Unconventional Liquid Crystals and Their Applications

Wei Lee, Sandeep Kumar, Wei Lee, Sandeep Kumar

• Publisher: De Gruyter
• Year: 2021
• ISBN:
  9783110583519
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

The work focuses on recent developments of the rapidly evolving field of Non-conventional Liquid Crystals. After a concise introduction it discusses the most promising research such as biosensing, elastomers, polymer films, photoresponsive properties and energy harvesting. Besides future applications it discusses as well potential frontiers in LC science and technology.

1 Introduction: from conventional to unconventional liquid crystals

Po-Chang Wu

Sandeep Kumar

Wei Lee

Abstract

Liquid crystals have received tremendous attention from both the academic and technological points of view by virtue of their unique molecular structures characterized by the mesogenic cores and fluidity but long-range orientational order and stimuli-responsive material properties. To date, a variety of liquid crystal materials have been systematically synthesized and extensively employed in a wide spectrum of daily products. Further potential applications have been suggested as well, based on their superior particularities and remarkable advances made in the twenty-first century. Among the thermotropic liquid crystals, rod-like and disk-like molecules are generally considered as conventional liquid crystals, whereas liquid crystals with other molecular shapes, showing unusual material properties distinct from those of conventional ones, are unconventional liquid crystals. In addition, liquid crystal-based mixtures and composites, comprising certain types of substances (e.g. dyes, monomers, and nanomaterials) as additives, can also be regarded as unconventional liquid crystals in that concern of modified and improved properties have led to a surge of research activities to discover and develop new material systems in attempt to tailor the host liquid crystals to extend their applications beyond displays. According to the contents of the book entitled Unconventional Liquid Crystals and Their Applications, this chapter is aimed at providing the reader with a background on the basis of what has been established about conventional rod-like liquid crystals and unconventional liquid crystals in terms of their structures, material properties, and potential applications. We start from an overview on the evolution of liquid crystal research, followed by introducing fundamental concepts, including types, structures, material properties, and well-known applications of various rod-like liquid crystal mesophases. Furthermore, an introduction to recent developments of specific hybrid liquid crystal systems with unusual or modified material characteristics, adoptable for revolutionary liquid crystal technologies, is presented to draw the reader’s attention from conventional to unconventional liquid crystals from an application perspective.

1.1 Overview

Matter in nature can commonly be classified into three states – solid, liquid and gas, according to the strength of intermolecular force and the mobility of the molecules. When substances have anisotropic properties, mesophases known as liquid crystals (LCs) can be obtained in between solid and liquid states. The material properties of LCs are intriguing because they combine features of both the solid and liquid states. That is, molecules in an LC phase are capable of flowing like liquids and can be self-assembled to form long-range orientational order and in some cases together with one- or two-dimensional translational order like solids. Moreover, LC molecules are susceptible to external stimuli so that their physical properties such as dielectric and optical anisotropy can be tuned by the electric field, magnetic field, temperature, acoustic waves or ultrasound pulses (Lee and Chen, 2001), and the like. This enables a variety of readily unique electro-optical responses of LCs and, thus, successful applications to ubiquitous flat-panel displays in this day.

The discovery of LCs could trace back to the year 1888 when an Austrian botanist by the name of Friedrich Reinitzer observed an intriguing phenomenon of double melting points during his study of the organic material cholesterol benzoate. After receiving samples sent from Reinitzer, the German physics professor Otto Lehmann then performed careful analysis on this unusual state at various temperatures by his state-of-art polarizing optical microscope equipped with a sample stage with precise control of temperature and eventually coined the term “liquid crystal” in 1904 as known today (Sluckin et al., 2004). Existing LCs can broadly be divided into lyotropic and thermotropic LCs, depending on the nature of their formation. Lyotropic LCs are typically obtained by dissolving amphiphiles with certain solvents. Molecules of such two-component systems are composed of a hydrophilic (polar) head group in connection with a hydrophobic tail. Varying the solvent concentration as well as the temperature results in the observation of various lyotropic mesophases, such as micellar, cubic (micellar cubic I and bicontinuous cubic V), hexagonal columnar (H) and lamellar (L_α) phases (Lagerwall and Scalia, 2012). Certain chromonic dyes (e.g. Sunset Yellow FCF), drugs (e.g. disodium cromoglycate) and short strands of nucleic acids form assemblages that give rise to LC phases in a dissimilar manner from amphiphilic molecules. When these flat molecules, usually containing two end polar groups, are mixed with a polar solvent, typically water, stacks of molecules construct sufficiently long assemblies, resulting in (lyotropic) chromonic LC phases, including the nematic (N) and columnar (M) phases, depending on the temperature and concentration (Tam-Chang and Huang, 2008). In contrast, thermotropic LCs possessing anisotropic molecular shape in their pure forms are stably obtained in given temperature ranges upon heating (cooling) from the solid (liquid) phase. The shape anisotropy of a molecule with a rigid core plays a crucial role in the design of a thermotropic LC and, in turn, its mesophases as well as material properties. In view of all existing thermotropic LCs with well-defined chemical/molecular structures, rod-like (or calamitic) and disk-like (or discotic) molecules are the two simple and common forms of LCs so that they are normally defined as conventional LCs. Other chemically synthesized LCs with unusual molecular shapes different from those of conventional LCs, such as bowl-like or bowlic (Wang et al., 2017b), bent (or banana)-shaped (Antal Jákli et al., 2018), hockey-shaped (Sarkar et al., 2011), λ-shaped (Ooi and Yeap, 2018), Y-shaped (Kashima et al., 2014), star-shaped (Vinayakumara et al., 2018) mesogenic compounds and the like (Li et al., 2018c; Osman et al., 2016; Radhika et al., 2013), are generalized as unconventional LCs, which are reviewed from the chemical aspect in Chapter 2 of this book.

To date, mesophases of conventional LCs – such as nematic, smectic, and cholesteric (or chiral nematic) phases in rod-like LCs as well as columnar and nematic phases in disk-like LCs – have been enormously explored as reported in many scientific and technological investigations. Among them, rod-like LCs with unique structural and material features are the most widely synthesized and used molecules for developing LC technologies. The best known is the application of nematic LCs in displays that have been commercialized in a wide spectrum of our daily seen information products, ranging from large-size TVs over medium-size laptops, monitors, and automotive panels to small-size mobile phones and watches (Chen et al., 2018a; Ko et al., 2018). This great achievement makes rod-like LCs fascinating in a variety of research fields, including chemistry, physics, materials science, and engi...

Table of contents

• Title Page
• Copyright
• Contents
• 1 Introduction: from conventional to unconventional liquid crystals
• 2 Unconventional liquid crystals: chemical aspects
• 3 Ferroelectric liquid crystals and their application in modern displays and photonic devices
• 4 Liquid crystalline materials for efficient solar energy harvesting
• 5 Liquid crystal-based biosensing: exploiting the electrical and optical properties of various liquid crystals in quantitative bioassays
• 6 Thermotropic liquids and liquid crystals from DNA and proteins
• 7 Liquid crystals doped with ionic surfactants for electrically induced anchoring transitions
• 8 Time-resolved dynamics of dye-doped liquid crystals and the origin of their optical nonlinearity
• 9 Light reconfigurable chiral liquid crystal superstructure for dynamic diffraction manipulations
• 10 Photoalignment of liquid crystalline polymers attained from the free surface
• 11 Photoresponsive liquid-crystalline block copolymers with hierarchical structures
• 12 Molecular modeling of liquid crystal elastomers
• 13 Liquid crystal polymer films with high reflectivity
• 14 Ultrathin films of nanomaterials: a lyotropic liquid crystalline system and its sensing application
• 15 Quantum-dot-dispersed liquid crystals: mesogenic science to smart applications
• Index