Abstract
The mechanism of particle nucleation has great influence on the kinetics and the particle size distribution of polymer latexes produced in emulsion polymerization. These are key aspects for the application of these products and for the development of emerging processes, such as living/controlled free-radical polymerizations. One very popular theory publish in the 80s establishes that the newborn (primary) particles undergo limited coagulation even when the concentration of surfactant is above its critical micellar concentration. In this communication, the experiment that was the base for the formulation of such theory is reexamined and discussed to determine whether it can be considered as ‘‘typical condition’’, such that the general character of their conclusions can be justified. In this context, the positive skewness of the PSD observed in that time and taken as an evidence of the coagulative nucleation is discussed as well.
Conclusions
The emulsion polymerization performed in a dilatometer in a very similar way as Lichti et al. did, is not, from the kinetic point of view, comparable to the run effected in the stirred reactor. It is very likely that because of the usage of a high shear device to emulsifier the mixture, monomer mini-droplets can be formed and that these can act as nucleation loci in competition with monomer-swollen micelles, contributing in this way to the positive skewness of the PSD. Therefore, the results highly suggest that the reaction conditions studied by Lichti et al. cannot be considered as typical of the conventional emulsion polymerizations effected in stirred reactors. Moreover, the positive skewness of the PSDs does not represent a validation of the coagulative-nucleation theory, because there are other phenomena that can lead to this shape of the PSD. However, notwithstanding these weaknesses of such theory, it seems that limited coagulation of primary particles might occur depending on the monomer and specific reaction conditions.
