Phase Behaviors and Molecular and Supramolecular Structural Identifications of a Liquid Crystalline Second Generation Monodendron

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• 作者：Zhen Liu ; Lei Zhu ; Wensheng Zhou ; Stephen Z. D. Cheng ; Vigil Percec ; and Goran Ungar
• 刊名：Chemistry of Materials
• 出版年：2002
• 出版时间：May 2002
• 年：2002
• 卷：14
• 期：5
• 页码：2384 - 2392
• 全文大小：479K
• 年卷期：v.14,no.5(May 2002)
• ISSN：1520-5002

文摘

A second generation monodendron (abbreviated as G₂NB) based on the AB₂ mesogenicmonomer, 13-hydroxy-1-(4-hydroxyphenyl)-2-(4-hydroxy-4' '-p-terphenylyl)tridecane buildingblocks, exhibits multiple mesophase transformations with varying temperatures. On thebasis of thermal transition behaviors studied by differential scanning calorimetry, structuralanalyses carried out by wide-angle and small-angle X-ray scatterings and electron diffractionexperiments, and morphologies observed in polarized light microscope, the equilibrium phasetransition sequence and phase structures have been identified. From the glass transitiontemperature at 28 C, the most ordered phase is a highly ordered liquid crystalline (LC)smectic B (28e10001.gif">) phase. This phase is defined on the basis of the relationship between the LCmesogenic orientation direction and the layer normal. Within the layers, a molecular lateralhexagonal packing exists. It is followed by a low ordered LC smectic C (S_C) phase appearedin a narrow temperature range of ~2 C and quickly entered a nematic phase. At a hightemperature of 92 C, G₂NB reaches the isotropic melt. An interesting observation is thatthe layer spacing of these 28e10002.gif"> and S_C phases are much larger than the chemical repeat unitof the LC monomer; rather, it is a representative of the whole size of the G₂NB monodendron.Therefore, the 28e10003.gif"> phase is a combined supramolecular layer structure and a lateralmolecular hexagonal packing, while the S_C phase is a pure supramolecular phase. Asurprising finding is that the supramolecular S_C phase formation possesses a stronger coolingrate dependence than that of the 28e10004.gif"> phase formation. This provides an opportunity tocontrol the appearance of the S_C phase formation by only varying the cooling rate. Possiblepacking structural model and formation mechanisms of this phenomenon are discussed.