Double Roles of Stabilization and Destabilization of Initiator Potassium Persulfate in Surfactant-Free Emulsion Polymerization of Styrene under Microwave Irradiation

Emulsifier-free emulsion polymerization of styrene in a water/acetone mixture under microwave irradiation resulted in narrowly distributed stable polystyrene nanoparticles with an averaged hydrodynamic radius 〈Rh〉 down to 35 nm when 50 wt % of acetone was added. For a given initiator potassium persulfate (KPS) concentration, 〈Rh〉 was proportional to the monomer concentration in the range 1.2−7.0 wt %; while for a given monomer concentration, 〈Rh〉 showed a minimum with an increasing initiator concentration. After both the roles of stabilization and destabilization of the initiator (KPS) were considered, we introduced a new parameter, the effective surface area stabilized per ionic group, and reformulated the Wu plot of 〈Rh〉 ∝ Wmonomer/Wstabilizer into 〈Rh〉/[k1(1 + k2WKPS)] ∝ Wmonomer/Winitiator. It can qualitatively explain both the monomer and initiator concentration dependence of the particle size over a wide concentration range. The study of the salt effect showed that 〈Rh〉 increased with the sodium sulfate concentration for fixed KPS and styrene concentrations.