Isothermal crystallization dynamics

The non-isothermal crystallization dynamics were performed using DSC, employing cooling rates of 2.5, 5, 10, 20, 25, 30, 35 and 40°C/min. The isothermal crystallization dynamics were studied for each sample heated to 290 °C, with a 5 min hold time, and cooled to the isothermal crystallization temperature using a cooling rate of 200°C/min, and then holding for 40 min to obtain the crystallization exotherm. 

The Ozawa equation of isothermal crystallization dynamics applied to non-isothermal crystallization assumes that the crystallization proceeds under a constant cooling rate, from the valid mathematical derivation of Evans [47], In... 

Figure 7. Dynamic shear response (expressed as G, G , J" and tanS) of isotactic polypropyienes of varying crystallinity (torsion pendulum, 1Hz, ref 6). O 65%, O 56%, A 50%, achieved by annealing of quenched specimen ( 45%) + 68%, isothermally crystallized. Curves are calculated from parameterized equation (8).

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...

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. 

The isothermal crystallization kinetics of the dynamically cured blends was analyzed using Avrami s equation as given below. 

Sherwin, M.B., Shinnar, R. and Katz, S. (1967) Dynamic behaviour of the well-mixed isothermal crystallizer. AIChEJ, 13, 1141-1154. 

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