Abstract
摘要:自然界中物質熔化的現象雖然都具有一般性,但其中有很多複雜的相變機制,仍不是瞭解得很清楚,尤其是低維度系統的相變過程與三維物體的相變行為仍有很大的不同,因此吸引了大量學術上的興趣和實際用途上的關注。利用液晶材料在其smectic相位下可以形成有層狀結構的懸浮液晶超薄膜,我們可以藉此來研究低維度系統下的相變行為以及其臨界現象,同時研究中實驗所得數據亦可用來檢驗許多統計力學上不同的物理模型,並可藉此對低維度系統的物理機制有更深入的瞭解。
本計畫是擬使用高解析液晶薄膜比熱儀,光反射儀,光學紋理觀測系統以及改裝型穿透式電子顯微鏡來研究諸多種類之液晶薄膜的熔化現象,前期計畫已成功建造以上量測儀器並使其運轉正常,同時亦有豐富的研究成果出現。此次申請的研究計畫中將承續並展開下列研究:
(1) 進行二維液晶晶體結構分析與電子繞射實驗以瞭解造成完美晶格次序在二維液
晶薄膜系統下存在的原因,並同時確認其薄膜層間之作用力大小。
(2) 檢驗二維液晶薄膜系統內所含之熵是否與其體積平方成正相關,同時並檢驗其相變序列之SmA-HexB臨界行為是否符合coupled XY模型的描述。
(3) 研究旋光型鐵電型液晶薄膜材料之SmA-SmC*,SmC*-SmCα*以及SmCα*-SmCFI*
等相變之臨界行為,並從光學紋理觀察判斷此類液晶之相轉換序列是否如理論所預測。
(4) 驗證locked liquid相位存在的可能性,並使用電子繞射技術開發新的彎型液晶相位
的誕生。並同時進行彎型液晶薄膜在熱學上的相變與臨界行為研究。
(5) 進行斜交液晶薄膜之光學紋理與相變序列的分析研究。並同時檢驗是否SmI-SmL、SmL-SmF與SmL1-SmL2之臨界行為屬於二維熔化理論所預測的KT型態之相轉變。
液晶超薄膜是目前唯一能提供substrate-free的低維度實驗系統,此系統使我們能展開許多低維度系統融化現象的研究,其中許多精細相變的物裡機制是本實驗一直有興趣的課題,藉由對液晶薄膜的研究本實驗室可以瞭解很多以前均未被探索過的問題,並且可對液晶薄膜在低維度下的物理性質有更進一步的瞭解。此次申請的研究計畫中亦將開展燕尾型與彎型液晶薄膜材料之結構分析研究,以期發現具有不同對稱性的新液晶相位,同時建立相關模型以瞭解前瞻性液晶材料排列與結構上變化的成因,實驗結果可提供合成化學家在合成材料時所需的重要參考依據。在學術上,實驗所得之相關結果可使我們進一步瞭解不同前瞻性液晶材料發生相轉換時之臨界行為,同時其結果亦可用來比對許多現存的物理模型。
Abstract: In nature, the melting phenomena of materials are common but not fully understood. To understand the melting process in reduced dimensions and near surfaces is of great academic and practical interest. Smectic liquid crystals (LCs) are uniquely suited for studying the phenomenon of melting in reduced dimensions, because of their ability to form stable free-standing films with uniform thickness consisting of two or more molecular layers. To date, the studies of smectic liquid-crystal systems have proven to be among the most fruitful to test the current models and theories in the physics of reduced dimensions and near surfaces.
This project is planned to use high-resolution ac calorimeter, optical reflectometer, polarized optical microscope, and transmission electron microscope to study the phenomena of melting in LC thin film system. In this project, we are going to conduct and develop following research topics:
1. Check if the system entropy of a LC film is proportional to its volume square in reduced dimensions; Examine if the critical behavior of the SmA-HexB transition of ultrathin LC films can be well described by coupled XY model.
2. Study the phase-transitional behavior of the SmA-SmC*,SmC*-SmCα*以及SmCα*-SmCFI* transitions in chiral ferroelectric and atiferroelectric LC films using optical textures and high-resolution ac calorimeter.
3. Conduct electron-diffraction experiments on ultrathin LC films to realize the cause of the
existence of a perfect ordering in 2D system; Confirm the molecular interactions in LC films from reflectivity and diffraction data.
4. Confirm the existence of the locked liquid phase in bent-like LC films; To conduct experiments on the structural analysis of frontier LC materials in order to develop future new LC compounds.
5. Conduct optical textural and heat capacity experiments on tilted hexatic LC thin films.
Free-standing smectic LC films represent an unusual system to study melting in which the spatial dimension can be varied continuously from three to two without the extraneous effects of substrates. Our goal in this project is to discover new frontier LC phase and to establish some related models that can explain how the structure, the thermal property, and type of optical textures of studied LC films are affected by the geometry of the LC molecule, the magnitude of spontaneous polarization, and the location and number of chiral center on the LC molecule. The obtained experimental data can not only provide us further realization on the critical behaviors of different phase transitions in some LC thin films, but also help us for the understanding of the physical mechanism of reduced-dimensional system. In addition, our experimental result can be used to check many different exisiting physical models on critical phenomenon.
Keyword(s)
液晶薄膜
低維度系統
臨界行為
AC比熱與光反射量測
電子繞射
結構分析
液晶光學紋理分析
前瞻型液晶材料
liquid crystal (LC) films
reduced dimensions
LC optical textures
frontier LCs
AC calorimetry & optical reflectivity
electron diffraction
structural analysis