Tactoidal solution

Smectic and nematic meso phases can be easily interconverted in the case of low-molar-mass compounds, and this gives melts of these compounds the characteristics of liquids. On the other hand, these melts are optically anisotropic because of their one-dimensional order, and so, have characteristic colors. Consequently, they are also known as liquid crystals. Solutions of rod-shaped macromolecules exhibit similar ordered behavior they are called tactoidal solutions. One-dimensional order can be induced in polymer melts by lowering the temperature below the melt or glass transition temperature, whereby the one-dimensional order is frozen in. The characteristic X-ray diagrams discussed above are then obtained. 

The layers of the smectic mesophase and the molecules in the nematic mesophase are easily displaced with regard to each other, so that melts of this type have a liquid character. Because of their ordered state, however, these melts are optically anisotropic and show characteristic colors. For this reason, such melts are also called liquid crystals." Similar phenomena of order can also occur in solutions of rod-shaped macromolecules, so that one also speaks of tactoidal solutions. 

Rod-like crystals (Eig. 4.15 b) are formed from partly neutralized Fe solutions (0 < OH/Ee < 3) (Mackay, 1962 Atkinson et al., 1977 Paterson Tait, 1977). They are usually monodisperse, around 50 nm long, 6 nm wide and also elongated in the [010] direction. In concentrated suspensions, these rods associate to form tactoids,... 

This has2been shown to be correct by using cis-Ru(bpy)2(H20)2 as catalyst then all the components for the reaction are within the interlamellar space and photooxidation proceeds at a rate comparable with that observed in solution (Figure 7c). These observations outline the importance of the textural arrangement of the clay sheets with respect to the availability of the surface sites, in line with what has been previously discussed in the paragraph dealing with the concept of tactoids. 

LMM, after dialysis against a solution of 0.05 M KC1-0.005 M tris-maleate buffer (pH 6.2), also exhibited a decreased capacity to form well-ordered paracrystals or tactoids as examined by electron microscopy (Figure 5). It is interesting that several half-reconstituted paracrystals (Figure 5b) were found after 6 weeks of frozen storage. 

FIGURE 10.4 Electron micrograph of a swollen n-butylammonium vermiculite prepared in a 0.1 M solution of n-butylammonium chloride. The scale is defined by the white bar, which represents 200 nm. The three large arrows indicate packets of vermiculite layers, or tactoids. In the region between the two smaller arrows, the layers appear to be individually separated. 

FIGURE 10.5 Schematic illustration of the swelling of n-butylammonium vermiculite in a 0.1 M n-butylammonium chloride solution (a) represents the n-butylammonium vermiculite crystal id =2 nm) prior to swelling, (b) the gel id =12 nm) formed by a homogeneous sixfold expansion in the range 0°C < T < 14°C, and (c) the tactoid formed when the gel collapses at T < 0°C or T > 14°C. In (c), the dashed line represents the fact that the tactoid structure occupies roughly the same volume as the gel structure. 

The larger particle sizes thus could be accretions of these basic units, and several such agglomerations were noted. The rods were arranged side by side, closely packed in bundles. Apparently the alkali peels these rods from the coal mass, and they subsequently agglomerate in solution, similar to tactoid formation (12), according to a crystal growth type of clustering theory (13, 14).)... 

When tropomyosin solutions are dialyzed against divalent cations such as Mg " or Ca +, fibrous paracrystals are formed. Paracrystalline tactoids under these conditions have a characteristic axial repeat period of 40 nm (Cohen and Longley, 1966). The axial periodicity is formed by the alternating arrangement of wide bright (27 nm) and narrow dark (13 nm) bands, each of which is surrounded by clear boundary lines. Two sets of oppositely directed tropomyosin filaments are arranged in the paracrystalline structure and each boundary line corresponds to the position of... 

Figure 1.15 Schematic representation of formation of polymer-based CMP systems viz. (a) conventional bulk CMP (tactoid), (b) intercalated NCs, and(c) exfoliated NCs, via physical mixing (dry/solution phase mixing or melt compounding) and in-situ polymerization routes.

FBj. 3. Sickled erythrocytes in oxygen-depleted whole blood from a patient with sickle-cell anemia, demonstrating the similarities in shape to tactoids formed in stroma-free solutions of their deoxygenated hemoglobin. Phase photomiaography, X 375. From John W. Harris, Proc. Soc. Bxptl. Biol, and Med. 75, 197 (1950). 

Fig. 9 Bright field TEM images of nylon-6/ montmorillonite composite nanofibers from (a) pure HFIP solution and (b) a solution of 95% HFIP and 5% DMF (a single sheet, b stacked sheets/tactoids). Reprinted with permission from [87], Copyright (2002) Elsevier...

Figure 17.8 Schematic of film drying, (a) Isotropic suspension, (b) Emergence of self-assembled tactoids at a critical concentration, (e) Alignment of tactoids in a highly concentrated solution, (d) Dried ehiral nematic film.

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