Particle growth termination

When an aqueous phase radical enters the polymer particles it becomes a polymer phase radical, which reacts with a monomer molecule starting a propagating polymer chain. This chain may be stopped by chain transfer to monomer, by chain transfer to agent or it may terminate by coupling. Small radicals in the particle may also desorb from or reenter the particle. In a batch reactor. Interval I indicates the new particle formation period, Interval II particle growth with no new particles, and Interval III the absence of monomer droplets. 

Condensation takes place in such a way as to maximize the number of Si-O-Si bonds and minimize the number of terminal hydroxyl groups by internal condensation. Thus rings are quickly formed to which monomers add, creating three-dimensional particles.12 By a variation of pH and salt addition the particle aggregation into secondary particles (A) or further particle growth (B) is controlled. Thus the particle size and pore structure of the silica is determined. 

The rate of growth of particles depends on the concentration of the reagents in the particles, radicals and monomer, and on the propagation rate constant. The gel effect, which causes the termination rate constant to be lower at higher conversions, can cause higher free radical concentrations in the particles and thus higher particle growth rates. This effect also contributes to conversion oscillations. 

The number-average particle diameter (dn) at different R values obtained after 21 and 160 h of reaction time are shown in Figure 2. As R increases in this range, the terminal particle size decreases sharply, reaches a minimum at R values of about 1.4, and then increases again. A similar trend is observed for the particle size at earlier reaction times. Thus, at up to 21 h of reaction time, particle growth is much faster at the lower R range. Particles produced at low R values (below about 1.3) have achieved about 80% of their final size in this time, whereas the percentage decreases to 75, 70, and 65% for R values of 1.73, 2.43, and 3.54, respectively. 

The chain of reactions can be accelerated by vibrational excitation of molecules. The typical reaction time is about 0.1 ms and is much faster than ion-ion recombination (1-3 ms), which determines the termination of the chain. As the negative cluster size increases, the probability of reactions with the vibrationally excited molecules decreases because of an effect of vibrational-translational (VT) relaxation on the cluster surface. When the particle size reaches a critical value (about 2 nm at room temperature) the cliain reaction of cluster growth becomes much slower and is finally stopped by the ion-ion recombination process. The typical time for 2 nm particle formation by this mechanism is about 1 ms at room temperature. A critical temperature effect on particle growth is partially due to VT relaxation, which depends exponentially on translational gas temperature according to the Landau-Teller effect (Section 2.6.2). Even a small increase of gas temperature results in a reduction of the vibrational excitation level and decelerates the cluster growth. 

Ligand functional groups on the polymer first form a soluble macromolecular complex. The loss of CO transforms complexes into polymer-immobilized cluster particles M . The subsequent growth of these particles leads to the formation of a nanoparticle. Initiation, growth of particles, and termination by simple disproportionation of metal carbonyl on the particle surface all occur. 

Interval III Particle Growth in the Absence of Monomer Droplets.—James and Sundberg have published the results of an experimental study of ideal and non-ideal behaviour in the seeded emulsion polymerization of styrene. Unlike the experiments on seeded emulsion polymerization reported in papers referred to above, the amounts of monomer added to the seed latices were less than those required to saturate the particles and form a separate monomer droplet phase. The reaction systems were therefore the seed analogues of Interval III of a conventional emulsion polymerization reaction. The results are found to be in good agreement with the predictions of the Stockmayer-O Toole theory, provided that allowance is made for the effect of monomer/polymer ratio at the reaction locus upon the rate coefficient for bimolecular mutual termination. A paper by Hamielec and Marten is concerned with the effects of chain entanglements and the rubber-glass transition... 

In parallel with crystallite growth there can also be interactions of the metal particles with polymeric molecules S, terminating particle growth ... 

After the particle growth stage of the emulsion polymerization is terminated, the monomer concentration within the latex particles decreases and so does the heat production rate. In the meantime, the temperature control system is able to reduce the temperature of the jacket. When the molar flow rate (mol s ) of the monomer... 

Barrett and Thomas (1) have made an extensive study of nucleation and particle growth in dispersion polymerisationo They conclude that for poly (methyl methacrylate) in aliphatic hydrocarbon the threshold degree of polymerisation is sufficiently low so that nearly all oligomers form nuclei or are captured by particles before their radicals are terminated. In consequence nearly all polymerisation takes place within the particles, the concentration of polymer in the diluent phase remains extremely low and nearly all radicals pass into the particles. Polymerisation proceeds in the swollen particles and is greatly accelerated because of the reduction in the diffusion controlled termination rate, due to the high viscosity or gel-like state of the particle. It is usual to add an... 

In emulsion polymerization, the molecular weights strongly depend on the average number of radicals per particle. Detailed mathematical models for the calculation of linear [23] and nonlinear [24-34] polymers for any value of n are available. A detailed discussion of this issue is outside the scope of this chapter. Instead, particular solutions for the limiting cases of Smith-Ewart [7] are presented in Table 4.1 where for Case 3, it was considered that the main chain growth termination event was bimolecular termination. 

The latter kind of reaction is the most common chain-terminating process in smog because NOj is a stable free radical present at high concentrations. Chains may terminate also by reaction of free radicals with NO or by reaction of two R radicals, although the latter is uncommon because of the relatively low concentrations of radicals compared to molecular species. Chain termination by radical sorption on a particle surface is also possible and may contribute to aerosol particle growth. 

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