Encapsulation inorganic particles

Figure 4.4 General synthetic strategy used for the elaboration of polymer-encapsulated inorganic particles through heterophase polymerization.

Keywords ATRP Capsules Composite materials Controlled radical polymerization Emulsion polymerization Encapsulation Inorganic particles Miniemulsion polymerization NMP RAFT... 

We first describe some general approaches to encapsulate inorganic particles other than clay by emulsion polymerization. Increased interest in this approach... 

Fig. 12.1.5 Polymer encapsulation of inorganic particles by polymerization of adsorbed surface active monomer.

Crosslinked polyacrylamide latexes encapsulating microparticles of silica and alumina have also been prepared by this method [179], Three steps are involved a) formation of a stable colloidal dispersion of the inorganic particles in an aqueous solution containing acrylamide, crosslinker, dispersant, and initiator b) HIPE preparation with this aqueous solution as the dispersed phase and c) polymerisation. The latex particles are polyhedral in shape, shown clearly by excellent scanning electron micrographs, and have sizes of between 1 and 5 pm. 

Erdem, B., E. D. Sudol, V. L. Dimonie, and M. S. El-Aasser, Encapsulation of inorganic particles via miniemulsion polymerization. I. Dispersion of titanium dioxide particles in organic media using OLOA 370 as stabilizer. J. Polym. Sci. A Polym. Chem. 38 (2000) 4419-4430. 

The miniemulsion process is also very suitable for the preparation of hybrid latex particles. One can have the inorganic particles already present in the droplets, then the polymerization reaction results in encapsulated nanoparticles (see [33]). 

Although some papers on encapsulation of inorganic particles can be found before 1980, most of the early work begins in the mid-1980s and, on average, about 50 papers per year were published on the topic of encapsulation using the emulsion polymerization technique. 

Encapsulation of Inorganic Particles with Emulsion Polymerization... 

Formation of a hemimicelle on the surface of the inorganic particle where, after swelling with monomer, the encapsulating polymer is formed. 

Polymerization on the surface is in competition with the process of new particle formation. Depending on the amount of surfactant, the type of monomer and the monomer droplet size any of the three nucleation mechanisms shown in Fig. 4 (micellar, homogeneous or droplet nucleation) can occur. In order to prevent micellar nucleation, the net surfactant concentration, after correction for the adsorbed amount on the surfaces, should be below the CMC. The presence of conventional surfactants in encapsulation reactions introduces the problem that a delicate balance between the stabilization of polymer particles and inorganic particles and the formation of new particles has to be maintained. 

According to Hergeth and coworkers [55], a minimum surface of the inorganic particles is needed to prevent secondary nucleation. To estimate this amount, a formula was derived for seeded emulsion polymerization with spherical particles and a water-soluble initiator [55]. This formula was based on the observation that primary particles are produced by a coUapse and micellization process of oligomeric chains. An upper limit for the particle size was estimated to be 100 nm for the encapsulation of silica with polyvinyl acetate. A relatively water-soluble monomer is applied here for more hydrophobic monomers this upper limit will be higher. Because the surface area needed to prevent secondary nucleation is proportional to the monomer conversion per unit of time, the encapsulation efficiency can be improved by using monomer-starved conditions. So far, mainly submicrometer particles have been encapsulated with this method. The encapsulation of the larger filler particles... 

In general, a strong interaction of the first layer of encapsulating polymer with the surface of the inorganic particle can lead to an increase in the glass transition temperature, 7 [57]. 

Erdem B, Sudol ED, Dimonie VL et al (2000) Encapsulation of inorganic particles via miniemulsion polymerization. III. Characterization of encapsulation. J Polym Sci Part A Polym Chem 38 4441-4450... 

Emulsion polymerization is the polymerization technique that starts with emulsified monomer in the continuous aqueous phase. Polymer formation takes place in the micelles and is initiated by water-soluble initiators. The monomers are insoluble or sparingly soluble in water. Emulsion polymerization is used very frequently in order to perform encapsulation of inorganic particles with polymers where water-based coatings are required. For the encapsulation of inorganic particles, seeded emulsion polymerization is performed hydrophobic inorganic particles are dispersed with normal surfactants or protective colloids in the aqueous phase. As polymerization on the surface of inorganic particles is always in competition with secondary particle formation, the concentration of the surfactants should be lower than their critical micelle concentration. However, homogeneous nucleation can also occur, which... 

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