Kinetics-controlled particle morpholog

It is possible that equilibrium morphology is not obtained because the movement of the polymer chains is not fast enough to reach that equilibrium within the time-frame of the reaction this is kinetic control of morphology. The kinetic parameters influence the rate of formation of a certain morphology [27, 28], which is basically determined by the interfacial tensions [29]. The parameters of importance are the rate of formation of the polymer (parameters are propagation rate coefficient, and the local monomer and radical concentrations) and the rate of diffusion of the polymer chains (parameters are viscosity in the locus of polymerization, molar mass and topology of the polymer chain). Both the rate of formation and the rate of diffusion of a polymer chain are, for example, affected by the mode of addition of the monomer and initiator. An increased rate of addition of the monomer will lead to a lower instantaneous conversion and thus a lower viscosity in the particle, which in turn increases the rates of diffusion and leads to different morphologies. 

Flocculation processes are complicated phenomena because of the varieties of both particle morphology and chemical reactions they encompass.34 A few concepts of a general nature have emerged, however, and they will be the focus of this chapter. From the perspective of kinetics, perhaps the most important of these broad generalizations is the distinction that can be made between transport-controlled and reaction-controlled flocculation, parallel to the classification of adsorption processes described in Section 4.5. Flocculation kinetics are said to exhibit transport control if the rate-limiting step is the movement of two (or more) particles toward one another prior to their close encounter and subsequent combination into a larger particle. Reaction control occurs if it is particle combination instead of particle movement (toward collision) that limits the rate of flocculation. 

A considerable amount of work has been published during the past 20 years on a wide variety of emulsion polymerization and latex problems. A list of 11, mostly recent, general reference books is included at the end of this chapter. Areas in which significant advances have been reported include reaction mechanisms and kinetics, latex characterization and analysis, copolymerization and particle morphology control, reactor mathematical modeling, control of adsorbed and bound surface groups, particle size control reactor parameters. Readers who are interested in a more in-depth study of emulsion polymerization will find extensive literature sources. 

The morphology of latex particles is controlled by the thermodynamic and kinetic factors. The thermodynamic factors determine the ultimate stability of the multiphase system, inherent in the production of a composite latex particle, while the kinetic factors determine the ease with which such a thermodynamically favored state can be achieved. The parameters affecting the thermodynamics of the system include the particle surface polarity, the relative phase volumes, and the core particle size. The parameters affecting the kinetics of the morphological development include the mode of monomer addition (monomer starved or batch) and the use of crosslinking agents. Of course, crosslinked core/shell latexes constitute IPNs, see Section 6.4.1. 

The kinetic mechanisms [35, 36] in the HIPS polymerization process and the complete process [37-41] have been mathematically modeled to a detailed level by different groups. Diverse aspects of the HIPS technology have been extensively studied in the past by many authors works that review several of these aspects are the texts of Scheirs and Priddy [42] (properties, applications, modeling, and later technologies), Echte [34] (particle morphology), Simon and Chappelear (industrial processes) [43], and Meira et al. [39] (process modeling and control). 

The above computations for the fiee energy changes for composite latex particles have shown that only core-shell, inverted core-shell and hemispherical particles are stable in a thermodynamic sense. All of the other reported morphologies (e.g. sandwich-like , raspberry or confetti-like particles or occluded domains) are non-equilibrium, kinetically controlled structures, prepared... 

The heterogeneity of emulsion polymerization systems offers unique possibilities of structural control of emulsion (co)polymers, on a molecular scale (intermolecular and intramolecular microstructure) as well as on a parficle-size scale (particle morphology). The kinetic and mechanistic features of emulsion (co)polymerization are strongly reflected in molecular size and its distribution, chemical conqxisition and its distribufion, particle moiphology, and product properties. A further fine tuning of polymer properties calls for advanced characterization techniques enable of revealing delicate structural details in emulsion (co)poIymers. 

Compared with the traditional sintering method, the molten salt method is one of the simplest methods for controlling the morphology of particles and obtaining highly reactive powders of a single phase at low temperatures in a short soaking time. This is because the molten salt can accelerate the reaction kinetics at this low temperature and facilitate the formation of ceramic particles. 

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