Emulsion polymerization particle morphology

As in the case of emulsion polymerization, particle morphology is ruled by the interplay between thermodynamics and kinetics. Equilibrium morphologies are reached when the internal viscosity of the polymer particle is low. Thus, due to the plasticizing effect of the alkyd resin, equilibrium morphologies are usually reached for alkyd/acrylic systems [96]. The equilibrium morphology is affected by the presence of graft copolymer that reduces the interfacial tension between the polymer phases in the particle. Methods to calculate the equilibrium morphology of multiphase polymer particles are available [43]. 

Research on the modelling, optimization and control of emulsion polymerization (latex) reactors and processes has been expanding rapidly as the chemistry and physics of these systems become better understood, and as the demand for new and improved latex products increases. The objectives are usually to optimize production rates and/or to control product quality variables such as polymer particle size distribution (PSD), particle morphology, copolymer composition, molecular weights (MW s), long chain branching (LCB), crosslinking frequency and gel content. 

Particle morphology, of fillers, 11 303-304 Particle nucleation, in emulsion polymerization, 14 713-714... 

Abstract Emulsion homopolymers and copolymers (latexes) are widely used in architectural interior and exterior paints, adhesives, and textile industries. Colloidal stabihzators in the emulsion polymerization strongly affect not only the colloidal properties of latexes but also the fdm and mechanical properties, in general. Additionally, the properties of polymer/copolymer latexes depend on the copolymer composition, polymer morphology, initiator, polymerization medium and colloidal characteristics of copolymer particles. 

Emulsion polymerization of ABS (241) gives a mbber-phase particle morphology which is mosdy determined by the mbber-seed latex. Since the mbber particle size, polydispersity, and cross-linking are established before the preparation, the main variables relate to grafting, molecular weight... 

Cross-linked polystyrene porous particles (with 21 mol% DVB) have been prepared by the concentrated emulsion polymerization method, using either toluene or decane as the porogen and an aqueous solution of SDS as the continuous phase. Since toluene is a good solvent for polystyrene while decane is a nonsolvent , the morphologies obtained in the two cases were different. The particles based on toluene (with a volume fraction of dispersed phase of 78%) have very small pores which could not be detected in the SEM pictures. The pore size distribution, which has sizes between 20 and 50 A and was determined with an adsorption analyzer, almost coincides with that in a previous study [49] in which porous polystyrene beads have been prepared by suspension polymerization. In contrast, the porous particles based on decane have pore sizes as large as 0.1-0.3 pm, which could be detected in the SEM pictures [44a], and also larger surface areas (47 m2 g ) than those based on toluene (25 m2 g ). The main difference between the concentrated emulsion polymerization and the suspension polymerization consists of the much smaller volume fraction of continuous phase used in the former procedure. The gel-like emulsion that constitutes the precursor in the former case contains polyhedral cells separated by thin films of continuous phase. The polymerization of the cells does not... 

Core-shell nanoparticles can also be fabricated using microemulsions. This was performed using a two-stage microemulsion polymerization beginning with a polystyrene seed [62]. Butyl acrylate was then added in a second step to yield a core-shell PS/PBA morphology. The small microlatex led to better mechanical properties than those of similar products produced by emulsion polymerization. Hollow polystyrene particles have also been produced by microemulsion polymerization of MMA in the core with crosslinking of styrene on the shell. After the synthesis of core-shell particles with crosslinked PS shells, the PMMA core was dissolved with methylene chloride [63]. The direct cross-... 

Although emulsion polymerization has been carried out for at least 50 years and has enormous economic importance, the detailed quantitative behavior of these reactors is still not well understood. For example, there are many more mechanisms and phenomena reported experimentally than have been incorporated in the existing theories. Considerations such as non-micellar particle formation, non-uniform particle morphologies, polymer chain end stabilization of latex particles, particle coalescence, etc. have been discussed qualitatively, but not quantitatively included in existing reactor models. 

It can be shown theoretically that the relative amounts of the two monomers in a two-stage emulsion polymerization can affect the final particle structure [ 19], with core-shell morphology being favored thermodynamically, as the amount of second-stage polymer is increased. 

Emulsion polymerization reactions are sometimes carried out with small seed particles formed in another reaction system. A number of advantages can he derived from using seed particles. In a batch reactor seed latex can he helpful hi controlling particle concentration, polymerization rate, particle morphology, and particle size characteristics. In a CSTR the use of a feed stream containing seed particles can also help to prevent conversion and/or surface tension oscillations, which are caused by particle formation phenomena, This factor will be discussed in more detail later in this chapter. 

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. 

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