Spherulites occurrences

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. 

It should be pointed out that there is no direct physical relation between the phenomenon of fractionated crystallization and the number and the size of spherulites in the pure polymer. Whereas the occurrence of fractionated crystallization is related to the ratio between the number densities of dispersed polymer particles and primary nuclei, the size and the number of spherulites are additionally influenced by the cooling rate and the crystallization temperature. There is, therefore, also no relation between the fractionated crystallization and the type of the arising crystalline entities (complete spherulites, stacks of lamellae,...) both in the pure and in the blended material. There is, finally, no relation between the scale of dispersion which is necessary for the occurrence of fractionated crystallization and the spherulite size in the unblended polymer. 

Fig. 6. (Opposite) Thin-section photomicrographs of textural occurrences of diagenetic carbonates in the Oseberg Formation. (A) Spherulitic precipitates of early diagenetic siderite (S) embedded in late poikilotopic calcite (C).

Several blends prepared by co-precipitation followed by crystallization from the melt exhibit a double melting behavior, due to the occurrence of the secondary crystallization process. The amorphous component causes a retarded crystallization of some of the crystallizable chains, which form lamellae smaller than and located between the primary ones constituting the spherulites (see Figure 3.22). 

Obviously, if spherulite boundaries are areas of weakness, then the occurrence of transcrystallinity must raise doubts. Although transcrystallinity enhances the physical bond between fiber and matrix, in prepregs with a high fiber content, where the fibers are in close proximity, adjacent transcrystalline regions may impinge and the boundary between transcrystalline regions and could act as a long spherulite boundary. 

It should be mentioned that the occurrence of a fractionated crystallization is related only to the number densities of dispersed polymer particles and primary heterogeneous nuclei. No direct physical relationship has been found with the number or size of spherulites. These parameters are additionally influenced by the cooling rate and the crystallization temperature (Frensch et al. 1989). 

The response of spherulites to a deformation appeared to result from a number of mechanisms including slip, tilt (rotation in the plane of orientation) and twinning of the crystalline lamellae. Besides this complexity, which is inherent in all spherulitic polymers, the plastic deformation of PP is affected by the early occurrence of a necking process. Ultrahigh molecular weight PP has also been studied and its colddrawing orientational behavior was observed to be very different from that of conventional PP. 

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