Particle Nucleation In Emulsion Polymerization Research Articles

Effect of Surfactant Concentration on Particle Nucleation in Emulsion Polymerization ofn-Butyl Methacrylate

The effect of surfactant concentration on particle nucleation in emulsion polymerization of n-butyl methacrylate has been studied using sodium dodecyl sulfate as the surfactant and potassium persulfate as the initiator. The rates of reaction as a function of time were determined for surfactant concentrations above and below critical micelle concentration of the surfactant. The order of dependence of the particle number on the surfactant concentration was found to be different in different ranges of surfactant concentration. This has been explained on the basis of the predominant phenomenon that affects particle nucleation in each range of surfactant concentration. In the range where the end of particle nucleation is determined by a critical surfactant coverage necessary for colloidal stability, the particle number is expected to increase in proportion to the third power of the surfactant concentration. It has been shown how a size-dependent volumetric growth-rate of particles can result in different exponents in the Smith−Ewart equation relating the particle number to surfactant concentration. A constant volumetric growth rate of particles is assumed in the Smith−Ewart theory of particle nucleation.

Modeling Particle Formation in Emulsion Polymerization: An Approach by Means of the Classical Nucleation Theory

A framework for modeling particle nucleation in emulsion polymerization has been developed on the basis of a combination of classical nucleation theory with radical polymerization kinetics and the Flory-Huggins theory of polymer solutions. The basic assumption is that water born oligomers form stable nuclei under critical conditions. The only adjustable model parameter is the activation energy of nucleation. The model allows us to calculate the chain length of the nucleating oligomers, the number of chains forming one nucleus, the diameter of the nucleus, the total number of nuclei formed, and the rate of nucleation.

Latex particle nucleation in emulsion polymerization

Although it has been generally accepted for the past 45 years that the latex particles which are the principal locus of the emulsion polymerization reaction are nucleated from monomer solubilized in emulsifier micelles, conclusive evidence to support this supposition has been lacking. Twenty years ago it became apparent that in the case of monomers such as methyl methacrylate and vinyl acetate which have significant solubility in water, latex particles are nucleated by precipitation of oligomeric radicals formed in aqueous solution. Then it was found that even styrene, which has a very low solubility in water, could be polymerized in emulsion without an emulsifier or below the critical micelle concentration of the emulsifier when the latex particles must also be nucleated by oligomeric precipitation. However, such ‘emulsifier-free emulsion polymerization’ proves impossible with p-tert-butyl styrene which has only one tenth of the water solubility of styrene. Subsequently much evidence has been published to show that the ultimate latex particles are formed by coalescence of unstable precursor particles. It is argued that in the case of water-insoluble monomers, precursor particles may be formed by coalescence of monomer-swollen micelles destabilized by polymerization.

SANS study of particle nucleation in emulsion polymerization

We describe the use of small-angle neutron scattering (SANS) to measure the average radius of polystyrene particles in the early stages of emulsion polymerization. Polymerization was started by placing monomer in contact with an aqueous polyacrylamide gel containing intiator, in the absence of sufractant. Reaction was allowed to occur for 12, 24 and 48 h, and slices were removed from the gels in each system at various distances (of order 10 mm) from the interface. SANS meausrements on these slices showed the presence of particles of radius 6 ± 1 nm. These experiments show that, in these systems, one can halt the growth (by propagation and coagulation) of very small colloidal particles for a sufficiently long time to measure physical properties such as the amount of monomer imbibed at equilibrium swelling, surface charge, etc.

The effect of emulsifier-polymer complex formation on particle nucleation in emulsion polymerization

The effect of emulsifier-polymer complex formation on particle nucleation in emulsion polymerization

The effect of desorption in micellar particle nucleation in emulsion polymerization

AbstractThe effect of desorption of monomer radicals from particles on the particle number generated in interval I of an emulsion polymerization is calculated by means of a semi steady state model. It is shown that desorption leads to an increase in the particle number, and at the same time the orders of particle number with respect to emulsifier and initiator are changing. The general equation of Nomura N∝Sz0ϱ1−zi is found to apply, z increasing from 0,6 to 1,0 with increasing desorption. The values of z and N are dependent on the exact reaction mechanisms in the monomer/polymer system and on rate constants of chemical reactions and diffusion processes involved in the particle formation.

Particle nucleation in emulsion polymerization. II. Nucleation in emulsifier‐free systems investigated by seed polymerization

AbstractExperiments with seeded polymerization in emulsifier‐free systems were carried out with styrene in order to test the theory of particle nucleation presented in the first article in this series. The effect of amount, size, and surface charge density of the seed particles on the formation of new particles was investigated. An expression for the capture rate of oligomeric radicals from the water phase was evaluated in which the rate of capture was considered to be governed by the absorption of oligomers with chain length one less than the critical chain length for precipitation of the oligomer. Coagulation of primary particles was also included in the expression for the number of new particles obtained in the system. Limited coagulation of primary particles with already formed particles and with seed particles was found to play an important role in determining the final number of new particles found at the end of the runs.

Particle nucleation in emulsion polymerization. III. Nucleation in systems with anionic emulsifier investigated by seeded and unseeded polymerization

AbstractStudies of seeded and unseeded polymerization of styrene using sodium dodecylsulfate as emulsifier have been carried out in order to investigate the mechanism of particle nucleation in such systems and to test the theory presented in Part I of this series. The rate of capture of water‐soluble oligomeric radicals was considered to be governed by absorption of oligomers with chain length one less than the critical chain length. It was concluded that the micelles became the dominating loci for particle nucleation above CMC for the emulsifier. A complete nonsteady‐state model for particle initiation above CMC which takes into account radical desorption and reabsorption has been developed. It was indicated that, even for styrene, desorption of radicals may play a role in controlling the radical and particle number of interval I under certain conditions. The model also showed that the efficiencies of particles in absorbing radicals could be calculated from physical parameters, such as diffusion constants and surface charge densities, which are available for the system.

Particle nucleation in emulsion polymerization. IV. Nucleation in monomer droplets

AbstractNucleation in finely dispersed monomer emulsions in competition with homogeneous and micellar mechanism was studied. The emulsions were prepared with a high‐pressure homogenizer under varying homogenizing conditions and made stable by the addition of hexadecane to the monomer. Sodium dodecyl sulfate was used as emulsifer. The number of particles in latexes polymerized with potassium persulfate and benzoyl peroxide initiators was measured and plotted as a function of the free emulsifier concentrations. With persulfate initiator the particle number achieved a minimum in the transition region between droplet and water phase–micellar nucleation mechanisms. With very fine emulsions Smith–Ewart case II kinetics with n = 0.5 applied. The reaction rate, which differed from conventional emulsion polymerizations, decreased with time up to the point at which n began to increase. In these runs the particle size distribution became nearly monodisperse.

Particle nucleation in emulsion polymerization. I. A theory for homogeneous nucleation

AbstractThis paper is the first in a series intended to clarify the particle nucleation mechanisms in emulsion polymerization. The theory for particle nucleation by precipitation of oligomeric radicals from the water phase is discussed and a model based on the diffusion, propagation and termination steps is presented. The physical factors that influence the capture rate of oligomers in particles are discussed, and qualitative expressions for the electrostatic repulsion and reversible diffusion are derived. These factors are shown to be able to explain the relatively slow absorption rate of oligomers in particles and micelles. A kinetic model for simultaneous particle nucleation and limited flocculation is presented. Numerical integration of this model shows that the particle number goes through a maximum and that simultaneous nucleation and flocculation of primary particles may take place after Interval I in an emulsion polymerization is finished.