Growth of polymer particles

Growth of Polymer Particles in Vinyl Chloride Emulsion Polymerization... 

The growth of polymer particles constitutes the driving force not only for mass transport of monomer to the main reaction site, but also for adsorption of surfactant onto the growing surface of the particles. Hence, micelles (if present) disaggregate and their concentration diminishes with time until they eventually disappear, that is, when the surfactant concentration falls below CMC at this point, micellar nucleation ceases. Only about 1/lOOOth of the micelles initially present act as nucleation sites, and the rest disaggregate to stabilize the growing particles. 

Despite several advantages (water as the dispersion medium is environmentally friendly) [272], i.e. higher polymerization rates [273] and the relative simplicity of the process, the emulsion polymerization involves many mechanistic events, and the understanding of the events that dictate the rate of formation and the growth of polymer particles is difficult [274]. The mechanism of emulsion polymerization is shown in Fig. 1.23. 

Goebel et al. [88] have found that the particle concentration decreases with conversion and the decrease is inversely proportional to emulsifier concentration. The surface tension parallels the number of particles and both abruptly increase with conversion up to 10% conversion. The higher the emulsifier concentration, the higher the number of particles which are observed. Keeping a constant concentration of emulsifier, the number of particles decreases with increasing monomer concentration. When the monomer pressure decreased by one third, the number of particles increased twice. This was ascribed to variation of the emulsifier adsorption activity with the monomer pressure. The particle concentration remains constant if the small amount of inhibitor is added. The growth of polymer particles was reported to proceed by the propagation of monomer as well as by the association of polymer particles. 

Fig. 3.12 Polymerization of acrylamide monomer (M) in AOT-stabilized w/o microemulsion a micelles and water-swollen AOT droplets (d 6 nm) before polymerization b growth of polymer particle—(a) by monomer (M) diffusion through toluene phase and (b) by collision between particles c polymer particles (d 40 nm) and small micelles (d 3 nm) (after [68])...

Figure 7.4, a summary of various studies, reveals that in the second stage, fewer primary particles are formed and polymerization occurs essentially by the growth of polymer particles. As the propagation continues, monomer molecules from the emulsified monomer droplets (see Fig. 7.5) diffuse toward the propagating chains within the polymer particles. The diffusion of the monomer to the polymer particles continues at a fairly rapid rate to maintain constant monomer concentration in the polymer particles. 

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