Microemulsion polymerization kinetics

The presence of a very large number micelles indicates that radicals are captured predominantly by the monomer-swollen micelles. Each entry of a radical to a monomer-swollen micelle leads to a nucleation event and therefore the particle number increases with conversion. The particle growth is supposed to be a result of propagation of monomer and the agglomeration of primary particles. The dead monomer-swollen polymer particles and uninitiated monomer-swollen micelles serve as a reservoir of monomer. Solution or bulk polymerization kinetics seem to govern the microemulsion polymerization process [30,31]. 

In contrast to emulsion polymerization, the reaction kinetics of microemulsion polymerization is characterized by two polymerization rate intervals the interval of constant rate characteristic of emulsion polymerization is missing [42,49,53], as shown in Fig. 2. Polymer particles are generated continuously during the reaction by both micellar and homogeneous mechanisms. As the solubility of the monomer in the continuous domain increases, homogeneous... 

Leong and Candau (18) obtained inverse latices of small size (<50nm) via photopolymerization of acrylamide in a microemulsion system of acrylamide, water, toluene and Aerosol OT. They observed that rapid polymerization and total conversion was achieved in less than 30 minutes. The microemulsions remained transparent and stable during polymerization. Candau et al. (19) also reported the results of a kinetic study of the polymerization of acrylamide in inverse microemulsions. Both oil soluble AIBN and water soluble potassium persulfate initiators were used. The rate was found to depend on the type of initiator, but in both cases neither autoacceleration nor dependence on initiator concentration was observed. An excellent review of microemulsion polymerization was published recently by Candau (20). 

Morgan, J.D., Lusvardi, K.M. and Kaler, E.W. (1997) Kinetics and mechanism of microemulsion polymerization ofhexyl methacrylate. Macro molecules, 30, 1897-1905. 

A growth in micellar size is always observed during the reaction due to the internal dynamics of microemulsions and inverse-microemulsions. This takes the form of either coagulation of active and inactive micelles or the diffusion of monomer from the unreacted micelles to the nucleated particles. Each final particle contains a number of macromolecules, on the order of one, in a collapsed state [33], with the particle size independent of the nature of the free radical initiator [34]. These features lead to a unique kinetic mechanism relative to the other heterophase polymerizations discussed herein [33, 35,36]. A more detailed discussion of microemulsion and inverse-microemulsion polymerizations are given in two recent reviews [37, 38]. 

The present review will mainly focus on inverse emulsion polymerization, the most commonly employed water-in-oil synthesis method and on inverse microemulsion polymerization which is more recent and offers some new prospects. The formulation components and their actions, the various structures of the colloidal dispersions prior to polymerization and some latex properties will be discussed. The kinetics and the mechanisms occurring in these water-in-oil systems will also be analysed and compared to the more conventional emulsion polymerization process. 

While inverse (mini)emulsion polymerization forms kinetically stable macro-emulsions at, below, or around the CMC, inverse microemulsion polymerization produces thermodynamically stable microemulsions upon further addition of emulsifier above the critical threshold. This process also involves aqueous droplets, stably dispersed with the aid of a large amount of oil-soluble surfactants... 

Most studies have dealt either with the free radical polymerization of hydrophobic monomers—e.g., styrene [56-89], methyl methacrylate (MMA) [68,73,74,84,86,90-93] or derivatives [2,94,97], and butyl acrylate (BA) [98-100]—within the oily core of O/W microemulsions or with the polymerization of water-soluble monomers such as acrylamide (AM) within the aqueous core of W/O microemulsions [101-123]. In the latter case, the monomer is a powder that has to first be dissolved in water (1 1 mass ratio) so that the resulting polymer particles are swollen by water, in contrast with O/W latex particles, where the polymer is in the bulk state. The polymerization can be initiated thermally, photochemically, or under )>-radiolysis. The possibility of using a coulometric initiation for acrylamide polymerization in AOT systems was also reported [120]. Besides the conventional dilatometric and gravimetric techniques, the polymerization kinetics was monitored by Raman spectroscopy [73,74], pulsed UV laser source [72,78], the rotating sector technique [105,106], calorimetry, and internal reflectance spectroscopy [95]. 

However, it is difficult to control the micelle formation during microemulsion polymerization, hi general, polymerization process is kinetically and thermodynamically unstable because of Ostwald ripening, the growth by collision between monomer droplets and monomer consumption during polymerization [154,155]. It is noteworthy that precise control of the micelle is essential to produce monodisperse and nano-sized conducting polymer nanomaterials. 

From a mathematical point of view, limiting cases of macroemulsion polymerization are mini- and microemulsion polymerizations. In miniemulsion polymerization, only small monomer droplets are present and these are also the main reaction locus. In microemulsion polymerization, the monomer droplets are also small and, in principle, reaction can take place in the monomer droplets as well as in the micelles and polymer particles. An important feature of a microemulsion is that it is thermodynamically stable, whereas the other emulsion types are only kinetically stable. However, if monomer is added very slowly and a small amount of surfactant is present, polymer particles gradually swell starting from a micelle population only. Thus, the emulsion polymerizations differ with respect to the populations present, but in all cases the latex obtained consists of segregated entities with a size at least one order of magnitude smaller than in suspension polymerization. In the most... 

Tablell.8 Variation of kinetic parameters in the microemulsion polymerization of acrylates [43]. 

In the second chapter (Preparation of polymer-based nanomaterials), we summarize and discuss the literature data concerning of polymer and polymer particle preparations. This includes the description of mechanism of the radical polymerization of unsaturated monomers by which polymer (latexes) dispersions are generated. The mechanism of polymer particles (latexes) formation is both a science and an art. A science is expressed by the kinetic processes of the free radical-initiated polymerization of unsaturated monomers in the multiphase systems. It is an art in that way that the recipes containing monomer, water, emulsifier, initiator and additives give rise to the polymer particles with the different shapes, sizes and composition. The spherical shape of polymer particles and the uniformity of their size distribution are reviewed. The reaction mechanisms of polymer particle preparation in the micellar systems such as emulsion, miniemulsion and microemulsion polymerizations are described. The short section on radical polymerization mechanism is included. Furthermore, the formation of larger sized monodisperse polymer particles by the dispersion polymerization is reviewed as well as the assembling phenomena of polymer nanoparticles. 

The inverse emulsion polymerization mechanisms and kinetics can be found in the literature [10,66-68]. The area of inverse emulsion polymerization has not been studied extensively, except perhaps for the inverse microemulsion polymerization of acrylamide. The most important applications for these acrylamide-based products are as polymeric flocculants in water treatment. The two major advantages of this polymerization process are the very high polymer molecular weight and a colloidal system that results in rapid dissolution of the polymer in water. 

Rgure 5.4. A schematic lepiesentation of typical polymerization rate as a function of monomer conversion profiles for (a) conventional emulsion polymerization (Interval II Smith-Ewart Case 2 kinetics), (b) miniemulsion polymerization, and (c) microemulsion polymerization. The distinct intenrals of the polymerization processes are also included in these plots. 

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