旋光基尾端具有巨型分子對旋光性材料的液晶相及其光電性質的影響

The Effect of Bulkyl Groups at the Chiral Tail of Chiral Materials on the Mesomorphic and Electro-Optical Properties

李冠達(Guan-Da Li)

指導教授 ： 吳勛隆

大同大學/工程學院/化學工程學系所/碩士(2010年)

摘要

本研究主要以一旋光中心化合物(s)-Propylene oxide為起始物合成五系列的旋光末端烷鏈具有巨型分子的新型旋光性液晶材料I-6(m)、Ⅱ-6(m)、III-6(m)、IV-6(m)和V-6(m)，探討藉著改變液晶材料分子結構上的非旋光末端烷鏈長度(m)及在旋光末端烷鏈導入巨型分子對液晶相種類變化及對其物理性質的影響。其結構如下:
合成出之材料的液晶相及其相轉移溫度的變化藉由偏光紋理圖及熱微差掃描卡計(DSC)的鑑定可分別得知，並經由電流轉換行為(switching current)及介電性質(dielectric constant)的量測可進一步鑑定誘電性液晶相的存在。
實驗結果顯示：五個系列旋光基尾端具有巨型分子的旋光性液晶材料皆具有SmA*和SmC*相，在化合物II-6(10)及IV-6(8-10)有SmCγ*相的生成。其中五個系列液晶化合物的澄清點溫度及化合物I-6(m)、III-6(m)及V-6(m)的SmA*液晶相的溫度範圍都隨著非旋光末端烷鏈(m)的增長而降低。分別比較在旋光末端烷鏈導入直鏈、燕尾及十字的化合物，可以發現隨著末端官能基尺寸大小的增加，SmA*液晶相的溫度範圍會降低，但熔點溫度卻會上升。
材料的物理性質如電流轉換行為、自發性極化值、傾斜角、和介電常數皆於誘電性SmC*液晶相下量測。五個系列液晶化合物的Ps最大值的範圍介於19.15-74.63(nC/cm2)之間。另一方面，五系列傾斜角最大值介於19.75°-34°之間，其值對於末端官能基尺寸大小及非旋光末端烷鏈(m)的長度改變無顯著規則性。實驗結果發現︰在旋光末端導入巨型分子，並沒有產生TGB*和SmCA*液晶相，也沒有層間的微相分離而得到較特別的挫敗相。

關鍵字

旋光性液晶材料 ； 誘電性液晶相 ； 光電性質 ； 電流轉換行為 ； 介電常數

並列摘要

The major investigation work of the study is an attempt to congeneric the structure-property relationship of the chiral liquid crystals in terms of alteration of chiral tail inducting bulkyl group and achiral alkyl chains in the chiral liquid crystal materials.
Thus, the optically active alcohols, 1-cyclopentyloxy-2-propanol,
1-(cyclopentylmethoxy)-2-propanol, 1-cyclohexylmethoxy-2-propanol,
1-neopentyloxy-2-propanol and 1-(norbornane-2-methanoxy)-2-propanol, were
designed and synthesized by reacting (S)-propylene oxide with bulkyl group alcohols under basic condition. In subsequence, five novel homologous series of chiral materials, (R)-4-(1-cyclopentyloxy-2-propyloxycarbonyl)phenyl 4’-alkoxybiphenyl-4-carboxylates, I-6(m), (R)-4-(1-cyclopentylmethoxy-2-propyloxycarbonyl)phenyl 4’-alkoxybiphenyl-4-carboxylates, II-6(m), (R)-4-(1-cyclohexylmethoxy-2-propyloxycarbonyl)phenyl 4’-alkoxybiphenyl-4-carboxylates, III-6(m), (R)-4-(1-neopentyloxy-2-propyloxycarbonyl)phenyl 4’-alkoxybiphenyl-4-carboxylates, IV-6(m) and (R)-4-(1-neobornane-2-propyloxycarbonyl)phenyl 4’-alkoxybiphenyl-4-carboxylates, V-6(m), derived from these alcohols were synthesized for the consideration of increasing the aliphatic alkyl chain length (m) and bulkyl groups size at the chiral tail on the mesomorphic and electro-optical properties.
The mesomorphic phases and their corresponding transition temperatures were capitally characterized by the microscopic textures and DSC thermograms. And the ferroelectric phases were further identified by the measurements of electric switching behavior and dielectric constant ε'.
The results show that, all compounds unveil enantiotropic mesophases of the SmA* and SmC* phases. The compound II-6(10) and IV-6(8-10) exhibit an additional SmCγ* phase. The clearing point decreases when increasing aliphatic alkyl chain lengths (m). Furthermore, the compounds I-6(m), III-6(m) and V-6(m) decrease the thermal stability of the SmA* phase when lengthen of achiral alkyl chain lengths (m).
The comparison of three kinds of materials PPmPPB(10) with linear chain, MEEmPB(10) with swallow-tailed group and IV-6(10) with tert-butyl group at the chiral moiety of the molecules, show that the linear chain compound displays wider temperature range of SmC* phase. The temperature range of SmA* phase is decreased when the terminal group size at chiral tail is ascended. But the milting point is increase when the terminal group size at chiral tail is upturned.
The physical properties such as switching current, spontaneous polarization tilt angle and dielectric constant in the ferroelectric SmC* phases of all chiral compounds were measured. The magnitudes of spontaneous polarization (Ps) rising with decreasing temperature. The maximum magnitudes of spontaneous polarization measured for the materials in the SmC* phase are in the range of 19.15 to 74.63 nC/cm2. These results shows that the maximum Ps values reveal no consequential correlation to the change of achiral alkyl chains, m.
The maximum apparent tilt angles (θ) measured for materials in the SmC* phase is in the range of 19.75 to 34°. These results also indicate that the optical tilt angles have no signification correlation to the variations of the bulkyl group size at chiral tail and achiral chain (R and m).

並列關鍵字

Chiral materials ； ferroelectric phase ； electro-optical property ； switching behavior ； dielectric constant ε'

參考文獻

[2] I. Dierking, Textures of Liquid Crystals. (Wiley-VCH) (2003).

[8] L.J. Yu, H. Lee, C.S. Bak, M.M. Labes. Phys. Rev. Lett., 36, 388 (1976).

[9] N.A. Clark, S.T. Lagerwall. Appl. Phys. Lett., 36, 899 (1980).

[12] S. Garoff, R. B. Meyer. Phys. Rev. Lett., 38, 848 (1977).

[13] S. Garoff, R. B. Meyer. Phys. Rev. A, 19, 338 (1979).