Emulsion and Miniemulsion Polymerization

These polymerizations normally utilize the formation of an emulsion from a mixture of water, monomer, and surfactant. The most common type of emulsion polymerization is an oil-in-water (0/W) emulsion, in which droplets of monomer (the oil) are emulsified (with surfactants) in a continuous phase of water. Water-soluble polymers, such as certain polyvinyl alcohols can also be used to act as emulsifiers/stabilizers for the monomer droplets. The polymerization takes place within the monomer latex particles, which are composed of individual polymer chains and are typically 100 nm in size. These latex particles form 

The major advantages of miniemulsion polymerizations over related emulsion systems are  

These features all enable more precise control over the resultant particle size and also facilitate the synthesis of a wider range of materials than conventional emulsion processes allow. 

The small droplet size (in the nm regime), high kinetic stability and optical transparency of miniemulsions enable the further application of these materials compared to conventional emulsion systems. In particular, miniemulsions have been primarily used to prepare well-defined nanometer-sized polymeric nanoparticles, using the monomer as the dispersant phase. Surfactants containing a polymerizable group (otherwise known as a surfmers) can be used to protect, stabilize and functionalize the polymer particles. 

One of the key features of miniemulsion droplets is their extremely high stability that prevents any exchange reaction occurring between the payloads of different droplets. This was elegantly demonstrated by the suppression of the formation of Prussian blue upon mixing of two droplets each of which contained a reactive precursor (K4[Fe(CN 6] and FeCls) to the blue pigment, compared to the formation of the complex (indicated by the formation of the bright blue color) when the droplets where formed by a micellar assembly or a conventional emulsion polymerization method. This is proposed to be very important in delivery applications (Landfester, 2006). 

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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. 

Any colloidal material provides an intrinsically favorable accessibility to its surface when compared to bulk material. Therefore, the availability of receptor binding sites should be facilitated by using colloidal MIPs. Submicron scale MIPs were prepared by precipitation polymerization, emulsion polymerization, and miniemulsion polymerization. Precipitation polymerization uses the insolubility of the formed polymer microgel in a certain solvent, whereas emulsion and miniemulsion polymerization employ two solvent phases for the preparation of the colloidal polymer. The latter methods offer the opportunity for tailoring the surface of the colloids exclusively, thereby enhancing the accessibility of the binding sites. Each of the three approaches has their own characteristics and will be described in the following sections. 

Scheme 7.2 Free radical suspension, emulsion and miniemulsion polymerization.

Ouzine K, Graillat C, McKenna T (2004) Continuous tubular reactors for latex production conventional emulsion and miniemulsion polymerizations. J Appl Polym Sci 91 2195-2207... 

Apphcation of ATRP to dispersed aqueous systems (emulsion and miniemulsion polymerization) has attracted attention in recent years as it may provide process and economic advantages over the traditional homogeneous bulk and solution polymerizations. However, adaptation of the ATRP process to aqueous dispersions poses several challenges that originate mainly from having more than one phase in the reaction mixture which lead to issues related to phase partitioning and trans-... 

Baruch-Sharon, S. and Margel, S. (2010). Synthesis and characterization of poly-chloromethylstyrene nanoparticles of narrow size distribution by emulsion and miniemulsion polymerization processes. Colloid and Polymer Science, 288, 869-877. 

A broad range of polymers are produced by polymerization in heterogeneous media, including polyolefins manufactured by slurry (high density polyethylene and isotactic polypropylene) and gas phase (linear low density polyethylene and high density polyethylene) polymerization coatings and adhesives produced by emulsion and miniemulsion polymerization flocculants obtained by inverse emulsion and microemulsion polymerization poly(vinyl chloride) (PVC) and polystyrene produced by suspension polymerization and toners synthesized by dispersion polymerization. As a whole, they represent more than 50% of the polymer produced worldwide [1]. 

Upon addition of the initiator, the polymerization reaction proceeds, the characteristics of the products depending on the initial monomer dispersion. Thus, in the case of the microemulsion polymerization, small particles (20-60nm) are formed emulsion and miniemulsion polymerizations lead to polymer dispersions of similar size (most often 80-3(X)nm), but miniemulsion polymerization allows the production of composite particles not attainable otherwise. Suspension polymerization yields relatively large particles (50-1000pm). 

The electrical conductivity of a fluid is a quantitative measure of its ability to carry an electrical current, and therefore depends to a large extent on the concentration of ionic species. Given that the conductivity of pure water is extremely low (limited to 0.0548 xScm" at 25 °C by the HjO dissociation constant into H and OH" when no added ions are present), this technique will be sensitive to changes in ionic concentration. So, while it is not impossible to be used for the online monitoring of solution or melt phase processes, it is better suited for use in emulsion and miniemulsion polymerization reactions where ionic surfactants and initiators are commonly employed. 

In conclusion, online conductivity is a useful tool for the online monitoring emulsion and miniemulsion polymerizations that are stabilized by anionic surfactants. If applied in a... 

As discussed, emulsion polymerization is an important indnstrial process. Miniemulsion polymerization is a rapidly anerging technology. Conversion of monomer to polymer is one of the most important process variables in batch or semibatch polymerization. This is affected by a series of variables among which impurities, oxygen in the system and poor inifiator (free radical) are typical ones. Attenuated total reflection (ATR)-UV spectroscopy was evaluated as a method for monitoring emulsion and miniemulsion polymerization of acrylates with excellent results [23]. 

It should be noted that the application of controlled-radical polymerization (CRP) techniques in emulsion polymerizations has not yet been particularly successful due to the relatively large size of the monomer droplets and insolubility of the radicals generated. However, Charleux and coworkers have reported the utilization of SGI-based alkoxyamines for the emulsion and miniemulsion polymerization of styrenes and acrylates to afford very well-defined and relatively small polymer particles (about 50 nm) (Figure 17.13) (Charleux and Nicolas, 2007). In fact, from a practical point of view very few experimental parameters had to be changed with respect to the classical solution polymerization recipe (Qi et al, 2001 Cunningham, 2002). Indeed, the recent advances in miniemulsion techniques have enabled the... 

Much has now been written on the use of RAFT in emulsion and miniemulsion polymerization, and many reviews relating to the use of RAFT in heterogeneous media have appeared. Our first communication on RAFT polymerization briefly mentions the successful emulsion polymerization of butyl methacrylate with cumyl dithiobenzo-ate as a table entry. Additional examples and brief discussion of some of the important factors for successful use of RAFT polymerization in emulsion and miniemulsion were provided... 

H) Homopolymerization and copolymerization Miniemulsion homopolymerizations of vinyl chloride, VAc, MMA, BA, styrene, VeovalO, dodecyl methacrylate, and stearyl methacrylate have been reported. °° ° In miniemulsion homopolymerization, once the polymer particles are formed, the process evolves in a similar manner as in interval III of a conventional emulsion polymerization, that is, in absence of monomer droplet phase. The differences in the polymerization rates observed when comparing conventional emulsion and miniemulsion polymerizations can be attributed to the different number of polymer particles formed in each process, which can be substantially different depending on the initiator systems employed. 

Figure 11 Evolution of the volume fraction of polymer in the particles as a function of conversion for emulsion and miniemulsion polymerization processes.

H) Emulsion and miniemulsion polymerization Emulsion-type polymerizations for ROMP of norbornene and its derivatives were first reported using hydrates of Ru, Ir, and Os four decades ago. " However, polymerization rates were very low. ROMP using water-soluble ruthenium carbene complexes as catalysts was used to polymerize functionalized 7-oxanorbornenes not only in water and methanol, but also in aqueous emulsions. Gationic water-soluble aliphatic phosphines were used in the synthesis of the ruthenium carbene complexes. The polymer polydispersity was low (1.1-1.3), although few details of the dispersed phase polymerization were reported. 

Recently, functionalized latexes have been obtained by means of CRP in an ab initio batch emulsion and miniemulsion polymerization process. Even though these woiks may not always intentionally be making surface-functional particles, it is a very efficient way of doing so. The approach consists in adding a hydrophilic and water-soluble control agent and in situ building up a polymeric surfactant, which after micellization causes the control agent to be inside the latex particle. 

Furthermore, this approach was also translated very effectively to nitroxide-mediated emulsion and miniemulsion polymerization [73-76] to prepare in situ PAA-based hairy nanoparticles using a water-soluble alkoxyamine initiator. 

There has been extensive interest in preparing polymer lay nanocomposites by using direct emulsion and miniemulsion polymerizations, which are covered in other chapters of this book. The focus of this section is on using nascent clay particles as stabilizing agents for direct emulsion polymerization. 

Mechanical Properties of the Latex Films. Figure 13.18 shows a noteworthy effect of the addition of Laponite on the mechanical properties of the nanocomposite film. A remarkable increase in the storage modulus at the rubbery plateau was observed with the addition of only 5 wt% Laponite. This improvement of the mechanical properties of the poly(styrene-co-butyl acry-late)-Laponite film was expected, considering the good dispersion of the clay platelets inside the polymeric matrix and the armored morphology of the hybrid particles. It is worth remembering here that similar results were reported by Ruggerone and co-workers" and Faucheu et al. for PS-Laponite films obtained from latexes prepared by emulsion and miniemulsion polymerization, respectively (see Sections 13.3.2.1 and 13.3.2.2). 

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