Section 4.4 Melt Crystallization

As pointed out above, the semicrystalline polymer can be considered as a two-phase composite of amorphous regions sandwiched between hard crystalline lamellae (Fig. 4.2(a)). Crystal lamellae ( c) are normally 10-25 nm thick and have transverse dimensions of 0.1-1 pm while the amorphous layer thickness, a, is 5-10 nm. As mentioned in the previous section, melt-crystallized polymers generally exhibit a spherulitic morphology in which ribbon-like lamellae are arranged radially in the polycrystalline aggregate (Bassett, 1981). Since the indentation process involves plastic yielding under the stress field of the indenter, microhardness is correlated to the modes of deformation of the semicrystalline polymers (see Chapter 2). These... 

A horizontal column is typified by the Brodie Purifier, which is shown schematically in Figure 26. Feed enters the column between recovery and refining sections, and crystals exit the refining section and pass through a purifying section. The purifying section is a wash column in which the crystals are contacted with melt generated at the bottom of the column. 

Purification of a chemical species by solidification from a liquid mixture can be termed either solution crystallization or crystallization from the melt. The distinction between these two operations is somewhat subtle. The term melt crystallization has been defined as the separation of components of a binary mixture without addition of solvent, but this definition is somewhat restrictive. In solution crystallization a diluent solvent is added to the mixture the solution is then directly or indirectly cooled, and/or solvent is evaporated to effect crystallization. The solid phase is formed and maintained somewhat below its pure-component freezing-point temperature. In melt crystallization no diluent solvent is added to the reaction mixture, and the solid phase is formed by cooling of the melt. Product is frequently maintained near or above its pure-component freezing point in the refining section of the apparatus. 

The many technological innovations in melt crystal growth of semiconductor materials all build on the two basic concepts of confined and meniscus-defined crystal growth. Examples of these two systems are shown schematically in Figure 1. Typical semiconductor materials grown by these and other methods are listed in Table I. The discussion in this section focuses on some of the design variables for each of these methods that affect the quality of the product crystal. The remainder of the chapter addresses the relationship between these issues and the transport processes in crystal growth systems. 

Because of chain folding, melt-crystallized polymers are not as strong as they could be. One can envisage that under a load a sample will at some point yield, with chains in the amorphous domains becoming oriented in the draw direction while the lamellar arms of the spherulite undergo shear and whole sections are pulled ont. This process is illustrated in Figure 8-65. 

Sample Preparation Melt-Crystallization, Sectioning and KMn04 Etching... 

CRYSTALLIZATION OF ORGANIC CHEMICALS. All the examples given in this chapter are for inorganic salts. The principles of crystaUization from solution, however, apply equally to organic materials. Walas lists a number of organic compounds for which crystallization data are reported in the literature. Organic compounds are also often purified by melt crystallization as discussed in the next section. 

Meteorites yield a variety of ages, each reflecting a specific episode in their histories. Some of these "ages" are indicated in Figure 1 the end of nucleosynthesis in a star (i) the first formation of solids in the Solar system (7) melt crystallization in parent bodies (75) excavation of meteoroids from these bodies and the meteorite s fall to Earth. Other events, like volcanism or metamorphism on parent objects can be established by gas retention as can formation intervals (based on extinct radionuclides) measuring the time between the last production of new nucleosynthetic material and mineral formation in early Solar System materials 14). In the following sections we discuss some of these ages and the information that they convey. 

An experimental intensity curve as a function of the scattering vector is produced in SAXS. From the isotropic patterns of a melt-crystallized polymer, a slice is taken and, when projected in the plane I q) against q, it shows a scattering maximum with a wide statistical distribution. The evaluation of structural parameters from the intensity curve requires the whole scattering curve. Nonetheless, the lower and upper ends of the curve cannot be determined because of the nature of the scattering process. Therefore, mathematical approximations are used to access both ends of the curve. According to the characteristics of the dispersion function measured experimentally, the intensity curve is divided into three parts, which are described in the following sections. 

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