Isothermal crystallization structural evolution

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. 

Figure 5.15 Model of structural evolution process in the isothermal crystallization of polyethylene chains. The random chain segments change to the locally regularized trans-form. The stacked lamellar structure is formed with the long period 800 A. The structure changes further to the stacked lamellar structure of 400 A period by inserting the new lamella into the original lamellae. It is noted that the radius of gyration is kept unchanged during this process. (From Reference 46 with permission from the Society of Polymer Science, Japan.)...

Figure 7.14 Structural evolution of 10/90 iPP/POE isothermally crystallized at 130°C showing both the sectorized curved single crystal and crosshatched structure of iPP.

Fig. 11.5 Simulation results for the time evolution of structure factors on the isothermal crystallization of homogeneous C2 (a) and C3 (b) solutions with the concentration 0.125 at the temperature. 5EJk (Time periods are labeled in parallel sequences with the curves, in the unit of Monte Carlo cycles). The dot segments represent the initial state (Zha and Hu 2007) (Reprinted with permission)...

If the homogeneous C2 and C3 solutions with the concentration of 0.125 are quenched to a low temperature of l.5EJk for isothermal crystallization, their structure factors as a function of time are shown in Fig. 11.5a, b. According to the introduction in the Sect. 9.2, Fig. 11.5a shows the typical evolution of structure... 

Figure 5.16 Structural evolution in the isothermal crystallization process of isotactic polypropylene revealed by a combination of infrared and WAXD/SAXS data. Parts of random coils in the melt are regularized to short helical segments, which form higher density domains in the melt. These domains approach gradually as shown by a correlation distance and change into stacked lamellar structure [67].

Figure 5.20 Structural evolution in the isothermal crystallization of polyoxymethylene at (a) 130°C and (b) 150°C. The difference between these two processes is seen in the generation of tie chains passing through the neighboring lamellae (ECC) in Figure 5.19 [49,65].

Figure 5.24 Illustration of the structural evolution process in the isothermal crystallization process of polyethylene. The radius of gyration (R ) was not changed remarkably as known from the SANS data analysis [72] (refer to Fig. 5.18).

---