Emulsion polymerizations of styrene and N-butyl acrylate in an automated reaction calorimeter.

摘要

Emulsion polymerizations of styrene and n-butyl acrylate using the nonionic surfactant Triton X-405 were studied in detail to understand the kinetics and the nucleation mechanisms of these homo- and co-polymerizations. The partitioning behavior of the surfactant between the phases was found to play the major role in determining the nucleation mechanisms.; In emulsion homopolymerizations of styrene, two separate nucleation periods were noted for Triton X-405 concentrations below 80 mM resulting in bimodal latex particle size distributions. Although the total concentration of the emulsifier in the recipe was above its critical micelle concentration (CMC), the portion of the surfactant initially present in the aqueous phase was below its CMC due to its partitioning. The first of the two nucleation periods was attributed to homogeneous nucleation while the second was attributed to micellar nucleation.; In contrast to the styrene homopolymerizations, in the emulsion polymerization of n-butyl acrylate, the surfactant partitioned primarily into the aqueous phase leading to nucleation in the presence of micelles and unimodal particle size distributions. Nucleation was accompanied by limited aggregation in the early stages of the reactions. Limited aggregation was thus shown to be an important mechanism in these polymerizations.; The copolymerizations of styrene and n-butyl acrylate, on the other hand, exhibited an in-between behavior. The kinetic behavior was found to vary widely depending on the emulsifier concentration. The results were attributed to the surfactant partitioning behavior in the system which led to homogeneous/coagulative nucleation at the lowest level (4.2 mM) to homogeneous/coagulative followed by micellar nucleation at the intermediate levels (6.3 and 8.4 mM) to micellar nucleation at the highest levels (12.5 and 16.2 mM). Unimodal particle size distributions were obtained at the highest and lowest levels whereas particle size distributions were bimodal at the intermediate levels due to two distinct nucleation periods.; Solids content was shown to be an important variable in studying the role of mixing on the kinetics of these emulsion polymerizations. At low solids (30%), impeller speed and type did not have any significant effect on the kinetics of polymerizations whereas at high solids (50%), reactions using different impellers exhibited different behaviors.