A water-insoluble dye was used as a probe to study the particle nucleation
mechanisms in the semibatch emulsion polymerization of methyl methacrylate.
The experimental results show that homogeneous nucleation plays a key role
in the particle formation period. When the surfactant concentration ([S])
is above its critical micelle concentration (CMC), the mixed modes of parti
cle nucleation (micellar and homogeneous nucleation) are operative in the p
olymerization system. In the absence of micelles, homogeneous nucleation is
the predominant mechanism. Limited flocculation of latex particles becomes
more important in controlling the particle concentration as the initiator
concentration ([I]) is increased from 2.66 to 31.92 mM. When [S] is kept co
nstant at 12 mM, the degree of formation of particle nuclei in the aqueous
phase increases with increasing [I]. Monomer droplet nucleation however, ca
nnot be ruled out. This is because diffusion of dye molecules from monomer
droplets to the growing latex particles originating from homogeneous nuclea
tion is prohibited and in the absence of micelles an appreciable amount of
dye still can be detected in the resultant latex particles. (C) 2000 Elsevi
er Science Ltd. All rights reserved.