Maltese cross pattern

A larger number of smaller spherulites are produced at larger undercoolings, a situation suggesting nucleation control. Various details of the Maltese cross pattern, such as the presence or absence of banding, may also depend on the temperature of crystallization. 

An optical microscope photograph taken at 200 X magnification using polarizing filters is shown in Fig. 21. The spherulites show a characteristic Maltese cross pattern produced by the interaction of the polarized light with the... 

Fig. 2 Visible light micrograph of spherulites from a crystalline diepoxide taken with a 5X lens and cross polarization. The observed maltese cross pattern arises from the spiral positioning of lamella along the radial growth direction. The high refractive index c-axis is tangential to the spherulite s radius.

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. 

Two important observations can be made from figs 3.13 and 5.13. The first is that a spherulite consists offibrils growing out in a radial direction the second is that each spherulite exhibits a Maltese-cross pattern. This pattern is shown particularly clearly in fig. 3.13. 

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. 

There is yet a larger scale of organization in many crystalline polymers, known as spherulites. These spherical structures are composed of many crystalline lamellae, which have grown radially in three dimensions and which are connected by amorphous molecular segments (Keith, 1969). Spherulites are easily seen with an optical microscope between crossed polarizers, and under these conditions they exhibit a characteristic pattern with circular birefringent areas possessing a Maltese cross pattern, as shown in Figure 1.11. 

Figure 1.11. Spherulites of low-density polyethylene as observed between crossed polaroids, revealing characteristic Maltese cross pattern. (Geil, 1963.)...

A spherullite is usually pictured as an array of lamellae radially disposed to one another and as a result is spherically birefringent. A spherullite has two unique refractive indices the radial (Ht) and the tangential ( t)- The refractive index ellipse can represent the variation in refractive index in the plane, where the length of the major axis of the ellipse is proportional to the maximum refractive index in the plane and the length of the minor axis is proportional to the minimum refractive index. If the larger refractive index is in the tangential direction, i.e. < rit, the spherulite is termed negative. Spherulites show a characteristic Maltese cross pattern with a maximum in the intensity in the direction at 45 ° to the polarizer/analyser pair. 

Fig. 1.3 A thin section of bulk crystal- lized nylon, in polarized light, reveals a bright, binefitigent and spherulitic texture. At high magnification a classic Maltese cross pattern is seen, with black crossed arms aligned in the position of the crossed polarizers. 

Figure 6.13 Spherulites of low-density polyethylene, observed through crossed polarizers. Note characteristic Maltese cross pattern (34).

One of the earliest and most commonly used microscopical methods of examining polymers is between crossed polarizers. Some of the earliest work was determining the birefringence of fibres, then came the study of spherulites in semi-crystalline polymers. Often the spherulites show a simple Maltese Cross pattern where the dark areas show zero-amplitude birefringence, which simply arises from the orientation of the crossed polars. In the polypropylene spherulites (Fig. 2.7a), radial growth has occurred along the a -axis which is the fastest crystal growth direction, while the and c-axis are effectively randomly oriented. 

Polarized light micrographs show details of the spherulitic structure in molded nylon (Fig. 5.38 A) which is similar to PBT [165]. The nonspherulitic skin, a transition zone and a spherulitic core region are observed. This is as expected, as the quench rate declines away from the surface e.g. in the transition zone, and thus there is some nu-cleation of spherulites. A polarized light micrograph shows the spherulitic core with a region near the skin in the lower left side (Fig. 5.38B). There are polygonal spherulites in the core (Fig. 5.38C) with classical Maltese cross patterns. 

Sap of lacquer trees, which appears dark under a polarized light optical microscope, showed a "Maltese cross" pattern with a number of circles of about 10 ym when it was dried into a film as shown in Fig. 9. This indicated the formation of considerable molecular orientation of the plant gum in the film, corresponding in pattern to the water droplets in the oil(urushiol) phase of sap. 

Typically, Maltese cross patterns were observed for blends with equimolecular-weight components. In such blends, the kinetics of crystal (spheruHte) formation depended on the of the parent components, whereas the Tn was dependent on... 

Figure 7.40 Origin of Maltese cross pattern from a negative spherulite. The orientations of the polarizer (P), the analyser (A) and the refractive index ellipses are shown.

Macrospherulites of polyethylene glycols that have diameters of up to about 2 cm (Figure 8) have been reported (88). When viewed between crossed Polaroids , the characteristic Maltese cross patterns and spherulitic banding that are seen in microscopic spherulites are apparent in these spherulites and can be seen with the naked eye (Figure 9). 

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