Crystallization phenomena

V. Palumbo-Romand, M. Ferriol, M. T. Cohen-Adad, N. Clavaguera. Nucleation and crystallization phenomena in stoichiometric monomethyl-hydrazine-water mixtures. J Cryst Growth 180 23%, 1997. 

Detonation in Crystals. Phenomena associated with detonation in large single crystals of explosives was discussed by T.E. Holland et al, in jApplphys 28, 1212(1957)... 

Haloes and other ice crystal phenomena are discussed by Minnaert (1954), Tricker (1970, 1979), and Greenler (1980). The book by the latter author is particularly recommended for its clarity and its superb color photographs. 

The Journal of the Optical Society of America for August 1979 is almost entirely devoted to meteorological optics it contains several papers on rainbows, the glory, and ice crystal phenomena. 

Certain polymers can be crystallized by mechanical stress. Namely, the stress induced elongation decreases the entropy of the chains. For this reason, an additional decrease in entropy, which is required for crystallization is comparatively small. Strain induced crystallization phenomena are of great practical importance since the elastomeric properties can be tailored in some way. In particular, PIB readily undergoes strain induced crystallization already close to room temperature. 

J.A.H. van Laarhoven, M.A.B. Kruft, and H. Vromans, Effect of supersaturation and crystallization phenomena on the release properties of a controlled release device based on eva copolymer, J. Controlled Release, 82(2-3) 309-317, August 2002. 

From previous discussion it is clear that this behavior must be ascribed to crystallization phenomena or at least to segmental motions which result in an increase of order in the polymer and hence a more efficient chain packing. 

Zeolite crystallization represents one of the most complex structural chemical problems in crystallization phenomena. Formation under conditions of high metastability leads to a dependence of the specific zeolite phase crystallizing on a large number of variables in addition to the classical ones of reactant composition, temperature, and pressure found under equilibrium phase conditions. These variables (e.g., pH, nature of reactant materials, agitation during reaction, time of reaction, etc.) have been enumerated by previous reviewers (1,2, 22). Crystallization of admixtures of several zeolite phases is common. Reactions involved in zeolite crystallization include polymerization-depolymerization, solution-precipitation, nucleation-crystallization, and complex phenomena encountered in aqueous colloidal dispersions. The large number of known and hypo-... 

Progress in zeolite crystallization in the last several years has been mostly in the experimental realm and has resulted in several new synthetic zeolites and compositional variants of previously known structures. Theoretical advances have come more slowly but are significant. Until the elucidation of zeolite structures in the last decade or so, structural interpretation of crystallization phenomena was not possible. As additional physical and chemical techniques are applied to the complex structural chemistry of zeolite crystallization, our understanding of the mechanism increases, and the extent of empiricism in synthesis decreases. Areas of investigation still unexplored that should add considerable understanding... 

It has proved to be possible to measure liquid-liquid phase-separation boundaries and crystallization phenomena in high viscous molten polymer mixtures. 

Hartley, F. D., F. W. Lord and L. B. Morgan Crystallization phenomena in polymers. III. Effect of melt conditions and the temperature of crystallization on the course of the crystallization in polyethylene terephthalate. Phil. Trans. 247 A, 23-34 (1954). 

To understand why solution experiments sometimes fail to produce cocrystal products, and why solvent-drop grinding experiments can work when performed on the same system, the 1 1 cocrystal formed by nicotinamide and frans-cinnamic acid (frans-(2E)-3-phenylacrylic acid) has been studied [56]. In this work ternary isothermal phase diagrams of the cocrystal system was used to understand the crystallization phenomena, and to deduce methodologies and for the experimental design of cocrystal preparation. Cocrystals are most likely to form from solutions in which the two reactants have similar degrees of solubility, and the success of solvent-drop grinding was explained in that crystallization took place in the region of low solvent mole fractions where the cocrystal would be more stable relative to the separated reactants. 

Supramolecular chemistry on crystalline surfaces is governed to a large extent by lateral interactions. Nevertheless, the substrate plays an important role in mediating these interactions. Intermolecular recognition on a surface, for example, can take place only if the adsorbate-substrate interaction allows the molecules to meet each other. Therefore, the choice of the substrate plays a decisive role. Too strong interactions immobilize molecules and do not leave room for 2D supramolecular chemistry or may even induce decomposition too weak adsorbate-substrate interactions cause high mobility and 2D crystallization phenomena will not occur. 

A better picture of the crystallization phenomena was obtained from the microscope examination of the crystals at different stages of their development. The first crystals of zeolite omega appeared as spheres with diameters around 0.5-1 pm (Fig.2). With the appearance of (001) faces, this form evolved into barrel-shaped particles (Fig.3). At a later stage of the syntheses, the habit evolved to euhedral hexagonal with the development of (100) faces (Fig.4). 

Alfrey, T., and H. Mark A statistical treatment of crystallization phenomena in high polymers. J. Phys. Chem. 46, 112—118 (1942). 

Crystallization phenomena in high polymers. Pure and appl. Chemistry 5, 743 -758 (1962). 

Apart from the purely thermodynamic analysis, the description of the -> electro crystallization phenomena requires special consideration of the kinetics of nucleus formation [i-v]. Accounting for the discrete character of the clusters size alteration at small dimensions the atomistic nucleation theory shows that the super saturation dependence of the stationary nucleation rate /0 is a broken straight line (Figure 2) representing the intervals of Ap within which different clusters play the role of critical nuclei. Thus, [Ap, Apn is the supersaturation interval within which the nc -atomic cluster is the critical nucleus formed with a maximal thermodynamic work AG (nc). 

Crystallization and processing of palm oil with satisfactory results in a scraped-surface heat exchanger line for margarine and shortening requires some attention due to the slow crystallization phenomena observed in palm oil. 

To investigate the crystallization phenomena induced by the external field, a cyclic force is imposed on a small portion of molecules in the amorphous structure, in addition to tiie inherent intermolecular forces. As a practical matter, this force may be applied to susceptor molecules (molecules witii dangling or floating bonds, or other defects) tiirough direct electric fields, electric fields induced by the external magnetic field, or other excitation sources. In tiiis study, it is assumed that some molecules can act as susceptors of the external field, for tiie sole purpose of inducing artificial molecular movements selectively and of observing tiie subsequent crystallization behavior. As discussed previously, the results of this study could provide a clue to mechanisms involved in the athermal crystallization of a-Si to poly-Si, which is induced by the field-enhanced molecular movements. 

Macromolecules are intricate physical-chemical systems whose properties vary as a function of environmental influences such as temperature, pH, ionic strength, contaminants, and solvent composition, to name only a few. They are structurally dynamic, often microhetero-geneous, aggregating systems, and they change conformation in the presence of ligands. Superimposed on this is the limited nature of our current understanding of macromolecular crystallization phenomena and the forces that promote and maintain protein and nucleic acid crystals. 

Figure 2-10 A hypothetical expanded map between supersaturation and various crystallization phenomena. When the system is highly supersaturated, it may form oil droplets, an amorphous solid, or nuclei from spontaneous nucleation. When the system is less supersaturated, it may form nuclei from secondary nucleation or just growth on the existing ciystal surface.

This survey on the literature indicates that only few data are available on the droplet crystallization phenomena in incompatible polymer blends. Moreover, these observations are partly not completely explained, and, where explained, these explanations are partly not satisfying or contradictory. In the next chapter, therefore, experimental results for some selected systems as investigated by the authors are presented with the aim to show all faces and properties of fractionated crystallization in detail, and to contribute to a better understanding of the origin of the effect. 

C in all cases, the commonly used frozen storage conditions (approximately — 18°C) would be located above the Tg temperatures thus systems stored at this temperature would be at the rubbery state and would have more molecular mobility. In this situation, deteriorative phenomena depending on diffusion could occur. As amylopectin and amylose retrogradation are crystallization phenomena occurring in starch systems, they were studied in the present work as examples of deteriorative changes. As observed in Table 18.1, values foimd for the onset temperature (T p varied between... 

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