Aggregate growth, diffusion-controlled

The state of aggregation of the polymerising system represents another important factor which may affect the kinetics of polymerisation. It is well known (96,97) that many radical polymerisations are enhanced by increase in the viscosity of the po-lymerisingsystem, and this phenomenon was explained by a decrease in the rate of termination step which may become diffusion-controlled. In fact, the effect of viscosity should be observed at any stage of radical polymerisation, and this problem has been discussed recently by Benson and North (98, 99). Of course, this type of acceleration cannot be observed when the growth involves living polymers and therefore such an explanation does not apply to polymerisation of NCA, particularly since no termination resulting from active end-active end interaction takes place in these processes. 

Other types of supports have other physical and mechanical properties and therefore different fragmentation behavior and kinetic characteristics. MgCl2 as a support fragments much earlier and extensively even at low polymer yields because it consists of loose agglomerations of many small crystalline subparticles [52-54], Hence, here the polymerization rate shows no initial period of low activity, but immediately rises steeply, passes through a maximum, and decelerates slowly in a diffusion-controlled manner. A similar kinetic behavior is observed when reversibly aggregated polymer latex nanoparticles are used as support. In ethylene slurry polymerization [55], the monomer at once has access to the primary latex particles so that polymerization and macroparticle growth start immediately and rise steeply. 

It is clear that if we wish to produce particles with a fairly uniform size, then one short burst of nucleation should occur in a short time interval, ti — ti. One way of achieving this is through the use of a fairly low reactant concentration. Furthermore, uniform growth of the particles requires that the solute be released slowly to allow diffusion to the particles without buildup of the solute concentration and further bursts of nucleation. This mechanism of nucleation followed by diffusion-controlled growth does not apply to the formation of particles that are aggregates of finer primary particles. Instead, it may apply only to the primary particles. 

Second, nucleation and growth of Stober silica particles is modeled by a controlled aggregation mechanism of subparticles, a few nanometers in size, as for example presented by Bogush and Zukoski (19). Colloidal stability, nuclei size, surface charge, and diffusion and aggregation characteristics are the important parameters in this model. 

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