (3) 將硬核連結基上的氫改為氘對其物理性質的影響。
藉由偏光紋理圖及DSC的鑑定可分別得知液晶相及相轉移溫度的變化，經由電流轉換行為及介電性質的量測可進一步鑑定誘電性及反誘電性液晶相的存在。
在第一系列(m=8-11)中出現SmA*、SmC*、SmCγ* 的液晶相，而在非旋光末端烷鏈碳數較長的材料(m=12)則只有SmA*-SmC*的液晶相。第二系列(m=8,10)中出現了SmC*-SmCA*- SmX* (SmI* or SmF*)的液晶相轉移，而在第二系列(m=9,11,12)中則只有SmC*相的生成。第三系列(m=8-12)與第一系列(m=8-11)比較可以發現所有材料均抑制了SmCγ*相的生成而生成SmA*和SmC*液晶相。
實驗結果顯示: 含半氟化旋光中心會抑制SmA*的生成，但有利於SmCA*的生成。將硬核連結基上的氫改成氘時，則會增加SmA*相及SmC*相溫度範圍。當硬核連結基由酯基改為醚基時 (1)含未氟化旋光中心之液晶分子其SmA*相及SmC*相溫度範圍均變窄 (2)含半氟化旋光中心之液晶分子其SmA*相消失且SmCA*相溫度範圍變窄但SmC*相溫度範圍變寬。
材料的物理性質皆於SmC*、SmCγ*、SmCA*及 SmX* (SmI* or SmF*)的液晶相下量測。化合物I(m=8-12), II(m=8-12)和 III(m=8-12)自發性極化值最大值各分別介於17.7-30.9nC/cm2, 80-120.4nC/cm2和8-20nC/cm2之間，顯示含半氟化旋光中心之液晶分子有較大的自發性極化值。而當硬核連結基上的氫被氘取代則會降低自發性極化值。比較I(m=8-12), PPmPPB(m=8-12) 和II(m=8-12), MPFPECPmBC(m=8-12)可以看出當硬核連結基由酯基改為醚基時會增加自發性極化值。化合物I(m=8-12), II(m=8-12)和 III(m=8-12)傾斜角的最大值範圍各分別介於18°-22°, 23°-34.5°和 21°-27.5°之間，顯示含半氟化旋光中心之液晶分子有較大的傾斜角，當硬核連結基上的氫被氘取代會增加傾斜角。比較I(m=8-12), PPmPPB(m=8-12) 和II(m=8-12), MPFPECPmBC(m=8-12)可以看出當硬核連結基由酯基改為醚基時對於傾斜角不會有太大的影響。
當液晶分子之連結基為-COO-或-CD2O-時，皆會抑制SmCγ*相的生成。而在第二系列中可以看出當液晶分子含有半氟化旋光烷鏈及醚基連結基時，會有利於SmCA*相的生成並抑制SmA*像的生成。當液晶分子含有半氟化旋光末端烷鏈，會明顯的提升自發性極化值。

關鍵字

醚基連結基 ； 半氟化旋光性液晶材料 ； 誘電性液晶 ； 反誘電性液晶

並列摘要

The purpose of this research work was an attempt to elucidate the structure-property correlation in the chiral smectic liquid crystals. Optically active (S)-1-propyloxy-2-propanol and (S)-1-(2,2,3,3,3-pentafluoropropyloxy)-2-propanol were designed and prepared by the treatment of (S)-propylene oxide with corresponding alcohols under basic condition. Three homologous series of chiral compounds derived from these alcohols were synthesized, and their mesophases and electro-optical properties were investigated. The effects of changing (1) the linking group in the core structure of the molecule changing from carboxylate group to methyleneoxy group, (2) the chiral group varied from non-fluorinated to semi-fluorinated alkoxy chain length and, (3) the linking groups modified from -CH2O- to deuterated –CD2O- core structure of the molecule were compared and discussed.
The mesomorphic phases and their corresponding transition temperatures were primarily characterized by the microscopic textures and DSC thermograms, and the ferroelectric and antiferroelectric phases were further identified by the measurements of electric switching behavior and dielectric constant ε'.
Compounds I(m=8-11) exhibited the SmA*, SmC* and SmCγ* phases, while compound (m=12) with longer alkyl chain exhibited the SmA*-SmC* phases. Compounds II(m=8,10) with even number of alkyl chain length displayed ferroelectric SmC*, antiferroelectric SmCA* and ferroelectric SmX* (SmI* or SmF*) phases, and compounds II(m=9,11,12) with odd number of alkyl chain length displayed exclusively SmC* phase. It is interesting to found that the deuterated compounds III(m=8-12) suppressed the formation of SmCγ* and resulted in the formation of SmA* and SmC* phases as compared to the non-deuterated compounds I(m=8-11). These results demonstrated that the semi-fluorinated compounds II(m=8-12) facilitate the formation of antiferroelectric phase, and suppressed the formation of SmA* phase. When the linking group changes from -CH2O- to deuterated –CD2O- core structure of the molecules, the temperature range of the SmA* phase and the SmC* phase increase. Comparing structurally similar non-fluorinated compounds PPmPPB(m=8-12) to I(m=8-12), it can be found that when the linking group changes from –COO- to -CH2O-, the temperature range of the SmA* phase and the SmC* phase decrease. Comparing structurally similar compounds II(m=8-12) to MPFPECPmBC(m=8-12), it can be found that compounds with -CH2O- linking group, suppresses the formation of SmA* phase and decreases the temperature range of the SmCA* phase, but increases the temperature range of the SmC* phase.
The physical properties of the chiral compounds in ferroelectric SmC*, SmCγ*, SmX* (SmI* or SmF*) and antiferroelectric SmCA* phases were also measured. The maximum magnitudes of the spontaneous polarization for compounds I(m=8-12), II(m=8-12) and III(m=8-12) are approximately in a range of 17.7-30.9nC/cm2, 80-120.4nC/cm2 and 8-20nC/cm2, respectively, demonstrating that the compounds with semi-fluorinated chiral group have larger Ps values. When the linking group of the chiral compounds changes from -CH2O- to deuterated –CD2O- in the core of the molecule, the maximum Ps values slightly decreases. Comparing I(m=8-12) to PPmPPB(m=8-12), and II(m=8-12) to MPFPECPmBC(m=8-12) indicate that the maximum Ps values are greater in the structurally similar compounds with methyleneoxy linking group than carboxylate group in the core of the molecules.
The maximum apparent tilt angles of series I(m=8-12), II(m=8-12) and III(m=8-12) are approximately in a range of 18°-22°, 23°-34.5° and 21°-27.5°. The results showed that the compounds with semi-fluorinated chiral group have larger apparent tilt angles. When the linking group changes from -CH2O- to deuterated –CD2O- in the core structure of the molecule, the optical tilt angles increase. Comparing I(m=8-12) to PPmPPB(m=8-12), and II(m=8-12) to MPFPECPmBC(m=8-12), it can be found that there has no any signification correlation of the maximum apparent tilt angles.
In conclusion, the chiral compounds with -COO- or -CD2O- linking group on the main core structure of the molecule suppress the formation of the SmCγ* phase. And when the chiral compounds with –CH2O- linking group on the main core structure of the molecule having semi-fluorinated alkyl chain in chiral tail facilitate the formation of SmCA* phase but suppress the formation of the SmA* phase. Regardless of the compounds with -COO- or -CH2O- linking group on the main core structure of the molecule, the results show that the compounds with semi-fluorinated alkyl chain in chiral tail largely enhance the maximum Ps value.

並列關鍵字

semi-fluorinated chiral liquid crystal ； ferroelectricity and antiferroelectricity ； Methyleneoxy linking group

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