Crystal nucleation, pure melt

The experimental conditions and results of the analysis of the purity of separated benzene crystals are shown in Table 1. In tests of No. 1-1 to 1-4, and 3-1 and 3-2, the melt was compressed to the pressure shown in Table 1 and kept on the same value, without seed crystals. Nucleation occured on the wall and crystals grew there. In tests of No.2-1, 2-2 and 3-3, seed crystals were made as described above they grew both inside the optical cell and on the wall. In these tests, since the melt around benzene crystals was replaced by the water, the crystals were taken out without serious destruction. The shapes of benzene crystals were dendritic, and purity of it was over 99.9 mole percent, independent from the operational conditions and the feed compositions as shown in Table 1. Therefore, crystals obtained by high pressxire crystallization is considered to be very pure due to the complete removement of mother liquid from crystal surface. 

C), it has been observed that its crystallization from the melt is enhanced [103-106], Melt crystallized polymers nucleated with n-s polymer-CD-ICs crystallize more rapidly, evidence greater levels of crystallinity, higher melt crystallization temperatures, and semicrystalline morphologies characterized by crystals which are smaller and more uniformly distributed than in un-nucleated pure bulk samples. 

The effectiveness/efficiency of nucleation with (n-s) polymer-CD-ICs was observed to be at least comparable or superior to that produced by more traditional nucleation agents, such as talc, and for that matter pure CDs. The alteration of crystalline morphology achievable by using n-s polymer-CD-ICs as nucleants in melt-crystallization is demonstrated in Fig. 19. It is very clear that the morphology of PCL melt-crystallized in the presence of a small amount of (n-s) a-PCL-C-IC is very distinct from the pure melt-crystallized PCL. Crystal sizes are drastically reduced and more homogeneously distributed for the nucleated PCL. 

Solidification. When pure water is cooled to below 0°C and heat is continuously removed, all of the water will crystallize (assuming for the moment that ice nucleation occurs readily). This is a case of crystallization from the melt, often called solidification. Crystallization is a cooperative transition and it completely occurs at the equilibrium temperature, i.e., 0°C. That means that we cannot speak of a supersaturation. Proceeding from Eq. (14.3), we derive the driving force for crystallization, i.e., the difference in chemical potential between ice and water, as... 

The formal basis for analyzing the kinetics of crystallization from the pure melt has been developed substantially. With appropriate modifications, crystallization of polymers has been shown to follow the general mathematical theory that was developed many years ago for the crystallization of metals and other low molecular weight substances. The most elementary form, developed by von Goler and Sachs [132] postulated a process of nucleation and growth. However, in the original formulation there was no termination step, or demarcation for the end of the transformation. To remedy this problem, it was proposed independently by several different investigators that, when two crystallites collided, or made contact. 

In comparison to other conventionally produced semicrystalline thermoplastics, PHB and the HB-HV copolymers have remarkably low nucleation densities in the absence of self-seeding or deliberately introduced nucleants (Table 5.6). This low nucleation density, which in practice means that it is possible to grow spherulites several millimetres in diameter on crystallization from the melt, is attributed to the purity of the fermentation-produced polymer, and in particular to the absence of inorganic catalysts residues. This absence of heterogeneous nucleation has led to a great deal of academic interest in the PHBV range of polymers as systems for the study of homogeneous nucleation kinetics, but the poor nucleation of the pure polymer is also of commercial... 

To illustrate the general Avrami equation, a particularly simple case is selected athermal nucleation followed by a spherical free growth in three dimensions. All nuclei are formed and start to grow at time t = 0. The spherical crystals grow at a constant rate f. It is an established fact that crystallization from a relatively pure melt occurs at a constant linear growth rate. All nuclei within the radius rt from... 

A comprehensive discussion of the many different aspects of the crystallization kinetics of homopolymers from the pure melt has been presented in this chapter. A comprehension of crystalhzation kinetics is central to understanding structure and properties in the crystalline states. A great deal of the observed phenomena can be explained by modifying conventional nucleation and growth processes, characteristic of low molecular weight substances, to the behavior of long chain molecules. Although a well-developed framework has been established, within which to view the crystallization kinetics of polymers, it is quite evident that there are still major problems that remain to be resolved. 

Cimmino et al. [25] reported that the radial growth rates of crystallization G, measured in sPS/PPE blends, decrease strongly with increase in PPE content (Figure 20.3). This effect might arise from an increase in the transport free energy of crystalline segments in the melt, due the larger Tg of the blend compared with pure sPS, or to a decreased capability of sPS to nucleate, induced by its dilution in PPE. 

Wu and Woo [26] compared the isothermal kinetics of sPS/aPS or sPS/PPE melt crystallized blends (T x = 320°C, tmax = 5 min, Tcj = 238-252°C) with those of neat sPS. Crystallization enthalpies, measured by DSC and fitted to the Avrami equation, provided the kinetic rate constant k and the exponent n. The n value found in pure sPS (2.8) points to a homogeneous nucleation and a three-dimensional pattern of the spherulite growth. In sPS/aPS (75 25 wt%) n is similar (2.7), but it decreases with increase in sPS content, whereas in sPS/PPE n is much lower (2.2) and independent of composition. As the shape of spherul-ites does not change with composition, the decrease in n suggests that the addition of aPS or PPE to sPS makes the nucleation mechanism of the latter more heterogeneous. 

The crystallization rate constant k) is a combination of nucleation and growth rate constants, and is a strong function of temperature (47). The numerical value of k is directly related to the half time of crystallization, ti/2, and therefore, the overall rate of crystallization (50). For example, Herrera et al. (21) analyzed crystallization of milkfat, pure TAG fraction of milkfat, and blends of high- and low-melting milk-fat fractions at temperatures from 10°C to 30°C using the Avrami equation. The n values were found to fall between 2.8 and 3. 0 regardless of the temperature and type of fat used. For temperatures above 25°C, a finite induction time for crystallization was observed, whereas for temperatures below 25°C, no induction time was... 

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