Immiscible monomers

Figure 4c illustrates interfacial polymerisation encapsulation processes in which the reactant(s) that polymerise to form the capsule shell is transported exclusively from the continuous phase of the system to the dispersed phase—continuous phase interface where polymerisation occurs and a capsule shell is produced. This type of encapsulation process has been carried out at Hquid—Hquid and soHd—Hquid interfaces. An example of the Hquid—Hquid case is the spontaneous polymerisation reaction of cyanoacrylate monomers at the water—solvent interface formed by dispersing water in a continuous solvent phase (14). The poly(alkyl cyanoacrylate) produced by this spontaneous reaction encapsulates the dispersed water droplets. An example of the soHd—Hquid process is where a core material is dispersed in aqueous media that contains a water-immiscible surfactant along with a controUed amount of surfactant. A water-immiscible monomer that polymerises by free-radical polymerisation is added to the system and free-radical polymerisation localised at the core material—aqueous phase interface is initiated thereby generating a capsule sheU (15). 

This study illustrates a particular use of FT-Raman spectroscopy (Section 2.4.2) to monitor an emulsion polymerization of an acrylic/methacrylic copolymer. There are four reaction components to an emulsion polymerization water-immiscible monomer, water, initiator, and emulsifier. During the reaction process, the monomers become solubilized by the emulsifier. Polymerization reactions were carried using three monomers BA (butyl acrylate), MMA (methyl methacrylate), and AMA (allyl methacrylate). Figure 7-1 shows the FT-Raman spectra of the pure monomers, with the strong vC=C bands highlighted at 1,650 and 1,630 cm-1. The reaction was made at 74°C. As the polymerization proceeded, the disappearance of the C=C vibration could be followed, as illustrated in Fig. 7-2, which shows a plot of the concentration of the vC=C bonds in the emulsion with reaction time. After two hours of the monomer feed, 5% of the unreacted double bonds remained. As the... 

Water-immiscible monomers, or not more than slightly water-soluble monomers, may be polymerized as a suspension of large droplets in water. The droplets are kept in suspension by agitation and by the use of stabilizers, such as gelatin, talc, or bentonite clay. The free radical initiator used must be soluble in the monomer. Droplet size is 0.01-0.5 cm in diameter in typical operating modes. Polymerization in this way can be pictured as the simultaneous operation of many droplet-sized reactors, which on completion give beads, or pearls, of polymer. In fact these are the names, which are sometimes applied to this method of polymerization and to the product obtained. 

In a typical aqueous emulsion polymerization, a water-immiscible monomer is polymerized in the presence of a surfactant and a water-soluble initiator, such as potassium peroxodisuUate. A water-soluble radical, e.g. -SOl formed by thermal decomposition of the initiator, grows by addition of monomer dissolved in the... 

Similar approaches can be used in the microfluidic synthesis of Janus particles where two compartments are formed within the particle that have different compositions. In this case immiscible monomers are passed along microfluidic channels in parallel streams and then subsequently polymerized. This approach allows precise control over the volumetric fraction of each monomer by controlling the relative flow rates of the two monomer streams [3]. Polymerization is usually by UV irradiation, and the surfaces of each compartment of the Janus particles can be subsequently functionalized or used for immobilization of biomolecules. They can also be loaded with dyes or other small molecules to act as colorimetric indicators or in microencapsulation of drugs [3]. 

Interfadal polycondensation n Involves polymer formation at or near the interface between two immiscible monomer solutions under very mild reaction conditions. 

Many molecules are composed of segments that if not bonded would phase separate. Amphiphiles made up of hydrophilic and hydrophobic moieties, and block copolymers formed of immiscible monomers are important examples. The interface that forms between oil and water is a natural habitat for amphiphiles similarly, block copolymers are preferentially adsorbed at the interface between phase separated monomers. 

Asami, R., Atobe, M. and Fuchigami, T. (2005) Electropolymeiisation of an immiscible monomer in aqueous electrolytes using acoustic emulsification. Journal of the American Chemical Society, 127, 13160-13161. 

Figure 1. An assembled test tube, post-launch, showing the polyethylene cap, the chorolastic septum and the polymerized sample. This tube contained the immiscible monomers, polyethyleneglycol 200 monomethacrylate (PEG-200) and 3-Methacryloxypropyl-pentamethyldisiloxane (disiloxane).

The polymers formed from the immiscible monomers seem to hold promise for the development of new materials that cannot be synthesized on the ground except, perhaps in the presence of a solvent. We are seeing a mixing that is driven, in large part, by surface tension differences. The benefit of mechanical stirring needs to be investigated further. 

Conventional emulsion polymerization comprises emulsification of a water-immiscible monomer in a continuous water medium using an oil-in-water emulsifier and polymerization using a water-soluble or oil-soluble initiator to give a colloidal dispersion of polymer particles in water. The average particle size of conventional latexes is usually 0.1-0.3ym in contrast to the original emulsion droplet size of 1-lOym. Thus the mechanism of polymerization is not a simple one of polymerization of the monomer droplets, and any mechanism proposed for the process must explain the order-of-magnitude reduction in particle size observed upon converting monomer to polymer. 

In this procedure, conceptually similar to suspension polymerization, the polymerization starts in the aqueous phase in presence of a water soluble initiator Systran, an immiscible monomer forming droplets upon agitation, and a surfactant, to stabilize the droplets in the emulsion. 

In addition to the synthesis of latexes with high solids content and small particle size, in the past decade, microemulsion polymerization has heen used to synthesize a wide range of materials. For instance, several works have incorporated inorganic materials such as carhon nanotuhes, " ZnO nanoparticles (UV absorption),montmorillonite clay, " and quantum dots (luminescence probes) " to produce nanocomposites. Furthermore, nanogels, " conductive polypyrrole and polyaniline latexes, " " polyurethanes using immiscible monomers, and polymers in water-in-scC02 microemulsions have been also prepared by microemulsion polymerization. 

According to the solubility of the core-forming monomer in the reaction media, two different methods—emulsion polymerization and dispersion polymerization— have been exploited to obtain self-assembled nanoparticles by PISA [37, 38]. The dispersion polymerization can be carried out either in water or in organic solvents. The emulsion polymerization starts from a monomer-in-water emulsion, where a water-soluble polymer precursor is chain-extended by polymerizing a water-immiscible monomer, resulting in self-assembled block copolymers. In contrast to the emulsion polymerization, dispersion polymerization is conceptually much simpler and the initial reaction solution is homogeneous. 

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