Biaxial nematic liquid crystals: fact or fiction?
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Thermotropic crystal
Biaxial nematic
Mesogen
Lyotropic
Columnar phase
In this article, we report synthesis and characterization of a novel thermotropic liquid crystals, namely, 4-bromo-[3′-nitro-4′(4″-n-alkoxybenzoyloxy)] phenyl azobenzenes. The first member of a homologous series is nonmesogenic in nature. The n-ethyl to n-decyl homologues exhibit enantiotropic nematic mesophases, while the n-dodecylton -hexadecyl homologues exhibit only smectic mesophases. The synthesized compounds were characterized by a combination of elemental analysis and standard spectroscopic methods. The mesogenic behavior of present series is explained by comparing each homologue of the related mesogenic series. The impact of the lateral nitro group on mesomorphism is also discussed.
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Regioselectively mesogen-incorporated cellulose derivatives, in which the hydroxy group at C-6 is displaced by a bulky and rigid mesogenic group such as {4-[(4-methoxyphenoxy)carbonyl]phenoxy}acetate or [(4′-cyanobiphenyl-4-yl)oxy]acetate, and the C-2 and C-3 groups are displaced by octanoyl or lauroyl groups, were synthesized. Thermogravimetric and differential scanning calorimetric analyses of the final products, along with polarized optical microscopic observations, revealed that the obtained cellulose derivatives have a thermotropic liquid-crystalline nature and transition from the mesomorphic to the isotropic phase over a wide range of temperatures. These derivatives could be considered to be main-chain liquid-crystalline cellulosic polymers.
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Abstract Thermotropic liquid‐crystalline polyesters, containing the 3,3′‐biphenylylene unit as part of a conformationally mobile mesogenic group, were synthesized. Preliminary investigation of polymers as to their ability for manifesting liquid‐crystalline properties was carried out by polarizing microscopy and DSC.
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Thermotropic liquid-crystalline polyesters based on mesogenic aromatic residues and intrinsically non-mesogenic chiral segments are described, and an analysis of the thermal and optical behaviour in the bulk and of the chiroptical properties in dilute solutions is presented.The polymers of interest comprise two major classes according to the nature of the diacid and dihydroxy component. Bipolyesters and cobipolyesters based on the (R)-3-methyladipyl residue and several dihydroxy aromatic mesogens belong to the first class, and in the second class are included polyesters derived from mesogenic bis(4-carboxyphenyl)terephthalate itself or mixtures with isophthalate or phthalate isomers and various symmetric or non-symmetric chiral diols.Structural effects on the properties in the bulk and in solutions are discussed. In particular, the nature of the different mesogenic groups and the structure and optical purity of the chiral component (diol or diacid), as well as the chemical composition and monomeric unit distribution, are considered. Convenient routes to the synthesis of mesomorphic polyesters with modulated thermotropic properties relevant to the onset, nature and breadth of the mesophase are indicated.
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The present state of the synthesis and studies of the structure and properties of thermotropic liquid crystal polymers and copolymers with mesogenic groups in the main chain is examined and the thermodynamic and structural principles of the identification of the polymeric mesophases as well as the influence of the chemical structure of the mesogenic blocks and flexible spacers on the mesomorphic properties of the polymers are discussed. The bibliography includes 93 references.
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Depending on the phase behaviour, liquid crystals (LC) can be classified into thermotropic and lyotropic liquid crystals. This chapter provides an overview of the various methods used in simulating self-assembly phenomena in liquid crystals. It describes prominent atomistic molecular dynamics (MD) studies of the LC phase behaviour of various rod-like molecules. The chapter also discusses how Monte Carlo simulations, along with coarse-grained models based on excluded volume interactions can be used to study the wide variety of phases that occur in systems consisting of a collection of bent-core or banana-shaped liquid crystal molecules. The chapter deals with chromonic liquid crystals, which are basically lyotropic in nature. It also discusses the thermotropic liquid crystalline phase formed by disk-like molecules and a special class of lyotropic liquid crystal phase, known as chromonic. The chapter also provides a summary of the status of the existing computational studies of various liquid crystal phases.
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