Origins of Birefringence

Fig. 7 Figure to show the origins of birefringence (left) and a schematic diagram of a polarised optical microscope (right)... 

Until now we have considered the basic origin of birefringence and some of the general techniques used for determining this optical parameter. It is necessary, however, to discuss certain limitations when interpreting this parameter. Until now no mention has been made of two or multiphase systems such as semicrystalline polymers, amorphous block copolymers or even plasticized or filled polymers. In such systems the measured birefringence can be expressed as... 

The molecular chain folding is the origin of the Maltese cross which identifies the spherulite under crossed Polaroids. The Maltese cross is known to arise from a spherical array of birefringent particles through the following considerations ... 

The most commonly encountered manifestations of chiroptical phenomena are circular birefringence (also known as optical rotation), optical rotatory dispersion (ORD), and circular dichroism (CD). An explanation as to the nature of circularly and linearly polarized light is provided, and the origins of the various chiroptical effects are discussed. In each instance, a concise summary of the calculations used by workers in the field to report the results of their investigations is provided. 

McCrone (1965) also noted that second-order phase transitions have been termed as pseudopolymorphic. Such transitions are difficult to detect by optical methods, because of the small structural changes that occur hence, the origin of the prefix pseudo sometimes used to describe them. However, the birefringence of the crystals changes during such phase changes (see Section 4.2), so the use of crossed polarizers makes the phase change readily detectable. 

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). 

When an electric field is applied to the liquid crystalline solution of polypeptide, the proton signal of a solvent molecule such as methylene bromide or methylene chloride splits into a doublet however, the center signal is still observed in the initial position, even in the steady state (Fig. 7). The origin of the splitting wfll be mentioned in Section V, and let us pay attention only to this center signal here. This signal corresponds to disordered solvent molecules which are free from the action of molecular fields caused by the oriented molecular clusters. These free solvent molecules are more in evidence in the ratio in less concentrated (but fuUy birefringent) liquid crystalline solution. When measured soon after the removd of the external field (the... 

When the literature is surveyed a number of different terms arise to describe the type of birefringence observed. We will try to establish at the onset what some of these categories are and to comment on their origin and relative importance in polymeric materials. 

Until recently, this origin of apparent optical anisotropy had been generally neglected for birefringence measurements on polymeric solids. The phenomenon arises when the medium contains at least two phases each having a different refractive index and at least one dimension... 

According to Eq. (5 A), any elements with < 23 °C created by rubbing will have relaxed quickly before any subsequent measurements are carried out. The relaxation of these elements is the origin of the transient birefringence right after each rubbing pass reported in subsection... 

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