Nonlinear Dielectric Relaxation Spectroscopy of Ferroelectric and Antiferroelectric Liquid Crystals

铁电和反铁电液晶的非线性介电弛豫光谱

基本信息

批准号：
03452092
负责人：
HAYAKAWA Reinosuke
金额：
$ 3.97万
依托单位：
The University of Tokyo
依托单位国家：
日本
项目类别：
Grant-in-Aid for General Scientific Research (B)
财政年份：
1991
资助国家：
日本
起止时间：
1991 至 1992
项目状态：
已结题

项目摘要

We developed the measurement system for the nonlinear dielectric relaxation spectrum in a wide frequency range and with high accuracy. This system consists of the wave generator, the charge amplifier, the digital storage oscilloscope and the personal computer. We measured the nonlinear spectra of some Ferroelectric Liquid Crystals (FLCs) in the SmC^* phase by use of this system. The obtained nonlinear dielectric constants of FLCs are much larger in magnitude by several decades than those obtained for ferroelectric polymers. The fact that the higher-order nonlinear spectrum has a counterpart in the linear one shows that the nonlinear mode is originated from the fluctuation of the helical structure, called the Goldstone mode. The negative sign of the third-order nonlinear dielectric constant implies that the origin of this nonlinearity is the orientational saturation of the dipolemoments by the applied electric field. The third-order dielectric constant is almost independent of the temperature in the SmC^* phase and shows the critical behavior in the vicinity of the SmA-SmC^* phase transition temperature. The critical behavior appears more remarkably in the nonlinear spectrum than in the linear one. The linear and nonlinear spectra are explainable by the phenomenological theory. We also measured the nonlinear dielectric spectra for an antiferroelectric liquid crystal MHPOBC in various smectic phases. In the ferrielectric phase, for example, we found a phase transition from a ferri-state to a ferro-state induced by the increase of electric field. From the saturated value of polarization in both the states, we estimated the molecular arrangement in the ferrielectric phase. Through the present study, this spectroscopy is found to be a useful tool to obtain the information on the structure and dynamics of the electro-active smectic liquid crystals.

我们开发了宽频率范围、高精度的非线性介电弛豫谱测量系统。该系统由波形发生器、电荷放大器、数字存储示波器和个人计算机组成。我们利用该系统测量了一些 SmC^* 相铁电液晶 (FLC) 的非线性光谱。获得的 FLC 非线性介电常数比铁电聚合物的非线性介电常数大数十倍。高阶非线性谱与线性谱存在对应物这一事实表明，非线性模式源于螺旋结构的涨落，称为戈德斯通模式。三阶非线性介电常数的负号意味着这种非线性的根源是外加电场导致偶极矩的取向饱和。三阶介电常数几乎与 SmC^* 相的温度无关，并在 SmA-SmC^* 相变温度附近表现出临界行为。临界行为在非线性谱中比在线性谱中表现得更显着。线性和非线性谱可以用现象学理论来解释。我们还测量了不同近晶相的反铁电液晶 MHPOBC 的非线性介电谱。例如，在亚铁电相中，我们发现电场增加会引起从亚铁态到铁态的相变。根据两种状态下的饱和极化值，我们估计了亚铁电相中的分子排列。通过目前的研究，发现这种光谱是获取电活性近晶液晶结构和动力学信息的有用工具。