Crystallization dynamics

The crystal stmcture of glycerides may be unambiguously determined by x-ray diffraction of powdered samples. However, the dynamic crystallization may also be readily studied by differential scanning calorimetry (dsc). Crystallization, remelting, and recrystallization to a more stable form may be observed when Hquid fat is solidified at a carefully controlled rate ia the iastmment. Enthalpy values and melting poiats for the various crystal forms are shown ia Table 3 (52). 

Dynamic crystal growth is initiated at several distant nucleation points, the individual monocrystalline areas grow in all directions until the front edges of neighboring crystals meet [138] Electrostatic interactions are the dominant binding forces [118,122]... 

Fig. 4 Reported dynamic crystallization temperatures for PEO isolated spheres as a function of their volume (bottom x-axis) and diameter (top x-axis). The inset shows dynamic (symbols with vertical bars) and isothermal crystallization (vertical dashed bars) temperature ranges for PEO spheres, see text...

The interphases between MDs should not be able to cause nucleation of the semicrystalline phase. If the interphase displays nucleation activity, first-order crystallization kinetics may be obtained however, the supercooling at which dynamic crystallization from the melt occurs will not be the maximum that could be possibly attained if homogeneous nucleation were in place. 

M.F. Horstemeyer et al A multiscale analysis of fixed-end simple shear using molecular dynamics, crystal plasticity, and a macroscopic internal state variable theory. Modell. Sim. Mat. Sci. Eng. 11, 265-286 (2003)... 

From a technological point of view, the dynamic crystallization of composites is of a great interest, because most of processing routes take place under these conditions. Generally, the crystallization and melting behaviour of polymer/CNT nanocomposites is analyzed by differential scanning calorimetry (DSC) analysis the transition temperatures are taken as the peak maximum or minimum in the... 

The dynamic crystallization-induced diastereomeric resolution was further investigated in order to develop practical approaches to obtain one diastereomer by exploiting more diversity of the diastereomeric nitroaldol adducts in the dynamic system [77]. A larger and more diverse dynamic nitroaldol system (CDS-5C) was generated by equimolar amounts of nine different benzaldehydes, nitroethane 38, and triethylamine was used as catalyst as shown in Scheme 15. A total of 36 nitroaldol diastereomers were formed under thermodynamic control in chloroform-d, and the reaction was followed by H-NMR spectroscopy. 

It was found that is a function of temperature but the model was found to give a better fit than analytical expressions like the Avrami model or the modified Gompertz model (Kloek, Walstra and Van Vliet 2000). The main advantage of this model is that as it is formulated as a differential equation, it can be used to predict isothermal as well as dynamic crystallization. However, this model does not consider the polymorphism of the material which is a critical point in the crystallization of cocoa butter. Another contribution is the model of Fessas et al. (Fessas, Signorelli and Schiraldi 2005) which considers all the transitions possible between each... 

Walker D., Powell M. A., Lofgren G. E., and Hays J. F. (1978) Dynamic crystallization of a eucrite basalt. Proc. 9th Lunar Planet Sci. Conf. 1369-1391. 

Lofgren G. (1996) A dynamic crystallization model for chondrules melts. In Chondrules and the Protoplanetary Disk (eds. R. H. Hewins, R. H. Jones, and E. R. D. Scott). Cambridge University Press, Cambridge, pp. 187-196. 

Lofgren G. E. and Lanier A. B. (1990) Dynamic crystallization study of barred olivine chondmles. Geochim. Cosmochim. Acta 54, 3537-3551. 

Loisel, C., G. Lecq, G. Keller, and M. Ollivon, Dynamic Crystallization of Dark Chocolate Influence of Temperature and Lipid Additives, J. Food. Sci. 63 T3-19 (1998). 

Polymorphism and kinetics of crystallization of TAG and fat under static conditions (e.g., in differential scanning calorimetry [DSC] apparatus) have been studied for a long time and are summarized in many reviews (2-6). Yet, these conditions are far-removed from industrial applications, where crystallization is usually achieved under shear (dynamic). Shearing has a major effect on crystallization kinetics it induces a faster and more homogeneous crystallization, often in the stable form and with a refined grain size. Yet, its effect is far from being fully understood. Recently, several studies of dynamic crystallization of lipids have been reported (7-12). 

Dynamic crystallization under precise control of shear and temperature was studied in a prototype apparatus specially developed. The whole cell, similar to a Couette viscometer, was made out of glass. The inner cylinder rotated at a controlled speed, CO, while the outer wall was fixed. A double-mantel with a circulation of water allowed precise control of the temperature. Temperature of cocoa butter in the cell was measured with a chromel-alumel thermocouple. Measures were recorded with a data acquisition system. Shear rate imposed to cocoa butter in the system could be estimated from the rotation speed of the inner cylinder, assuming that the fluid is Newtonian and incompressible. There is no normal speed, only tangential speed. The shear rate, y> in the specimen is a single function of the radiu. In the rest of this work, shear in the cell was characterized by its average value, y, calculated by integration over the cell thickness (7). 

Three different isothermal crystallization experiments were performed in this work classical static (i.e., quiescent) crystallization in the DSC apparatus, dynamic crystallization with the apparatus described above, and dynamic-static crystallization. Dynamic isothermal crystallization consisted in completely solidifying cocoa butter under a shear in the Couette apparatus. Comparison of shear effect with results from literature was done using the average shear rate y. This experiment did not allow direct measurement of the solid content in the sample. However, characteristic times of crystallization were estimated. The corresponded visually to the cloud point and to an increase of the cocoa butter temperature 1 t) due to latent heat release. The finish time, was evaluated from the temperature evolution in cocoa butter. At tp the temperature Tit) suddenly increases sharply because of the apparition of a coherent crystalline structure in cocoa butter. This induces a loss of contact with the outer wall and a sharp decrease in the heat extraction. 

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