Continuous particle nucleation

The idea of gradual addition of monomer to polymerizing micro emulsions is not new and has been attempted for methylmethacrylate [31] and styrene [32-34] to make more efficient use of the surfactant. For example, Gan et al. have performed styrene microemulsion polymerizations where monomer was added gradually either dropwise or using a hollow-fiber membrane [32,35]. They observed relatively large ( 40 nm) and uniform particle sizes with no indications of continuous particle nucleation. However, this result is more likely due to a combination of fast monomer addition relative to the polymerization rate and depletion of the redox initiator system employed rather than to the growth of glassy particles. 

Continuous particle nucleation with a few polymer chain per particle (2-3) and some coagulation in the latter stages of the reaction [20],... 

A continuous particle nucleation mechanism was further confirmed by transmission electron microscopic (TEM) experiments performed on polyacrylamide samples taken at various degrees of conversion [25]. The number of polymer particles was shown to increase proportionally with conversion (Fig. 6a), whereas the size remained roughly constant. 

The mechanism of polymerization in ternary and quaternary oil-in-water microemulsions has become understood only in recent years. The onset of turbidity upon polymerization and the lack of stability with time observed by most authors, particularly for MM A monomer, is likely the reason for the slow progress in the comprehension of the mechanism of O/W systems. Only slight changes in the formulation are sufficient to significantly affect the polymerization process and to induce particle coagulation at any stage of the reaction. This may explain the disparity in the kinetic data reported by some authors for very similar systems. With this remark in mind, one can, however, conclude that the scheme that is now well accepted is that of a continuous particle nucleation mechanism as in the case of inverse systems. This view is supported by several features. 

It should be noted that the probability for the continuous particle nucleation throughout the emulsion polymerization increases with increasing surfactant concentration. For constant monomer weight, the higher the surfactant concentration, the smaller the latex particles produced in the emulsion polymerization system. In addition, the longer the particle nucleation period, the broader the residence time distribution of particle nuclei within the reactor (i.e., the broader the resultant particle size distribution). These rules of thumb, based on a large number of fundamental studies on nucleation and growth of particle nuclei, have been widely used in industry to effectively use surfactant to stabilize various latex products with balanced performance properties. 

The inverse microemulsion polymerization process appeared to combine high rate with high molecular weights (up to 10 ) and displayed a novel feature that each final latex particle consisted of one single polymer molecule in a collapsed state, suggesting that the kinetics of the reaction did not follow the Smith-Ewart theory but were characterized by continuous particle nucleation. 

On the basis of the above results, it was postulated that particle nucleation took place continuously throughout the polymerization process, in sharp contrast with conventional emulsion polymerization where particle nucleation takes place only in the initial period (interval I)- The results of TEM experiments performed on polyacrylamide samples taken at different extents of conversion [53] also supported the theory of continuous particle nucleation since the number of polymer particles was found to increase proportionally with conversion while the size remained approximately constant. 

The debate as to which mechanism controls particle nucleation continues. There is strong evidence the HUFT and coagulation theories hold tme for the more water-soluble monomers. What remains at issue are the relative rates of micellar entry, homogeneous particle nucleation, and coagulative nucleation when surfactant is present at concentrations above its CMC. It is reasonable to assume each mechanism plays a role, depending on the nature and conditions of the polymerization (26). 

Achieving steady-state operation in a continuous tank reactor system can be difficult. Particle nucleation phenomena and the decrease in termination rate caused by high viscosity within the particles (gel effect) can contribute to significant reactor instabilities. Variation in the level of inhibitors in the feed streams can also cause reactor control problems. Conversion oscillations have been observed with many different monomers. These oscillations often result from a limit cycle behavior of the particle nucleation mechanism. Such oscillations are difficult to tolerate in commercial systems. They can cause uneven heat loads and significant transients in free emulsifier concentration thus potentially causing flocculation and the formation of wall polymer. This problem may be one of the most difficult to handle in the development of commercial continuous processes. 

One of the most promising ways of dealing with conversion oscillations is the use of a small-particle latex seed in a feed stream so that particle nucleation does not occur in the CSTRs. Berens (3) used a seed produced in another reactor to achieve stable operation of a continuous PVC reactor. Gonzalez used a continuous tubular pre-reactor to generate the seed for a CSTR producing PMMA latex. 

Miniemulsion polymerization involves the use of an effective surfactant/costabi-lizer system to produce very small (0.01-0.5 micron) monomer droplets. The droplet surface area in these systems is very large, and most of the surfactant is adsorbed at the droplet surfaces. Particle nucleation is primarily via radical (primary or oligomeric) entry into monomer droplets, since little surfactant is present in the form of micelles, or as free surfactant available to stabilize particles formed in the continuous phase. The reaction then proceeds by polymerization of the monomer in these small droplets hence there may be no true Interval II. 

Beyond the nucleation stage, size change in the system can occur by a number of mechanisms as depicted in Fig. 3.2. The prevailing mechanism depends on such factors as the feed particle size and other solid properties, liquid surface tension and viscosity and the mode of operation (batch or continuous). After nucleation has occurred, the predominating growth mechanisms are ... 

In Stage II (referred to as Stage II to differentiate it from the classical Interval II), the rate of polymerization and the number of polymer particles continue to increase but at a slower rate. Polymer particles are formed by homogeneous nucleation as long as monomer droplets and enough emulsifier (>0.05 mM) are present in the system. The end of this stage is marked by the disappearance of monomer droplets, but particle nucleation may or may not end at this time. 

Candau and co-workers were the first to address the issue of particle nu-cleation for the polymerization of AM [13, 14] in an inverse microemulsion stabilized by AOT. They found that the particle size of the final microlatex (d 20-40 nm) was much larger than that of the initial monomer-swollen droplets (d 5-10 nm). Moreover, each latex particle formed contained only one polymer chain on average. It is believed that nucleation of the polymer particle occurs for only a small fraction of the final nucleated droplets. The non-nucleated droplets also serve as monomer for the growing particles either by diffusion through the continuous phase and/or by collisions between droplets. But the enormous number of non-nucleated droplets means that some of the primary free radicals continuously generated in the system will still be captured by non-nucleated droplets. This means that polymer particle nucleation is a continuous process [ 14]. Consequently, each latex particle receives only one free radical, resulting in the formation of only one polymer chain. This is in contrast to the large number of polymer chains formed in each latex particle in conventional emulsion polymerization, which needs a much smaller amount of surfactant compared to microemulsion polymerization. 

In addition to the practical interest, the process presents challenges encouraging further fundamental exploration. A thorough study not reported here, has been performed on the mechanism and kinetics of the polymerization of acrylamide in AOT/water/toluene microemulsions (Carver, M.T.r Dreyer, U. Knoesel, R. Candau, F. Fitch, R.M. J. Polym. Sci. Polym. Chem. Ed., in press. Carver, M.T. Candau, F. Fitch, R.M. J. Polym. Sci. Polym. Chem. Ed., in press). The termination reaction of the polymerization was found to be first order in radical concentration, i.e. a monoradical reaction instead of the classical biradical reaction. Another major conclusion was that the nucleation of particles is continuous all throughout the polymerization in contrast to conventional emulsion polymerization where particle nucleation only occurs in the very early stages of polymerization. These studies deserve further investigations and should be extended to other systems in order to confirm the unique character of the process. 

Note that left-hand side of this expression is, in fact, a continuity equation for which states that the multi-particle joint PDF is constant along trajectories in phase space. The term on the right-hand side of Fq. (4.32) has a contribution due to the Alp-particle collision operator, which generates discontinuous changes in particle velocities Up" and internal coordinates p", and to particle nucleation or evaporation. The first term on the left-hand side is accumulation of The remaining terms on the left-hand side represent... 

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. 

The ultrasonification process is connected with the rapidly increased oil-water interfacial area as well as the significant re-organization of the droplet clusters or droplet surface layer. This may lead to the formation of additional water-oil interface (inverse micelles) and, thereby, decrease the amount of free emulsifier in the reaction medium. This is supposed to be more pronounced in the systems with non-ionic emulsifier. Furthermore, the high-oil solubility of non-ionic emulsifier and the continuous release of non-micellar emulsifier during polymerization influence the particle nucleation and polymerization kinetics by a complex way. For example, the hairy particles stabilized by non-ionic emulsifier (electrosteric or steric stabilization) enhance the barrier for entering radicals and differ from the polymer particles stabilized by ionic emulsifier. The hydro-phobic non-ionic emulsifier (at high temperature) can act as hydrophobe. 

---