On the Reaction Characteristics of Miniemulsion Polymerization with Aqueous Phase Initiation – Experiments and Modeling

Abstract

Miniemulsions, consisting of submicron droplets of very hydrophobic lauryl methacrylate or 4-tert-butyl styrene, are successfully polymerized using water-soluble sodium persulfate. Monitoring the calorimetric profile as well as the droplet and particle size distribution with conversion manifests a process of monomer redistribution, droplet disappearance, and narrowing of the particle size distribution. The observed reaction characteristics could be modeled adequately, using thermodynamic principles. The results of the work presented do not only have predicting value, but also enfeeble the idea of a one-to-one translation of monomer droplets into polymer particles in miniemulsion polymerization.

On the miniemulsion polymerization of very hydrophobic monomers initiated by a completely water‐insoluble initiator: thermodynamics, kinetics, and mechanism

“…It is apparent from Figure and the measured polydispersities that the PSDs of the latex products are significantly broader than their aqueous phase initiated counterparts . The droplet diameters and their poly values, measured for both TBS and LMA, agree very well with the values measured on similar miniemulsions that have been polymerized using sodium persulfate, see Table .…”

“…The overall rates of polymerization for miniemulsion systems initiated by LPO are undoubtedly lower than for similar miniemulsion systems initiated by sodium persulphate . Moreover, the conversion‐time histories of these LPO initiated polymerizations do not reflect the typical S‐shape often observed in (mini)emulsion polymerizations with (long) nucleation periods and in the presence of monomer droplets.…”

“…For all polymerizations, a continuous decrease of the polymerization rate after a steep initial rise can be observed. Albeit this continuous decrease is less pronounced for LMA and TBS than for ODA and STY, the difference with the plateau (or even increase) in the polymerization rates measured for aqueous phase miniemulsion polymerization is apparent . Bulk behavior in (mini)emulsion polymerization is usually indicative for either a Smith–Ewart case I or case III situation, in which the average number of radicals ( ) per particle is considerably lower or higher than 0.5 .…”

“…Above this conversion, a gel effect occurs and as a result, the radical concentration is no longer constant and the pseudo first order assumption is no longer obeyed. For aqueous phase initiated miniemulsion polymerizations instead, the presence of droplets, and therefore a constant monomer concentration inside the polymer particles, results in a pseudo zeroth order behavior in the conversion‐time histories . The bulk‐like behavior for LPO initiated miniemulsion polymerization is also visible from the rate of polymerization [Fig.…”

“…The control over the nucleation process and the number of reaction loci can result in reduced reactor fouling compared to conventional emulsion polymerization using the same formulation . However, in previous work we demonstrated that particle concentration and particle size distribution of the final latex do not necessarily agree with the droplet concentration and the droplet size distribution of the miniemulsion when water‐soluble initiators are used . Different volumetric growth rates between nanoparticles result in a thermodynamic driving force for monomer transfer, thereby narrowing the resulting particle size distribution.…”