Polymerization miniemulsion

Miniemulsion polymerization enables to incorporate water-insoluble materials such as resins, organic pigments, polymers, etc into the polymer matrix. The additive seed allows to control the particle number and particle size during the production process. Furthermore, miniemulsion polymerizations and copolymerizations carried out with acrylic and methacrylic monomers in the presence of unsaturated alkyd resins lead to the production of stable hybrid latex particles containing grafted and crosslinked alkyd resin/acrylic products as coating polymer [114]. In the reaction, the multifunctional resin acts as a hydrophobe as well as the costabilizer of the miniemulsion. 

The miniemulsion polymerization can be applied for the preparation of the composite particles containing the hydrophobic and/or hydrophilic domains and/or phases. The hydrophobized material, which is to be incorporated, has to be dispersed in the monomer phase. Then, miniemulsification in the water phase has to be carried out. The hydrophilic additives themselves require a hydrophobic surface so that they can be dispersed into the hydrophobic monomer phase. Erdem et al. described the encapsulation of Ti02 particles via miniemulsion in two steps mentioned above. First, TiOs was dispersed in the monomer using OLOA 370 (poly(butene 

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Different sized nanocapsules are formed by a miniemulsion polymerization of variety of monomers in the presence of larger amounts of hydrophobe [117]. Hydrophobe and monomer form a common miniemulsion before polymerization, whereas the polymer is immiscible with the hydrophobe and phase-separates throughout the polymerization to form particles with a morphology consisting of a hollow polymer structure surrounding the hydrophobe. Differences in the hydrophilicity of oil and polymer turned out to be the driving force for the formation of nanocapsules. In the case of poly(methyl methaciylate) (PMMA) and hexadecane (HD), the pronounced differences in hydrophilicity are suitable for direct nanocapsule formation. In the case of styrene as the monomer, the hydrophilicity of the polymer phase has to be adjusted in order to favor the nanocapsule structure, which is done either by the addition of an appropriate comonomer or initiator. 

Principles and Applications of Emulsion Polymerization, by Chorng-Shyan Chern Copyright 2008 by John Wiley Sons, Inc. 

Because nucleation of particles is often irreproducible, commercial emulsion polymerizations are often seeded with polymer particles of known size and concentration. The seed particles when exposed to monomer in the form of monomer droplets swell to an equilibrium size. Under proper polymerization conditions no new particles form, and the polymerization consists of growing these seed crystals. Thus the polymerization consists of Intervals II and III only. 

As discussed, miniemulsions are characterized by a rather large surface area, sufficient to alter the principal mechanism of nucleation. In miniemulsion polymerization, nucleation occurs predominantly by radical entry into monomer in the interior of the miniemulsion droplets. In addition, because of the improved physical stability of the miniemulsion droplets, it is possible to adjust the total surfactant concentration so as to limit the total number of micelles in solution, thereby limiting aqueous phase nucleation. For example, Fontenot and Schork studied the batch polymerization of methyl methacrylate with SDS as the surfactant and hexadecane as the secondary disperse phase component. The mechanism of particle nucleation could be altered from droplet control to micellar control as the concentration of SDS was increased. Relative to micelles, droplets can have higher radical numbers (due to their larger size), and can, therefore, significantly enhance the early stages of the polymerization process. The rate of polymerization per particle is faster, and the systems are converted faster. Following nucleation, the reaction proceeds with polymerization of the monomer in the miniemulsion droplets. Thus, there is no distinct Interval II. Droplet nucleations have been found 

4 Use of Composition Ripening to Produce Large Monodisperse Latex Particles 

Among different alternatives, a very effective way to operate an LRP in segregated systems is indeed miniemulsion. In this case, small monomer droplets are the primary locus of reaction and all the difficulties from interphase transfer vanish, since monomer and all the other hydrophobic species required to run an LRP are already in the main reaction locus. However, further difficulties have been reported, such as incomplete droplet nudeation and colloidal stability problems [74, 82, 85]. More subtle is the evidence of instabilities in the miniemulsion due to the kinetics of LRP. In contrast to conventional systems, where long chains are created from the beginning, in LRP all the polymer chains are short initially. This might lead to superswelling states of the droplets and, eventually, to destabilization [86]. 

Despite these difficulties, literature abounds with examples of styrene miniemulsion polymerizations by NMP all show good control of the MWD and the aforementioned problem of slow polymerization rates [76]. Due to the improvements in nitroxide efficiency, studies involving polymerization of acrylates have also appeared [87]. However, a fast buildup of nitroxides is often observed in this case, which depresses the polymerization rate. A possible solution is represented by the removal of the excess of nitroxides, which has also given good results in styrene polymerization [87]. 

ATRP is also very effective when applied to miniemulsion systems. Still, care is needed in the choice of surfactant (nonionic) and ligand (not too water-soluble). Also, it proved effective to run a so-called reverse ATRP , that is, starting from the metal in the oxidized form, since the original metal complex (such as Cu(I)) is rather sensitive to oxidation during miniemulsion formation by sonication [88]. 

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Microemulsion and miniemulsion polymerization differ from emulsion polymerization in that the particle sizes are smaller (10-30 and 30-100 nm respectively vs 50-300 inn)4" and there is no monomer droplet phase. All monomer is in solution or in the particle phase. Initiation takes place by the same process as conventional emulsion polymerization. 

Microemulsion and miniemulsion polymerization processes differ from emulsion polymerization in that the particle sizes are smaller (10-30 and 30-100 nm respectively vs 50-300 ran)77 and there is no discrete monomer droplet phase. All monomer is in solution or in the particle phase. Initiation usually takes place by the same process as conventional emulsion polymerization. As particle sizes reduce, the probability of particle entry is lowered and so is the probability of radical-radical termination. This knowledge has been used to advantage in designing living polymerizations based on reversible chain transfer (e.g. RAFT, Section 9.5.2)." 2... 

NMP in miniemulsion has been more successful. In miniemulsion polymerization nuclealion lakes place directly in the monomer droplets that become the polymer particles. Particle sizes are small (<100 nm). Most w ork has used TEMPO and high reaction temperatures (120-140 °C) with S or BA as monomer. 

Various initiation strategies and surfactant/cosurfactant systems have been used. Early work involved in situ alkoxyamine formation with either oil soluble (BPO) or water soluble initiators (persulfate) and traditional surfactant and hydrophobic cosurfactants. Later work established that preformed polymer could perform the role of the cosurfactant and surfactant-free systems with persulfate initiation were also developed, l90 222,2i3 Oil soluble (PS capped with TEMPO,221 111,224 PBA capped with 89) and water soluble alkoxyamines (110, sodium salt""4) have also been used as initiators. Addition of ascorbic acid, which reduces the nitroxide which exits the particles to the corresponding hydroxylamine, gave enhanced rates and improved conversions in miniemulsion polymerization with TEMPO.225 Ascorbic acid is localized in the aqueous phase by solubility. 

Asua, J.M. (2002) Miniemulsion polymerization. Progress in Polymer Science, 27, 1283-1346. 

Tiarks, F., Landdfester, K. and Antonietti, M. (2001) Preparation of polymeric nanocapsules by miniemulsion polymerization. Langmuir, 17, 908-918. 

Galindo-Alvarez, J., Boyda, D., Marchal, Ph., Tribet, Ch., Perrin, P., Begue, E.M., Durand, A. and Sadder, V. (2011) Miniemulsion polymerization templates a systematic comparison between low energy emulsification (Near-PIT) and ultrasound emulsification methods. Colloids and Surfaces A Physicochemical and Engineering Aspects, 374 (1—3), 134—141. 

H.J. Barraza, F. Pompeo, E.A. O Rear, and D.E. Resasco, SWNT-filled thermoplastic and elastomeric composites prepared by miniemulsion polymerization. Nano Lett. 2, 797-802 (2002). 

Taniguchi T, Takeuchi N, Kobaru S, Nakahira T (2008) Preparation of highly monodisperse fluorescent polymer particles by miniemulsion polymerization of styrene with a polymerizable surfactant. J Colloid Interface Sci 327 58-62... 

T.R. McCaffery and Y.G. Durant, Apphcation of low-resolution Raman spectroscopy to onhne monitoring of miniemulsion polymerization, J. Appl. Polym. Sci., 86, 1507-1515 (2002). 

One disadvantage of the miniemulsion polymerization is the low rate of polymerization. Even if almost the same number of droplets exist in the miniemulsion polymerization compared with the number of micelles in general emulsion polymer-... 

When chloro-octadecane was found to give the same result as a so-called cosurfactant, an argument arose in terms of the real role of this highly hydrophobic compound because it is not surface active and has no cooperation with surfactant. Taking account of these systems, the definition of miniemulsion polymerization will be revised to the polymerization in which a water-insoluble compound in the dispersed phase retards or inhibits diffusion degradation of the emulsion. ... 

Miniemulsion Polymerization Technology edited by Vikas Mittal. Forthcoming summer 2010. 

The book is a ready reference for the background information as well as advanced knowledge regarding the applications of miniemulsion polymerization technology. 

Miniemulsion polymerizations follow a different mechanism from the conventional (macroemulsion) emulsion polymerizations. Radicals generated in... 

Microemulsion polymerizations follow a different mechanism from the conventional emulsion polymerizations. The most probable locus of particle nucle-ation was suggested to be the microemulsion monomer droplets [27], although homogeneous nucleation was not completely ruled out. The particle generation rate in microemulsion polymerization is given by an expression similar to Eq. (21), which was used for the miniemulsion polymerization of styrene [28] ... 

McCaffery, T.R. 8c Durant, Y.G. Application of Low-Resolution Raman Spectroscopy to Online Monitoring of Miniemulsion Polymerization /. Appl. Polym. Sci. 2002, 86, 1507-1515. 

Miniemulsion polymerization involves the use of an effective surfactant/costabi-lizer system to produce very small (0.01-0.5 micron) monomer droplets. The droplet surface area in these systems is very large, and most of the surfactant is adsorbed at the droplet surfaces. Particle nucleation is primarily via radical (primary or oligomeric) entry into monomer droplets, since little surfactant is present in the form of micelles, or as free surfactant available to stabilize particles formed in the continuous phase. The reaction then proceeds by polymerization of the monomer in these small droplets hence there may be no true Interval II. 

The size of the monomer droplets plays the key role in determining the locus of particle nucleation in emulsion and miniemulsion polymerizations. The competitive position of monomer droplets for capture of free radicals during miniemulsion polymerization is enhanced by both the increase in total droplet surface area and the decrease in the available surfactant for micelle formation or stabilization of precursors in homogeneous nucleation. 

Research (Fontenot and Schork 1993a, b) indicates that miniemulsion polymerization can provide benefits over the current process technology of conventional emulsion polymerization. Among these are a process which is much more robust to contamination and operating errors, a more uniform copolymer composition when used for copolymerization, and a final product which is far more shear-stable than the product of conventional emulsion polymerization. 

Mouron, D., Reimers, J., and Schork, F.J., Miniemulsion polymerization of methyl methacrylate with dodecyl mercaptan as cosurfactant. J. Polym. Sci. (Polym. Chem.), 34, 1073-1081 (1996). 

Gooch, J. W., Dong, H Schork, J. F., Poehlein, G. W., Wang, S. T., Wu, X., and Schork, J. F Novel Water-Borne Coatings via Hybrid Miniemulsion Polymerization, ACS Symposium Series, 2000. 

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