Spherulites birefringence structure

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. 

Fully developed spherulites are usually microscopic and exhibit radial symmetry [19,35-38] a schematic of a birefringent structure of a spherulite is shown in Figure 16.2 [28]. Nudeation and growth are the key mechanisms of spherulite formation [30,32,34,39,40]. Internal fluctuations in the system cause homogeneous... 

Figure 16.2 Schematic representation of the birefringence structure of spherulites. The high-power image shows the helical chains stacked together in a well-ordered lattice of crystalline...

In addition to microphase structures, MDI/BDO-based polyurethane systems have exhibited spherulitic superstructure. Characterization of the birefringence of the spherulites was used to determine the orientation of the hard-segment domains (7). However, because of the sensi-... 

Microscopic examination of these samples show for the PET-rich blends the presence of very small spherulites. The PBT-rich blends show very low birefringence, low turbidity, and no organized structures. The SALS patterns are circularly symmetrical. 

Microscopic examination of the samples crystallized at 130°C shows very low turbidity and birefringence for the PBT samples the turbidity in the blends increased, and small spherulites were present for PET. The samples crystallized at 110°C again showed small spherulites for PET, and no organized structures were observed in the blends of intermediate composition although their turbidity was quite high with samples of very high PBT composition, the turbidity was lost. 

A dispersion of spherulitic liquid crystalline particles in brine exists between 0.8 gm/dl NaCl (Figure 2(a), first sample on the left) and 1.2 gm/dl. As the salinity is increased to about 1.4 gm/dl NaCl, the amount of liquid crystals as well as the birefringence increase, and the texture observed using PLS is intermediate between those of the spherulite (S) and lamellar (L) structures. The aqueous solution is a homogeneous lamellar phase between 1.6 and 1.8 gm/dl NaCl. The surfactant molecules form bilayers with their polar heads toward the brine. Figure 3(a) shows the lamellar structure as observed by polarized microscopy at 1.6 gm/dl salt and without any polymer. The bands represent "oily streaks" in a planar background. 

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. 

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. 

Spherulites show an imperfect crystalline structure, since the melting point of the spherulite usually lies considerably below the thermodynamic melting point (see Chapter 10). Even then, a further increase in X-ray crystallinity can also be observed when the spherulites have filled the volume. Localized orientation of the crystalline region leads to the characteristic optical properties of spherulites. If spherulites are cross-linked by radiation, the identity of the individual spherulite is retained even after they have been heated above the melting point. The birefringence of oriented... 

Polymer structure n. (1) A general term referring to the relative positions, arrangements in space, freedom of motion of atoms in a polymer molecule, and orientation of chains. Such structural details have important effects on polymer properties such as the second-order-transition temperature, flexibility, and tensile strength. (2) The microstructure of a polymer, as observed by light- or electro-microscopic techniques, and including crystalline structure, birefringence, distribution of sizes of filler particles and spherulites, and distribution of reinforcement directions. These, too, have important influences on macroscopic properties and behavior. 

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