Polarized light microscopy crossed polarizers

The use of a single polar is compatible with phase contrast microscopy. Crossed polars produce a dark field in which fine fibers will not be seen. If a compensator such as a first order red plate is also used, most of the fine fibers will be seen provided the light source (31) is sufficiently intense. 

The characterization of liquid crystals by polarized light microscopy is the most straightforward method available and, whenever possible, it should be carried out in the initial stages of an investigation on new polymers. Thermal analyses alone can be misleading. In this procedure, a thin layer of the melt is kept at constant temperature on a hot-sta and obsawed between crossed polars. The appearance or texture of the melt is dependent on the structure of the mesophase, and, therefore, it is often possible to directly identify the type of mesophase present by this method. A good review of the microscopy of liquid crj b ajqjears in the books by Hartshome and by Demus and Richter... 

If the microstructure is too fine, and the material is pleochroic, one can attempt to identify the azimuths for maximum and minimum absorption of linearly polarized light (171 in optically biaxial material, these need not coincide with the local extinction directions. The use of optical pleochroism as just described is one example of how studies limited to observations between crossed polars do not exhaust the information available from light microscopy. 

Figure 1.37 Intensity change of polarized light passing through an analyzer when the elliptically polarized light changes. The polarizer and analyzer are in a crossed position. (Reproduced with permission from D.B. Murphy, Fundamentals of Light Microscopy and Electronic Imaging, Wiley-Liss. 2001 John Wiley Sons Inc.)...

Figure37. (a) Crystalline and right-handed helical fibersmadeofGlc-NC(12)CN-Glc (116, n= 12) observed using polarized light microscopy (at 25 C in water). Periodical structures of the fibers are denoted by arrows, (b) Polarized light micrographs of representative dehydrated and right-handed fibers from Glc-NC(12)CN-Glc (116, rt = 12), (top) photographed trough cross-polarized filters and (bottom) through plane-polarized filters. Reproduced from ref. 338 (Shimizu and Masuda, J. Am. Chem. Soc. 1997, 119,28)2) with permission of the American Chemical Society.

The optical anisotropy of cokes gives rise to a characteristic pattern of extinction contours when a polished surface is examined by polarized light microscopy using crossed polars, or reflection interference colours if the polars are parallel and a half wave plate is inserted into the optical system (, 12) ... 

The effect of additives on the asphaltene from the Catalytic Incorporated (Cat. Inc.) coal liquid product was studied. Asphaltene is defined as the pentane insoluble but benzene soluble part of the coal liquid. The fractionation procedure has been described in detail elsewhere(l) and is shown schematically in Figure 1. Some work was also done with A240 petroleum pitch. Elemental analysis for the Wyoming sub-bituminous coal. Cat. Inc. coal liquid product, and Cat. Inc. asphaltene and A240 petroleum pitch are shown in Table I. Measured amounts of the additive compounds to be studied were added to the Cat. Inc. asphaltene and petroleum pitch. The samples were pyrolyzed and the pyrolysis residues examined by cross polarized light microscopy. Elemental analyses of the residues were done. 

The polymer network structure can be studied by various means. Optical characterization is particularly versatile, since it can probe the composites directly and test whether, and to what degree, the network is oriented (75, 27, 30, 31), Hot-stage cross polarized light microscopy can be used to test the influence of monomer or polymer on LC phase transitions of these composites. Measurement of the birefringence of the bare polymer network, or of the LC composite in the isotropic state, yields information concerning anisotropy of the polymer network and of the type and strength of interaction between the network and LC matrix (75, 27, 30, 31). 

Starch can be destructurized using extrusion technologies in specific conditions. Sufficient work, heat and time have to be applied to a cereal-based starch product in the presence of plasticizers to destructurize it. The best plasticizer for starch is water in quantities lower than 45%. Other plasticizers are glycols such as glycerol and sorbitol. Whereas thermoplastic starch can contain a certain amount of granular residue and a few Maltese crosses can be detected in polarized light microscopy, destructurized starch is substantially free from those features. 

Figure 5.24 Schematic representation of carbon fiber structures obtained from Courtelle precursor, (a) Isotropic center—with an outside skin of oriented crystalline material, (b) Double cross— with the outside showing a different orientation to that of the center, (c) Single cross—where the complete fiber shows one type of preferred orientation. Source Reprinted from Knibbs RH, The use of polarized light microscopy in examining the structure of carbon fibres, J Microscopy, 94(3), 273-281, 1971.

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