On the colloidal stability of poly(methyl methacrylate) and polystyrene particles prepared with surface-active initiators

The stability of poly(methyl methacrylate) (PMMA) latexes prepared with surface-active initiators (inisurfs) of the 2,2′-azobis(N-2-methylpropanoyl-2-amino-alkyl-1)sulfonate series (AAS) has been investigated in comparison with polystyrene (PS) dispersions. These initiators act in aqueous emulsion polymerizations two-fold as radical source to start the polymerization and as stabilizers for the colloidal particles. There exist strong correlations between the surface activity of the AAS on the one hand and the rate of polymerization and the stability of the latex particles on the other hand. For both polymers, it was found that the alkyl chain length of the inisurfs determines the zeta-potential, the hydrophilicity of the particles surfaces, and the colloidal stability of the dispersions. Moreover, the calculated barrier height of the overall interaction energy between two latex particles fits nicely with the experimentally observed stability. But also the hydrophilicity of the polymers influences the stability of the latexes. As shown by our experiments the more hydrophobic PS has a greater zeta-potential and hence, stronger repulsive forces between the polymeric particles exist compared with the more hydrophilic PMMA.