Pickering polymerization

Pickering polymerization. Redrawn after reference 58. (b) SEM image of Laponite clay-armored polystyrene latex. Scale bar indicates 200 nm. Reprinted with permission from reference 58. 2005 American Chemical Society, (c) MMT-armored polyacrylamide latex particles. Scale bar indicates 1 im. Reprinted from reference 60 with permission of the American Chemical Society. 

Wolters, D., Meyer-Zaika, W., Bandermann, F., Suspension of polymerization of styrene with pickering emulsifiers, Macromol. Mater. Eng. 286 (2001) 94. 

After completion of phase inversion, the polymer solution is dispersed in a 2-4-fold amount of water. Suspension aids used are water-soluble organic polymers, such as poly(vinyl alcohol) or polyvinylpyrrolidone, or inorganic compounds, such as Pickering systems. In order to achieve a final conversion of 99.5 %, initiator combinations with different decomposition times are used, and the polymerization follows a defined temperature-time profile. The suspension is then centrifuged, dried and compounded. 

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. 

In the past, many groups have tried to encapsulate clay platelets inside latex particles. This encapsulation poses some extra challenges because of the tendency of the clay platelets to form stacks and card-house structures. Most of the attempts resulted in the so-called armored latex particles, i.e. clay platelets in the surface of the latex. Recently, natural and synthetic clays were successfully encapsulated. The anisotropy of the clay resulted in non-spherical latex particles (Figs. 5 and 6), either peanut-shaped [63] or flat [64]. Clay platelets also turned out to be good stabilizing agents for inverse Pickering emulsion polymerizations [65]. 

Polymerization of Emulsion Droplets Armored with Inorganic Nanoparticles Pickering Suspension and Miniemulsion... 

Emulsion Polymerization Solids-Stabilized, or Pickering, Emulsion Polymerization. 42... 

We have seen from the above discussion that solid particles can adhere to a soft interface, and thus to monomer droplets. The effect of Pickering stabilization protects the droplets from coalescence. The use of solid particles as stabilizers in emulsion-based polymerization techniques was first described in open literature by... 

A variety of other nanoparticles have been used in Pickering suspension polymerizations, including magnetic Fc304 [98-100] and CdS nanoparticles stabilized by poly(ethylene glycol- /ock-styrene-fctock-2-(dimethylamino) ethyl methacrylate) [101]. 

Wu and coworkers [102] reported on the inverse Pickering suspension polymerization of N-isopropyl acrylamide (NIPAM). They used various sizes of silica particles, ranging from 53 to 962 nm in diameter, as solids stabilizer. Suspension polymerizations were successful for the silica particles of diameter <500 nm. Similar work was reported by Wang and coworkers. [103]. 

Bon, Keddy, and coworkers [109] demonstrated that soft armored polymer latex made via Pickering miniemulsion polymerization [i.e., poly(lauryl acrylate) armored with Laponite clay discs] could be used as a nanocomposite additive in standard poly(butyl acrylate-co-acrylic acid) waterborne pressure-sensitive adhesives (PSAs), leading to marked mechanical property enhancements (see Fig. 13). 

Voorn and coworkers demonstrated the inverse Pickering miniemulsion polymerization of aqueous acrylamide and 2-hydroxyethyl methacrylate in cyclohexane using hydrophobically modified Montmorillonite platelets (cloisite 20A) as solids stabilizer [110]. 

The use of a high-energy homogenization step to prepare the submicrometer-sized monomer droplets for the Pickering miniemulsion process could be a drawback for industrial scale-up. A better outcome could be achieved by the equivalent emulsion polymerization process in which solid nanoparticles were used as solids stabilizer. 

Muller and coworkers prepared disc-like polymer Janus particles from assembled films of the triblock copolymer SBM and, after hydrolysis of the ester groups into methacrylic acid units, used these as Pickering stabilizer in the soap-free emulsion polymerization of styrene and butyl acrylate [111]. Armes and coworkers described the synthesis of PMMA/siUca nanocomposite particles in aqueous alcoholic media using silica nanoparticles as stabilizer [112], extending this method to operate in water with a glycerol-modified silica sol [113, 114]. Sacanna showed that methacryloxypropyltrimethoxysilane [115] in the presence of nanosized silica led to spontaneous emulsification in water, which upon a two-step polymerization procedure afforded armored particles with an outer shell of PMMA [116]. Bon and coworkers demonstrated the preparation of armored hybrid polymer latex particles via emulsion polymerization of methyl methacrylate and ethyl methacrylate stabilized by unmodified silica nanoparticles (Ludox TM O) [117]. Performance of an additional conventional seeded emulsion polymerization step provided a straightforward route to more complex multilayered nanocomposite polymer colloids (see Fig. 14). 

The use of either styrene or butyl methacrylate as monomer led to stable latexes that were not covered by silica particles. Bon and coworkers proposed a mechanism for the solids-stabilized, or Pickering, emulsion polymerization that effectively combines coagulative nucleation with heterocoagulafion throughout the polymerization process. The growing latex particles become unstable and collide irreversibly with the nanoparticles that are dispersed in the water phase. The key to successful polymerization is that this collision process is fast with respect to the timescales of particle nucleation and growth. 

Fig. 14 TEM images of (a) PMMA latex rumored with sdica nanopaiticles obtained by Pickering emulsion polymerization. Multilayered nrmocomposite polymer colloids with (b) a hairy outer-layer of poly(acrylonitrile) and (c) a soft shell of poly(n-butyl acrylate). Scale bars 100 nm. Figure and legend are taken from [117]...

Ma and Dai [121] reported the synthesis of polystyrene latexes armored with silica nanoparticles (10-15nm in diameter, PA-ST silica sol, Nissan Chemicals) via solids-stabilized emulsion polymerization. They used VA-086, 2,2 -azobis [2-methyl-lV-(2-hydroxyethyl)propionamide], as nonionic initiator. Whereas we found that Pickering emulsion polymerization of styrene using Ludox TM-40 and a low flux of radicals generated from potassium persulfate did not result in an armored latex, the hydroxyethyl groups probably enhance the wettability of the surface of the latex particles to promote silica adhesion. This was confirmed by a... 

Bon SAF, Colver PJ (2007) Pickering miniemulsion polymerization using Laponite clay as a stabilizer. Langmuir 23(16) 8316-8322... 

Cauvin S, Colver PJ, Bon SAF (2005) Pickering stabriized miniemulsion polymerization preparation of clay armored latexes. Macromolecules 38(19) 7887-7889... 

Chen T, Colver PJ, Bon SAF (2007) Organic-inoigtinic hybrid hollow spheres prepared from Ti02-stabilized Pickering emulsion polymerization. Adv Mater 19(17) 2286-2289... 

Wang C, Zhang C, Li Y, Chen Y, Tong Z (2009) Fadle fabrication of nanocomposite microspheres with polymer cores and magnetic shells by Pickering suspension polymerization. React Funct Polym 69(10) 750-754... 

Duan L, Chen M, Zhou S, Wu L (2009) Synthesis and characterization of poly(N-isopropy-lacrylamide)/silica composite microspheres via inverse Pickering suspension polymerization. Langmuir 25(6) 3467-3472... 

Gao Q, Wang C, Liu H, Wang C, Liu X, Tong Z (2009) Suspension polymerization based on inverse Pickering emulsion droplets for thermo-sensitive hybrid microcapsules with tunable supracoUoidal structures. Polymer 50(12) 2587-2594... 

Voorn DJ, Ming W, Van Herk AM (2006) Polymer-clay nanocomposite latex particles by inverse Pickering emulsion polymerization stabilized with hydrophobic montmorillonite platelets. Macromolecules 39(6) 2137-2143... 

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