Mosaic texture

The process of development of a mosaic texture was observed by Taylor (2) first in thermally metamorphosed coals and then in partially carbonized vitrinite. He noted the appearance of spherical bodies in the plastic vitrinite, their growth, and the final development of the mosaic texture which is characteristic of the walls of the vesicular coke structure. From their appearance and behavior in polarized light it was deduced that the spherical bodies probably had a single plane of preferred orientation—i.e., a lamellar structure. 

Figure 3. Liquid-crystal textures of the methyl-substituted model ester viewed through crossed polarizers, a, Smectic C-to-nematic transitional phase and b, smectic mosaic texture at 160 °C. Original magnification, 320x.

Some materials crystallize to give a microstructure that can be mistaken for a smectic mosaic texture. A simple test involves reheating the sample as soon as the "mosaic texture" has formed if the transition is not reversed at close to the same temperature, we are dealing with crystallization (Neubert, M.E., Kent State University, personal communication, 1989). A reversible transition would be inconclusive, i.e. it would be consistent with the formation of either a crystalline or a liquid crystalline phase. However, reversibility is expected over a greater range of cooling rates in the latter case. 

These different contrast mechanisms can all be used to reveal the scale of liquid crystalline polymer microstructures. In specimens that exhibit a mosaic texture, and in those that contain predominantly planar defects, domain size is easily defined in terms of areas that uniformly show extinction between crossed polars. However, if the defects are predominantly linear, as in specimens that exhibit schlieren textures, such simple characterization of microstructural scale is no longer possible. Here it is more convenient to look at the length of disclination line per unit volume, which is equivalent to the number of lines intersecting unit area, and analogous to the dislocation density as defined for crystalline solids. Good contrast is essential in order to obtain an accurate count. Technologically, microstructural scale is of growing interest because of its relevance to processability, mechanical properties and optical transparency. 

Figure 7. Cross section of a well de- Figure 6. Thin spot in a nonuniform veloped mosaic texture showing elastic sulfur coating...

Figure 12. Coated urea granule show- Figure 13. Exterior surface of a coating coalescence of sulfur droplets on ing with intergranular pores and poor the exterior surface mosaic texture...

When urea is coated with pressure-atomized sulfur at about 180 °F and quickly cooled, a mosaic texture is developed, as in the cross section of the sulfur coating in Figure 15. This mosaic texture is well developed and is characterized by high ratios of the areas of crystalline to carbon disulfide-insoluble sulfur. The mosaic texture in such samples is different... 

Figure 14. Partially coated urea gran- Figure 15. Cross section of well de-ule showing positive contact angle he- veloped mosaic texture in higher tem-tween sulfur droplets and the exterior perature pressure-atomized coating surface of the granule cooled quickly...

Figure 16. Cross section showing poorly developed mosaic texture and low elasticity of intergranular sulfur in pressure-atomized sulfur coating...

Figure 19. Layer of sulfur with poorly developed mosaic texture covered with layer of massive sulfur crystals in a cross section...

Figure 20. Cross section showing dis- Figure 21. Pinhole pores in an extinct layers of sulfur with a feathered terior surface with mosaic texture edge...

Fissure propagation through the pore wall material of coke is also dependent on the size and orientation of anisotropic components present fissures propagate easily through large optical textures (>60 ym) while mosaic textures (1 to 10 ym) tend to act as fissure stops. Experimental verification of this aspect of fissure generation and propagation will be discussed below ... 

The cross polarized micrographs of the pyrolyzed residues of the Cat. Inc. original coal liquid and Cat. Inc. asphaltene are shown in Figure 2. The microstructures are primarily coarse mosaic textures. The following subsections describe the different additives of elemental and organic sulfur, organometallics and their combined effects on Cat. Inc. asphaltene during pyrolysis. 

Smectic phases show a number of characteristic textures including (1) the focal conic and fan texture, characteristic of smectic A, and often formed from the coalescence of batoimets (2) the mosaic texture observed when the smectic B phase is formed and (3) the broken focal conic structure resulting from smectic C phases that can also show the Schheren patten described earho-. Examples of focal conic textnres arc shown in Figure 11.13. 

By studying paramorphotic patterns and the way that they appear in focal-conics, phase identification can be accomplished and information on mesophase structure can be obtained. However, the problems of phase identification are greatly cased when the focal-conic texture is accompanied by a homeotropically oriented texture. For example, the smectic A phase can exhibit the unbroken focal-conic and optically extinct homeotropic textures together, whereas the smectic C phase exhibits broken focal-conic and schlieren textures, and the E phase exhibits banded focal-conic and mosaic textures. Thus, the... 

Mosaic texture of the orthogonai smectic B and crystai B phases... 

Figure 10 Photomicrograph of the mosaic texture of a crystai J phase formed in a free-standing fiim seen in the poiarizing microscope (100 x).

Mosaics can be observed for a number of phases, for example, smectics F and hexatic B, and crystals B, E, G, J, K, and H, all exhibit mosaics of one form or another (see Figure 10). It is very difficult to identify these phases from their mosaic textures as they all look very similar. Two exceptions are worthy of note the E phase, because it is biaxial, has a very characteristic mosaic pattern where the domains tend to overlap to give a ghost-like appearance and the B phases, where the mosaic texture is often accompanied by a homeotropic texture that remains optically extinct on rotation of the microscope stage. Figure 11 shows a variety of mosaic textures where the molecules are either tilted or perpendicular with respect to the layer planes. 

Figure 11 The mosaic textures of smectic phases showing the side elevations of the domains (A) the orthogonal smectic phases and (B) the mosaic texture of a smectic phase where the long axes of the molecules are tilted with respect to the layer planes.

Figure 8). The enthalpy and entropy changes associated with these transitions are summarized in Table 4. Presently we do not know the degree of crystallinity of these polyethers. However, the fact that the enthalpy and entropy changes of all transitions are larger than those of melting seems to indicate that the polyethers exhibit multiple smectic mesophases. Additional support for smectic type mesophases are provided by their high viscosity and by the optical polarizing micrographs which exhibit mosaic textures, typical of higher ordered smectic phases, or batonnets textures,...

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