Aggregate of crystallites

Portiand cement clinker structures (18,19) vary considerably with composition, particle size of raw materials, and burning conditions, resulting in variations of clinker porosity, crystallite sizes and forms, and aggregations of crystallites. Alite sizes range up to about 80 p.m or even larger, most being 15—40 )J.m. 

Most undrawn crystalline polymers possess spherulite morphology with a radial arrangement of fibrils which are complex aggregates of crystallites and amorphous regions. 

Second, there is material precipitated with lime from seawater and brines with high magnesium content. The magnesium is precipitated as the hydroxide using slaked lime or dolomite. Seawater is pretreated with sulfuric acid to remove bicarbonate salts. The precipitate is washed, filtered, and dried. The products tend to comprise small aggregates of crystallites yielding high surface areas. 

When a polymer crystallizes from the melt without disturbance, it normally forms spherical structures that are called spherulites [1,2]. The dimensions of spherulites range from micrometers to millimeters, depending on the structure of the polymer chain and the crystallization conditions, such as cooling rate, crystallization temperature, and the content of the nucleating agent. The structure of spherulites is similar regardless of their size they are aggregates of crystallites [1-6]. 

The morphology of a crystallizable polymer is a description of the forms that result from crystallization and the aggregation of crystallites. The various morphological features which occur in bulk crystallized polymers are reviewed in this section. 

Incorporating a plasticizer into the RIM formulations as a nonvolatile residual solvent should at least partially inhibit the formation of the fringed micelle crystallites and thus depress the build-up of physical crosslinks by die aggregation of crystallites. 

In practice there are several difficulties. Small amounts of contamination by oils degrade the adhesion, and the crystals will slide rather than being stretched to failure. Because the solvent fills the gap between the two sides of the specimen holder, polymeric debris may be drawn into the gap and remain. Such bridges formed in this way are extremely difficult to observe but of course will contribute to the tensile data. At present the acquisition of tensile data is limited by the aggregation of crystallites and the noncrystalline material that bridges the gap. Crystals < 10 jw,m are difficult to handle. 

FIGURE 39.2 Schematic presentation of poorly crystalline carbon, (a) Single-layer plane, (b) parallel layers in a crystallite, (c) unassociated carbon, (d) an aggregate of crystallites, single layers and unassociated carbon. (From Bokros, J.C. 1972. Chem. Phys. Carbon. 5 70-81, New York, Marcel Dekker. With permission.)... 

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. 

For preciseness, let us begin by considering an aggregate of crystallites, i.e. an aggregate of small regions each defined by a crystal unit cell. We then ask how we would define the orientation of a single imit cell of lowest symmetry, i.e. a triclinic unit cell. 

Spherulite An aggregation of crystallites as a spherical cluster, consisting of fibrillar crystalline lamellae radiating from the center of the spherulite. 

The aggregates of crystallites also have widely different sizes and properties. Some, such as soot, are extremely small and contain only afew small crystallites. In such cases, the properties are mostly related to the surface area (see Ch. 10). 

The use of nonlocal density functional theory (NLDFT) for modeling adsorption isotherms of Lennard-Jones (LJ) fluids in porous materials is now well-established [1-5], and is central to modem characterization of nanoporous carbons as well as a variety of other adsorbent materials [1-3]. The principal concept here is that in confined spaces the potential energy is related to the size of the pore [6], thereby permitting a pore size distribution (PSD) to be extracted by fitting adsorption isotherm data. For carbons the slit pore model is now well established, and known to be applicable to a variety of nanoporous carbon forms, where the underlying micro structure comprises a disordered aggregate of crystallites. Such slit width distributions are then useful in predicting the equilibrium [1-5] and transport behavior [7,8] of other fluids in the same carbon. 

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