Miniemulsion droplets

Considering theoretically a copolymerization on the surface of a miniemulsion droplet, one should necessarily be aware of the fact that this process proceeds in the heterophase reaction system characterized by several spatial and time scales. Among the first ones are sizes of an individual block and macromolecules of the multiblock copolymer, the radius of a droplet of the miniemulsion and the reactor size. Taking into account the pronounced distinction in these scales, it is convenient examining the macrokinetics of interphase copolymerization to resort to the system approach, generally employed for the mathematical modeling of chemical reactions in heterophase systems [73]. 

It is accepted that the radical entry rate coefficient for miniemulsion droplets is substantially lower than for the monomer-swollen particles. This is attributed to a barrier to radical entry into monomer droplets which exists because of the formation of an interface complex of the emulsifier/coemulsifier at the surface of the monomer droplets [24]. The increased radical capture efficiency of particles over monomer droplets is attributed to weakening or elimination of the barrier to radical entry or to monomer diffusion by the presence of polymer. The polymer modifies the particle interface and influences the solubility of emulsifier and coemulsifier in the monomer/polymer phase and the close packing of emulsifier and co emulsifier at the particle surface. Under such conditions the residence time of entered radical increases as well as its propagation efficiency with monomer prior to exit. This increases the rate entry of radicals into particles. 

Nevertheless, for inverse miniemulsions the surfactant is used in a very efficient way, at least as compared to inverse micro emulsions [47,48] or inverse suspensions [49] which are used for subsequent polymerization processes. Again, the surface coverage of the inverse miniemulsion droplets with surfactant is incomplete and empty inverse micelles are absent. Again this is important for the interpretation of the reaction mechanism. 

The interfacial energy between the oil and water phase in a miniemulsion is significantly larger than zero. The surface coverage of the miniemulsion droplets by surfactant molecules is incomplete. 

The stability of miniemulsion droplets against diffusional degradation results from an osmotic pressure in the droplets, which controls the solvent or monomer evaporation. The osmotic pressure is created by the addition of a substance, which has extremely low water solubility, the so-called hydrophobe. This crucial prerequisite is usually not present in microemulsions, but... 

The first interval is the interval of particle nucleation (interval I) and describes the process to reach an equilibrium radical concentration within every droplet formed during emulsification. The initiation process becomes more transparent when the rate of polymerization is transferred into the number of active radicals per particle n, which slowly increases to n 0.5. Therefore the start of the polymerization in each miniemulsion droplet is not simultaneous, so that the evolution of conversion in each droplet is different. Every miniemulsion droplet can be perceived as a separate nanoreactor, which does not interact with others. After having reached this averaged radical number, the polymerization kinetics is slowing down again and follows nicely an exponential kinetics as known for interval III in emulsion polymerization or for suspension polymer-... 

The process of miniemulsion allows in principle the use of all kinds of monomers for the formation of particles, which are not miscible with the continuous phase. In case of prevailing droplet nucleation or start of the polymer reaction in the droplet phase, each miniemulsion droplet can indeed be treated as a small nanoreactor. This enables a whole variety of polymerization reactions that lead to nanoparticles (much broader than in emulsion polymerization) as well as to the synthesis of nanoparticle hybrids, which were not accessible before. 

The hydrophobic monomer vinylnaphthalene also forms at temperatures above its melting temperature miniemulsions and the miniemulsion droplets... 

Miniemulsion copolymerization of a 50 50 styrene/methyl methacrylate monomer mixture, using hexadecane as hydrophobe, was carried out by Rodriguez et al. [78]. The mechanism of mass transfer between miniemulsion droplets and polymer particles in the miniemulsion copolymerization of styrene-methyl methacrylate (AIBN as initiator, hexadecane as hydrophobe) was studied, analyzing the mass transfer of highly water-insoluble compounds from miniemulsion droplets to polymer particles by both molecular diffusion and collisions between droplets and particles [79,80]. 

In my opinion, the field of miniemulsion is still on its rise in polymer and material science since there are numerous additional possibilities both for fundamental research and application. As a vision one may think of single molecules trapped and crystallized in each small droplet, which enables new types of physico-chemical experiments and handling of complex matter [132]. Since miniemulsions allow a very convenient and effective separation of objects in compartments of the size of 30-300 nm in diameter, some general new perspectives for polymer chemistry are opened. In miniemulsion droplets, it is in principle possible to isolate complex polymers or colloids strictly from each other and to react each single molecule for itself with other components, still working with significant amounts of matter and technically relevant mass fluxes. This... 

As a basis for hydrogels, hyperbranched polymers [41] can also be used. These polymers can be connected by click chemistry in miniemulsion droplets in order to obtain hyperbranched polyglycerol (HPG)-based particles. Such materials are of great interest for drug release because they are nontoxic [42, 43] and show similar behavior to poly(ethylene glycol)s. 

As a third possibility, nanocapsules in a miniemulsion system could be achieved using different interfacial reactions in inverse miniemulsions. The formation of polyurea, polythiourea, and polyurethane nanocapsules synthesized through the polyaddition reaction has been described in detail [110-112], The size of the nanocapsules could be controlled by the amount of surfactant used and the addition time of the diisocyanate. The wall thickness was adjusted by the amount of employed monomers. dsDNA molecules were successfully encapsulated into poly-butylcyanoacrylate (PBCA) nanocapsules by anionic polymerization, which took place at the interface between the miniemulsion droplets and the continuous phase [113]. 

Paiphansiri U, Tangboriboonrat P, Landfester K (2006) Polymeric nanocapsules containing an antiseptic agent obtained by controlled nanoprecipitation onto water-in-oil miniemulsion droplets. Macromol Biosci 6(1) 33—40... 

Crespy D, Stark M, Hoffmann-Richter C, Ziener U, Landfester K (2007) Polymeric nanoreactors for hydrophilic reagents synthesized by interfacial polycondensation on miniemulsion droplets. Macromolecules 40(9) 3122-3135... 

Landfester et al. [ 143] studied the miniemulsion polymerization of styrene using hexadecane as the costabilizer. When styrene miniemulsions were subjected to varying sonication times (see Table 5), very similar trends are seen as for the MMA miniemulsions. The particle size and the polydispersity of miniemulsion droplets rapidly polymerized after sonication either do not depend on the amount of the costabihzer, or are very weak functions of the amount of costabilizer (see Table 6). It was found that doubhng the amount of costabilizer does not decrease the radius nor have any effect on the polydis-... 

Another type of emulsion-like process is called miniemulsion polymerization. Miniemulsions are stable oil in water emulsions with average droplet diameter of 80-400 nm, prepared using a mixture of an anionic emulsifier and a cosurfactant such as a long-chain fatty alcohol or n-alkane. The polymer latexes are prepared by initiation of polymerization in the miniemulsion droplets. 

The synthesis of stable latexes requires suitable nucleation of primary particles and subsequent stabilization. In classical free-radical emulsion polymerization water-soluble initiators are used. Chain growth initially affords water-soluble oligomeric radicals, which can nucleate particles by collapsing upon themselves or by entering a surfactant micelle (cf. Section 7.1). Similar considerations appear reasonable for the aforementioned catalytic polymerization to stable latexes by the water-soluble complex 6a (Scheme 7.7) [65, 71]. As a different strategy, a very fine initial dispersion of a hydrophobic catalyst precursor can be achieved as a solution in a large number of toluene/hexadecane miniemulsion droplets (0 ca. 100 nm), dispersed in the continuous aqueous phase [77, 82]. 

A. D. Crespy, M. Stark, C. Hoffmann-Richter, U. Ziener, K. Landfester, Polymeric nanoreactors for hydrophilic reagents synthesized by interfacial polycondensation on miniemulsion droplets. Macromolecules, 2007, 40, 3122 b) O. Gazit, R. Khalfln, Y. Cohen, R. Tannenbaum, Self-assembled diblock copolymer nanoreactors as catalysts for metal nanoparticle synthesis, J. Phys. Chem. C, 2009, 113, 576. 

A. Musyanovych, V. MaUander, K. Landfester, Miniemulsion droplets as single molecule nanoreactors for polymerase chain reaction, Biomacromolecules, 2005, 6, 1824. 

Since the reaction is conducted in the small miniemulsion droplets and effective diffusion does not take place, miniemulsion polymerization is not restricted to radical polymerization. Several examples underline that other types of polymerizations can also be carried out in miniemulsion (see Fig. 2) ... 

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