Kinetics, nucleation

The isothermal crystallization of PEO in a PEO-PMMA diblock was monitored by observation of the increase in radius of spherulites or the enthalpy of fusion as a function of time by Richardson etal. (1995). Comparative experiments were also made on blends of the two homopolymers. The block copolymer was observed to have a lower melting point and lower spherulitic growth rate compared to the blend with the same composition. The growth rates extracted from optical microscopy were interpreted in terms of the kinetic nucleation theory of Hoffman and co-workers (Hoffman and Miller 1989 Lauritzen and Hoffman 1960) (Section 5.3.3). The fold surface free energy obtained using this model (ere 2.5-3 kJ mol"1) was close to that obtained for PEO/PPO copolymers by Booth and co-workers (Ashman and Booth 1975 Ashman et al. 1975) using the Flory-Vrij theory. 

Although the kinetic nucleation theory of Hoffman and co-workers was developed for homopolymers, it has been applied to the crystallization of block copoly-... 

Times for molecular dynamic calculations are thus not well suited for calculation of hydrate kinetic nucleation phenomena, which can have metastability lasting hours or days, while the simulation is typically limited to 10-9 s. 

Giddings, J. C., and H. Eyeing Multi-barrier kinetics nucleation. J. Phys. Chem. 62, 305-308 (1958). 

Nucleation and growth kinetics — Nucleation-and-growth is the principal mechanism of phase transformation in electrochemical systems, widely seen in gas evolution, metal deposition, anodic film formation reactions, and polymer film deposition, etc. It is also seen in solid-state phase transformations (e.g., battery materials). It is characterized by the complex coupling of two processes (nucleation and phase growth of the new phase, typically a crystal), and may also involve a third process (diffusion) at high rates of reaction. In the absence of diffusion, the observed electric current due to the nucleation and growth of a large number of independent crystals is [i]... 

Deposition potential — is the required value to observe the appearance of a new phase in the course of a -> electrocrystallization process. See, - equilibrium forms of crystals and droplets, - nucleation and growth kinetics, -> nucleation overpotential. 

NucleBtion Kinetics Nucleation rate is generally measured as the rate of formation of nuclei (numbers formed per unit volume per unit time). Sometimes the induction time, or the time necessary for the onset of nucleation once the subcooled state has been attained, is used for calculation of nucleation rate because the actual rate is often very difficult to measure. Induction time for nucleation will be reviewed later in this section. 

The concentration threshold above which crystallization is observed at times shorter than the processing time or desired product shelf-life or GI transit time, is determined by the kinetic stability of supersaturated states and is regulated by the nucleation mechanisms and kinetics. Nucleation phenomena are equally important in the control of micrometric properties and in the selective crystallization of a particular polymorph. 

Based on the work of Avramit and Frenkel, Tomellini et al. developed a kinetic nucleation model to describe the time evolution of the... 

Figure 1. Schematic diagram of the kinetic nucleation model developed in Ref. 217. (Reproduced with permission.)...

Girshick, S. L., and Chiu, C.-P. (1990) Kinetic nucleation theory A new expression for the rate of homogeneous nucleation from an ideal supersaturated vapor, J. Chem. Phys. 93, 1273-1277. 

In conclusion, although the most active catalyst would be one that contains both PdO and Pd sites, this situation can only be achieved close to the PdO decomposition temperature, in a metastable condition, which is limited by kinetics (nucleation rate). At both extremes the combustion activity is low. Without PdO species present, metallic Pd is not active. Similarly, completely oxidized PdO would not be active unless it contained oxygen vacancies. 

Critical Phenomena in Dusty Plasma Kinetics Nucleation of Nanoparticles, Winchester Mechanism, and Growth of First Generation of Negative Ion Clusters... 

Hanzu, L, Djenizian, T, Ortiz, G. F, and Knauth, P. (2009). Mechanistic Study of Sn Electrodeposition on TiOj nanotube layers thermodynamics, kinetics, nucleation, and growth modes. J. Phys. Chem. C, in press. 

When a homogeneous aqueous phase becomes supersaturated with respect to a solid phase, the rate of appearance of the new solid phase is controlled by nucleation kinetics. Nucleation occurs whenever enough of the solute species come together to create a particle that can grow spontaneously. Classical models of nucleation kinetics are based on a combination... 

Major topics include rate equations, reactor theory, transition state theory, surface reactivity, advective and diffusive transport, aggregation kinetics, nucleation kinetics, and solid-solid transformation rates. The theoretical basis and mathematical derivation of each model is presented in detail and illustrated with worked examples from real-world applications to geochemical problems. The book is also supported by online resources self-study problems put students new learning into practice and spreadsheets provide the full data used in figures and examples, enabling students to manipulate the data for themselves. 

Interestingly, within the cell, the polymer remains in a non-crystalline, amorphous state. On disruption of the cell, for example when the polymer is extracted, rapid crystallization occurs and high levels of crystallinity are developed. Some work [4] has suggested that this may be explained by a kinetic nucleation mechanism. This implies that the polymer granules within the cells are very small and that the probability of a nucleation event triggering crystallization is very low. It is only when the cell is disrupted and the granules are allowed to coalesce that rapid heterogeneous nucleation becomes possible. 

---