Emulsion polymerization nucleation mechanisms

Emulsion Polymerization. Emulsion SBR was commercialised and produced in quantity while the theory of the mechanism was being debated. Harkins was among the earliest researchers to describe the mechanism (16) others were Mark (17) and Elory (18). The theory of emulsion polymerisation kinetics by Smith and Ewart is still vaUd, for the most part, within the framework of monomers of limited solubiUty (19). There is general agreement in the modem theory of emulsion polymerisation that the process proceeds in three distinct phases, as elucidated by Harkins (20) nucleation (initiation), growth (propagation), and completion (termination). 

This paper presents the physical mechanism and the structure of a comprehensive dynamic Emulsion Polymerization Model (EPM). EPM combines the theory of coagulative nucleation of homogeneously nucleated precursors with detailed species material and energy balances to calculate the time evolution of the concentration, size, and colloidal characteristics of latex particles, the monomer conversions, the copolymer composition, and molecular weight in an emulsion system. The capabilities of EPM are demonstrated by comparisons of its predictions with experimental data from the literature covering styrene and styrene/methyl methacrylate polymerizations. EPM can successfully simulate continuous and batch reactors over a wide range of initiator and added surfactant concentrations. 

Emulsifier is not a necessary component for emulsion polymerization if ihe following conditions are satisfied The particles are formed by homogeneous nucleation mechanism, and the particles are stabilized by factor(s) olher than emulsifier. As to the latter, the sulfate end group that is the residue of persulfate initiator serves for stabilization of dispersion via interparticle electrorepulsive force (20). When the stabilization mechanism works well, a small number of particles grow during polymerization without aggregation, keeping the size distribution narrow. Finally stable, monodisperse, anionic particles are obtained. 

In accordance with the Smith-Ewart theory, the nucleation of particles takes place solely in the monomer-swollen micelles which are transformed into polymer particles [16]. This mechanism is applicable for hydrophobic (macro)mon-omers (see Scheme 2). The initiation of emulsion polymerization is a two-step process. It starts in water with the primary free radicals derived from the water-soluble initiator. The second step occurs in the monomer (macromonomer)-swollen micelles by entered oligomeric radicals. 

Thus in the emulsifier-free emulsion copolymerization the emulsifier (graft copolymer, etc.) is formed by copolymerization of hydrophobic with hydrophilic monomers in the aqueous phase. The ffee-emulsifier emulsion polymerization and copolymerization of hydrophilic (amphiphilic) macromonomer and hydro-phobic comonomer (such as styrene) proceeds by the homogeneous nucleation mechanism (see Scheme 1). Here the primary particles are formed by precipitation of oligomer radicals above a certain critical chain length. Such primary particles are colloidally unstable, undergoing coagulation with other primary polymer particles or, later, with premature polymer particles and polymerize very slowly. 

Microemulsion polymerizations follow a different mechanism from the conventional emulsion polymerizations. The most probable locus of particle nucle-ation was suggested to be the microemulsion monomer droplets [27], although homogeneous nucleation was not completely ruled out. The particle generation rate in microemulsion polymerization is given by an expression similar to Eq. (21), which was used for the miniemulsion polymerization of styrene [28] ... 

The C12-(EO)9-MA macromonomer was found to be a very effective emulsifier for BzMA in water even at a concentration less than 5 wt%, to give a stable milky emulsion [42,96]. Table 3 shows that the rate of polymerization depends on the initiator type and polarity of continuous phase. In water solution, the rates are several times higher than in heptane. The rate of polymerization increases with increasing macromonomer concentration in systems with KPS and AIBN, and it is constant with AVA. The higher the macromonomer concentration, the higher the particle concentration and rate of polymerization. These results indicate that distribution of the initiator between the phases influences in complex way the polymerization and nucleation mechanism. 

In the emulsifier free-emulsion polymerization the reaction loci are formed by nucleation of amphiphilic macromomer micelles (micellar mechanism) or by... 

Instead of conventional surfactant molecules, amphiphilic water soluble macromonomers, especially PEO macromonomers, have been used extensively as a reactive emulsifier and as steric stabilizer polymer, as summarized in Table 5. Generally speaking, however, the mechanism for the particle nucleation in the emulsion polymerization systems using macromonomers has been poorly established when compared to the dispersion copolymerizations with macromonomers as mentioned earlier. 

Unzueta et al. [18] derived a kinetic model for the emulsion copolymerization of methyl methacrylate (MMA) and butyl acrylate (BA) employing both the micellar and homogeneous nucleation mechanisms and introducing the radical absorption efficiency factor for micelles, F, and that for particles, Fp. They compared experimental results with model predictions, where they employed the values of Fp=10 and Fn,=10", respectively, as adjustable parameters. However, they did not explain the reason why the value of Fp, is an order of magnitude smaller than the value of Fp. Sayer et al. [19] proposed a kinetic model for continuous vinyl acetate (VAc) emulsion polymerization in a pulsed... 

The kinetics and mechanisms of particle growth and polymer structure development are comparatively well understood compared to those of particle nucleation. Therefore, the rate of polymerization and the properties of the polymer produced can be (roughly) estimated as long as the number of polymer particles produced is known (for example, in seeded emulsion polymerization). However, the prediction of the number of polymer particles produced is still far from being an estabUshed technique. Therefore, further efforts are needed to qualitatively and quantitatively clarify the effects of numerous factors that affect the process of particle formation in order to gain a more quantitative understanding of emulsion polymerization. 

Micellar nucleation may not be the only, or even primary process of nuclea-tion and growth. Other mechanisms are discussed later. To provide a comprehensive model for emulsion polymerization, the apphcability of each mechanism must be considered. 

As pointed out above, particle nucleation includes all three mechanisms -micellar, homogeneous, and droplet, since these mechanisms may compete and coexist in the same system. Often one will dominate. Therefore, any general model of emulsion polymerization should include all three mechanisms. Hansen and Ugelstad [31] and Song [10] have presented probabilities for each of these mechanisms in the presence of all three. 

Reimers and Schork [94, 95] report the use of PMMA to stabihze MM A miniemulsions enough to effect predominant droplet nucleation. Emulsions stabilized against diffusional degradation by incorporating a polymeric costabilizer were produced and polymerized. The presence of large numbers of small droplets shifted the nucleation mechanism from micellar or homogeneous nucleation, to droplet nucleation. Droplet diameters were in the miniemulsion range and reasonably narrowly distributed. On-hne conductance measurements were used to confirm predominant droplet nucleation. The observed reaction rates were dependent on the amount of polymeric costabilizer present. The latexes prepared with polymeric costabilizer had lower polydispersities (1.006) than either latexes prepared from macroemulsions (1.049) or from alkane-stabilized miniemulsions (1.037). 

Besides giving latices of narrow particle size distribution, mixed surfactant systems have shown several other interesting characteristics which lighten some aspects concerning the mechanism of particle nucleation in emulsion polymerization process. 

In the polymerization of styrene, using potassium persulfate as initiator. Roe ( ) observed that the total number of particles in latices depended on the composition of the mixed surfactants and not on the total number of micelles. Therefore, he devaluated the micellar nucleation mechanism for emulsion polymerization as proposed by Harkins( ) -Smith-Ewart(j). 

In addition to the practical interest, the process presents challenges encouraging further fundamental exploration. A thorough study not reported here, has been performed on the mechanism and kinetics of the polymerization of acrylamide in AOT/water/toluene microemulsions (Carver, M.T.r Dreyer, U. Knoesel, R. Candau, F. Fitch, R.M. J. Polym. Sci. Polym. Chem. Ed., in press. Carver, M.T. Candau, F. Fitch, R.M. J. Polym. Sci. Polym. Chem. Ed., in press). The termination reaction of the polymerization was found to be first order in radical concentration, i.e. a monoradical reaction instead of the classical biradical reaction. Another major conclusion was that the nucleation of particles is continuous all throughout the polymerization in contrast to conventional emulsion polymerization where particle nucleation only occurs in the very early stages of polymerization. These studies deserve further investigations and should be extended to other systems in order to confirm the unique character of the process. 

Usually, monomer droplets are bdieved not to play any role in emulsion polymerization other than as a source of monomer. Ugelstad and associates have shown, however, that in cases with very small monomer droplets, these may become an important, or even the sole, loci for particle nucleation. The system may then be regarded as a microsuspension polymerization with water-soluble initiators. It has therefore been pointed out (Hansen and Ugelstad, 1979c) that particle nucleation mndels should include all three initiation mechanisms— micellar, homogenous, and droplet—since all these mechanisms may compete and coexist in the same system, even if one of them usually dominates. 

The prediction of the evolution of the PSD in Interval II is simpler than that in the ether intervals and it was for this reason that it was discussed first. Even the qualitative features of particle formation in Interval I are in doubt and the relative importance of homogeneous (ije., oligomeric precipitation) versus heterogeneous (i.e., micellar) nucleation mechanisms are not fully understood. For tbis reason, detailed solutions to Eq. (S) in this Interval, when c is nonzero, appear to be premature. Moreover, in many emulsion polymerizations, the precise details of events occurring in Interval I are masked by the subsequent particle growth in Intervals II and III. 

---