Maltese Cross appearance

Some polymers, when they are suitably prepared in thin slices or as thin films, exhibit circular features when they are viewed in the optical microscope (fig. 3.13), whereas others show less regular patterns, depending on the polymer and the method of preparation of the sample. In order to see these features the polarising microscope with crossed polarisers (see section 2.8.1) is used. The circular features shown in fig. 3.13 are caused by spherical structures called spherulites which are a very important feature of polymer morphology, the subject of much of chapter 5, where the Maltese cross appearance seen in fig. 3.13 is explained. Each spherulite consists of an aggregate of crystallites arranged in a quite complicated but regular way. 

Spherulites are in some ways similar to the grain structure in metals. They are typically about 0.01 mm in diameter and have a Maltese cross appearance between crossed Polaroids. Large spherulites contribute to brittleness in polymers. To avoid this, nucleating agents are often added or the polymer is shock cooled to promote smaller spherulites. 

It is easy to observe spherulite growth in a thin film of low molecular weight polyethylene oxide, melt between a microscope slide and a cover slip, using polarised light microscopy. The spherulites grow as discs once their diameter exceeds the film thickness of about 0.1 mm. The discs have a radiating fibrous appearance and a Maltese cross pattern with arms parallel to the crossed polarising filters below and above the specimen (Fig. 3.24b). However, these two-dimensional spherulites are a rarity in nearly all cases the spherulites are three-dimensional with polyhedral boundaries. 

Fig. 5.14 Formation of the Maltese cross for a spherullte (a) the directions of transmission of the polariser and analyser, (b) a schematic representation of the orientation of the indicatrices in the spherullte and (c) the appearance of the Maltese cross. In (b) the indicatrix orientations are shown for only eight radial directions, for clarity, and should be imagined forthe remainder.

Consider a spherulite observed between the crossed polariser and analyser in a polarising microscope (see fig. 5.14). Assume that the crystallites within the spherulite have a constant orientation with respect to the radius vector. The corresponding orientations of the indicatrices are then as shown in fig. 5.14(b) and, according to the principles of the polarising microscope explained in section 2.8.1, the Maltese cross will appear in the orientation shown in fig. 5.14(c). Even if the shorter axis of the indicatrix is parallel to the radius vector the orientation of the cross will not change. All that matters for the field to appear dark is that one of the principal axes of the indicatrix should be parallel to the axis of the polariser. 

A number of terms have been used to describe inclusions, some of which are self-explanatory, such as bubbles, Ijords (parallel channels), veils (thin sheets of small bubbles), clouds (random clusters of small bubbles), negative crystals (faceted inclusions) and so on. Most frequently inclusions are distributed randomly throughout the crystal, but sometimes they show a remarkable regularity, e.g. as in hexamine (Denbigh and White, 1966 Bourne and Davey, 1977) and ammonium perchlorate (Williams, 1981). Sometimes hour-glass or Maltese cross patterns may appear, e.g. as in sucrose (Powers, 1969/70 Man-tovani et al., 1985). Several examples of different types of inclusion in crystals are illustrated in Figure 6.46. 

The appearance of the domains depends on frequency when u < Uc = Airale fingerprint domains are observed [76, 114, 115, 116], while at the frequencies above the critical value cylindrical domains in the form of cono-scopic Maltese crosses are observed [76, 110, 114-116]. 

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