pH Effects on the Synthesis of Nanocapsules via Interfacial Miniemulsion Polymerization Mediated by Amphiphilic RAFT Agent with the R Group of Poly(methyl acrylic acid-ran-styrene)

Interfacial radical miniemulsion polymerization was constructed to be a general process for nanoencapsulation and the preparation of nanocapsules by using an amphiphilic reversible addition−fragmentation transfer (RAFT) agent with the R group of poly(methyl acrylic acid-co-styrene) as a reactive surfactant. Styrene and n-nonadecane were used as a model of monomer and core materials, respectively. It was found that the fragmental R radicals, which exited out of the droplets into the water phase, could trigger the homogeneous nucleation. When the homogeneous nucleation rather than the monomer droplet nucleation became the dominant mechanism of the particle formation, a large fraction of solid particles would be coformed with the nanocapsules. By decreasing pH values, we could decrease the hydrophilicity of R radicals and thus suppress the formation of the solid particles from the homogeneous nucleation. For the current model system, when values were decreased from pH 8.12 to 6.45, the fraction of nanocapsules linearly increased from 18 to 94%. In the middle pH values very uniform nanocapsules were obtained accompanying a few of the solid particles.