Laboratory of Macromolecular and Organic Chemistry, Eindhoven University of Technology , Eindhoven, The Netherlands.
J Phys Chem B. 2012 Apr 5;116(13):3928-37. doi: 10.1021/jp300008f. Epub 2012 Mar 21.
The hydrogen-bonded hexagonal columnar LC (Col(hd)) phases formed by benzene-1,3,5-tricarboxamide (BTA) derivatives can be aligned uniformly by an electric field and display switching behavior with a high remnant polarization. The polar switching in three symmetrically substituted BTAs with alkyl chains varying in length between 6 and 18 carbon atoms (C6, C10, and C18) was investigated by electro-optical switching experiments, dielectric relaxation spectroscopy (DRS), and solid-state NMR. The goal was to characterize ferroelectric properties of BTA-based columnar LCs, which display a macroscopic axial dipole moment due to the head-to-tail stacking of hydrogen-bonded amides. The Col(hd) phase of all three BTAs can be aligned uniformly by a dc field ∼30 V/μm. Moreover, C10 and C18 display extrinsic polar switching characterized by a remnant polarization and coercive field of 1-2 μC/cm(2) and 20-30 V/μm, respectively. In the absence of an external field, the polarization is lost in 1-1000 s, depending on device details and temperature. DRS revealed a columnar glass transition in the low-temperature region of the LC phase related to collective vibrations in the hydrogen-bonded columns that freeze out below 41-54 °C. At higher temperatures, a relaxation process is present originating from the collective reorientation of amide groups along the column axis (inversion of the macrodipole). Matching activation energies suggest that the molecular mechanism underlying the polar switching and the R-processes is identical. These results illustrate that LC phases based on BTAs offer the unique possibility to integrate polarization with other functionalities in a single nanostructured material.
由苯-1,3,5-三甲酰胺（BTA）衍生物形成的氢键六方柱状 LC（Col(hd)）相可以通过电场均匀排列，并具有高剩余极化的开关行为。通过电光开关实验、介电松弛谱（DRS）和固态 NMR 研究了三个对称取代的 BTA 中烷基链长度在 6 至 18 个碳原子之间（C6、C10 和 C18）的极性开关。目标是表征基于 BTA 的柱状 LC 的铁电性质，这些 LC 由于氢键酰胺的头尾堆积而显示宏观轴向偶极矩。所有三种 BTA 的 Col(hd)相都可以通过约 30 V/μm 的直流场均匀排列。此外，C10 和 C18 显示出具有 1-2 μC/cm(2)和 20-30 V/μm 的剩余极化和矫顽场的外部分极开关特性。在没有外部场的情况下，根据器件细节和温度的不同，极化在 1-1000 秒内消失。DRS 揭示了 LC 相低温区与氢键柱中集体振动有关的柱状玻璃化转变，该转变在 41-54°C 以下冻结。在较高温度下，存在源于酰胺基团沿柱轴集体重排的弛豫过程（宏观偶极子的反转）。匹配的激活能表明，极性开关和 R-过程的分子机制是相同的。这些结果表明，基于 BTA 的 LC 相提供了将极化与单个纳米结构化材料中的其他功能集成的独特可能性。
