Particle generation homogeneous nucleation

The particle generation rate was calculated by a step mechanism, namely formation of primary precursor particles by homogeneous nucleation (JLQ.) followed by coagulation to latex particles (8-9). This homogeneous nucleation mechanism is often referred to as the HUFT mechanism for its originators Hansen, Ugelstad, Fitch, and Tsai. 

Deriving an expression for f(t) a considerable simplification occurs if one takes all polymer particles to be nucleated at the same size dp(t,t). The generation of new polymer particles in an emulsion system is basically due to two mechanisms micellar and homogeneous particle production. Then, the rate of particle nucleation, fit), can be expressed as (12) ... 

The number of polymer particles is the prime determinant of the rate and degree of polymerization since it appears as the first power in both Eqs. 4-5 and 4-7. The formation (and stabilization) of polymer particles by both micellar nucleation and homogeneous nucleation involves the adsorption of surfactant from the micelles, solution, and monomer droplets. The number of polymer particles that can be stabilized is dependent on the total surface area of surfactant present in the system asS, where as is the interfacial surface area occupied by a surfactant molecule and S is the total concentration of surfactant in the system (micelles, solution, monomer droplets). However, N is also directly dependent on the rate of radical generation. The quantitative dependence of N on asS and R,- has been derived as... 

Microemulsion polymerizations follow a different mechanism from the conventional emulsion polymerizations. The most probable locus of particle nucle-ation was suggested to be the microemulsion monomer droplets [27], although homogeneous nucleation was not completely ruled out. The particle generation rate in microemulsion polymerization is given by an expression similar to Eq. (21), which was used for the miniemulsion polymerization of styrene [28] ... 

The primary requirement is a source of vapor and a less volatile material that serves for condensation nuclei. Sodium chloride is commonly used to produce nuclei in the 10-nm-size range. This can be accomplished by heating the bulk material or by atomizing a dilute solution and drying the aerosol. The nuclei source can be a component (impurity) of the bulk material used, such as in the Rapaport-Weinstock generator, in which the impurity in the nebulized liquid serves as the condensation nuclei [9], The use of heterogeneous nucleation for the formation of particles leads to a substantially more controllable and monodisperse aerosol than without the use of nuclei, that is, homogeneous nucleation. 

The ultrafine range is usually composed of emissions from local combustion sources or particles generated by atmospheric photochemical activity that leads to homogeneous nucleation. The principal mechanism of decay of the ultrafine range is attachment to particles in the accumulation mode by diffusion. Neglecting the Brownian motion of the coarse particles compared with the fine particles, the fractional rate of decay of particles in the ultrafine range is given by (Chapter 7)... 

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. 

SDSX]>310 mM. These data also can be used to explain the decreased Rp/Npf with [SDSX]. As more latex particles are generated by homogeneous nucleation, both the cage effect associated with the initiator radicals produced in pairs inside the particles and segregation of the absorbed radicals among the discrete particles will become more pronounced. As a result, the number of radicals which can be accommodated into the particles will decrease with increasing [SDSX]. 

Formation of latex particles can proceed via the micellar nucleation, homogeneous nucleation and monomer droplet nucleation. The contribution of each particle nucleation mechanism to the whole particle formation process is a complex function of the reaction conditions and the type of reactants. There are various direct and indirect approaches to determine the particle nucleation mechanism involved. These include the variations of the kinetic, colloidal and molecular weight parameters with the concentration and type of initiator and emulsifier. There are some other approaches, such as the dye method where the latex particles generated via homogeneous nucleation do not contribute to the amount of dye detected in the latex particles since diffusion of the extremely hydrophobic dye molecules from the monomer droplets to the latex particles generated in water is prohibited. On the contrary, nucleation of the dye containing monomer droplets leads to the direct incorporation of dye into the polymer product. However, the dye also act as a hydrophobe and enhances the stability of monomer droplets as well as the monomer droplet nucleation. 

PRIMARY NUCLEATION. In scientific usage, nucleation refers to the birth of very small bodies of a new phase within a supersaturated homogenous existing phase. Basically, the phenomenon of nucleation is the same for crystallization from solution, crystallization from a melt, condensation of fog drops in a supercooled vapor, and generation of bubbles in a superheated liquid. In all instances, nucleation is a consequence of rapid local fluctuations on a molecular scale in a homogenous phase that is in a state of metastable equilibrium. The basic phenomenon is called homogeneous nucleation, which is further restricted to the formation of new particles within a phase iminfluenced in any way by solids of... 

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