Isothermal crystallization measurements

Fig. 5 Schematic plot showing reported crystallization temperatures for PEO in the bulk and as a component of block copolymers of varying compositions. The morphology of the PEO block is indicated on the x-axis. The filled bars are for data where isothermal crystallization measurements were performed and Avrami indexes of 1 or less were reported. The horizontal lines indicate the maximum temperature range that can be associated with PEO homogeneous nucleation, see text...

Fig. 12.4. Degree of crystallinity Dc and average number of crystalline alkyl carbons p>er side chain vs. melting tempjerature Tm from isothermal crystallization measurements. The sjun-bols correspond to that one used for the different isotherms in Fig. I2.3b. The inset shows schematically the directions of crystal growth during primary (1st) and secondary (2nd) crystallization...

Transport Activation Energy of Polymer Segments During Isothermal Crystallization Measured by D and Polarizing Optical Microscopy... 

The quality of the baseline is also important in isothermal measurements. In isothermal crystallization measurements the sample is melted at a temperature beyond the equilibrium melting point, held isothermally at this temperature (Ti), and then quickly cooled to the temperature of isothermal crystallization (T ), and held there until the crystallization process is completed. A sloping isothermal baseline may indicate the absence of thermal equilibrium. This may take place, for example, if one of the cell covers is tilted, resulting in erroneous heat loss. In such a case, the baseline is tilted at both Ti and T. On the other hand, if the baseline is horizontal at T), but slopes at after the crystallization is completed, secondary crystallization may take place (see Section 2.7). 

Verhoyen, 0., Dupret, F. and Legras, R., Isothermal and non-isothermal crystallization kinetics of polyethylene terephthalate mathematical modeling and experimental measurement, Polym. Eng. Sci., 38, 1592-1610 (1998). 

The experimental desorption isotherms for randomly methylated -cyclodextrin (RAMEB) and RAMEB-enriched minerals are presented in Figure 1. The isotherms were measured for the RAMEB in the forms of powder and crystals however, these were the same within the range of experimental error. (From Jozefaciuk et al., 2001)... 

A DSC heating thermogram for the same fat sample can conveniently be measured as soon as isothermal crystallization has gone to completion. 

Isothermal crystallization Injection molded sPS/PPE blends having different composition [19], melted at Tmax = 300 °C for tmax = 5 min and then crystallized isothermally for 60 min at various Tc (from 232 to 244 °C), were investigated by means of DSC and WAXD and compared with pure sPS. DSC measurements do not show a melting peak for sPS < 40 wt%, suggesting the absence of crystallinity. In contrast, at higher contents (80 wt%) three separate melting endotherms (labeled I, II and III) between 260 and 271 °C are clearly found, as in pure sPS (Figure 20.2a). 

Answers to these questions can be obtained if one performs small angle scattering measurements during isothermal crystallization. Such measurements have been performed on different materials. The results were observed to depend strongly on the material used. So, for example, with increasing crystallization time, the long period increased with polyethylene, it decreased with polyethylene terephthalate and it stayed constant with poly-P-hydroxybutyrate. 

Measurements were performed up to now during isothermal crystallization at 117 °C of an initially amorphous film. Fig. 43 shows the scattering power Q = S I(s) ds of the small angle scattering as well as the degree of crystallinity obtained from wide angle scattering as a function of crystallization time. 

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. 

In the work reported by Ng and Oh (2), an Avrami exponent of three was observed for palm oil from solid fat content (SFC) measurements. Using differential scanning calorimetry (DSC) to monitor the isothermal crystallization of palm oil, Kawamura (3) reported an Avrami exponent of four for palm oil. 

Observed good fit of SFC data was reported by Herrera et al. (6) for the fitting of SFC data of hydrogenated sunflower oil using Avrami exponent of three and one. In the work reported by Dibildox-Alvarado and Toro-Vazquez (7) on the isothermal crystallization of tripalmitin in sesame oil using ultraviolet (UV) measurement, the tripalmitin solutions were observed to crystallize with Avrami exponents of three and four in the temperature range of 285 K to 295 K. 

Induction times determined by pNMR, turbidity, and light-scattering measurements are compared to those determined using PLM in conjunction with image analysis. Isothermal DSC was attempted as a fifth method for comparison. However, because of the inherent lack of sensitivity at the high cooling rates required to obtain isothermal crystallization conditions, it was abandoned. This research was carried out in the context of our milk-fat minor components study (2). 

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