曲棍型液晶分子之研究-分子結構與液晶相及性質之探討

本研究的主要目的為合成七系列不同苯環數及不同連接基之曲棍形液晶分子，討論分子結構及尾端烷氧鏈長度對液晶相及相關溫度的影響並探討各種液晶相相關性質。實驗中顯示，增加鏈長度導致澄清點下降，向列相(N)溫度範圍縮小而層相溫度範圍增加，同傾(synclinic)層相(SmC)變寬而反傾(anticlinic)層相(SmCa)變窄。苯環數增加，使液晶相溫度範圍增加，液晶相趨於穩定。加入聯苯基修飾核心部分，能夠使液晶相溫度範圍變寬，其中向列相溫度範圍縮小，並使SmCa相溫度範圍變寬。曲棍形液晶之SmA-SmC相轉移焓值隨鏈長增加，由微弱地一級相轉移(weakly first order)趨於一級相轉移(first order)。SmC-SmCa相轉移焓值隨鏈長增加而變小，由一級相轉移(first order)趨於微弱地一級相轉移(weakly first order)。此現象不同於典型桿形液晶分子，其SmA-SmC及SmC-SmCa相轉移焓皆為二級相轉移(second order)。將具有負介電異向性(Δε＜0)曲棍形液晶分子注入兩片經平行配向處理的導電玻璃中間，使分子長軸做平行方向的排列。於向列相施加一垂直玻璃基板方向的直流電場，會出現成對且具有等間距的筆直條痕(straight stripes)，條痕平行基板配向方向，旋轉樣品條痕會有明暗變化，甚而成對的條痕明暗對調，其與桿形液晶形成的Williams Domains紋理條痕方向以及光學上的變化皆不相同。於交流電場下，可觀察到不同於桿形分子所形成之週期性紋理圖。曲棍形分子SmC紋理圖中schlieren區域可觀察到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紋理圖中扇形區域在不同強度的電場下會產生顏色變化，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。 mBB–B2n及mBⅡB–B2n系列具有SmCa相，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。SmCa紋理圖中扇形區域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。

關鍵字

曲棍形液晶分子；同傾層相(SmC) ；反傾層相(SmCa)

並列摘要

The purpose of this research is to probe into “hockey stick ”shaped mesogens. The mesophase were studied by optical polarizing microscopy. Differertial scanning calorimetry and powder x-ray diffraction. The mesogens were obtained by esterification of 3-alkoxy-4-methoxybenzoic acid, 3-alkoxycinnamic acid, 3-alkoxy-(4’-phenolyl)benzoate and 3-alkoxy-4’-hydroxylstilbene with 4-alkoxy-(4’-phenolyl)benzoate, 4-alkoxy-(4’-(4’’-phenolyl)benzoyloxy)benzoate and 4-alkoxy-4’-biphenyl-carboylic acid. The resultant mesogens mBIIABBn, Me-mBBBn, Me-mBBBBn, mBIIABBBn, Me-mBB-B2n, mBB-B2n and mBIIB-B2n, exhibit nematic, SmA, SmC and SmCa phases. The texture of nematic exhibit schlieren textures with 2-brush or 2-and 4-brush defects. For a homogeneously aligned (EHC cell) nematic phase of hockey stick mesogen, with the alignment direction parallel to one of the crossed polarizers, a uniform dark texture with strong optical fluctuation was observed. Upon application of a DC field, pairwise and uniformly spaced birefringent straight stripes developed with the stripe long axis parallel to the rubbing direction. The pairwise birefringent stripes turned color when the sample was rotated, and the colors were exchanged with a reversing rotation. The textures of SmC phase exhibit schlieren and broken focal conic textures. The schlieren texture displayed 2- and 4-brush defects. Defects of 2-brushes have been considered as evidence for the anticlinic arrangement of the neighboring layers in the smectic phase of calamitic mesogens. There were elongated domains of various sizes with long axes parallel to the rubbing direction and with disclinations between them. Thedomains were of two different colors when the sample was rotated or the polarizers were decrossed. The domain colors changed with application of DC fieids. The textures of SmCa exhibit schlieren and broken focal conic textures. The schlieren texture containing plenty of 2-brush defects. The optical appearance across this schlieren texture was more uniform than that of SmC phase. There were floating liquids sweeping through the sample, a behavior easily observed during temperature variation. Some periodic parallel stripes, rather similar to the pitch band observed for chiral smectic phases, occurred in the broken focal conic areas. The color chwnges and variation of textures under electric fields resembled that of SmC phase.

並列關鍵字

hockey stick mesogens ； synclinic ； anticlinic

參考文獻

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曲棍型液晶分子之研究-分子結構與液晶相及性質之探討

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