Molecule biaxial

Luckhurst G R 1985 Molecular field theories of nematics systems composed of uniaxial, biaxial or flexible molecules Nuclear Magnetic Resonance of Liquid Crystals ed J W Emsiey (Dordrecht Reidel)... 

Disk-shaped molecules based on a metal atom possess discotic Hquid crystal phases. An example is octasubstituted metaHophthalocyanine. FiaaHy, metallomesogens which combine both rod-like and disk-like features iato a single molecule adopt the biaxial nematic phase. In addition to there being a preferred direction for orientation of the longest molecular axis as is tme for the nematic phase, perpendicular to this direction is another preferred direction for orientation of the shortest molecular axis (12). NonmetaHomesogens which combine both rod- and disk-like features iato a single molecule also adopt a biaxial nematic phase, but at least ia one case the amount of biaxiaHty is very small (15). 

Deliberately oriented polystyrene is available in two forms filament (mono-axially oriented) and film (biaxially oriented). In both cases the increase in tensile strength in the direction of stretching is offset by a reduction in softening point because of the inherent instability of oriented molecules. 

In addition to orientation in one direction (mono-axial orientation), biaxial orientation is possible. This is achieved when sheet is stretched in two directions resulting in layering of the molecules. This can increase the impact strength, tensile strength and solvent cracking resistance of polymers and with crystalline plastics the polymer clarity may also be improved. 

In this review the definition of orientation and orientation functions or orientation averages will be considered in detail. This will be followed by a comprehensive account of the information which can be obtained by three spectroscopic techniques, infra-red and Raman spectroscopy and broad line nuclear magnetic resonance. The use of polarized fluorescence will not be discussed here, but is the subject of a contemporary review article by the author and J. H. Nobbs 1. The present review will be completed by consideration of the information which has been obtained on the development of molecular orientation in polyethylene terephthalate and poly(tetramethylene terephthalate) where there are also clearly defined changes in the conformation of the molecule. In this paper, particular attention will be given to the characterization of biaxially oriented films. Previous reviews of this subject have been given by the author and his colleagues, but have been concerned with discussion of results for uniaxially oriented systems only2,3). 

The structures of phases such as the chiral nematic, the blue phases and the twist grain boundary phases are known to result from the presence of chiral interactions between the constituent molecules [3]. It should be possible, therefore, to explore the properties of such phases with computer simulations by introducing chirality into the pair potential and this can be achieved in two quite different ways. In one a point chiral interaction is added to the Gay-Berne potential in essentially the same manner as electrostatic interactions have been included (see Sect. 7). In the other, quite different approach a chiral molecule is created by linking together two or more Gay-Berne particles as in the formation of biaxial molecules (see Sect. 10). Here we shall consider the phases formed by chiral Gay-Berne systems produced using both strategies. 

Due to the biaxiality of the molecules in the C2v structure, the refractive index for light polarized parallel to the layers is different depending upon... 

Note 2 Short board-like shaped molecules usually form biaxial nematic mesophases. It is recommended that the use of the term disordered sanidic mesophases for such mesophases be discontinued (see Definition 3.3.1, Note 5). 

The PET studied was Mylar C film (DuPont) of 12 to 25 micron thickness. Such films are biaxially oriented and are about 50% crystalline. The glass transition and melting temperatures are about 70° and 250°C., respectively. The polymer contains several parts per thousand of inorganic materials, moisture, and 1 to 2% cyclic oligomers (mostly trimer) (8). The number-average molecular weight of the polymer molecules is... 

Before blowing the parison into the cavity, it can be mechanically stretched to orient molecules axially, Fig. 3.61 [25], The subsequent blowing operation introduces tangential orientation. A container with biaxial molecular orientation exhibits higher optical (clarity)... 

Infrared spectroscopy has been a common tool for the study of solid surfaces (3). As in any surface spectroscopy, the number of adsorbed molecules and the surface area of the solid determines the sensitivity needed for IR studies. For low area surfaces, reflection techniques have been used to measure IR spectra of adsorbed monolayers on metal surfaces (7). However, for nonmetallic surfaces such as mica, the low reflectivity of mica makes reflection techniques less suitable for IR measurements. At the same time, the biaxial properties of mica, the parallel nature of the surfaces, and the absorbance of the mica itself present difficulties in IR spectroscopy (8). 

The bent core molecules do not only exhibit spontaneous resolution in smectic phases. One achiral derivative resolves in a nematic phase in this fluid state [ 145], while a substituted oxadiazole which forms a biaxial nematic phase also segregates [ 146]. The bent core clearly has a special stereochemical influence as a result of the effects it induces beyond the molecule, at least for liquid crystals. 

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