Scattering spherulitic structure

Figure 15 Morphological map of linear polyethylene fractions. Plot of molecular weight against crystallization temperature. The types of supermolecular structures are represented by symbols. Patterns a, b and c represent spherulitic structures with deteriorating order from a to c. Patterns g and d represent rods or sheet-like structures whose breadth is comparable to their length g or display a different aspect ratio d. Pattern h represents randomly oriented lamellae. Neither h nor g patterns have azimuthal dependence of the scattering. Reproduced with permission from Ref. [223]. Copyright 1981 American Chemical Society. (See Ref. [223] for full details.) Note the pattern a is actually located as o in the figure this was an error on the original.

Electron microscopy (5,40,52,63, 64), on the other hand, can provide direct information on the domain structure under favorable conditions, such as when the domains are crystalline. When the samples exhibit a semicrystalline superstructure, small-angle light scattering and polarized microscopy have been used in addition to electron microscopy to study the spherulitic structure. These methods are complemented by differential scanning calorimetry, and various techniques for studying dynamic mechanical behavior which can be interpreted to give additional, if somewhat less direct, information on domain structure. 

Morphology changes were observed by optical microscopy and small-angle light scattering. The pure components exhibit spherulitic structures, each with different orientation of the optic axis with respect to the spherulite radius. Spherulites become disordered and larger with the introduction of small amounts of the second component. Larger amounts of the second component result in a loss of spherulitic order. 

By means of several optical techniques, viz. small angle laser light scattering (SALLS), optical microscopy, etc, the spherulite structure can be studied. From the photographic scattering pattern the spherulitic radius, R, can be calculated as a function of the crystallization time and/or blend composition [Stein, 1964] ... 

The two types of scattering patterns are illustrated in Figure 6.15 (54). These patterns arise from the spherulitic structure of the polymer, which is optically anisotropic, with the radial and tangential refractive indexes being different. 

Table 6.3 shows the relation of the polymer structures and their sizes, superimposed on the range of structural sizes observable by the various microscopy and scattering techniques. An important point is the overlap among the various techniques, which makes complementary analyses possible. For example, study of spherulitic structures is shown to be possible by optical, SEM or TEM methods. A flow chart (Fig. 6.1) is provided at the end of this section as an aid in the final selection of a characterization technique. 

In order to discuss the time variation of the internal spherulitic structure, it is convenient to employ the scattering profiles at an azimuthal angle 45° using a reduced scattering angle w [13]. 

As was discussed in detail in Sect. 5.1, X-ray scattering experiments in the small-angle range (SAXS) can be used for an investigation of crystalline-amorphous superstructures. The conversion of the spherulitic structure com-... 

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