Effect of Functional Monomers and Initiators on Particle Nucleation

3 Effect of Functional Monomers and Initiators on Particle Nucleation 

The presence of hydrophilic, functional monomers such as 2-hydroxyethyl methacrylate, acrylic acid, and methacrylic acid can play an important role in the free radical polymerization taking place in the continuous aqueous phase. Chem and Sheu [54] studied the effect of using a small quantity of 2-hydroxyethyl methacrylate in the styrene miniemuision polymerization system with sodium dodecyl sulfate and long chain alkyl (lauryl and stearyl) methacrylates as the surfactant and costabilizers, respectively. Both the populations of latex particles originating from monomer droplet nucleation and homogeneous nucleation in the miniemuision polymerization system in the presence of 2-hydroxyethyl methacrylate become larger as compared to those in the absence of 2-hydroxyethyl methacrylate. Nevertheless, the fraction of 

The influence of the type of initiators (sodium persulfate versus 2,2 -azobisisobutyronitrUe) on the particle nucleation mechanisms and kinetics involved in the styrene miniemulsion polymerizations has also been studied [39]. As expected, the oil-soluble 2,2 -azobisisobutyronitrile promotes nucleation in the homogenized monomer droplets. On the other hand, formation of particle nuclei in the continuous aqueous phase becomes more important when water-soluble sodium persulfate is used. This result is consistent with the vinyl chloride work of Saethre et al. [44]. The number of polyvinyl chloride latex particles generated by mechanisms other than monomer droplet 

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. 

Beyond Interval II, the second maximal polymerization rate can be attributed to the gel effect. The bimolecular termination reaction becomes diffusion-controlled in the latex particles and the average number of free radicals per particle increases signihcantly in the latter stage of polymerization, thereby leading to an acceleration of the free radical polymerization. The rate of polymerization then decreases continuously toward the end of polymerization due to the depletion of monomer and/or the diffusion-controlled propagation reaction in the reaction loci. 

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