Bulk Crystallization Kinetics

Isofliermal.— With bulk isothermal crystallization, the conversion-time inter- 

Invariably the fractional n values are close to 2 and this has been attributed to the growth of rods whose numbers are increasing with time and inherent in the structure of spherulites. Calvert et al. using a u.v. optical microscope, have obtained some evidence from the variation in amorphous content within spherulites of the complexity of the micro-structure of spherulites. The central regions of the spherulites contained less amorphous-regions than the outer, and 

Obviously spherulites cannot be assumed to be homogeneous spherical particles, and the fractional n values observed in their crystallization kinetics is a result of their complex structure. With such crystallizations, however, the rate parameter, Zj, must at the moment be of little mechanistic significance as are parameters such as induction times and half-lives derived from it. Determination of surface free energies from their temperature dependence must also be suspect until the exact significance of the Zi rate parameter be known. 

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In addition to induction time measurements, several other methods have been proposed for determination of bulk crystallization kinetics since they are often considered appropriate for design purposes, either growth and nucleation separately or simultaneously, from both batch and continuous crystallization. Additionally, Mullin (2001) also describes methods for single crystal growth rate determination. 

Isothermal Bulk Crystallization Kinetics of Isotactic Polypropylene Component... 

Jaffe melted and isothermally recrystallized fibers spun under different stress levels. The bulk crystallization kinetics was measured with both DSC and optical microscopy techniques. Table 3.30 lists the samples studied. The film samples were included to ensure the absence of spurious DSC effects due to fiber packing. The samples were heated from 50°C to the melt temperature at 80°C/min. The melt temperatures ranged from 170 to 230°C. The samples were held in the melt for a specified time and then cooled to the 130°C crystallization temperature at 40°C/min. 

Figure 10.17 Primary nucleation density of iPP crystals in iPP/LDPE blends isothennaUy crystallized from melt, as a function of LDPE concentration. Values calculated from bulk crystallization kinetics, according to Equation (10.15), per volume of blend (squares) and per volume fraction of iPP in the blend (circles). Reprinted from Galeski et al. [77], Copyright 1984, with permission from Elsevier.

The effect of the blocky chain architecture on spherulite growth rate and bulk crystallization kinetics of novel ethylene-octene block copolymers is described. These copolymers form space-filling spherulites even when the crystdlinity is as low as 7 %. Spherulite growth rates were analyzed by Lauritzen-Hoffman theory and the bulk crystallization kinetics were subjected to Avrami analysis. Comparison with random copolymers showed that the blocky architecture imparts a substantially higher crystallization rate. 

The sphemlite growth rate and bulk crystallization kinetics of OBCs was studied. Rejection of the non-crystallizable soft block from the gro face decreases the crystallization kinetics of OBCs with increasing soft block content. The sphemlite growth rate conformed to the LH analysis and it was shown that a more disordered fold surface is obtained with increasing soft block content. Avrami analysis of bulk crystallization kinetics confirms sphemlitic growth and heterogeneous nucleation. It was also shown that statistical sequences crystallize much slower than ethylene blocks in OBCs. 

Roboshot injection molding machine with different mold temperatures, and to measure bulk crystallization kinetics by DSC. 

The classic Avrami Equation (1) is used to study the isothermal bulk crystallization kinetics of PLA. 

Both thermodynamic and kinetic factors need to be considered. Take, for instance, acetic acid. The liquid contains mostly dimer but the crystal contains the catemer and no (polymorphic) dimer crystal has ever been obtained. Various computations (R. S. Payne, R. J. Roberts, R. C. Rowe, R. Docherty, Generation of crystal structures of acetic acid and its halogenated analogs , J. Comput. Chem, 1998, 19,1-20 W. T. M. Mooij, B. P. van Eijck, S. L. Price, P. Verwer, J. Kroon, Crystal structure predictions for acetic acid , J. Comput. Chem., 1998, 19, 459-474) show the relative stability of the dimer. Perhaps the dimer is not formed in the crystal because it is 0-dimensional and as such, not able to propagate so easily to the bulk crystal as say, the 1-dimensional catemer. 

In semi-crystalline polymers the interaction of the matrix and the tiller changes both the structure and the crystallinity of the interphase. The changes induced by the interaction in bulk properties are reflected by increased nucleation or by the formation of a transcrystalline layer on the surface of anisotropic particles [48]. The structure of the interphase, however, differs drastically from that of the matrix polymer [49,50]. Because of the preferred adsorption of large molecules, the dimensions of crystalline units can change, and usually decrease. Preferential adsorption of large molecules has also been proved by GPC measurements after separation of adsorbed and non-attached molecules of the matrix [49,50]. Decreased mobility of the chains affects also the kinetics of crystallization. Kinetic hindrance leads to the development of small, imperfect crystallites, forming a crystalline phase of low heat of fusion [51]. 

The influence of plastic deformation on the reaction kinetics is twofold. 1) Plastic deformation occurs mainly through the formation and motion of dislocations. Since dislocations provide one dimensional paths (pipes) of enhanced mobility, they may alter the transport coefficients of the structure elements, with respect to both magnitude and direction. 2) They may thereby decisively affect the nucleation rate of supersaturated components and thus determine the sites of precipitation. However, there is a further influence which plastic deformations have on the kinetics of reactions. If moving dislocations intersect each other, they release point defects into the bulk crystal. The resulting increase in point defect concentration changes the atomic mobility of the components. Let us remember that supersaturated point defects may be annihilated by the climb of edge dislocations (see Section 3.4). By and large, one expects that plasticity will noticeably affect the reactivity of solids. 

Kinetics of crystallization. Trick (79) has reported a dilatometric study of the bulk crystallization of PTHF. The rates he observed for a polymer of Mw = 130,000 (Polymer A) are shown in Fig. 27. He also found that a lower molecular weight polymer (Polymer B, Mn = 6760) crystallized to a higher degree of crystallinity, whereas the introduction of comonomer units (Polymer C) decreased the degree of crystallinity (Fig. 28). From attempts to fit the Avrami Equation to the experimental data in the early stages of crystallization, a tentative value of n = 3 was... 

The crystallization kinetics of bulk triglycerides and oil-in-water emulsions has been characterized by both NMR imaging and localized spectroscopy. The rate of lipid crystallization in an oil-in-water emulsion was affected by the addition of a second homopolymer (addition of trilaurin to trimyristin in this case). The addition of the second homopolymer of higher chain length was observed to slow the rate of crystallization [26]. 

Epitaxy has been used to stabilize films with crystal structures that are metastable in the bulk phases. Kinetic stabilization is obtained when the growth is performed under conditions of high surface diffusion, but low bulk diffusion. In this way, crystallographi-cally oriented film growth occurs while phase transformations are prohibited. The circumstances under which thermodynamic stabilization can be achieved have also been enumerated. Namely, these are by minimizing ... 

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