Initiators aqueous phase, particle formation

Emulsion polymerization is the polymerization technique that starts with emulsified monomer in the continuous aqueous phase. Polymer formation takes place in the micelles and is initiated by water-soluble initiators. The monomers are insoluble or sparingly soluble in water. Emulsion polymerization is used very frequently in order to perform encapsulation of inorganic particles with polymers where water-based coatings are required. For the encapsulation of inorganic particles, seeded emulsion polymerization is performed hydrophobic inorganic particles are dispersed with normal surfactants or protective colloids in the aqueous phase. As polymerization on the surface of inorganic particles is always in competition with secondary particle formation, the concentration of the surfactants should be lower than their critical micelle concentration. However, homogeneous nucleation can also occur, which... 

Emulsion polymerizations most often involve the use of water-soluble initiators (e.g. persulfate see 33.2.6.1) and polymer chains are initiated in the aqueous phase. A number of mechanisms for particle formation and entry have been described, however, a full discussion of these is beyond the scope of this book. Readers are referred to recent texts on emulsion polymerization by Gilbert4 and Lovell and El-Aasser43 for a more comprehensive treatment. 

Radicals typically are generated in the aqueous phase and it is now generally believed that formation of an oligomer of average chain length z (z-mer P/) occurs in the aqueous phase prior to particle entry.44 The steps involved in forming a radical in the particle phase from an aqueous phase initiator are summarized in Scheme 3.17. The length of the z-mcr depends on the particular monomer and is shorter for more hydrophobic monomers. 

Various initiation strategies and surfactant/cosurfactant systems have been used. Early work involved in situ alkoxyamine formation with either oil soluble (BPO) or water soluble initiators (persulfate) and traditional surfactant and hydrophobic cosurfactants. Later work established that preformed polymer could perform the role of the cosurfactant and surfactant-free systems with persulfate initiation were also developed, l90 222,2i3 Oil soluble (PS capped with TEMPO,221 111,224 PBA capped with 89) and water soluble alkoxyamines (110, sodium salt""4) have also been used as initiators. Addition of ascorbic acid, which reduces the nitroxide which exits the particles to the corresponding hydroxylamine, gave enhanced rates and improved conversions in miniemulsion polymerization with TEMPO.225 Ascorbic acid is localized in the aqueous phase by solubility. 

Rate of Formation of Primary Precursors. A steady state radical balance was used to calculate the concentration of the copolymer oligomer radicals in the aqueous phase. This balance equated the radical generation rate with the sum of the rates of radical termination and of radical entry into the particles and precursors. The calculation of the entry rate coefficients was based on the hypothesis that radical entry is governed by mass transfer through a surface film in parallel with bulk diffusion/electrostatic attraction/repulsion of an oligomer with a latex particle but in series with a limiting rate determining step (Richards, J. R. et al. J. AppI. Polv. Sci.. in press). Initiator efficiency was... 

The emulsion polymerization system consists of three phases an aqueous phase (containing initiator, emulsifier, and some monomer), emulsified monomer droplets, the monomer-swollen micelles, and monomer-swollen particles. Water is the most important ingredient of the emulsion polymerization system. It is inert and acts as the locus of initiation (the formation of primary and oligomeric radicals) and the medium of transfer of monomer and emulsifier from monomer droplets or the monomer-swollen particle micelles to particles. An aqueous phase maintains a low viscosity and provides an efficient heat transfer. 

In the case of more water-soluble monomers and (amphiphilic) macromonomers, the Smith-Ewart [16] expression does not satisfactorily describe the particle nucleation. The HUFT [9,10] theory, however, satisfactorily describes the polymerization behavior or the particle nucleation of such unsaturated hydrophilic and amphiphilic monomers. The HUFT approach implies that primary particles are formed in the aqueous phase by precipitation of oligomer radicals above a critical chain length. The basic principals of the HUFT theory is that formation of primary particles will take place up to a point where the rate of formation of radicals in the aqueous phase is equal to the rate of disappearance of radicals by capture of radicals by particles already formed. Stabilization of primary particles in emulsifier-free emulsion polymerization may be achieved if the monomer (or macromonomer) contains surface active groups. Besides, the charged radical fragments of initiator increases the colloidal stability of the polymer particles. 

Some polymer-composition vs. conversion curves were obtained for the copolymerizations with different f s (Figure 2), and all of them seem to intersect the ordinate at 1.0. From the initial slope of the curves and the monomer ratio in the aqueous phase the monomer reactivity ratio was calculated, but the calculation resulted in a negative r2. Therefore, it was concluded that the copolymerization could not be regarded as a homogeneous one even just after the beginning of the reaction. The first stage was considered to be a transitional stage to establish the particle formation. 

For the styrene/hexadecane system, the amount of initiator does not have an effect on the particle number, but in the case of more water-soluble monomers, for example MMA and vinyl chloride [67], secondary particle formation was observed. Here, the amount of new particles increases with the concentration of the water-soluble initiator. Homogeneous nucleation in the water phase can be restrained by using a water-soluble redox initiator, e.g., (NH4)S208/NaHS03 at lower temperature (45°C) [68] or even more efficiently by using an interfacial acting redox initiator (cumene hydroperoxide/Fe2+/ethylenediamine tetraacetate (EDTA)/sodium formaldehyde sulfoxylate (SFS)) [69, 70] to initiate the miniemulsion polymerization. The hydrophobic radicals decrease the homogeneous nucleation in the aqueous phase. 

Polymerization. Figure 1- shows how the formation of particles during the polymerization depends on the initial concentration of SDS in the aqueous phase. In order to achieve a monodisperse latex, the particle nucleation must be confined to the initial stage of the polymerization. No new particles can be permitted to form during the reaction, and agglomeration of latex particles must be prevented- Thus both a too high and a too low emulsifier concentration must be avoided (I6). This was accomplished by careful postaddition oT emulsifier. 

We know that the particle formation mechanisms can be much more complex, especially for monomers such as MMA and VA. The sulfate ion radical formed from persulfate initiator is not likely to be strongly attracted to either micelles or polymer particles due to its charge and hydrophilic nature. Instead, this radical will begin to polymerize monomer which is dissolved in the water phase. As monomer units are added, the aqueous phase ion radical will become less hydrophilic and more surface active. During the period of surface activity, it may well adsorb on a micelle, a polymer particle, or a monomer drop. 

Once a micelle is stung, polymerization proceeds very rapidly. The particle can accommodate more monomer as its polymer content increases and the water-polymer interfacial surface increases concuirently. Tlie new surface adsorbs emulsifier molecules from the aqueous phase. This disturbs the equilibrium between micellar and dissolved soap, and micelles will begin to disintegrate as the concentration of molecularly dissolved emulsifier is restored to its equilibrium value. Thus the formation of one polymer particle leads to the disappearance of many micelles. The initial latex will usually contain about 10 micelles per milliliter water, but there will be only about 10 particles of polymer in the same volume of the final emulsion. When all the micelles have disappeared, the surface tension of the system increases because there is little surfactant left in solution. Any tendency for the mixture to foam while it is being stirred decreases at this time. 

The equations for dN/dt and dRj/dt, as well as for dV dt [Eq. (39)] are solved by numerical integration for the polymerization stem MMA-K2S20e water, with rate constants obtained from the literature. The initiator efficiency was set equal to unity. Particle numbers between 10 and 10 were drained for initiator concentrations of 10 -10 mol/dm. The calculations showed that N should be almost independent of the chosen value offor values between 5 and 70 (in strong contrast to our calculations). The reason for this is probably that aqueous-phase termination with subsequent precipitation is the dominant particle-formation mechanism in Aral s model, even more so with increasing initiator concentration. The theoretical particle-formation time was on the order of 2 sec, a veiy low value compared to the experimental results of Fitch and Tsai. Aral et at. found that their calculated particle numbers were approximately in accordance with the experimental results of Yamazaki et al. (1968) for emulsifier-free polymerizations. Aral s model does not inclnde any coagulation mechanisms. It will therefore have the same shortcomings as most other models, namely that the strongly increased particle number in... 

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