Isothermal Crystallization Process

Also, after determining the temporal distribution of crystallization peaks in a complete isothermal experiment, the melting thermogram of a particular polymorph can be determined. First, the sample is remelted completely, and held to erase its thermal memory. Then, the isothermal crystallization process is repeated, but only up to the time at which the polymorph of interest has completely solidified (exothermic peak fully formed). A heating scan is then performed immediately (Kawamura, 1980, 1981). 

The MD simulations on isothermal crystallization processes of a-Si are introduced. To obtain a realistic a-Si structure, the /-Si prepared at 3500 K is rapidly quenched to 500 K at a cooling rate of 10 K/s. During the cooling process the structural change is observed by the Voronoi polyhedron analysis. It is shown that the unit cell of the amorphous structure becomes similar to that of crystalline structure with decrease of temperature, although its phase is still amorphous. 

Figure 5.10 Time dependence of the integrated intensity estimated for the infrared bands characteristic of the partially disordered trans-form (1368cm ) and the regular trans-zigzag form (728 cm ) in the isothermal crystallization process from the melt. The time 0 s was defined as the time when the temperature reached just the predetermined crystallization point (Tc). (From Reference 44 with permission from the Society of Polymer Science, Japan.)...

Figure 5.11 Time-dependent wide-angle X-ray scattering pattern taken for LLDPE(2) in the isothermal crystallization process.

Figure 5.12 Time dependence of small-angle X-ray scattering patterns measured for (a) LLDPE(2) and (b) DHDPE in the isothermal crystallization process from the melt. It should be noticed that the intensity exchange (at (b)) can be observed between the firstly appeared peak and the secondly appeared peak in the crystallization process.

Figure 5.13 Comparison of vibrational spectroscopic data with SAXS data collected for (a) LLDPE (2) and (b) DHDPE samples in the isothermal crystallization process from the melt. Q. invariant, d) lamellar thickness, Li and I(Li) the long period and the corresponding SAXS peak intensity of lamellar stacking structure LI for 800 A period and L2 for 400 A period.

Figure 5.17 Comparison of crystallization rate between the pure components and their blend sample in DHDPE/LLDPE(2) system in the isothermal crystallization process from the melt. (Erom Reference 38 with permission from the American Chemical Society.)...

The type of dependences on the composition, on the crystallization temperature, and on the chemical nature and molecular mass of the components observed in kinetic and thermodynamic properties, relative to the isothermal crystallization process, the final overall morphology, and the thermal behaviour, are all to be related to the physical state of the melt, which at is in equilibrium with the developing solid phase. 

Fig. 11 Degree of crystallization a as a function of crystallization time during an isothermal crystallization process. Shrinkage V = const, (memory effect), not to confuse with volume contraction where V / const...

Fig. 23.10. iPS, quenched to the glassy state and then crystallized at various temperatures (Left) Variation of during a subsequent heating, obtained by SAXS experiments. Crystallization line and melting line. (Right) DSC thermograms of samples, measured after isothermal crystallization processes with a heating rate of 0.5 K min [9.20]... 

Figure 7.5 shows different stages of attachment of the crystallizing chain to the nucleating surface. The attractive van der Waals interactions controlled the adsorption and preorientation of PE on SWNT, and the single PE chains with different chain lengths were aligned parallel to the SWNT axis under isothermal crystallization process. ... 

The kinetics of the isothermal crystallization of polymers have been thoroughly studied. However, practical processes such as extrusion, molding, injection and film production usually proceed under non-isothermal crystallization conditions. In order to obtain products with better properties, it is necessary to have quantitative evaluations of the non-isothermal crystallization process. A few methods have been developed to study the kinetics of non-isothermal crystallization by DSC for polymers. 

Figure 15 Dependence of the dielectric loss on the logarithm of frequency during isothermal crystallization process for the elapsed times of 0,60,100, and 420 min. The open circles represent observed values, and the solid line overlapping with the open circles is given by a fit to the Havriliak-Megami equation. The dotted line, dotted-broken line, and broken line are the contributions from the a-process, oo-process, and p-process, respectively. The contribution on the lower frequency side is that from the DC conductivity due to impurities. This is represented by the solid line. With permission from Fukao, K. Miyamoto, Y. Phys. Rev. Lett. 1997, 79, 4613. ...

Matusita, K., Komatsu, T. Yokota, R. (1984). Kinetics of non-isothermal crystallization process and activation energy for crystal growth in amorphous materials. Journal of Materials Science, 19, pp. 291-296,0022-2461... 

The crystallite formation during isothermal crystallization was studied in time-dependent SAXS experiments. Subsequent to the isothermal crystallization the melting was monitored in temperature dependent SAXS measurements up to the melting point. Fig. 1 displays as an example the curves K"(z) for s-P(P-co-0)4 and s-P(P-co-0)15 during and subsequent to isothermal crystallization processes. 

DRS is not only able to determine the degree of miscibility of polymer blends but also provides an accurate technique to study the crystallization kinetics of amorphous/crystalltne polymer blends. The crystallization kinetics are identified by the significant changes in molecular dynamics of the amorphous phase during the isothermal crystallization process [10-18]. It is assumed that the degree of... 

In order to describe the non-isothermal crystallization process more effectively for comparison, Liu et al. [75] suggested a convenient procedure for characterizing non-isothermal crystallization kinetics by combining the Avrami and Ozawa equations based on the assumption that the degree of crystallinity was correlated to the cooling rate and crystallization time. Therefore, their relationship for non-isothermal crystallization can be derived by combining the equations (3) and (4) as follows ... 

Figure 1.21 Time-resolved SANS study on isothermal crystallization process of C12D25C192H384DC11D23 at (a)121, (b)115, (c)107, and (d)102°C [117,118].

Figure 5.14 Time dependence of the various structural parameters estimated in the isothermal crystallization process of isotactic polypropylene. The FTIR data show the growth of regular helical segments. The Rg, and L are the radius of gyroid of the higher-density domains, the correlation distance between the neighboring domains, and the long period of the stacked lamellae, respectively, revealed by the SAXS data analysis. The Q is the invariant and is approximately proportional to the degree of crystallinity Xc, which was evaluated also using the WAXD data [67].

Generation of Disordered Phase in Isothermal Crystallization of Polyethylene We perform a similar experiment for PE. Figure 5.17 shows the time-resolved measurement of infrared spectra in the isothermal crystallization process from the melt, where we use a linear low-density PE with 17 ethyl branches per... 

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].

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