Nucleation effective rates

The greater the undercooling, the more rapidly the polymer crystallizes. This is due to the increased probability of nucleation the more supercooled the liquid becomes. Although the data in Fig. 4.8 are not extensive enough to show it, this trend does not continue without limit. As the crystallization temperature is lowered still further, the rate passes through a maximum and then drops off as Tg is approached. This eventual decrease in rate is due to decreasing chain mobility which offsets the nucleation effect. 

The temperature dependence of the reaction was studied, and the activation energy of the reaction was calculated to be approximately 100 kj mol The exponent n was found to lie in the range 1-2, which is consistent with a 2D diffusion controlled reaction mechanism with deceleratory nucleation. The rate of reaction increases markedly with the amount of water added to the LDH with very small amounts of water added, the deintercalation process does not go to completion. This effect is a result of the LiCl being leached into solution. An equilibrium exists between the LDH and gibbsite/LiCl in solution. The greater [LiCl], the further to the LDH side this lies. 

The explanation is that Cu (or Pt, or Pd) produces spillover hydrogen which considerably accelerates the nucleation of nickel metal in the reduction conditions. At low loading, the high dispersion of NiO makes that nucleation is rate-limiting if NiO is pure copper permits nucleation and, consequently, reduction. The effect is proportionally smaller for high NiO loading, because the NiO crystallites are larger and can nucleate more easily (see Sections 2.2.2.0.B.a and 2.2.2.1, and Ref. 3). 

Addition of starch has a nucleating effect, which increases the rate of crystallisation. The rheology of starch/PCL blends depends on the extent of starch granule destruction and the formation of thermoplastic starch during extrusion. Increasing the heat and shear intensities can reduce the melt viscosity, but enhance the extrudate-swell properties of the polymer. 

In a number of other cases, however, notably in the exothermic decompositions of solids which can become explosive and in the endothermic transformation of hydrates of salts and of carbonates (to oxides), the slow processes seem to be nucleation. The rate laws for such nucleation-con-trolled processes can be very complex, and the rate studies are difficult to make and to reproduce. In many of these cases smj amounts of impurities play an important role in governing the de fed t nff and growth of nuclei, and there are a number of instances of sm bA ounts of water vapor having a significant catalytic effect. ... 

Mouran et al. [105] polymerized miniemulsions of methyl methacrylate with sodium lauryl sulfate as the surfactant and dodecyl mercaptan (DDM) as the costabilizer. The emulsions were of a droplet size range common to miniemulsions and exhibited long-term stability (of greater than three months). Results indicate that DDM retards Ostwald ripening and allows the production of stable miniemulsions. When these emulsions were initiated, particle formation occurred predominantly via monomer droplet nucleation. The rate of polymerization, monomer droplet size, polymer particle size, molecular weight of the polymer, and the effect of initiator concentration on the number of particles all varied systematically in ways that indicated predominant droplet nucleation. 

Fat crystallization has been extensively studied in bulk fats and, to a lesser extent, in emulsified fats. It has been shown that the crystallization behavior of a fat will proceed quite differently, depending on whether it is in bulk or emulsified form (4,5). Authors have examined the effect of the state of dispersion on the crystallization mechanisms (nucleation, crystallization rate) and polymorphic behavior (6-11) of partial- and triglycerides in bulk and emulsified form. Understanding the mechanisms of emulsion nucleation and crystallization is one of the first steps in understanding the destabilization of emulsions and partial coalescence, e.g., stabilization of liquid fat emulsions by solid particles (fat) or control of the polymorphic form of crystals during the process of partial coalescence to control the size of aggregates and textural properties. 

As mentioned earlier, the nucleation rate is actually not a simple power law model of growth rate and slurry density, but should contain terms dependent upon contact nucleation effects caused by the pump or propeller circulator and the motion of crystals striking each other within the crystallizer suspension. This is shown algebraically in Eq. (5.12). 

The melting and crystallization rates of oligomers and polymers were first measured by microscopy in the presence of remaining crystal, to eliminate nucleation effects. Figure 3.94 illustrates data for poly (oxyethylene) as a function of molar mass. On extrapolation to monomer dimensions, the metastabdity gap disappears and one... 

The degree of exfoliation also depends on the melt flow rate of the polymer used. It was found that PP with high melt flow rate of 25 g/10 min results in the formation of well-exfoliated nanocomposites than with polymers possessing low melt flow rate [27]. The presence of PP-g-MA in CPN has the potential to create the heterogeneous nucleation effect, which alters the crystallization temperature [25,29]. 

This is explained by a nucleation effect of the microsilica particles present. After several days, however, the hydration settles down to normal rates (Fidjestol and Lewis,... 

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