Spherulites Maltese cross

Figure 4.12 Spherulites of poly( 1-propylene oxide) observed through crossed Polaroid filters by optical microscopy. See text for significance of Maltese cross and banding in these images. [From J. H. MaGill, Treatise on Materials Science and Technology, Vol. lOA, J. M. Schultz (Ed.), Academic, New York, 1977, with permission.]...

The individual spherulite shows up by the characteristic Maltese cross optical pattern under crossed Polaroids, although the Maltese cross is truncated in the event of impinging spherulites. 

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. 

The molecular chain folding is the origin of the Maltese cross which identifies the spherulite under crossed Polaroids. The Maltese cross is known to arise from a spherical array of birefringent particles through the following considerations ... 

If the Polaroid filters are held fixed and the sample rotated between them, the Maltese cross remains fixed because of the symmetry of the spherulite. 

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... 

The morphology of the spherulites was in the form of a Maltese Cross , which was confirmed by the Avrami exponent value in the DSC study. The spherulite size of the binary blends was smaller than that of pure PET and PEN. 

Figure 6.103 Schematic of Maltese cross produced by spherulites in crosspolarized filters. Reprinted, by permission, from Strobl, G., The Physics of Polymers, 2nd ed., p. 146. Copyright 1997 Springer-Verlag.

The microscopic examination shows for PBT small spherulites which become immediately volume filling, yielding a circularly symmetrical SALS pattern. For PET we can observe volume-filling spherulites with distinct maltese crosses (Figure 5). When the PBT percentage increases,... 

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.

Stirring 0.7 wt% surfactant in water for several hours produced a turbid bluish-white system. A specimen under the microscope and between crossed polarizers revealed numerous birefringent spherulites 5 to lOOy in diameter, with characteristic Maltese crosses. Similar textures were also observed with 0.3 or 1.0 wt% NaCl present. Figure 1 is a photomicrograph of typical spherulites together with so-called myelinic figures (17b), which are caused by interference phenomena. 

Crosspolarized photomicrographs of PEO 1, EO-Is-EO 2, EO-Is-EO 3, and EO-Is-EO 4 films cast from 1% benzene solutions at 30° C are presented in Figure 1. The spherulitic texture with negative birefringence became less perfect and led to a less clear Maltese cross as the fraction of amorphous Is segment increased. When the EO fraction constituted less than 50%, the texture was not clearly resolved by light microscopy. 

An ordered packing of macromolecules may also cause an optical anisotropy and birefringence, which are characteristic, for instance, of polymer spherulites. Because of the radial anisotropy of a spheruHte and the convergence of beams in the spherical structure, the interference picture represents the so-caUed Maltese cross, the center of which is located in the center of spherulite. No calculations are performed using such a picture but the photoelasticity method is very efficient in revealing qualitatively the presence of any spherulites, or a mesomorphic or ordered sate of polymeric chains. 

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. 

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. 

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. 

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. 

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. 

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