Molecular origin of ferroelectricity in induced smectic-C* liquid crystalline phases.
Molecular origin of ferroelectricity in induced smectic-C* liquid crystalline phases.
复制标题
诱导近晶-C*液晶相中铁电性的分子起源。
DOI：
--
复制
发表时间：
1996
期刊：
影响因子：
--
通讯作者：
A. Sprick
中科院分区：
文献类型：
--
作者：
Mikhail A. Osipov;H. Stegemeyer;A. Sprick
基于最近的实验结果，根据手性分子偶极和四极有序之间的耦合，提出了诱导近晶-${C}^{*}$相中铁电有序的分子模型。对自发极化的值和符号与多种手性掺杂剂的分子结构之间的相关性的分析表明，一些新的实验结果无法在现有铁电有序理论的框架中解释。特别是，现实理论有望解释分别在柔性链或刚性核中拥有手性中心的两种不同类型的掺杂剂的性质之间的质的差异。发展了非手性近晶-$C$主相中手性掺杂剂诱导的铁电排序的一般统计理论，并用于获得自发极化的表达式，包括掺杂剂摩尔分数、四极有序参数、分子手性以及表征分子电偶极子和空间偶极子相对于分子刚性核的相对取向的几何参数。详细讨论了手性分子之间不同类型的极性相互作用，这些相互作用可能导致自发极化的出现。在该模型的背景下，可以定性地解释实验中使用的两种手性掺杂剂之间的性质差异。该理论的结果使人们能够理解为什么只有当掺杂剂分子在刚性核中具有手性中心时自发极化才对主体相的分子结构敏感。在同一模型的背景下，还可以解释由具有两种不同分子偶极子取向的近晶-$C$主体相中相同手性掺杂剂引起的极化的相反符号。后一个结果还使人们能够理解最近观察到的由手性掺杂剂浓度变化引起的偏振符号反转的起源。本文提出的一些最新实验数据支持了理论结论。
Based on recent experimental results, a molecular model of the ferroelectric ordering in the induced smectic-${C}^{*}$ phase is proposed in terms of a coupling between dipole and quadrupole ordering of chiral molecules. The analysis of the correlations between the value and sign of the spontaneous polarization and the molecular structure of a broad variety of chiral dopants indicates that several new experimental results cannot be explained in the framework of the existing theory of ferroelectric ordering. In particular, the realistic theory is expected to account for the qualitative difference between the properties of the two different types of dopants that possess a chiral center either in the flexible chains or in the rigid core, respectively. The general statistical theory of ferroelectric ordering induced by a chiral dopant in the nonchiral smectic-$C$ host phase is developed and used to obtain the expression for the spontaneous polarization in terms of the dopant molar fraction, quadrupole order parameter, molecular chirality, and the geometrical parameters that characterize the relative orientation of molecular electric and steric dipoles with respect to the molecular rigid core. The different kinds of polar interactions between chiral molecules, which can be responsible for the appearance of the spontaneous polarization, are discussed in detail. In the context of this model it is possible to explain qualitatively the difference in properties between the two types of chiral dopants used in the experiment. The results of the theory enable one to understand why the spontaneous polarization is sensitive to the molecular structure of the host phase only if the dopant molecule possesses a chiral center in the rigid core. In the context of the same model one explains also the opposite signs of the polarization induced by the same chiral dopant in the smectic-$C$ host phases with the two different orientations of the molecular dipole. The latter result enables one also to understand the origin of the recently observed polarization sign inversion induced by a change of concentration of the chiral dopant. The theoretical conclusions are supported by some recent experimental data presented in this paper.