Mini emulsion polymerization

Suspension polymerization may be the most important particle-forming polymerization from an industrial viewpoint. The system is very simple, composed of monomer, initiator, stabilizer, and medium (water in most cases). The monomer droplets with dissolving initiator are dispersed in water and the stabilizer exists at the interface. But suspension polymerization is regarded as a kind of homogeneous polymerization because the polymerization occurs only in monomer droplets and water does not affect the polymerization. Water contributes only to dividing the polymerization locus into small droplets and absorbing the heat evolved by polymerization. On the contrary, in emulsion polymerization, which is another type of polymerization performed in water and as practically important as suspension polymerization, water affects the polymerization significantly. In this section, emulsion polymerization is first discussed, and then some modified emulsion polymerizations such as soap-free emulsion polymerization and micro and mini emulsion polymerizations are described. 

McCaffery etal. discuss the use of a low-resolution Raman spectrometer to directly monitor a batch mini-emulsion polymerization.41 While this kind of equipment is unlikely to be installed in an industrial facility, the article raises several important points. In order to compensate for laser-power fluctuations, a functional group present in both the reactants and the product, the phenyl ring in styrene, was used as an internal standard. Since internal standards cannot be added to industrial reactions, this approach can be quite helpful. However, scientists must be certain that the internal standard will remain unchanged by the reaction and that changes in its signal only reflect laser-power fluctuations. 

Various papers on hybrid mini emulsion polymerization have used alkyd [98, 99], polyester [100] or polyurethane [101] as both the costabihzer and a component of the hybrid particle. Since most of these materials were added far in excess of the levels normally used as costabihzers, it is not surprising that they are effective. 

Wang et al. [98] carried out macroemulsion and mini emulsion polymerization of acrylic monomers in the presence of alkyd resins. Miniemulsion and macroemulsion polymers were produced using a commercial medium soya-linseed alkyd and a mix of acrylic monomers consisting of 50% BA, 49% MMA, and 1% acrylic acid (AA). PMMA polymer with a weight average molecular weight of 100,000 was used as the costabilizer. Alkyd levels were 5, 30, 60 or... 

Wang et al. [183] carried out hybrid mini emulsion polymerization of acrylates in the presence of polyurethane. The polyurethane was used as the costabilizer, and SLS as the surfactant. When MMA was used as the monomer, some homogenous nucleation was observed. This is in agreement with Tsavalas [179] who reported evidence of homogenous nucleation in the hybrid miniemulsion of MMA in the presence of alkyd. 

From the above summary of experimental investigations into Uving miniemulsion polymerization, we can see that controlled mini emulsion polymerization (SFRP, ATRP, and RAFT) is less colloidal-stable during polymerization than its non-living counterpart. Colloidal instability leads to phase separation in the worst cases. In improved cases, the latex that results from the controlled mini-... 

Keywords. Mini-emulsion polymerization, Monomer droplets, Particle size, Emulsifier, Co-emulsifier/Hydrophobe... 

The high ratio of emulsifier to water ensures that a large fraction of the emulsifier is undissociated and located at the droplet surface or even inside the droplets. Two features of mini-emulsion polymerization are different from those of conventional emulsion polymerization ... 

In mini-emulsion polymerization, the particle nucleation mechanism may be evaluated by the ratio of the final number of polymer particles to the initial number of monomer droplets (Np f/Nm i). If the particle nucleation process is primarily governed by entry of radicals into the droplets, then the value of Np>f/Nm>i should be around 1. A lower value of Np f/Nm i may imply incomplete droplet nucleation or coalescence. On the other hand, a higher value of Npf/Nm>i may indicate that the influence of micellar or homogeneous nucleation comes into play in the particle formation process, since one droplet feeds monomer to more than one micelle in the classical emulsion polymerization. For pure micel-... 

Kinetics of Mini-Emulsion Polymerization of Conventional Monomers... 

Scheme 3. The four rate regions involved in mini-emulsion polymerization...

Table 1. Kinetic parameters in the mini-emulsion polymerization initiated by KPS in the presence of SDS/classical coemulsifierM...

The dependence of the maximal Rp on [KPS] is stronger for mini-emulsion polymerization (Rp°c[KPS]x, x=0.6) as compared with the conventional emulsion polymerization Rp°c[KPS]°36 which is quite close to the micellar nuclea-tion model (0.4), even though Rp is always faster in emulsion polymerization within the initiator concentration range studied. The exponent 0.6 is a result of two contributions ... 

The influence of the emulsifier (SHS) concentration on Np is more pronounced in the conventional emulsion polymerization system (Rp°c[SHS]y, y= 0.68) than in mini-emulsion polymerization (y=0.25). This result is caused by the different particle formation mechanism. While homogeneous nucleation is predominant in the conventional emulsion polymerization, monomer droplets become the main locus of particle nucleation in mini-emulsion polymerization. In the latter polymerization system, most of the emulsifier molecules are adsorbed on the monomer droplet surface and, consequently, a dense droplet surface structure forms. The probability of absorption of oligomeric radicals generated in the continuous phase by the emulsifier-saturated surface of minidroplets is low as is also the particle formation rate. 

In conventional emulsion polymerization, the disappearance of the VAc/BA droplets at ca. 25% conversion results from the transfer of monomer from monomer droplets to the locus of polymerization (monomer-swollen polymer particles). The presence of HD in the minidroplets reduces the free energy of mixing of the constituent monomers in the droplets. Therefore, the difference in the free energy of mixing between the monomer and polymer (particles) in mini-emulsion copolymerization is less than that in conventional emulsion polymerization. As a consequence, a smaller flux of monomer from the monomer droplets to polymer particles is achieved during mini-emulsion polymerization. In addition, HD cannot be transported from the droplets to particles because of its extremely low water solubility. Thus, the HD concentration in the droplets is greater than that in the particles, and monomer is retained in the droplets to minimize the HD concentration gradient. 

It was shown that the basic principle of mini-emulsion polymerization can be extended to the reaction systems stabilized by cationic and non-ionic emulsifiers, leading to a narrow particle size distribution [ 102]. Besides, the effect of the... 

The polymer latex stability obtained from the mini-emulsion polymerization with various ratios of SDS/CA decreases in the series l/3>l/10>l/l>l/6>l/0, which is consistent with the stability of monomer droplets reported by Ugelstad (l/3>l/2>l/l>l/6>l/0) [106]. The latex particle size decreases with increasing CA concentration. Furthermore, a two-dimensional hexagonal packing of surface-active molecules has been reported to be formed at a molar ratio of SDS/CA=l/3 in the colloidal system [107]. The good packing of the oil-water interfacial zone leads to satisfactory stability of monomer droplets, and it remains intact throughout the polymerization. 

The dependence of the maximal Rp on [KPS] is quite similar for both the MMA mini-emulsion polymerization with HD (x=0.4) and the conventional emulsion polymerization (x=0.39) but different on [SDS] (y=0.16,ME) and (y= 0.24, CE) [ 108]. The reaction orders x and y are a complex function of the radical entry (particle nucleation) and the extent of compartmentalization of radicals. The radical entry or particle nucleation increases Rp. Np increases with increasing [KPS] and the degree of increase is more pronounced for the MMA emulsion polymerization (Np°c[KPS]x, x =0.28) as compared with that for the MMA mini-emulsion polymerization (x =0.11) (Table 1). The radical entry events are restricted due to the close-packed droplet surface layer, but the pseudo-bulk ki-... 

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