Morphology nucleation

Another type of remarkable double crystalline materials that have been synthesized and characterized by Balsamo et al. are ABC triblock terpolymers composed of polystyrene, polyethylene and poly(ir-caprolactone) (PS-5-PE-6-PCL or SEC) [51-54,57,63]. The morphology, nucleation and crystallization of such copolymers have been recently reviewed [3]. It is interesting to mention that in such terpolymers the PE block induces an antinucleation effect [3,63] on its covalently bonded neighboring PCL block, a remarkable effect that has only been observed in this tjq >e of triblock terpolymer. Diblock copolymers or triblock terpolymers with two crystallizing blocks can display aU possible effects from the nucleation point of view of one crystallizing block on the other. One block can cause nucleation of the other, or cause no effect, or in the other extreme of behavior induce antinucleation. In addition to the... 

Qualitative examples abound. Perfect crystals of sodium carbonate, sulfate, or phosphate may be kept for years without efflorescing, although if scratched, they begin to do so immediately. Too strongly heated or burned lime or plaster of Paris takes up the first traces of water only with difficulty. Reactions of this type tend to be autocat-alytic. The initial rate is slow, due to the absence of the necessary linear interface, but the rate accelerates as more and more product is formed. See Refs. 147-153 for other examples. Ruckenstein [154] has discussed a kinetic model based on nucleation theory. There is certainly evidence that patches of product may be present, as in the oxidation of Mo(lOO) surfaces [155], and that surface defects are important [156]. There may be catalysis thus reaction VII-27 is catalyzed by water vapor [157]. A topotactic reaction is one where the product or products retain the external crystalline shape of the reactant crystal [158]. More often, however, there is a complicated morphology with pitting, cracking, and pore formation, as with calcium carbonate [159]. 

The nucleation, growth, and morphology of crystals are influenced by both temperature and stress. 

Nucleation of particles in a very short time foUowed by growth without supersaturation yields monodispersed coUoidal oxide particles that resist agglomeration (9,10). A large range of coUoidal powders having controUed size and morphologies have been produced using these concepts (3,14). 

Crystal Morphology. Size, shape, color, and impurities are dependent on the conditions of synthesis (14—17). Lower temperatures favor dark colored, less pure crystals higher temperatures promote paler, purer crystals. Low pressures (5 GPa) and temperatures favor the development of cube faces, whereas higher pressures and temperatures produce octahedral faces. Nucleation and growth rates increase rapidly as the process pressure is raised above the diamond—graphite equiUbrium pressure. 

Several features of secondary nucleation make it more important than primary nucleation in industrial crystallizers. First, continuous crystallizers and seeded batch crystallizers have crystals in the magma that can participate in secondary nucleation mechanisms. Second, the requirements for the mechanisms of secondary nucleation to be operative are fulfilled easily in most industrial crystallizers. Finally, low supersaturation can support secondary nucleation but not primary nucleation, and most crystallizers are operated in a low supersaturation regime that improves yield and enhances product purity and crystal morphology. 

X-ray difl raaion (structure grain size preferred orientation stress) Scanning laser microscopy Optical microscopy Oocnl thickness topography nucleation general morphology internal oxidation) l.R. spectroscopy (specialised analysis and applications)... 

The premise that nucleation was always the rate controlling factor in kinetic theories was first disputed by Sadler in 1983 [44]. The disagreement arises from a comparison of the morphologies which would be obtained using the free energies from the Lauritzen-Hoffman theory with those observed experimentally. The... 

There have been questions raised as to the suitability of using nucleation to explain, for example, the morphology or concentration behaviour. However, Toda... 

Microtubule-organizing centers (MTOCs) serve to nucleate, stabilize, and organize the arrangement of microtubules, both in vivo and in vitro (Brinkley, 1985 Joshi, 1994). Two large morphological groups of MTOCs are identified ... 

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