Preface

Liquid crystals (LCs) are amazing compounds that self-organize into ordered soft states, being presently indispensable for nanotechnological applications. Molecular shape and intermolecular interactions, together with nanosegregation of the molecular structure, contribute thereby to their self-assembly into thermodynamically stable functional materials. *Liquid Crystals - Self-Organized Soft Functional Materials for Advanced Applications* is focused on both theoretical models and experimental results, pointing out the chemical and physical properties (thermodynamics, electro-optic switching behavior, and non-linear optic phenomena) of LCs that could be applicable to a wide range of devices. In this respect, the chapters cover the following topics:


The chapter by Scutaru et al. presents the structure and supramolecular ordering behavior of symmetrical and asymmetrical bent-core LCs. The purpose was to design LCs with large mesophase intervals on low transition temperatures, which might expand the field of electro-optical applications. The chapter by Popa-Nita and Repnik shows the theoretical model for the phase behavior and orientational properties of a binary mixture composed of CNTs into thermotropic nematic LCs. The chapter by Mochizuki describes the relationship between tilted smectic layer structure and electro-optic properties, considering in-plane and out-of-plane retardation switching behavior. The last chapter by Sasaki et al. reviews the effect of LC mixtures on response time and gain coefficient, required for the formation of a refractive index grating in a photorefractive device.

Because of the possibility of using such materials for a wide range of applications, the study of LCs presents both theoretical and practical importance. It is expected that the book will be of interest for researchers in academia and industries, as well as advanced students.

I would like to express my gratitude to all authors for their contributions to this book.

**1**

Section 1

Introduction

Section 1 Introduction

**3**

**Chapter 1**

Crystals

*Irina Carlescu*

**1. Introduction**

templating [1–4].

more important.

Introductory Chapter: Liquid

The domain of liquid crystals represents an actual and dynamic scientific area, directly implied in top technologies as nanotechnologies, aerospace domain, microelectronics, and molecular biology [1]. For about 130 years, liquid crystals have been the subject of study for fundamental science and in many fields of research such as chemistry, physics, medicine, and engineering as well, which contributed to the progress in materials science and to innovative applications. In addition, as a result of the recent development of advanced synthetic methods and characterization techniques, the nanostructured liquid crystalline compounds that display special ordering properties and assign new functions have been highlighted, such as electro-optical effects, actuation, chromism, sensing, or

Compared to other solid-state materials, liquid crystals present unique attributes, because they easily respond to external stimuli such as surfaces, light, heat, mechanical force, or electric and magnetic fields and eliminate defects by self-healing [5–9]. Thus, the understanding of the relationship between chemical structures of liquid crystalline compounds and their specific functions is becoming

Liquid crystals are quintessential soft matter materials that present one to several distinct phases between the crystalline solid phase (Cr) and the isotropic liquid phase (Iso) [10–13]. These intermediary phases or mesophases not only possess some typical properties of a crystalline elastic solid like positional and orientational order as well as anisotropy of optical, electrical, and magnetic properties but also have the characteristic properties of an ordinary viscous liquid such as fluidity, formation and fusion of droplets, or mechanical properties [14–19]. Consequently, the compounds that present mesophases have intermediary symmetry properties, between an isotropic liquid and a crystalline solid; hence, a viscoelastic nature can be attributed to liquid crystals. Moreover, the unique combination of order and mobility represents the basis for self-assembly and supramolecular structure formation in technical systems. Generally, liquid crystals are elongated shape molecules, which are more or less parallel to each other in mesophases, contributing to anisotropic physical properties. These properties associated with their viscoelastic nature induce in liquid crystals the ability to eas-

ily respond under external stimuli and to change their configuration [20].

Depending on particular conditions where mesophases arise, liquid crystals have been classified into two classes: lyotropic and thermotropic. Thermotropic liquid crystals can be obtained either by heating a crystalline solid or by cooling an isotropic liquid. In the case of thermotropic liquid crystals, when the crystalline state is heated, the positional ordering of molecules vanishes but not the orientational one and so the

## **Chapter 1**
