Abstract: Dopamine (DA) has been proved to be able to oxidize and polymerize into polydopamine (PDA) on the surface of a variety of materials, however, the mechanism of the DA polymerization on strongly electronegative surfaces and the formed morphologies of PDA are not clear yet. In order to investigate the influence of the surface electronegativity and oxidation conditions on the rate of the oxidized self-polymerization and the morphology of the PDA layer, herein, polystyrene/polyacrylic acid nanoparticles (PS/PAA NPs) with PS as the core and PAA as the shell were prepared by soap-free emulsion polymerization. Then, PDA was coated on the surface of the PS/PAA NPs and the effect of different pH buffers, reaction time and the amount of DA on the polymerization process and the PDA morphologies were investigated. The morphology and size of the nanoparticles were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Zeta potential. The results show that small PDA NPs are polymerized on the PS/PAA NPs surface to form the raspberry structure in the Tris buffer solution with a pH of 8.5, and the mass ratio of DA to PS/PAA NPs is 1∶1. The formed PDA NPs become bigger with higher DA contents and longer reaction time. When DA is excess, it eventually forms a uniform PDA shell on the surface of microspheres. Zeta potential results show that the raspberry-like structure is formed due to the balance between electrostatic interaction and charge repulsion in the surface polymerization of PAA. A larger amount of DA and a longer reaction time result in the reduction of charge repulsion to form a dense uniform PDA shell layer. In addition, uniform PDA shell layers form on the surface of PS/SDS microspheres with less electronegativity and nonionic PS microspheres. Therefore, the PDA deposition on the surface of the anionic materials is influenced by the negative value of the surface.

Key words: ionic polymer nanoparticles, polydopamine, morphology regulation, raspberry-like structure