Salt effects on controlled coagulation in emulsion polymerization

The effect of (1.0–10.0)×10−4 M electrolyte (NaCl and Na2SO4) addition on the electrostatically controlled coagulation process during the seeded emulsion polymerization was investigated. A particular seeded emulsion polymerization composed of an anionic poly(methyl acrylate-co-methyl methacrylate) seed and a mixture of styrene and cationic dimethylaminoethyl methacrylate (DM) was carried out using a cationic initiator, 2,2′-azobis(2-amidinopropane)·2HCl (V-50). The particle size and zeta-potential of the enlarged composite particles were measured in order to clarify the effects of compression of electrical double layer as well as the counter ion interactions between the anionic seeds and the cationic species. Increasing a concentration of electrolyte added in the seeded polymerization showed a trend of decreasing the absolute value of zeta-potential. With an addition of NaCl or Na2SO4, a low concentration (1.0×10−4 M) promoted a further particle growth. However, the higher concentrations (5.0–10.0)×10−4 M deteriorated the stability of the latices during the polymerization. The final particle size with an addition of mono-valence salt, NaCl, was larger than that with using a divalent Na2SO4, indicating more favorable controlled coagulation. It was concluded that the dissociation of DM and V-50 at pH 9.4 already compressed the electrical double layer before an addition of electrolyte. The more mobile cationic ion (Na+) was considered to be preferentially located in the Stern layer surrounding the polymer particles and effectively neutralized the anionic charge on the surface, thus reducing the stability of the particles to promote a controlled coagulation process.