Maltese-cross extinction

The major feature of polymers that have been bulk crystallized under quiescent conditions are polycrystalline structures called sphemlites. These are roughly spherical supercrystalline structures which exhibit Maltese cross-extinction patterns when examined under polarized light in an optical microscope. Spheruliies are characteristic of semicrystalline polymers and are also observed in low-molecular-weight materials that have been crystallized from viscous media. Sphemlites are aggregates of lamellar crystallites. They are not single crystals and include some... 

It is well known that PEO spherulites have a Maltese cross extinction pattern and a veiy fine spherulite texture as observed under POM (Ketabi and Lian, 2012 Choi and Kim, 2004 Xi et al., 2005 Choi, 2004) (refer to Figure 9). Meanwhile, under SEM, the image of pure PEO shows waving smooth and uniform stmeture without any phase separation (Johan et al., 2011). 

Fig. 10.18 Illustration of (a) the lamellar crystal, (b) the optical indicatrix and (c) the Maltese-cross extinction in polymer spherulites...

Fig. 5.42 Polarized light micrographs of a molded nylon cross section show a nonspherulitic skin (top in A) and rounded isolated spherulites in the transition zone. A classical Maltese cross extinction pattern is observed with black brushes showing the radial texture within the spherulites (B). (See also Fig. 1.3 in color section.)...

Fig. 4.3. Ordering of the indicatrices in a spherulite and the resulting Maltese cross extinction pattern, as observed between cross nicols. The orientation of the Maltese cross coincides with the directions of polarizer and analyzer...

This birefringence coupled with spherical geometry produces light extinction along the axis of each of the Polaroid filters, hence the 90° angle of the Maltese cross. 

Examination of thin sections of semicrystalline polymers reveals that the crystallites themselves are not arranged randomly, but form regular birefringent structures with circular symmetry. These structures, which exhibit a characteristic Maltese cross-optical extinction pattern, are called spherulites. Although spherulites are characteristic of crystalline polymers, they have also been observed to form in low-molar-mass compoimds that are crystallized from highly viscous media. 

Spherulites are remarkably easy to grow and observe in the laboratory (36). Simple coohng of a thin section between crossed polarizers is sufficient, although controlled experiments are obviously more demanding. It is observed that each spheruhte exhibits an extinction cross, sometimes called a Maltese cross. This extinction is centered at the origin of the spherulite, and the arms of the cross are oriented parallel to the vibration directions of the microscope polarizer and analyzer. 

Perhaps the most widely used application of polarised light microscopy in the study of polymers is in connection with crystalline morphologies [21-30]. Figure 10.5 shows typical spherulites of the a crystal form of poly(vinylidene fluoride), as imaged in polarised light. The extinction pattern seen in Figure 10.5(a) is made up of a black Maltese cross, a feature that is typical of a... 

Figure 3.15. Two images of a thin melt cast film of high density polyethylene the region is 200 x 250//m. The left hand image was taken in crossed polars. The radial Maltese cross is due to the extinction position. The spherulites in this material have dark circumferential bands. The crystals twist as they grow, and their orientation in these bands has the optic axis perpendicular to the specimen plane. The right hand image is the same area when a first order red plate is also used. The blue and yellow colors show that the spherulites are negative. (See color Insert.)...

Figure 3.67 Polarized optical micrograph of a doubly banded spherulite in the chiral polymer poly(R-3-hydroxybutyrate). Two closely spaced extinction bands are separated by a wider light region the crystal strncture is orthorhombic and the indicatrix is an ehipsoid with two optic axes. Close inspection of the vertical arms of the Maltese cross reveals dark bands crossing the light regions from left to right, indicating that the twist is left handed. Scale bar is 25 mm. From Saracovan et al. [102] with permission from the American Chemical Society.

Figure 3.2 Maltese extinction cross in spherulites and the optical properties of spherulites. (A) Direction of slow vibration is radial (B) direction of fast vibration is radial.

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