Model system for surfactant-free emulsion copolymerization of hydrophobic and hydrophilic monomers in aqueous solution

Polymeric nanoparticles were formed when a carboxylated polystyrene solutio n in tetrahydrofuran was added dropwise into water. The hydrophobic associa tion of the chain backbones was balanced by the hydrophilic repulsion of th e carboxylic groups. The resultant particles were very stable in water and resembled those formed in surfactant-free emulsion copolymerization. The fo rmation and stabilization of such particles were studied by a combination o f static and dynamic laser light scattering. The particle size decreases as the carboxylation extent [COOH] increases in the range 7-26 mol %. Further increase of [COOH] leads to an increase of the particle size. On the other hand, as [COOH] increases, the monotonic decrease of the average particle density [p] and the increase of the ratio of the average radius of gyration to the average hydrodynamic radius ([R-g]//[R-h]) reveal the swelling of t he particles, indicating a gradual change of their structure from spherelik e to clusterlike. Using longer polymer chains or more ionizable counterions results in smaller particles. Our results indicate that the formation and stabilization are controlled by a delicate balance between hydrophobic and hydrophilic interactions. More hydrophilic groups stabilizing a larger tota l interfacial area generally lead to smaller particles.