Particle formation mechanism

Particle Formation, Electron microscopy and optical microscopy are the diagnostic tools most often used to study particle formation and growth in precipitation polymerizations (7 8). However, in typical polymerizations of this type, the particle formation is normally completed in a few seconds or tens of seconds after the start of the reaction (9 ), and the physical processes which are involved are difficult to measure in a real time manner. As a result, the actual particle formation mechanism is open to a variety of interpretations and the results could fit more than one theoretical model. Barrett and Thomas (10) have presented an excellent review of the four physical processes involved in the particle formation oligomer growth in the diluent oligomer precipitation to form particle nuclei capture of oligomers by particle nuclei, and coalescence or agglomeration of primary particles. 

Nomura and Fujita (12), Dougherty (13-14), and Storti et al. (12). Space does not permit a review of each of these papers. This paper presents the development of a more extensive model in terms of particle formation mechanism, copolymer kinetic mechanism, applicability to intervals I, II and III, and the capability to simulate batch, semibatch, or continuous stirred tank reactors (CSTR). Our aim has been to combine into a single coherent model the best aspects of previous models together with the coagulative nucleation theory of Feeney et al. (8-9) in order to enhance our understanding of... 

The physical picture of emulsion polymerization is complex due to the presence of multiple phases, multiple monomers, radical species, and other ingredients, an extensive reaction and particle formation mechanism, and the possibility of many modes of reactor operation. 

No version of micellar entry theory has been proposed, which is able to explain the experimentally observed leveling off of the particle number at high and low surfactant concentrations where micelles do not even exist. There is a number of additional experimental data that refute micellar entry such as the positively skewed early time particle size distribution (22.), and the formation of Liesegang rings (30). Therefore it is inappropriate to include micellar entry as a particle formation mechanism in EPM until there is sufficient evidence to do so. 

It has been found that the "unattached" fraction is an ultrafine particle aerosol with a size range of 0.5 to 3 nm. In order to initiate studies on the formation mechanism for these ultrafine particles, a series of experiments were made in the U.S. Bureau of Mines radon chamber. By introducing SO into the chamber, particles were produced with an ultrafine size distribution. It has been found that the particle formation mechanism is supressed by the presence of radical scavengers. These experiments suggest that radiolysis following the decay of Rn-222 gives rise to the observed aerosol and the properties of the resulting aerosol are dependent on the nature and the amount of reactive gas present. 

It has been found that the activity which is conventionally referred to as the "unattached" fraction is actually an ultrafine particle aerosol with a size range of 0.5 to 3 nm. The hydroxyl radical from water molecule radiolysis is a key element to the particle formation mechanism. By injecting different concentrations of S02 into the test chamber, a possible particle formation mechanism has been suggested as follows Oxidizable species such as S02 reacts promptly with hydroxyl radicals and form a condensed phase. These molecules coagulate and become ultrafine particles. 

How this smoke effect varies with inert addition is best explained by considering the results of many early investigators who reported that incipient soot formation occurred in a very narrow temperature range. The various results are shown in Table 8.6. Since, as stated earlier, the incipient particle formation mechanisms for various fuels follow quite similar routes, it seems appropriate to conclude that a high activation energy process or processes control the incipient particle formation. The best concept and evidence to date is that given by Dobbins [77], It is likely that the slight variation of temperatures shown in Table 8.6 is attributable to the different experimental procedures... 

Particle size distributions of smaller particles have been made using electrical mobility analyzers and diffusion batteries, (9-11) instruments which are not suited to chemical characterization of the aerosol. Nonetheless, these data have made major contributions to our understanding of particle formation mechanisms (1, 1 ). At least two distinct mechanisms make major contributions to the aerosols produced by pulverized coal combustors. The vast majority of the aerosol mass consists of the ash residue which is left after the coal is burned. At the high temperatures in these furnaces, the ash melts and coalesces to form large spherical particles. Their mean diameter is typically in the range 10-20 pm. The smallest particles produced by this process are expected to be the size of the mineral inclusions in the parent coal. Thus, we expect few residual ash particles smaller than a few tenths of a micrometer in diameter (12). 

Numerous techniques have been applied for the characterization of StOber silica particles. The primary characterization is with respect to particle size, and mostly transmission electron microscopy has been used to determine the size distribution as well as shape and any kind of aggregation behavior. Figure 2.1.7 shows a typical example. As is obvious from the micrograph, the StOber silica particles attract a great deal of attention due to their extreme uniformity. The spread (standard distribution) of the particle size distribution (number) can be as small as 1%. For particle sizes below SO nm the particle size distribution becomes wider and the particle shape is not as perfectly spherical as for all larger particles. Recently, high-resolution transmission electron microscopy (TEM) has also revealed the microporous substructure within the particles (see Fig. 2.1.8) (51), which is further discussed in the section about particle formation mechanisms. 

This chapter focuses on silica synthesis via the microemulsion-mediated alkoxide sol-gel process. The discussion begins with a brief introduction to the general principles underlying microemulsion-mediated silica synthesis. This is followed by a consideration of the main microemulsion characteristics believed to control particle formation. Included here is the influence of reactants and reaction products on the stability of the single-phase water-in-oil microemulsion region. This is an important issue since microemulsion-mediated synthesis relies on the availability of surfactant/ oil/water formulations that give stable microemulsions. Next is presented a survey of the available experimental results, with emphasis on synthesis protocols and particle characteristics. The kinetics of alkoxide hydrolysis in the microemulsion environment is then examined and its relationship to silica-particle formation mechanisms is discussed. Finally, some brief comments are offered concerning future directions of the microemulsion-based alkoxide sol-gel process for silica. 

The present review paper, therefore, refers firstly to the particle formation mechanism in emulsion polymerization, the complete understanding of which is indispensable for establishing a correct kinetic model, and then, deals with the present subject, that is, what type of reactor and operating conditions are the most suitable for a continuous emulsion polymerization process from the standpoint of increasing the volume efficiency and the stability of the reactors. 

A negative or zero interfacial tension is a necessary condition for the emulsification which leads to the phase inversion reported by Bender (5). This is one of the most important criteria for the particle formation mechanism. Unfortunately this surface energetics aspect has not been discussed by previous workers. 

The results of the present Investigation of particle formation mechanism by mecms of electron microscopy correspond to the data of Huxm and Chong (26) who proved, by using the DLVO theory of stability (27), that very small particles (with diameter less tEan 20 nm) formed during polymerization of vinyl acetate are less stable, so that their flocculation with larger particles Is possible Immediately after they... 

We know that the particle formation mechanisms can be much more complex, especially for monomers such as MMA and VA. The sulfate ion radical formed from persulfate initiator is not likely to be strongly attracted to either micelles or polymer particles due to its charge and hydrophilic nature. Instead, this radical will begin to polymerize monomer which is dissolved in the water phase. As monomer units are added, the aqueous phase ion radical will become less hydrophilic and more surface active. During the period of surface activity, it may well adsorb on a micelle, a polymer particle, or a monomer drop. 

Another feature of the Smith-Ewart theory is that the reaction rate at the end of the nudeation perind is expected to he higher than in the steady state because n is higher than the steady-state value of O.S (Smith-Ewart Case 2 kinetics). There is little experimental evidence for such a maximum in rate (Ugelstad and Hansen, 1976), and this discrepancy may be explained by more details about the radical absorption rates in micelles and particles. Before any further discussion of particle-formation mechanisms, it therefore seems logicaHo review the mechanisms responstUe for radical absewption. 

The equations for dN/dt and dRj/dt, as well as for dV dt [Eq. (39)] are solved by numerical integration for the polymerization stem MMA-K2S20e water, with rate constants obtained from the literature. The initiator efficiency was set equal to unity. Particle numbers between 10 and 10 were drained for initiator concentrations of 10 -10 mol/dm. The calculations showed that N should be almost independent of the chosen value offor values between 5 and 70 (in strong contrast to our calculations). The reason for this is probably that aqueous-phase termination with subsequent precipitation is the dominant particle-formation mechanism in Aral s model, even more so with increasing initiator concentration. The theoretical particle-formation time was on the order of 2 sec, a veiy low value compared to the experimental results of Fitch and Tsai. Aral et at. found that their calculated particle numbers were approximately in accordance with the experimental results of Yamazaki et al. (1968) for emulsifier-free polymerizations. Aral s model does not inclnde any coagulation mechanisms. It will therefore have the same shortcomings as most other models, namely that the strongly increased particle number in... 

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