探討氟化程度對曲棍形液晶分子的液晶相影響

Effect of Flurorinated Extent on the Mesophases of Hockey Stick Mesogens of Liquid Crystals.

莊宗翰(Tsung-Han Chuang)

指導教授 ： 吳勛隆 ； 余良杰

大同大學/工程學院/化學工程學系所/碩士(2009年)

摘要

本研究所探討的液晶衍生物之硬核部分，是以一個聯苯加上一個苯環所組成。聯苯與單苯環之間是以一個酯基官能基在對位的方向連接而成，而柔軟的烷氧鏈以間位連接單苯環；在對位方向的烷氧鏈長度，碳數由8增加至12，而間位方向的烷氧鏈長度，碳數是6與8（系列III、IV），另外以9與13個氟原子取代（系列I、II），可形成曲棍形分子（系列I-IV），此類分子具有液晶相，分別由偏光顯微鏡、微差熱分析儀、X-ray粉末繞射與電場效應來鑑定其液晶相。
可以觀察到未氟化的曲棍形液晶分子（系列III、IV），具有SmA、SmC及SmCa液晶相，但是在氟原子取代後的曲棍形液晶分子（系列I、II），SmCa相消失，只觀察到SmA與SmC相。
曲棍形分子SmC紋理圖中schlieren區域可觀察到少許2-brush defects，表示液晶相具有反傾斜的層相結構，扇形區域中具有兩種不同光學性質區塊(domain)。SmC紋理圖中旋轉樣品時，會發現兩區塊顏色互換，這樣的現象表示在單一的區塊中分子均排列堆疊一致，而在相鄰的區塊內分子傾斜方向相反。
系列III及IV系列具有SmCa相，在紋理圖中schlieren區域大部分皆為2-brush defects，且schlieren紋理圖較SmC相均勻。扇形紋理圖亦具有兩種不同光學性質區塊(domain)，扇形區塊中出現等間距平行線條其類似於旋光層列相(chiral smetic phase)所出現的pitch band紋路。
曲棍形液晶分子之Iso.-SmA 相轉移，為一級相轉移(first order)，同時也可得知在SmA –SmC與SmC-SmCa 相轉移，為微弱地一級相轉移(weakly first order)，此現象不同於桿形液晶分子。除此之外，曲棍形液晶相在熱力學性質方面皆無法分辨出與桿形液晶相之差異。
X-ray 粉末繞射數據中得知，層相（smetic）為單層分子結構，而在SmA轉SmC相的傾斜角度變化不大。
將樣品灌入在2μm厚度的平行配向cell中，觀察到的SmA紋理圖中施以電場，紋理圖上並未有任何顯著的變化。SmA紋理圖中施以電場，可以觀察到分子排列較無施予電場的整齊，而且旋轉液晶樣品，觀察到兩區域顏色互換。SmCa紋理圖中施以不同強度的電場，類似pitch band的紋路消失，兩種不同掌性區域依舊存在，旋轉樣品時兩區塊顏色互換。嘗試測量曲棍形分子SmC及SmCa相之Ps值，但無法成功。
本實驗利用偏光顯微鏡、熱差掃瞄分析儀、粉末X光繞射以及電場效應，來獲得相關數據。

關鍵字

曲棍形液晶分子 ； 同傾層相(SmC) ； 反傾層相(SmCa) ； 液晶分子

並列摘要

Hockey stick mesogens were constructed by attaching flexible chains laterally (at meta-position) to a rod-like mesogenic core—3-hydroxyphenyl 4’-alkyloxy biphenyl-4-carboxylate. While the para-positioned chain were varied from octyloxy to dodecyloxy in length, the meta-positioned chains, hexyloxy and octyloxy (series III and IV), were replaced with nonafluorohexyloxy and tridecafluorooctyloxy chains (series I and II), respectively. The obtained mesogens and mesophases were studied with optical polarizing microscopy (OPM), differential scanning calorimetry (DSC), X-ray diffraction (XRD), and electric field effect.
Mesophases of smectic A (SmA), smectic C (SmC) and anticlinic smectic Ca (SmCa) were observed for the non-fluoro-derivatives, and only SmA and SmC were observed for the fluoro-derivatives. The OPM observation showed that the textures of SmA and SmC resemble those of rod-like mesogens, but occasionally with 2-brush defects in the schlieren textures of SmC. For the SmCa phase, the schlieren texture was dominated with 2-brush defects and the focal conic fans were overlapped with equal-distant parallel lines. The DSC study showed that the phase transitions of SmA-SmC and SmC-SmCa were all weakly first-order. Preliminary XRD results showed that these smectic phases were monolayer structure, and only very small layer contraction across the SmA-SmC transition was observed.
Elongated domains were observed for SmC and SmCa phases of these compounds in liquid crystal cell with surfaces treated for parallel alignments, 2μ thickness. The neighboring domains exchanged their dark and light appearance when the sample was rotated to opposite direction, but remained unchanged with inversion of the polarity of electric field. The SmCa phase exhibited similar behavior except that the parallel lines across the domains would gradually disappear with increasing field strength. The polarization of these smectic phases remain to be studied.

並列關鍵字

anticlinic ； liquid crystals ； hockey stick mesogens ； synclinic

參考文獻

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