Donald H. Napper

A model, together with a comparison with previously published experimental data, is presented for initiator efficiency in seeded styrene emulsion polymerization systems in the absence of secondary particle formation. The data had shown that a number of previous models are inapplicable, viz., those assuming that the rate-determining step for free-radical entry into a particle is either diffusional capture, surfactant displacement, or colloidal entry. The data support the supposition that the rate-determining step for free-radical capture by latex particles is aqueous-phase propagation to a critical degree of polymerization, whereupon capture (irreversible adsorption) of the resulting oligomeric free radical by a particle is essentially instantaneous. Mutual aqueous-phase termination of smaller species also occurs. When account is taken of the fact that the rate coefficients for (a) the first aqueous-phase propagation step and (b) aqueous-phase termination are both in the diffusion limit, this model is in qualitative and quantitative accord with the experimental dependences of the entry rate coefficient on the concentrations of initiator, of surfactant, of aqueous-phase monomer, and of latex particles as well as on particle size and on ionic strength. For styrene emulsion polymerization initiated by persulfate, the critical oligomer size for entry was found to be dimeric.

A detailed theory is presented for nucleation kinetics in emulsion polymerization systems based on the coagulation of precursor particles (which may themselves be formed by either homogeneous nucleation or micellar entry). These precursor particles differ from true latex particles by a slower rate of polymerization and the lack of stability against coagulation. The coagulative nucleation theory combines extended Müller-Smoluchowski coagulation kinetics with DLVO theory. Expressions are provided for the time evolutions of the nucleation rate, particle number, and particle size distribution (PSD). With physically reasonable values for the parameters for the coagulation kinetics, agreement is obtained with data for styrene emulsion polymerization systems. In particular, excellent accord is obtained with the early-time evolution of the PSD, such data being especially sensitive to assumptions as to the nucleation mechanism. In addition, agreement is obtained with data on the dependence of particle number on surfactant and initiator concentrations.