Nano-emulsions polymerization

FIGURE 21.12 Schematic representation of heterophase polymerization processes (a) Emulsion polymerization, (b) nano-emulsion polymerization, and (c) microemulsion polymerization. (From Antonietti, M. and Landfester, K., Prog. Polym. ScL, 2002, 27, 689-757. With permission.)... 

The initiator is another important component in a polymerization system. In the early stages, the initiation was started by applying thermal energy to a free radical generator of the persulfate type. Oil-soluble initiators such as 2,2 -azobis(2-methylbutyronitrile) (AMBN) have also been used and have made it possible to explain thoroughly the kinetics of the process [102]. Nano-emulsion polymerization in the presence of stable radicals, so-called living radical polymerization, has been reported to reduce the polydis-persity of the final latex [103,104]. 

Use of Nano-emulsions as Templates for the Preparation of Polymeric Nanoparticles 167... 

Sadurm, N., Solans, C., Azemar, N. and Garci a-Celma, M.J. (2005) Studies on the formation of O/W nano-emulsions, by low-energy emulsification methods, suitable for pharmaceutical aplications. Emulsion polymerization initiation of polymerization in monomer droplets., 26, 438-445. 

Caldero, G., Garcfa-Celma, M.J. and Solans, C. (2011) Formation of polymeric nano-emulsions by a low-energy method and their use for nanoparticle preparation. Journal of Colloid and Interface Science, 353, 406-411. 

Many commercially important polymers are produced via emulsion polymerization. This is also one of the most common methods to produce dye-doped beads. A dye is added to the mixture of monomers prior to initiating the polymerization and is either noncovalently entrapped or is copolymerized. The second method ensures that no leaching will occur from the particle but requires modification of the dye (typically by providing it with a double bond). This method is most common for preparation of pH-sensitive beads where a pH indicator is entrapped inside cross-linked polyacrylamide particles. The size of the beads can be tuned over a wide range so that preparation of both nano- and microbeads is possible. Despite thorough washing the surfactants are rather difficult to remove completely and their traces can influence the performance of some biological systems. 

The emulsion polymerization methodology is one of the most important commercial processes. The simplest system for an emulsion (co)polymerization consists of water-insoluble monomers, surfactants in a concentration above the CMC, and a water-soluble initiator, when all these species are placed in water. Initially, the system is emulsified. This results in the formation of thermodynamically stable micelles or microemulsions built up from monomer (nano)droplets stabilized by surfactants. The system is then agitated, e.g., by heating it. This leads to thermal decomposition of the initiator and free-radical polymerization starts [85]. Here, we will consider a somewhat unusual scenario, when a surfactant behaves as a polymerizing comonomer [25,86]. 

Emulsion polymerization was successfully employed for the preparation of nano-scale MIPs by synthesizing core-shell latexes with an imprinted shell. The use of a template with surfactant properties led to enhanced surface imprinting. Magnetic cores were synthesized to render MIPs which could be manipulated by magnetic fields in suspension, thereby facilitating the separation of the colloidal solid phase from the suspending solution. 

NaN, is used as an initiator for emulsion polymerization (Ref 137), as a cellulating agent (Ref 134) and as a retarder (Ref 185) in the manuf of sponge rubber. The addn of NaN, an alkali bicarbonate and an alkali to form a compn of pH 9-12 prevents or reduces plating out or coagulation of styrene and butadiene latexes stored in contact with metals (Ref 162). NaN, is used also to de-comp nitrites in the presence of nitrates (Ref 172). The rate of nitrite decompn is increased with an increase in azide concn. Acosta (Ref 172) detd the optimum ratio to be CNaN,/ NaNO, " 3.9- Compds of the structure R,R (- 0) (- NH) have been prepd from the corresponding sulfoxide and NaN, H,SO, in chlf soln (Ref 171) ... 

There are four main types of liquid-phase heterogeneous free-radical polymerization microemulsion polymerization, emulsion polymerization, miniemulsion polymerization and dispersion polymerization, all of which can produce nano- to micron-sized polymeric particles. Emulsion polymerization is sometimes called macroemulsion polymerization. In recent years, these heterophase polymerization reactions have become more and more important... 

In another interesting development, Yei et al. [124] prepared POSS-polystyrene/clay nanocomposites using an emulsion polymerization technique. The emulsion polymerization for both the virgin polystyrene and the nano composite started with stirring a suspension of clay in deionized water for 4h at room temperature. A solution of surfactant ammonium salt of cetylpyridinium chloride or POSS was added and the mixture was stirred for another 4 h. Potassium hydroxide and sodium dodecyl sulphate were added into the solution and the temperature was then raised to 50 °C. Styrene monomer and potassium persulfate were later on added slowly to the flask. Polymerization was performed at 50 °C for 8 h. After cooling, 2.5% aqueous aluminium sulphate was added to the polymerized emulsion, followed by dilute hydrochloric acid, with stirring. Finally, acetone was added to break down the emulsion completely. The polymer was washed several times with methanol and distilled water and then dried overnight in a vacuum oven at 80 °C. The obtained nanocomposite was reported to be exfoliated at up to a 3 wt % content of pristine clay relative to the amount of polystyrene. 

Emulsion polymerization is a powerful technique for developing products for a wide range of industry. The process can be used for the development of structured products in the nano- to microscales. Some of the key advantages of the process are the use of mild conditions, the near-complete conversion of monomers, the minimization of separation and recycling, the improved heat and mass transfer, and the improved environmental benefits due to the use of a water medium. However, the process is complex and requires careful modeling and scale-up. The prediction of process behavior, the key product properties, and the control of emulsion polymerization systems in particu-... 

Kriwet, B. Walter, E. Kissel, T. Synthesis of bioadhesive polyfacrylic acid) nano- and microparticles using an inverse emulsion polymerization method for the entrapment of hydrophilic drug candidates. J. Control. Release 1998, 56 (1-3), 149-158. 

Zhang K, Wu W, Meng H, Guo K, Chen J-F. Pickering emulsion polymerization Preparation of polystyrene/nano-SiOj composite microspheres with core-shell structure. Powder Technology. 2009 190(3) 393-400. 

Yu C-L, Kang J-S, Zhang F-A (2009) The effect of nano-Si02 coUoid on soap-free emulsion polymerization of methyl methacrylate and hydroxyethyl methacrylate. J Macromol Sci A Pure Appl Chem 46(9) 870-875... 

Han, Y.K., Yih, J.N., Chang, M.Y., Huang, W.Y., Ho, K.S., Hsieh, T.H., Lou, J.G., 2011. Facile synthesis of aqueous-dispersible nano-PEDOT PSS-co-MA core/shell colloids through spray emulsion polymerization. Macromol. Chem. Phys. 212,361-366. 

The conductive and biodegradable nano composite was made of PPy and polylactide [264,265]. PPy nanoparticles were incorporated into the poly-lactide matrix via the emulsion polymerization of pyrrole in the aqueous... 

PS-TiOa nanohybrid particles were synthesized in high yield via in-situ polymerization of styrene in the presence of Ti02 nanoparticles surface-modified with 3-(trimethoxysilyl)-propylmethacrylate (MPS) [173]. The encapsulation of nano-Ti02 by PS via mini-emulsion polymerization was also reported. In this case, polybutene-succinimide pentamine was used as stabilizer at the oil-water interface [174-176]. 

Nature of nanoparticle Emulsification method Nature of nano-emulsion Monomers Surfactants Polymerization and emulsification parameters Particle size (nm) References... 

Multifunctional biotinylated and streptavidin-coated polyurethane-urea nanoparticles have also been engineered from OAV nano-emulsions." " In these studies, biotin or streptavidin reacts with diisocyanate at the droplet interface and are successfully attached to the nanoparticle polymeric matrices. These nanoparticles exhibit diameters around 110-140mn (biotin nanoparticles) and 70-74nm (streptavidin nanoparticles). In addition, streptavidin-coated polyurethane-urea nanoparticles were functionalized with biotin anti-VCAM-1 and anti-ICAM-1 antibodies for specific targeting. Both nanoparticulate systems showed no cytotoxicity in healthy endothelial cells and therefore good biocompatibility properties. Figure 7.1 shows a schema of the synthesis of different polyurethane nanoparticles obtained from 0/W nano-emulsions by interfacial polymerization and images obtained by transmission electron microscopy. 

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