Nucleation Definition
Nucleation is the process where droplets of liquid can condense from a vapor, or bubbles of gas can form in a boiling liquid. Nucleation can also occur in crystalsolution to grow new crystals. It is seen in gases when tiny bubbles coalesce into larger ones. In general, nucleation is a self-organizing process that leads to a new thermodynamic phase or a self-assembled structure.
Nucleation is affected by the level of impurities in a system, which can provide surfaces to support assembly. In heterogeneous nucleation, organization begins at nucleation points on surfaces. In homogeneous nucleation, organization occurs away from a surface. For example, sugar crystals growing on a string is an example of heterogeneous nucleation. Another example is the crystallization of a snowflake around a dust particle. An example of homogeneous nucleation is growth of crystals in a solution rather than a container wall.
Examples of Nucleation
- Dust and pollutants provide nucleation sites for water vapor in the atmosphere to form clouds.
- Seed crystals provide nucleation sites for crystal growing.
- In the Diet Coke and Mentos eruption, the Mentos candies offer nucleation sites for the formation of carbon dioxide bubbles.
- If you place your finger in a glass of soda, carbon dioxide bubbles will nucleate around it.
Sources
- Pruppacher, H. R.; Klett J. D. (1997). Microphysics of Clouds and Precipitation.
- Sear, R.P. (2007). 'Nucleation: theory and applications to protein solutions and colloidal suspensions' (PDF). Journal of Physics: Condensed Matter. 19 (3): 033101. doi:10.1088/0953-8984/19/3/033101
Emulsion Polymerization
D. Distler, in Encyclopedia of Materials: Science and Technology, 2001
3 Model of Batch Emulsion Polymerization
As a result of the early work of Harkins in the late 1940s, emulsion polymerization as batch process is usually divided into three intervals:
Interval I: the particle formation or nucleation period. In this period micelles of the emulsifier, monomer droplets and polymer particles exist.
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Interval II: the particle growth period. Excess surfactant is adsorbed onto the latex particles, micelles disappear, the monomer droplets shrink because the monomer diffuses to the growing polymer particles. The rate of polymerization Rp is approximately constant:
where kInterval III: there are no monomer droplets. The remaining monomer is in the particles. Radical numbers per particle can increase because of high viscosity within the particles towards the end of the polymerization.
As described in the laboratory recipe, emulsion polymerization today is mainly performed as a semi-continuous process. To control particle size more precisely, a seed (i.e., preformed polymer dispersion) is used. As a result, new particle formation in micelles is avoided. Care has to be taken also, that secondary particle nucleation by coagulation or phase separation of oligomers to form new particles is avoided. If monomers with higher water solubility are used, partitioning of monomers between emulsion droplets, the water phase, and polymer particles usually has to be taken into account. Additionally, chain transfer, grafting, and cross-linking have to be considered.
The first step in the emulsion polymerization process is the generation of radicals by decomposition of the initiator.
The radicals can (see Fig. 6):
terminate with existing radicals;
start initiation within the water phase;
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enter monomer emulsion droplets and initiate polymerization;
initiate monomers in micelles or polymer particles; or
undergo transfer.