Micro-emulsion polymerization

Block or graft copolymers in a selective solvent can form structures due to their amphiphilic nature. Above the critical micelle concentration (CMC), the free energy of the system is lower if the block copolymers associate into micelles rather than remain dispersed as single chains. Often the micelles are spherical, with a compact core of insoluble polymer chains surrounded by a corona of soluble chains (blocks) [56]. Addition of a solvent compatible with the insoluble blocks (chains) and immiscible with the continuous phase leads to the formation of swollen micelles or polymeric micro emulsion. The presence of insoluble polymer can be responsible for anomalous micelles. 

The idea of gradual addition of monomer to polymerizing micro emulsions is not new and has been attempted for methylmethacrylate [31] and styrene [32-34] to make more efficient use of the surfactant. For example, Gan et al. have performed styrene microemulsion polymerizations where monomer was added gradually either dropwise or using a hollow-fiber membrane [32,35]. They observed relatively large ( 40 nm) and uniform particle sizes with no indications of continuous particle nucleation. However, this result is more likely due to a combination of fast monomer addition relative to the polymerization rate and depletion of the redox initiator system employed rather than to the growth of glassy particles. 

C02-philic molecules have been utilized for the design of metal-mobilizing ligands to be used in SCCO2 [67-69,135-137], e.g., as shown in Fig. 7a [55] and for the synthesis of surfactants that form micelles, emulsions, and micro emulsions in CO2, e.g., as shown in Fig. 7b. [70] Polymer solubility in SCCO2 has been studied [71] and utilized for polymer synthesis [72-74]. Recently, DeSimone and co-workers synthesized high-molar-mass fluoropolymers in SCCO2, and studied the polymerization kinetics [75]. 

Another interesting heterogeneous polymerization using macromonomers is a microemulsion copolymerization to produce particles 10-100 nm in diameter. Gan and coworkers [150] have prepared transparent nanostructured polymeric materials by direct polymerization of bicontinuous micro emulsions consisting... 

There is a considerable patent art concerning preparation of transparent mixtures of water with low molecular weight silicone oils using polymeric silicone surfactants. Some representative early references are Keil [47], Gee [48, 49], Gum [50] and Terae [51]. These compositions are called micro emulsions in the patents in the sense of being transparent mixtures of water, surfactant and oil - but note that they are transparent because of small particle size or because of index of refraction matching. 

The review provides recommendations to prevent early Surfmer polymerization and the consequent burying, so as to achieve a high degree of Surfmer incorporation at the end of the polymerization process. There are also hints on the possible use of Surfmers in dispersion and micro emulsion polymerization. 

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. 

As compared to classical inverse heterophase polymerization techniques such as polymerization in inverse micro emulsions [47] or dispersion polymerization [75,76], polymerization of inverse miniemulsions is favored by the very efficient use of surfactant and the copying process from the droplets to the par-... 

Fig. 2 Experimental and model rate versus conversion profiles for the polymerization of hexylmethacrylate in a microemulsion stabilized by the surfactant DTAB. The two curves are for initiator concentrations of 0.045 wt% (top) and 0.015 wt% (bottom) relative to the amount of monomer in the micro emulsion. The solid lines are predictions from the Morgan model [56]...

Fig. 4 Schematic presentation of the set-up (not to scale) for the polymerization of monomer in a micro emulsion via hollow-fiber feeding of additional monomer...

The polymerization of styrene in Winsor I-like systems by semi-continuous feeding of monomer stabilized by either DTAB, TTAB or CTAB has been systematically investigated by Gan and coworkers [69a]. Rather monodisperse polystyrene microlatexes of less than 50 nm with molecular weights of over one million were obtained at a polymer/surfactant weight ratio of 14 1. The Winsor I-like (micro)emulsion polymerization of styrene stabilized by non-ionic surfactant and initiated by oil-soluble initiators has also been reported very recently [69b]. The sizes of the large monomer-swollen particles decreased with conversion and they merged with growing particles at about 40-50% conversion. 

Fig. 10 The effect of potassium ferrocyanide on the luminescence spectra of [Ru(dip)3] in polymerized cationic micro emulsion (1)0 ppm, (2) 1 ppm, (3) 3 ppm, (4) 5 ppm, (5) 7 ppm and (6) 9 ppm. The contact time between the potassium ferrocyanide solution and the polymer film was 10 min. The insert is the Stern-Volmer plot for the quenching of [Ru(dip)3] + by potassium ferrocyanide...

Some non-oxide nanoparticles such as PbS and CdS can be used to prepare polymer-inorganic nanocomposites by a double-microemulsion process [103]. In this case, two precursor micro emulsions must be prepared separately first and then mixed together for polymerization. Using CdS-polymer nanocom-... 

Figure 23 Diagram of the three intervals of micro-emulsion polymerization, according to Candau [258]...

When the appropriate precautions are taken the method appears particularly suited for measuring very low tensions 10 mN m sometimes even as low as 10 mN m ). Such ultralow tensions are for example encountered in micro-emulsion systems and in just phase-separated polymeric or micellar solutions. For phase-separated colloid-polymer systems de Hoog and Lekkerkerker ) even reported values down to a few pN m , reproducibly being obtained after implementing a number of methodical improvements. (Alternatives for low tensions are the sessile and pending (micro-) drop but these do not usually go below 10 mN m ) Commercial apparatus are nowadays available. A variant proposed by Than et al. J employs a thin rod in the axis of the cylinder, to reduce spin-up time and suppress drift. Another variant, proposed by Kokov, analyses the centrifugal field required to squeeze liquid out of an orifice" ). 

Two characteristics of the o/w micro-emulsion polymerization are not found in conventional emulsion polymerization ... 

Liu T, Schuch H, Gerst M, Chu B. Laser light scattering study of micro-emulsion-like polymerization processes with block copolymers as dispersants. Macromolecules 1999 32 6031—6042. 

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