Emulsions concentrated systems

The monomer concentration within the forming latex particles does not change for a long period due to the diffusion of monomer from the droplets to the polymerization loci. Therefore, the rate of the propagation reaction does not change and a constant polymerization rate period is observed in a typical emulsion polymerization system. 

The largest portion of the monomer (>95%) is dispersed as monomer droplets whose size depends on the stirring rate. The monomer droplets are stabilized by surfactant molecules absorbed on their surfaces. Monomer droplets have diameters in the range 1-100 pm (103-105 nm). Thus, in a typical emulsion polymerization system, the monomer droplets are much larger than the monomer-containing micelles. Consequently, while the concentration of micelles is 1019-1021 the concentration of monomer droplets is at most 1012-1014 L 1. A further difference between micelles and monomer droplets is that the total surface area of the micelles is larger than that of the droplets by more than two orders of magnitude. The size, shape, and concentration of each of the various types of particles in the... 

Polymerization of the monomer in solution undoubtedly takes place but does not contribute significantly, since the monomer concentration is low and propagating radicals would precipitate out of aqueous solution at very small (oligomeric) size. The micelles act as a meeting place for the organic (oil-soluble) monomer and the water-soluble initiator. The micelles are favored as the reaction site because of their high monomer concentration (similar to bulk monomer concentration) compared to the monomer in solution. As polymerization proceeds, the micelles grow by the addition of monomer from the aqueous solution whose concentration is replenished by dissolution of monomer from the monomer droplets. A simplified schematic representation of an emulsion polymerization system is shown in Fig. 4-1. The system consists of three types of particles monomer droplets, inactive micelles in which... 

Regarding compressed emulsions, however, the above analysis cannot be applied since the droplets are distorted by the compressive force into complex, irregular shapes. Variables such as the area of the film between adjacent droplets and the disjoining pressure become extremely difficult to calculate. Nevertheless, calculations can be performed for the particular case of compressed, extremely concentrated systems, where <)> approaches unity. This is because, as - 1, the droplets more or less resemble the polyhedron of the unit cell. 

Such analysis requires a thorough study of the variables in the emulsion breaking system These include settling time, temperature, emulsion breaker chemical concentration (and type/, and electrostatic field. 

The major disadvantage of the laser diffraction and electrical pulse counting techniques is that they are only directly applicable to dilute emulsions or emulsions that can be diluted without disturbing the particle size distribution. However, many food emulsions are not dilute and cannot be diluted, either because dilution alters the particle size distribution or because the original sample is partially solid. For concentrated systems it is belter to use particle-sizing instruments based on alternative technologies, such as ultrasonic spectrometry or NMR (Dickinson and McClements, 1996). 

Photon correlation spectroscopy, carried out under very dilute conditions, has unambiguously demonstrated the expansion of carboxylic emulsion polymers at high pH, but it may not always be useful in predicting properties of practical interest. Of special concern is the apparent decrease in the intrinsic ionization constant of surface carboxyls at very low concentration. Since most uses of emulsion polymer occur at high concentrations, the measurement of particle-particle interactions is of great practical importance (21J. It has been found that the sedimentation and viscometric techniques closely reflect viscosity changes in latexes at much higher solids. Extension of the PCS approach to more concentrated systems is underway but not without problems (22). 

The results are generally consistent with a broader treatment of the techniques for measuring particle swelling of carboxylic emulsion polymer latexes reported elsewhere in this Monograph (9). The broader study, which was carried out independently but concurrently, has shown that the magnitude and pH of maximum expansion depends on dilution and ionic strength. Studies of the concentration dependence in the dilute regime and more concentrated systems are underway. 

The available data from emulsion polymerization systems have been obtained almost exclusively through manual, off-line analysis of monomer conversion, emulsifier concentration, particle size, molecular weight, etc. For batch systems this results in a large expenditure of time in order to sample with sufficient frequency to accurately observe the system kinetics. In continuous systems a large number of samples are required to observe interesting system dynamics such as multiple steady states or limit cycles. In addition, feedback control of any process variable other than temperature or pressure is impossible without specialized on-line sensors. This note describes the initial stages of development of two such sensors, (one for the monitoring of reactor conversion and the other for the continuous measurement of surface tension), and their implementation as part of a computer data acquisition system for the emulsion polymerization of methyl methacrylate. 

A kind of foam in which the gas bubbles have an unusually thick stabilizing film and exist clustered together as opposed to either separated, nearly spherical bubbles or the more concentrated, system-filling polyhedral bubbles. A microgas emulsion will cream to form a separate phase from water. Also termed aphrons or colloidal gas aphrons . 

The aim of this first section is to describe the rupturing mechanisms and the mechanical conditions that have to be fulfilled to obtain monodisperse emulsions. A simple strategy consists of submitting monodisperse and dilute emulsions to a controlled shear step and of following the kinetic evolution of the droplet diameter. It will be demonstrated that the observed behavior can be generalized to more concentrated systems. The most relevant parameters that govern the final size will be listed. The final drop size is mainly determined by the amplitude of the applied stress and is only slightly affected by the viscosity ratio p. This last parameter influences the distribution width and appears to be relevant to control the final monodispersity. 

Emulsion Solvent Evaporation The basic concept of the emulsion solvent evaporation technique producing nanoparticles is very straightforward. The particles are formed as an emulsion of a polymer-surfactant mixture and dispersed in an organic solvent. The solvent is then evaporated to leave behind the individual emulsion droplets which form stable free nanoparticles [203], This method is far easier and more preferable over methods such as spray drying and homogenization and operates under ambient conditions and mild emulsification conditions. The size and composition of the final particles are affected by variables such as phase ratio of the emulsion system, organic solvent composition, emulsion concentration, apparatus used, and properties of the polymer [204],... 

Procedure 3 [29] - the preparation and partial-polymerization of a concentrated emulsion of VDC was the same as in Procedure 1. The BMA monomer containing the redox system was introduced dropwise at room temperature into the partially polymerized VDC emulsion. Because of the presence of water and surfactant in the emulsion, the system remained a concentrated emulsion after the addition of the BMA monomer. This concentrated emulsion was introduced into a water bath at 45 °C to complete the polymerization. 

As is clear from Eq. l,the rate of particle growth (R /Nr) is proportional to the monomer concentration, [M]p and the average number of radicals per particle, n, respectively. Thus, n is one of the basic parameters that characterize the kinetic behavior of particle growth in an emulsion polymerization system. Early researchers devoted their efforts to deriving a quantitative description of n by solving Eq. 3 for n defined by Eq. 2 [4,119,120]. 

In an emulsion copolymerization, monomer partitioning between the monomer droplet, polymer particle and aqueous phases plays a key role in determining the rate of copolymerization and the copolymer composition. Two approaches (empirical and thermodynamic) have been proposed to predict the monomer concentrations in the polymer particles in an emulsion copolymerization system. In the emulsion copolymerization of St and MMA, Nomura et al. [45,122,140] first proposed an empirical approach for predicting the saturated concentration of each monomer in the polymer particles as a function of the monomer composition in the monomer droplets, as shown by... 

The basic concept of the present study was to show, other things being equal, that the rate of polymerization is affected by the size of the micelles and not by the total surfactant concentration as expressed by Equation (l). This micellar size effect was believed to be the reason why a nonlinear, i.e., a convex curve, relationship between In Rp and In Cg was obtained with emulsion polymerization systems of changing surfactant... 

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