Polymerisation Emulsion... Interfacial

For both types of polymerisation mechanisms, different polymerisation processes can be used ranging from simple bulk and solution polymerisation processes to more sophisticated ones such as suspension, emulsion, interfacial, plasma,... polymerisation processes. 

Strategems to overcome some of these basic problems include (i) the use of a high viscosity oil phase in w/o/w emulsions to prevent or decrease diffusion of individual surfactant molecules and water molecules, (ii) the polymerisation of interfacially adsorbed surfactant molecules, and (iii) the gelation of the oily or aqueous phases of the emulsions. 

Emulsions made by agitation of pure immiscible liquids are usually very unstable and break within a short time. Therefore, a surfactant, mostly termed emulsifier, is necessary for stabilisation. Emulsifiers reduce the interfacial tension and, hence, the total free energy of the interface between two immiscible phases. Furthermore, they initiate a steric or an electrostatic repulsion between the droplets and, thus, prevent coalescence. So-called macroemulsions are in general opaque and have a drop size > 400 nm. In specific cases, two immiscible liquids form transparent systems with submicroscopic droplets, and these are termed microemulsions. Generally speaking a microemulsion is formed when a micellar solution is in contact with hydrocarbon or another oil which is spontaneously solubilised. Then the micelles transform into microemulsion droplets which are thermodynamically stable and their typical size lies in the range of 5-50 nm. Furthermore bicontinuous microemulsions are also known and, sometimes, blue-white emulsions with an intermediate drop size are named miniemulsions. In certain cases they can have a quite uniform drop size distribution and only a small content of surfactant. An interesting application of this emulsion type is the encapsulation of active substances after a polymerisation step [25, 26]. 

Knowledge of surfactant equilibration and interactions will probably lead to improved formulations of multiple emulsions. Failing this the use of polymerisable surfactants can lead to obvious strengthening of interfacial barriers and allow control of stability and drug release. Nonetheless further detailed work on both w/o/w and o/w/o systems is justified. 

Microcapsules containing polymer and pigment were prepared in [299] by dispersing a viscous suspension of pigment and oil-soluble shell monomer forming o/w emulsions. Subsequently, a water-soluble shell monomer was added to the emulsion droplets, encapsulating them via interfacial polycondensation. These microcapsules were then heated for free radical polymerisation of the core monomers. It has been shown that polyvinyl alcohol (PVOH) used as stabiliser reacts with the oil-soluble shell monomers. The decrease of PVOH concentration as result of this interaction leads to coalescence of the particles and to the increase of their equilibrium particle size, however, methods are proposed to prevent the depletion of PVOH. 

Micro-emulsion is another variant of emulsion polymerisation. Such emulsions are thermodynamically stable systems including swollen monomer micelles dispersed in a continuous phase. In general, they require fairly large concentrations of surfactants to be produced compared with the other dispersed polymerisation systems. Hence, the interfacial tension of the oil/water is generally close to zero. Polymers with ultra-high molecular weight, i.e. above 10 g/mol, can be obtained, as can copolymers with a very well-defined, homogenous composition. Whereas polymerisation can take 24-48 h in the normal emulsion process, it proceeds at a fast rate in micro-emulsion, as total conversion can be obtained in less than 30 min. Polymer particles of very small size (diameter < 100 nm) and narrow distribution can be obtained by this process. 

Similarly to suspension and emulsion polymerisation, the system used in interfacial polymerisation is heterogeneous, but polymerisation takes place at the interface between both phases. Such a system can be easily illustrated in a practical laboratory course by reaction of a diamine soluble in an aqueous alkaline medium present in the upper part of a beaker, with a diacid chloride soluble in a non-miscible organic solvent such as chloroform present in the... 

Water-containing nanocapsules can be obtained by interfacial polymerisation of ACA in water-in-oil micro-emulsion. In such a system, water-swollen micelles of surfactants of small and uniform size are dispersed in... 

Emulsion droplets provide the large interfacial area necessary for efficient mass transfer during emulsion polymerisation. Most monomers have slight solubility in water so that they may be transported across the aqueous phase from the monomer droplets to the sites of polymerisation (i.e., the polymer particles). During polymerisation, the monomer concentration gradient will overcome Ostwald ripening forces, and diffusion of monomer from large drt lets to smaller monomer-swollen particles will occur. 

Details are given of an emulsion polymerisation that involves an encapsulation of a nonsolvent hydrocarbon for the polymer being formed. The phase separation of PS within a styrene-isooctane dispersion was modelled with the Flory-Huggins theory. The encapsulation is discussed in terms of interaction parameters, transport processes, polymer molecular weight, and interfacial tension effects. 38 refs. 

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