ABSTRACT:The mechanisms of microemulsion polymerizations stabilized by sodium dodecyl sulfate in combination with pentanol were investigated with a water-insoluble dye as the probe. The major parameters chosen for study were the types of initiators [water-soluble sodium persulfate (SPS) vs oil-soluble 2,2Ј-azobisisobutyronitrile (AIBN)] and the polarity of the monomers [relatively hydrophobic styrene (ST) vs relatively hydrophilic methyl methacrylate (MMA)]. Both continuous particle nucleation and limited particle flocculation had a significant influence on the polymerization kinetics. For the polymerizations investigated in this work, the relatively low initiation efficiency of AIBN resulted in a reaction system showing a quite different particle nucleation mechanism than that of the ST polymerization with SPS. The formation of particle nuclei in water was suppressed to some extent, and microemulsion droplet nucleation predominated in the ST polymerization initiated by AIBN. Homogeneous nucleation played an important role, and a mixed mode of particle nucleation (microemulsion droplet nucleation and homogeneous nucleation) was operative in the MMA polymerization. The MMA polymerization experienced stronger particle flocculation than its ST counterpart.

A water-insoluble dye was used to study nucleation and growth of particle nuclei in styrene microemulsion polymerizations, stabilized by sodium dodecyl sulfate/1-pentanol and initiated by sodium persulfate (SPS). Both the rate of polymerization and concentration of latex particles increase with an increase in the initial SPS concentration ([SPS] 0 ). Least-squares best fitting experimental data with a modified Morgan-Nomura model leads to the conclusion that the limited flocculation of latex particles plays an important role in the reaction system. The weight percentage of dye incorporated into latex particles y Dedicated to Dr. Gary W. Poehlein in token of respect and gratitude.(P dye ) increases with the progress of polymerization. This implies that particle nucleation takes place continuously throughout the reaction. At constant monomer conversion, P dye decreases with increasing [SPS] 0 , which is attributed to the increased probability of particle formation in the aqueous phase with [SPS] 0 . Measurements of zeta potential of latex particles and polymer molecular weight were also conducted to gain a better understanding of polymerization mechanisms and kinetics.