Crystal mechanisms

Table 4.3 Summary of Exponents in the Avrami Equation for Different Crystallization Mechanisms...

Ice formation is both beneficial and detrimental. Benefits, which include the strengthening of food stmctures and the removal of free moisture, are often outweighed by deleterious effects that ice crystal formation may have on plant cell walls in fmits and vegetable products preserved by freezing. Ice crystal formation can result in partial dehydration of the tissue surrounding the ice crystal and the freeze concentration of potential reactants. Ice crystals mechanically dismpt cell stmctures and increase the concentration of cell electrolytes which can result in the chemical denaturation of proteins. Other quaHty losses can also occur (12). 

To avoid misunderstanding, it should be emphasized that if the transition from one type of crystallization to the other one is considered, this does not imply a transformation of crystals of one type into the other one during stretching. In contrast, if the molecule enters a folded-chain crystal, it is virtually impossible to extend it. In this case, we raise the question, which of the two crystallization mechanisms controls the process at each given value of molecular orientation in the melt (this value being kept constant in the crystallization process during subsequent cooling of the system). At /J < /3cr, only folded-chain crystals are formed whereas at / > only fibrillar crystals result at /8 /3cr, crystals of both types can be formed. 

Sugar (sucrose) is obtained from either sugar beets or sugarcane. Sugar beets are traditionally diffused with water to extract the sugar from the pulp. The sugar is then crystallized, mechanically separated, and washed to produce white sugar. 

Tosheva, L. and Valtchev, V.P. (2005) Nanozeolites synthesis, crystallization mechanism, and applications. Chemistry of Materials, 17, 2494-2513. 

We note here that all the information presently available on high molecular weight polymer crystal structures is compatible with the bundle model. While very nearly all crystalline polymer polymorphs involve all-parallel chain arrangements, even the only known exception, namely y-iPP [104,105], where chains oriented at 80° to each other coexist, is characterized by bilayers of parallel chains with opposite orientation. This structure is thus easily compatible with crystallization mechanisms involving deposition of bundles of 5-10 antiparallel stems on the growing crystal surface. Also the preferred growth... 

But there are two important unresolved questions regarding the mechanism of crystallization the first question is are the primary nuclei actually formed from the earliest stage ( induction period ) of crystallization and the second is what is the role of the topological nature of polymers in the polymer crystallization mechanism ... 

On the basis of the concept described above, we propose a model for the homogeneous crystallization mechanism of one component polymers, which is schematically shown in Fig. 31. When the crystallization temperature is in the coexistence region above the binodal temperature Tb, crystal nucleation occurs directly from the melt, which is the well-known mechanism of polymer crystal nucleation. However, the rate of crystallization from the coexistence region is considered to be extremely slow, resulting in single crystals in the melt matrix. Crystallization at a greater rate always involves phase separation the quench below Tb causes phase separations. The most popular case... 

Our knowledge of crystallization mechanisms is still quite rudimentary. However, crystal strudures such as 3-nitro-4-hydroxy-p-nitrostyrene quinoxa-line and morellic acid provide snapshot views of some of the events that may occur during crystallization of more normal compounds. One attempts to get a better appredation of the dynamic processes that occur during crystallization events by studying crystal structures which though static as are all crystal structures, are still suggestive... 

Porous property of these particles was considered to be based on crystallization mechanism of calcium phosphate, in which fine crystals formed in the bottom of fluidized bed attached to the fluidized seeds by driving force of supersaturation and fluid conditions. 

W. Brostow (Ed.). Polymer Liquid Crystals Mechanical and Thermophysical Properties. Chapman Hall, London (1996). 

All of our observations lead us to believe that the crystallization of VPI-5 involves a solid phase reordering process. This solid phase transformation does not involve TBA as either a template or a space-filler. TBA may be present in the solid phase in very low concentrations that are below detection limits by IR and NMR and may still play some role in the crystallization mechanism. Also, it may serve to moderate the pH of the reaction mixture during its formulation and heating such that certain aluminophosphate precursor species are present in the solid phase. Here we will explore these two possibilities in order to further understand the crystallization process for forming VPI-5. 

If sintering occurs by diffusion through the bulk crystal (mechanism BS-XL), all the ratios will be the same as for surface diffusion except for the cross-sectional area Ab/As, which will scale as A2. Therefore,... 

Keywords Liquid crystal Mechanical stimuli Phase transition Photoluminescence Stimuli-responsive liquid crystal Thermal stimuli... 

Keywords wrinkling Thin-film Elastomeric polymer Polydimethylsiloxane Patterns Deformation Surfaces Self-assembly Polyelectrolyte multilayer films Thin-films Polymer brushes Colloidal crystallization Mechanical-properties Assembled monolayers Buckling instability Elastomeric polymer Tobacco-mosaic-virus Soft lithography Arrays... 

In 1848, the French scientist Louis Pasteur prepared the sodium ammonium salt of racemic tartaric acid and allowed it to crystallize in large crystals which are visually distinctive from hemihedral forms.4 By discriminating the asymmetric faces of the crystals, he picked out the two kinds of crystals mechanically with a pair of tweezers and a loupe. Finally he obtained two piles of crystals, one of (+) and one of (-)-sodium ammonium tartrate. This was the first separation of optically active compounds from their racemate. 

Figure 1.1 Scheme for the crystallization mechanism of Si-TPA-MFI. Reproduced from Corma and Davis1281 by permission of Wiley-VCH... 

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