Preparation of monodisperse polymer particle via an in situ charge neutralization-controlled particle coagulation mechanism in macroemulsion polymerization

Abstract

Styrene emulsion polymerizations using sodium dodecyl sulfate as an anionic surfactant and in the presence of cationic initiators 2, 2′- Azobis (2-methylpropionamidine) dihydrochloride (AIBA) and 2,2′-azobis(N,N′-dimethyleneisobutyramidine) dihydrochloride (ADIBA) were studied. In this work, an explanation for in situ charge neutralization is proposed, and the effect of in situ charge neutralization on the particle size distribution of the ultimate polymer particles is discussed. Charges of cationic radicals from initiator decomposition shield the anionic charges of surfactant molecules adsorbed on the particle surface and induce particle coagulation during the early particle formation period. As the occurrence of in situ charge neutralization, the particle number decreases and the rate of micelle capture for the initiator radical is enhanced. Thus, the particle nucleation period is shortened, and monodisperse polymer particles with a diameter of ～200−300 nm are prepared using this simple emulsion polymerization process. In addition, hydrolysis of the initiator AIBA at different pH conditions is used to control particle size and stability. Compared to conventional emulsion polymerization theories, the emulsion polymerization in the presence of in situ charge neutralization presents unique kinetics.