Spherulitic Aggregates

Spherulites are polycrystaUine aggregates with spherical symmetry. For more than a century it has been known that spherulites are favored by crystallization from impure, viscous media [5]. While polymer spherulites can be grown from solution under certain condi- 

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Johannsenite, Ca(Mn,Fe )Si206, is a species in whieh p = 0. There is little additional trivalent ion (Al, Fe, or Cr ) substitution. The mineral is most commonly found as spherulitic aggregates of brittle fibers or prisms. Another pyroxene, acmite (NaFe Si206), has been found as large (100 X 35 X 20 cm) single crystals of prismatic shape, as needles, as felted aggregates of fine fibers, and as capillary crystals. 

Silicate minerals that usually occur as spherulitic aggregates of fibers have formed as a result of the alteration of the many minerals subsumed within the category of biopyriboles. Alteration of the micas under hydrothermal conditions produces compositional variants on recrystallization such as hydrous muscovite. Some of these samples have been labeled asbestiform, probably because they are found in veins that criss-cross rock masses. Fibrous micaceous minerals also occur as discrete disseminated particles, although few detailed analyses of crystallites from the disperse occurrences have been made. Fibrous mica found in veins usually grades (composition-ally) into members of the serpentine mineral group, the clays or the chlorites. 

Of the 273 varieties of whiskers included in Appendix 2, most are singlecrystal whiskers. Polycrystal fibers and aggregates composed of fibrils are also known and have been produced. Stellate or spherulitic aggregates are common, although they are mostly unplanned—the result of rapid growth during experimental synthesis studies and in manufaeturing. 

Figure 15.8 SEM image of spherulite aggregates of jet cooked high amylose starch dispersion slowly cooled without stirring. (Reprinted from Fanta et al., 2008, with permission from Elsevier).

Monohydrocalcite has been observed rarely in lake sediments and biologic material (Hull and Turnbull, 1973). It generally is found as spherulitic aggregates which may slowly convert to aragonite or calcite. Like ikaite, its formation is favored by low temperatures and high hydrostatic pressures. 

Crystallization of D-fructose is usually effected at ice-box temperature from an ethanol solution, and further purification and recrystallization is performed at room temperature. The spherulitic aggregates of fine needles obtained by this technique are those of the hemihydrate, as shown by Young and coworkers. This possibility had been put forward by Honig and. lesser, but sufficient proof was lacking at that time. The anomalous x-ray diffraction data reported by Wolfrom and Thompson for their preparation of i fructose, in comparison with those of the normal form of D-fructose, may be explained by hemihydrate formation. Indeed, purification with ethanol at 25° results in dehydration, with formation of the... 

Figure 1. Optical micrograph of a thin PC-film containing spherulites and spherulite aggregates (magnification 250 x)...

It should be noted that dendrites and spherulitic aggregates are not completely crystalline. Certain amorphous regions remain even if macroscopically crystalline aggregates appear to have impinged. In some extreme cases of branched PE, the amorphous portions in spherulites can be as high as 90%. Two different types of amorphous regions can exist in these semicrystalline polymers,... 

In unfilled polymers the crystallites generally seem to be present in spherulitic aggregates [86]. Because of hindered motion, thermoplastics have a marked tendency to snper cool before crystallising. The degree of super cooling will itself determine the nnmber of... 

Figures 42 and 43 show the effects of blending and chemical interesterification on lard fat crystal morphology. PLM examination of native lard (Fig. 42a) shows that it consisted of dense, asymmetrical spherulitic particles and small crystals. Interesterification caused a decrease in the number of spherulites, and low density aggregates of fine crystals were observed instead (Fig. 42b). The grainy texture of native lard is often attributed to the presence of large spherulites, or spherulitic aggregates, and more specifically to the presence of tristearin (Hannewijk, 1972). Many studies have shown that interesterification reduces the spherulitic nature of the native lard crystal structure (Wiedermann et al., 1961 Herb et al., 1956 Hoerr and Waugh, 1950).

It should be re-emphasized that although our block copolymers do not display spherulitic morphology when they are compression molded, they are nevertheless crystalline. Hence, this indicates that under this mode of film preparation, aggregation into well developed superstructure is apparently kinetically limited. 

Gauffre, F. and Roux, D. (1999) Studying a new type of surfactant aggregate ( Spherulites ) as chemical microreactors. A first example Copper ion entrapping and particle synthesis. Langmuir, 15, 3738-3747. 

In addition to the main types described in Table 2, crystals may form treelike patterns known as dendrites, with such aggregates being termed dendritic or arboraceous. If an aggregate is composed of tiny crystals radiating from a center, it is termed a spherulite or rosette. 

Alternatively, a fiber can be one member of a group or aggregate, as a fibril is one part of a fiber. Spherulites (Fig. 1. IH), fan-shaped fibrous crusts or rosettes, and the veins depicted in Fig. 1.1A are examples of aggregates created when many crystalline fibers grow in close proximity. During fibrous growth, lateral interference may produce planar but irregular bound-... 

If crystallization is carried out from concentrated solutions, multilamellar aggregates are formed. In particular, melt crystallization of polyethylene gives bunched-up lamellae with an overall spherical symmetry. The space between the lamellae contains uncrystallized amorphous polymer. These objects are called spherulites, and their radii grow linearly with time, in spite of their intricate morphological features [9]. Another remarkable feature of spheruhtes formed by linear polyethylene is that they are gigantically chiral, although the molecules are achiral. 

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