Nucleation, crystallization behaviors

Since excellent reviews on block copolymer crystallization have been published recently [43,44], we have concentrated in this paper on aspects that have not been previously considered in these references. In particular, previous reviews have focused mostly on AB diblock copolymers with one crystal-lizable block, and particular emphasis has been placed in the phase behavior, crystal structure, morphology and chain orientation within MD structures. In this review, we will concentrate on aspects such as thermal properties and their relationship to the block copolymer morphology. Furthermore, the nucleation, crystallization and morphology of more complex materials like double-crystalline AB diblock copolymers and ABC triblock copolymers with one or two crystallizable blocks will be considered in detail. 

Several block copolymer systems have shown only domains I and III upon self-nucleation. This behavior is observed in confined crystallizable blocks as PEO in purified E24EP57EO1969 [29]. Crystallization takes place for the PEO block at - 27 °C after some weak nucleating effect of the interphase. Domain II is absent and self-nucleation clearly starts at Ts = 56 °C when annealed crystals are already present, i.e., in domain III (Fig. 17b). The absence of domain II is a direct consequence of the extremely high... 

Polymorphs and solvated crystals is generally observed in pharmacentical indnstry [1], The bioavailability, stability, solnbility, and morphology of the pharmacentical products are very influenced by polymorphs [2-7], therefore the control of the polymorphic crystallization is very important. The crystallization process of polymorphs and solvated crystals is composed of competitive nucleation, growth, and transformation from a meta-stable form to a stable form [4], Furthermore, the crystallization behavior is influenced by various controlling factors such as temperature, supersaturation, additives and solvents [8], In order to perform the selective crystallization of the polymorphs, the mechanism of each elementary step in the crystallization process and the key controlling factor needs to be elucidated [8], On the other hand, we reported for L-Glutamic acid and L-Histidine system previously [4] that the nucleation and transformation behaviors of polymorphs depend on the molecular stractures. If the relationship between molecular stmcture and polymorphic crystallization behavior is known, the prediction of the polymorphism may become to be possible for the related compound. However, detail in such relationship is not clearly understood. 

Batch crystallizers are often used in situations in which production quantities are small or special handling of the chemicals is required. In the manufacture of speciality chemicals, for example, it is economically beneficial to perform the crystallization stage in some optimal manner. In order to design an optimal control strategy to maximize crystallizer performance, a dynamic model that can accurately simulate crystallizer behavior is required. Unfortunately, the precise details of crystallization growth and nucleation rates are unknown. This lack of fundamental knowledge suggests that a reliable method of model identification is needed. 

In the optical resolution of DL-threonine mixtures by batch preferential crystallization, changes of solution concentration and crystal purity were measured. The mechanism of nucleation of the un-seeded enantiomer was discussed to explain the purity decrease of the resolved crystals. From the observation of crystallization behavior of the seed crystals of L-threonine, it was concluded that the existence of the D-enantiomer on the surface of the seed caused the sudden nucleation when they grew to attain sufficient amounts. 

The crystallization behavior of milk fat (which contains minor lipids) and a pure triacylglycerol fraction of milk fat were compared by Herrera et al. (1999). The results suggested that minor lipids delay nucleation but promote crystal growth. Other workers who examined the effects of added phospholipids on palm oil, suggested that some phospholipids delayed nucleation while others had more significant effects on the rate of growth of fat crystals (Smith, 2000),... 

Overall Crystallization Behavior. If instead of plotting the crystallization half times vs. blend composition we plot the crystallization rates for each component in the blends vs. temperature of crystallization, we obtain a series of curves which show a maximum. The right side of these curves is nucleation controlled while the left side is diffusion controlled (Figures 18 and 19). 

The influence of methanol proportions in solvents, and temperature, on the solubility and the transformation behavior of 2-(3-cyano-4-isobutyloxyphenyl) -methylthiazole-5-carboxylic acid (BPT) was investigated. The transformation behavior was explained by the chemical potential difference between the stable and metastable forms. It was shown that a specific solute-solvent interaction contributes to the preferential nucleation and growth of the stable or metastable forms and influences the transformation behaviors, and the solubility of the solvated crystals is much more influenced by the solvent compositions than the true polymorphs. The solubility ratio of the solvated crystals depends on the solvent composition, whereas the solubility ratio of the true polymorphs is considered to be independent of the solvents. The crystallization behavior was also investigated. The transformation rate after crystallization appeared to depend on the initial concentration of BPT and the addition rate of the antisolvent. The cause of this phenomenon was presumed to be a slight inclusion of the stable form in the metastable form <2005PAC581>. 

It is assumed that the crystallization behavior of milkfat is different between emulsion and bulk, and the lack of nucleation centers in the emulsion droplets may delay the nucleation, making less stable a form nucleated in the first. The occurrence of multiple forms of double-chain-length and triple-chain-length structures may be caused by segregated crystallization of multicomponent TAGs exhibiting complicated mixing behavior, but its details are open to fumre study. 

Nucleation outcomes from solutions with initially the same composition may vary as a consequence of impurities, rates at which supersaturation was created, thermal histories, experimental techniques employed to detect precipitation, and solution volumes in which nucleation occurred. This is illustrated by comparing results of the selective crystallization of buffer components during freezing from various labora-tories.f The initial salt concentrations and the crystallization behavior of disodium phosphate during the freezing of sodium phosphate buffer solutions are shown in Table 1. Murase et report... 

When water is replaced by nucleation gel to prepare synthesis mixture, the different crystallization behavior may be attributed to either the chemical composition factor or the nature factor of nucleation gel, or both. As discussed earlier, the change in chemical composition of synthesis mixture due to use of nucleation gel is not significant. So, the chemical composition factor is not expected to be significant. 

Fat crystallization has been extensively studied in bulk fats and, to a lesser extent, in emulsified fats. It has been shown that the crystallization behavior of a fat will proceed quite differently, depending on whether it is in bulk or emulsified form (4,5). Authors have examined the effect of the state of dispersion on the crystallization mechanisms (nucleation, crystallization rate) and polymorphic behavior (6-11) of partial- and triglycerides in bulk and emulsified form. Understanding the mechanisms of emulsion nucleation and crystallization is one of the first steps in understanding the destabilization of emulsions and partial coalescence, e.g., stabilization of liquid fat emulsions by solid particles (fat) or control of the polymorphic form of crystals during the process of partial coalescence to control the size of aggregates and textural properties. 

The reader will note many uses of qualitative terms to predict the behavior of these complex systems. As in the entire field of crystallization, these wide brackets around possibilities (e.g., will it crystallize, will it form an oil first, will it stay amorphous, will it grow, will it nucleate, what is good mixing, what is low supersaturation, etc. ) are necessary because of the extreme species and conditions dependency of the crystallization of organic molecules. The guidelines offered are intended as such and, in addition, to provide a framework for experimentation to determine where a particular system may fit in the wide scope of crystallization behavior possibilities. 

Crystallization rate, nucleation, size of crystalline units, crystalline structure, crystal modification, transcrystallinity, and crystal orientation are the most relevant characteristics of crystallization behavior in the presence of fillers. Here the discussion is focused on crystallization rate. The other topics are discussed in the following sub-chapters. 

It should also be noted that the crystallization behavior of melt-spun ribbons may be different on both ribbon sides [4.18]. Nucleation for primary crystallization of the transition metals is observed on both sides of the glassy ribbons, while other crystallization reactions have been observed to prefer usually either the free surface or the contact side of the ribbon [4.77]. This phenomenon may lead to different structural and chemical properties of the two ribbon sides, and consequently also to large anisotropy in the catalysts prepared from such ribbons [4.18, 19]. 

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