Bradley R. Morrison

Modelling particle size distributions and secondary particle formation in emulsion polymerisation

An extensive model is given for the particle size distribution (PSD), particle number, particle size and amount of secondary nucleation in emulsion polymerisations. This incorporates what are thought to be all of the complex competing processes: aqueous phase kinetics for all radical species arising from both initiator and from exit (desorption), radical balance inside the particles, particle formation by both micellar and homogeneous nucleation mechanisms, and coagulation (the rate of which is obtained using the Healy-Hogg extension of DLVO theory). The predictions of the model are compared to extensive experimental results on rates, time evolution of the particle size distribution, and relative amounts of secondary nucleation, for styrene initiated by persulfate with sodium dodecyl sulfate and with sodium dihexyl sulfosuccinate as surfactants. For this system values of almost all of the many parameters needed for the model are available from independent measurements, and thus no significant parameter adjustment is plausible. Accord with experiment is imperfect but quite acceptable, supporting the validity of the various mechanisms in the model. Effects such as the experimental variation of particle number with ionic strength, as well as calculated coagulation rate coefficients as functions of particle size, suggest that coagulation of precursor (i.e., newly-formed) particles is a significant effect, even above the cmc. The modelling also suggests why secondary nucleation occurs readily in systems stabilised with polymeric surfactant.

The role of aqueous-phase kinetics in emulsion polymerizations

The kinetics of species in the aqueous phase control many events in emulsion polymerization: the rate of entry of free radicals into particles (equivalent to initiator efficiency), the rate of exit (desorption) of free radicals from particles, the fate of desorbed free radicals and of free-radical species derived directly from aqueous-phase initiator. Aqueous-phase kinetics also dominate particle nucleation and re-seeding (secondary nucleation), and the in situ formation of surfactant. The mechanisms of each of these events are discussed, and it is shown how general methods can be constructed to deduce the rate-determining events for each of these. The methodology is then applied extensively to styrene, which leads to the following conclusions. (a) The aqueous-phase events which govern entry (initiator efficiency) are propagation and termination, with entry occurring irreversibly when a critical degree of propagation z is reached so that the resulting species (a di- or tri-styrenesulfonate species in the case of styrene with persulfate initiator) is sufficiently surface-active that, once adsorbed onto the particle it does not desorb before it propagates; the actual adsorption event is sufficiently rapid so as not to be rate-determining except during nucleation. (b) Exit of free radicals is governed by transfer inside the particle to form a monomeric radical which may desorb and diffuse irreversibly away from the parent particle before it propagates therein. (c) The fate of desorbed free radicals in the wide range of styrene systems examined is to re-enter another particle and remain therein, rather than the other possible fates (aqueous-phase termination or re-exit until intra-particle termination eventually occurs). (d) Below the cmc, nucleation is by the homogeneous-coagulative mechanism, while above the cmc, nucleation is through a process which combines the essential features of both homogeneous-coagulative and micellar-entry models. (e) Analysis of the aqueous-phase products produced in an emulsion polymerization shows that the species involved in termination, entry and exit also undergo subsequent reactions: hydrolysis and reaction with persulfate.

Radical Capture Efficiencies in Emulsion Polymerization Kinetics

Experimental data and models are presented for initiator efficiency in emulsion polymerization systems in the absence of particle formation. The data show that a number of 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 however support the model (first suggested by Priest) of aqueous phase propagation to a critical degree of polymerization, whereupon capture of the resulting oligomeric free radical by a particle is instantaneous. Mutual aqueous phase termination of smaller species also occurs; one must take account of the fact that the rate coefficient for this is some two orders of magnitude greater than that for low molecular weight species in polymeric systems. This model is in quantitative and qualitative 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, on ionic strength and on temperature.