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6816 results in Condensed Matter Physics, Nanoscience and Mesoscopic Physics

Page 81 of 341

  • 2 - Calamitic-Calamitic LC Dimers

  • Sandeep Kumar, Santanu Kumar Pal
  • Book:
    Liquid Crystal Dimers
    Published online:
    23 July 2017
    Print publication:
    06 January 2017, pp 10-58

  • Summary

    INTRODUCTION

    Scientific community has done a lot of efforts for controlling molecular ordering of polymer chains through various supramolecular approaches. Among them, a famous approach is keeping the LC molecules a repeating unit of polymer chain. The self-assembling tendency of LC molecules tends to assemble polymer chains into well-ordered structures. A variety of liquid crystalline polymers (LCPs) have been synthesized since the discovery of LCPs in 1923 by Vorlander et al. to achieve the highly ordered polymers.1 The understanding of self-assembly in condensed phases is challenged by these polymers whose interpretation at a molecular level requires experimental investigations as well as the development of new molecular theories. However, the structural heterogeneity inherent in a polymeric system complicates these tasks. An alternative approach is the use of monodisperse low molar mass compounds whose behaviour encapsulates the essential physics of polymeric system for developing a molecular understanding of polymers.

    Catalyzed with the motivation to understand LCPs at a molecular level, a wide variety of nonconventional low molar mass compounds were synthesized and shown to support liquid crystallinity. The fact that these compounds were LCs was very surprising because at that time most of the known low molar mass LCs were composed of molecules consisting of a single semirigid anisometric core with alkyl chains. Indeed, it had been widely assumed for many years that such a molecular structure was a prerequisite for the observation of liquid crystallinity. We now know that 1980s evidenced the beginning of the discovery of a rich diversity of structures capable of supporting mesogenic behaviour that has continued to the present day.

    Of all these new low molar mass LC discovered during the 1980s, one class that attracted particular attention and which still remains the focus of much research are the so-called LC dimers. An LC dimer is composed of molecules containing two conventional mesogenic groups linked by a flexible spacer. Thus, LC dimers contravened the accepted structure–property relationships for low molar mass mesogens by consisting of molecules having a highly flexible core rather than a semirigid central unit. In this respect, LC dimers represent an inversion of the conventional molecular design for low molar mass mesogens. Several names have been used to refer to these materials including dimesogens or Siamese twins but these have all now been superseded by the preferred term LC dimer.

  • 5 - Bent-Core LC Dimers

  • Sandeep Kumar, Santanu Kumar Pal
  • Book:
    Liquid Crystal Dimers
    Published online:
    23 July 2017
    Print publication:
    06 January 2017, pp 185-224

  • Summary

    Over the past two decades, bent-core liquid crystals (BLCs) have provided fascinating results to the scientific arena of LC research. The sharp bend within the linkage of the core group leads to the unique physical properties having no complement in conventional calamitic LCs such as occurrence of chiral phases and/ or polar phases even though the constituent molecules are achiral. Specially the bent (angular) molecular architecture leads to the variation in self-assembly processes that demonstrate as the unconventional phase structures. BLCs themselves are primarily comprised of three parts: bent core, rigid arm and the flexible tail chain. Up to now, a rich variety of BLCs with different structures and unique functions have been developed and their phase structures are also even more enriched. LC dimers obtained from monomeric BLC units, interconnecting two mesogenic units through flexible spacers (such as alkylene, siloxane, carbosilane, oxyethylene) are relatively new. The inherent motivation behind the preparation of bent-core dimers was to use the microsegregation to manipulate the phase structure of bent-core molecules. Based on the adjoined mesogenic units, the bent-core dimers can be categorically divided into three sub units:

  • 5.1 Symmetrical bent-core LC dimers

  • 5.2 Nonsymmetrical bent-core LC dimers

  • 5.3 Unconventional bent-core LC dimmers

    • SYMMETRICAL BENT-CORE LC DIMERS

    Introduction

    In symmetrical bent-core dimers, both the two mesogenic units joined via flexible spacer are BLC unit. The possible different types of arrangements possible for the bent-core dimers that can be achieved by connecting through a flexible spacer with possible combinations of the bent-core mesogenic units are presented in Figure 5.1.

    Structure–Property Relationships

    Dimers Based on Siloxane and Alkylene Spacer

    The first attempt of bent-core LC dimers consisting of two BLC units connected by flexible oligosiloxane spacer (dimethylsiloxane) units, shown in Figure 5.2, was reported almost a decade ago by G. Dantlgraber and his coauthors.1 The motivation behind the work was initiated by the observation that decoupling of the layers by the microsegregated oligosiloxane units disfavours an antiferroelectric (AF) assembly of the bentcore molecules. Therefore, antiferroelectricity is not solely arising by the compensation of the layer polarization but interlayer fluctuations of the molecules also play a vital role. Hence anticlinic layer arrangement or ferroelectricity can be induced by suppressing of the layer fluctuation. The dimers were synthesized by hydrosilylation of terminally unsaturated bentcore mesogens with hexamethyltrisiloxane (1) or octamethyltetrasiloxane (2) using Karstedt's catalyst.

  • 1 - Introduction

  • Sandeep Kumar, Santanu Kumar Pal
  • Book:
    Liquid Crystal Dimers
    Published online:
    23 July 2017
    Print publication:
    06 January 2017, pp 1-9

  • Summary

    LIQUID CRYSTALS

    Liquid crystals (LCs) are distinctive functional soft materials with a combination of order and mobility on a molecular, supramolecular and macroscopic level. Hierarchical self-assembly in LCs offers a powerful strategy for producing nanostructured mesophases. Molecular shape, microsegregation of incompatible parts, specific molecular interaction, self-assembly and selforganization are important factors that lead to the formation of various LC phases. LCs are accepted as the fourth state of matter after solid, liquid and gas. This fourth state of matter is intermediate between the solid and the liquid state. For this reason, they are referred to as intermediate phases or mesophases. This is a real thermodynamic stable state of matter where the constituents of a mesophase are called mesogens. A rigid core of the mesogen (which often consists of aromatic rings) induces structural order whereas the flexible parts (e.g., alkyl chains) provide the necessary mobility within the LC phase.

    The unique feature of LCs is the presence of both order and high degree of mobility in the mesophase that leads to the self-healing, adaptive and stimuli-responsive behaviour of these supramolecular systems and because of this, LCs have become the quintessential self-assembling molecular materials of the modern era. LCs have made huge impact on the development of the human societies. LCs are the advanced technological material found in lowpower- consuming LC displays (LCDs) which are being used in the last decades for the development of mobile data processing and communication tools. It is quite possible that LCDs might be replaced by other technologies in the future but, the fundamental knowledge gained with LCs can be used for the self-assembly of a huge variety of other materials.

    In 1888, Friedrich Reinitzer, Professor of Botany and Technical Microscopy at the German Technical University in Prague found that the compound cholesteryl benzoate which he had extracted from carrots exhibited two melting points, one at 145.5 °C and other at 178.5 °C. Between these two melting points, there was a milky liquid phase. Above 178.5 °C, the phase became clear. He observed distinct violet and blue colour phenomena at both these two different melting points under polarizing optical microscope. After having similar observations with a further derivative cholesteryl acetate that has a monotropic cholesteric phase, he contacted physicist Otto Lehmann.

Modern Techniques of Surface Science
  • 3rd edition
  • D. Phil Woodruff
  • Published online:
    20 October 2016
    Print publication:
    06 October 2016
  • This fully revised, updated and reorganised third edition provides a thorough introduction to the characterisation techniques used in surface science and nanoscience today. Each chapter brings together and compares the different techniques used to address a particular research question, including how to determine the surface composition, surface structure, surface electronic structure, surface microstructure at different length scales (down to sub-molecular), and the molecular character of adsorbates and their adsorption or reaction properties. Readers will easily understand the relative strengths and limitations of the techniques available to them and, ultimately, will be able to select the most suitable techniques for their own particular research purposes. This is an essential resource for researchers and practitioners performing materials analysis, and for senior undergraduate students looking to gain a clear understanding of the underlying principles and applications of the different characterisation techniques used in the field today.

Page 81 of 341