Abstract
Particle nucleation in emulsifier-free emulsion polymerization of styrene using potassium persulfate as initiator is thought to involve an in situ micellization mechanism. The particle nucleation period can be divided into two stages. Stage 1 is short (less than 10 min) and large amounts of oligomer micellar particle nuclei is produced during this stage, which consist mainly of oligomers of molecular weight ca. 1000 and are stable due to surface charge generated by the initiator end groups. As these particle nuclei capture free radicals from the aqueous phase and produce internal high molecular weight polymers, the surface charge density decreases dramatically. The particle nuclei then lose their stability and begin to undergo coagulation, which marks the beginning of Stage 2. The critical chain length for in situ micellization, n∗, decreases during Stage 1 due to the increase of oligomer concentration through significant aqueous-phase termination of radicals. Particle size increases during Stage 2 through coagulation and polymerization. Radical capture by particles begins to be more significant than the aqueous-phase termination during Stage 2. The concentration of oligomer stops increasing and n∗ becomes constant. Particle number decreases toward a constant value with time in Stage 2 because of coagulation. A model has been developed and used to simulate particle nucleation during the transient period (both Stage 1 and Stage 2) described above. The criterion used to identify the beginning of Stage 2 was arbitrarily selected as that point when the ratio of the rate of radical capture by the particles to the rate of aqueous-phase termination becomes 2.
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