Molecules, birefringence shapes

As to infrared spectroscopy - and the same holds good for other spectral ranges -the orientational order is readily observable in form of dichroism Being related to the molecular shape, the molecular polarizability is anisotropic as well. By the alignment of the molecules this anisotropy is transferred to the sample, however damped due to the imperfect order as described by the order parameters. As a consequence, the dielectric function and furthermore the (complex) refractive index are anisotropic, so that eventually (linear) dichroism and birefringence occur. 

It is interesting to observe that rod-shaped molecules such as tobacco mosaic virus or flat plate particles such as bentonite show the so-called coacervation at rather low concentrations at which the second phase solution shows a spontaneous birefringence. Oparin discussed the possible connection of such a phenomenon with the origin of life (25). 

As is known, the polypeptide a-helix molecules are rod-shaped if the helical internal structures are smeared out. Therefore, we may expect a phase separation in their solutions also. Indeed, Robinson (27) found in 1956 a phase separation in several solutions of the a-helix, poly-y-benzyl-L-glutamate, in which the second phase, separated out as small droplets, showed an optical birefringence. ITie critical concentration is of course a function of the molecular length. 

Other methods of molecular-weight determination, such as diffusion, sedimentation, flow birefringence, and light scattering (all of which are considered later in this chapter) involve terms related to the size and shape of the molecules and give a... 

Among these techniques, the addition of small molecules is relatively easier than the others due to good miscibility with polymers. In contrast, the combination of species is limited for the polymer blend and copolymerization technique. Therefore, addition of small molecules is becoming a good candidate these days. Addition of needle-shaped solid particles having strong optical anisotropy also modifies the orientation birefringence and its... 

There is ample evidence that small cycloalkanes and many of their derivatives with similar sphere-like shapes can exist as plastic crystals [21,22]. They are not birefringent and exhibit fewer X-ray diffraction lines in the plastic phases than in the corresponding (lower temperature) normal crystalline phases. Many plastic phases have cubic lattices like those of the closely related smectic D phases [33] which are comprised of aggregates of rod-like molecules. In both cases, more than one molecule can constitute the unit cell or fundamental lattice unit so that several molecules may tumble in a correlated motion. A similar microscopic model explains the lack of optical birefringence in the related cholesteric blue liquid crystalline phases from some chiral... 

However, when applying results of the type (57), it must be borne in mind that such equations apply to the amorphous constituent only, and must fail at advanced degrees of stretch where crystalliisation sets in since the influence of crystallisation on the double refraction of rubber-like polymers is considerable In addition to the intrinsic double refraction of the gel frame, the swollen gel may show structural double refraction and adsorption double refraction. The structural, or textural , birefringence is caused by the difference between the refractive indices of polymer substance and solvent. It was estimated by Wiener for rod-shaped particles and for platelets. Wiener s theory is not applicable to rods of molecular thickness, and the structual double refraction caused by oriented long-chain molecules has not yet been accessible to a reliable theoretical treatment. 

The birefringence of flow in solutions of long-chain molecules was discussed in Section 7. Other optical methods for the determination of particle si e were mentioned in Volume I. One of them is the direct photographic observation in the electron microscope. By way of example we may mention the experiments with the rod-shaped molecules of tobacco-mosaic-virus Attempts to apply this method to the spherical molecules of glycogene, a polysacharide, were made by Htjsemann and Rtjska. ... 

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