Liquid crystalline materials: physical properties and intermolecular interactions

1983 ◽  
Vol 309 (1507) ◽  
pp. 217-229 ◽  
Keyword(s):  
Molecular Structure ◽  
Physical Properties ◽  
Short Range ◽  
Liquid Crystalline ◽  
Phase Behaviour ◽  
Qualitative Model ◽  
Dielectric Studies ◽  
Crystalline Materials ◽  
Liquid Crystalline Materials ◽  
Dipole Correlation

The physical properties and the phase behaviour of some nematic liquid crystals are discussed, with emphasis on the influence of short-range antiparallel dipole correlation that occurs in mesogenic compounds with a strong terminal dipole moment. Evidence for this effect, which stems especially from dielectric studies, is summarized. Variations of the dipole correlation with molecular structure can explain the sometimes unexpected phase behaviour and physical properties of these substances. A qualitative model is given in terms of a monomer-dimer equilibrium.

Liquid crystalline polymers

2004 ◽  
Author(s):  
Sangdil I. Patel ◽  
Fred J. Davis
Keyword(s):  
Liquid Crystalline ◽  
Main Chain ◽  
Liquid Crystalline Polymers ◽  
Phase Behaviour ◽  
Side Chain ◽  
Polymer Backbone ◽  
Crystalline Materials ◽  
Crystalline Polymers ◽  
Anisotropic Behaviour ◽  
Liquid Crystalline Materials

The idea of combining the anisotropic behaviour of liquid crystalline materials with the properties of macromolecular systems was first suggested by Onsanger and subsequently Flory. The actual realization that such systems could exist came from studies of natural polymers such as the tobacco mosaic virus. Interest in these systems intensified with the development of highstrength systems, based on rigid-rod systems, notably the aramid fibres, however, liquid crystallinity in such systems occurs only at high temperatures, usually close to the decomposition point of the polymer. It was only in the late 1970s that the design criteria for liquid crystalline polymers became apparent, the secret being largely in the decoupling of the rigid aromatic groups which give rise to the anisotropic behaviour. As a result of these ideas two classifications of liquid crystalline materials were described. Main-chain liquid crystalline polymers, are those in which rigid aromatic molecules form part of the polymer backbone, either as a continuous chain or separated by a series of methylene groups in order to lower temperature at which liquid crystalline phase behaviour is observed. Side-chain systems resemble the comb-like systems studied by Shibaev and Plate, and have the rigid aromatic groups attached as a side-chain. In general, the monomer systems required for main-chain liquid crystalline polymers are relatively simple; synthetically these systems are prepared by step-growth methods and the main challenge is often maintaining sufficient solubility to allow suitable chain-lengths to be grown (an example of how such problems might be overcome is given in Chapter 4). Side-chain systems tend to be produced from more complex structural sub-units, and may be produced either by polymerization of the appropriate monomer or by functionalization of a preformed polymer backbone. Examples of both approaches are given in this chapter. From a practical viewpoint, the advantage of side-chain systems is that they tend to be much more soluble in common organic solvents and also that thermal phase transitions occur at reasonable temperatures (reasonable being well below the temperature at which the polymer decomposes). A further advantage of such side-chain systems is that the phase behaviour can be effectively tuned through the chemical modifications of the three components, namely the side-group, the flexible coupling chain and the polymer backbone.

Dendritic and Multipodal Liquid-Crystalline Materials based on Silsesquioxane and Siloxane Cores

2000 ◽  
Vol 628 ◽  
Author(s):  
Georg H. Mehl ◽  
Ralf Elsäβer ◽  
Keith J. Shepperson ◽  
Adam Thornton ◽  
John W. Goodby
Keyword(s):  
Crystalline Phase ◽  
Self Assembly ◽  
Liquid Crystalline ◽  
Phase Behaviour ◽  
Liquid Crystalline Phase ◽  
Stability Range ◽  
Inorganic Hybrid ◽  
Crystalline Materials ◽  
Liquid Crystalline Materials ◽  
Made In

ABSTRACTIn recent years great advances have been made in the synthesis of organic-inorganic hybrid materials, whether they are oligomeric, multipodal, polymeric or dendritic in structure. Of particular interest are liquid-crystalline systems, as the control of the soft self-assembly behaviour of these systems allows for the addressing of the macroscopic properties of these materials.The investigations of silsesquioxane and siloxane cores decorated with suitable organic groups of linear and branched structures leading to liquid-crystalline phase behaviour is presented. The structural and chemical features which govern the phase behaviour will be discussed. Important features are the size, geometry, structure and flexibility of the inorganic core, the flexibility, branching and length of the spacer linking inorganic cores and the organic groups, which promote liquid-crystalline phase behaviour. The effects of a variety of mesogenic structures on the liquid-crystalline phase behaviour and stability range will be discussed.

On resolving fine periodic microstructures in the optical microscope

1989 ◽  
Vol 47 ◽  
pp. 364-365
Author(s):  
W.S. Putnam ◽  
C. Viney
Keyword(s):  
Liquid Crystalline ◽  
Numerical Aperture ◽  
Polarized Light ◽  
Normal Incidence ◽  
Optical Microscope ◽  
Angle Of Incidence ◽  
Resolution Limit ◽  
Crystalline Materials ◽  
Periodic Microstructures ◽  
Liquid Crystalline Materials

Many sheared liquid crystalline materials (fibers, films and moldings) exhibit a fine banded microstructure when observed in the polarized light microscope. In some cases, for example Kevlar® fiber, the periodicity is close to the resolution limit of even the highest numerical aperture objectives. The periodic microstructure reflects a non-uniform alignment of the constituent molecules, and consequently is an indication that the mechanical properties will be less than optimal. Thus it is necessary to obtain quality micrographs for characterization, which in turn requires that fine detail should contribute significantly to image formation.It is textbook knowledge that the resolution achievable with a given microscope objective (numerical aperture NA) and a given wavelength of light (λ) increases as the angle of incidence of light at the specimen surface is increased. Stated in terms of the Abbe resolution criterion, resolution improves from λ/NA to λ/2NA with increasing departure from normal incidence.

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