Birefringence identification

Optical properties of fibers are measured by light microscopy methods. ASTM D276 describes the procedure for fiber identification using refractive indexes and birefringence. Other methods for determining fiber optical properties have been discussed (3,38—44). However, different methods of determining optical properties may give different results (42). 

This principle of compensation enables the microscopist to measure the thickness of a particle if the birefringence is known or to measure the biregringence of a particle if the thickness is known. The birefringence of a particle is useful for particle identification by consulting tables of optical constants. 

Natural anhydrite is orthorhombic with good cleavage. It is almost always seen as blocky crystals with 90° comers. The refractive index is 1.570 to 1.614, which is much higher than gypsum, so it stands out when we mount the specimen in 1.528 RI to make the gypsum disappear (Fig. 23). The birefringence is 0.044 RI, which means that particles as small as 10 p.m will show some color. As a matter of fact anhydrite is very colorful in the sizes we ordinarily encounter and is easy to recognize. It has parallel extinction that helps to confirm our identification (Fig. 15). 

The optical microscope is a valuable tool in the laboratory and has numerous applications in most industries. Depending on the type of data that is required to solve a particular problem, optical microscopy can provide information on particle size, particle morphology, color, appearance, birefringence, etc. There are many accessories and techniques for optical microscopy that may be employed for the characterization of the physical properties of materials and the identification of unknowns, etc. Utilization of a hot-stage accessory on the microscope for the characterization of materials, including pharmaceutical solids (drug substances, excipients, formulations, etc.), can be extremely valuable. As with any instrument, there are many experimental conditions and techniques for the hot-stage microscope that may be used to collect different types of data. Often, various microscope objectives, optical filters, ramp rates, immersion media, sample preparation techniques, microchemical tests, fusion methods, etc., can be utilized. 

The identification of n" and n in a crystal of unknown orientation may be obtained with the help of a crystal plate of known orientation and birefringence (Fig. 4.29). The polarizing microscope is equipped with a gypsum plate with birefringence d n" — n ) = 551 nm (red in the first order). Colors of addition and subtraction are obtained according to the orientation of the unknown crystal with respect to the plate. 

Fibers represent a special case, as such optical properties as refractive index and birefringence are important not so much for their influence on the appearance or performance of the product but as an aid to fiber identification. Fiber optical properties are considered in Chapter 19. Although similar identification techniques are applicable to transparent plastics in general, such tests are not widely used outside the forensic. science... 

Low- and high-powered microscopes are used to examine the morphological features of the fibres and the initial determination of whether the fibre is natural or man-made. FTIR microscopy can be used on a synthetic fibre to provide information in relation to the functional groups present this can be used to pinpoint which synthetic fibre it is. Polarising light microscopy is used with synthetic fibres plane-polarised Ught interacts with the fibres in order to provide refractive index values (many of these fibres have two refractive indices due to the chemical structure of the fibre and are said to be birefringent). This helps in the identification of the synthetic fibre. 

Powders are examined in their original form by dispersing them in either aqueous or nonaqueous mountants, depending on their solubility and dispersibility. The mounting media may include stains to increase the contrast of individual components for structural observation or discrimination on the basis of chemical and morphological characteristics. The detection of birefringence in powders is also useful for identification. 

Identification of solid form based on birefringence and morphology... 

Liquid crystal materials may exhibit birefringence (discussed in Section 2.5), so polarized optical microscopy is an ideal technique for visualization of liquid crystal textures. Samples are prepared as a thin film ( 2-20 pm thick) between glass plates. By observing the interesting defect textures that may form in each phase, it is often possible to make accurate phase identifications by microscopy alone, even though the microscope is unable to resolve the actual molecular packing structure. 

For phase identification, therefore, it is essential to be familiar with the different birefringence textures each phase can exhibit. These textures depend on molecular alignment with respect to the optical axis and the polarizers. To illustrate this, we can consider the Schlieren texture of the nematic phase shown in Figure 2.22. 

The identification of the appropriate order parameter for nematic liquid crystals is aided by a consideration of the observed structure and symmetry of the phase. As in any liquid, the molecules in the nematic phase have no translational order i.e., the centers of mass of the molecules are distributed at random throughout the volume of the liquid. Experiments of many varieties, however, do demonstrate that the nematic phase differs from ordinary liquids in that it is anisotropic. The symmetry, in fact, is cylindrical that is, there exists a unique axis along which the properties of the phase display one set of values, while another set of values is exhibited in all directions perpendicular to this axis. The symmetry axis is traditionally referred to as the director . The optical properties of nematics provide an example of how the cylindrical symmetry is manifest. For light passing parallel to the director, optical isotropy is observed, while for all directions perpendicular to the director, optical birefringence is observed. Rays polarized parallel to the director have a different index of refraction from those polarized perpendicular to the director. 

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