ABSTRACT:The initial droplet size distribution of miniemulsions has not yet been measured. It is deduced from previous experimental results that the distribution is broad. Furthermore, the small fraction of the distribution, depending on the nonideality of the cosurfactant-monomer system, may not be stable. This may lead to a rearrangement of the distribution leading to a bimodal distribution. The stability criterion is based on the phenomenon of molecular diffusion or Ostwald ripening. Experimental proof in support of the conclusion regarding the bimodal distribution is cited. The practical significance of the conclusion for making emulsion polymers with high solid contents is given.

Summary: Miniemulsion polymerization is a relatively new process that can produce polymer latexes of unusual characteristics. The initial droplet size distribution and the final particle size distribution in miniemulsion polymerization are broad. This has not been addressed by the present models that assume monodisperse droplet size and particle size distributions. A population balance framework for the miniemulsion polymerization that incorporates both the droplet size distribution and the particle size distribution is developed in this work.

A mathematical model to predict the evolution of the latex particle size distribution in an emulsion polymerization reactor was developed. The mathematical framework is based on the population balance approach. It is general in framework, readily expandable to incorporate the physiochemical phenomena of interest to the reacting system of interest. The model includes such mechanistic details as (1) particle generation from radicals entering micelles; (2) particle size dependence of the radical entry mechanism; (3) coupling of the radical concentration in the aqueous phase and the particle phase; (4) determination of the particle phase radical concentration by radical entry into, exit from, and termination inside the particle; and (5) thermodynamic equilibrium between the monomer concentration in the aqueous phase and the particle phase. The model was solved efficiently with orthogonal collocation. Dynamic simulations were compared with experimental data taken from the literature for the emulsion polymerization of styrene (monomer), potassium persulfate (initiator), and sodium dodecyl sulfate (emulsifier). The variables considered were the total number of particles formed, duration of the nucleation period, conversion at the end of the nucleation period, variation of the monomer volume fraction in the particles with time, and conversion-time curves for different monomer, initiator, and emulsifier concentrations. Close agreement was found between the simulations and the experimental data.

Summary: The predictions of the model developed in Part 1 of this series are compared with experimental values taken from literature. Initially, the method of solution of the population balance equation and the simulation algorithm are given. Various radical entry mechanisms are discussed in adequate detail. Plausible arguments are given to identify the correct radical entry mechanism. An expression to evaluate the radical exit coefficient is given. Model predictions of a number of variables are discussed. These include average number of radicals per particle, particle phase monomer volume fraction, average number of radicals averaged over all particles, monomer volume fraction averaged over all particles, variation of nucleation rate, variation of fraction of droplets nucleated, variation of average diameter, variation of standard deviation, variation of polydispersity index, and development of particle size distribution with time. Finally, model predictions for the variation of conversion with time for five different initiator concentrations, number average diameter, standard deviation and full distribution are compared with experimental values.

Effects of the operating policies-the initial initiator amount; the initial emulsifier amount; the monomer addition mode: batch or semibatch; and the monomer addition rate under "monomer-starved conditions" for the control of particle size distribution (PSD)-were studied through a model that simulates batch and semibatch reactor operations in conventional emulsion polymerization. The population balance model incorporates both the nucleation stage and the growth stage. The full PSDs were reported, which have normally been omitted in earlier studies. It was shown through simulations that the broadness of the distributions, both initial (obtained after the end of nucleation) and final (after complete conversion of monomer), can be controlled by the initial initiator amount and the emulsifier amount. The higher initiator amounts and the lower emulsifier amounts favor narrower initial and final distributions. The shape of the initial PSDs and the trends in the average size and range were preserved with subsequent addition of monomer in the batch or in the semibatch mode, although the final PSD was always considerably narrower than that of the initial PSD. The addition of monomer in the semibatch mode gave narrower distribution compared to that of the batch mode, and also, lower monomer addition rates gave narrower distributions (larger average sizes), which was a new result. It was further shown through simulations that, under monomer-starved conditions, the reaction rate closely matched the monomer feed rate. These conclusions are explained (1) qualitatively-the shorter the length of the nucleation stage and the larger the length of the growth stage (provided the number of particles remains the same), the narrower is the distribution; and (2) mathematically-in terms of the "self-sharpening" effect. Experimental evidence in favor of the self-sharpening effect was given by analyzing the experimental particle size distributions in detail. The practical significance of this work was proposed.