Preparation of fluorescent polymer particles by emulsion polymerization

Core-shell type fluorescent polymer particles were produced in multi-step emulsion polymerization. Cross-linking was introduced to the shell to suppress leakage of fluorescent dyes contained in the core particles. First, fluorescent polymer particles were synthesized by emulsion copolymerization of styrene (St) and butyl acrylate (BA) with an anionic surfactant (SE1025A) initiated by ammonium persulfate (APS) in the presence of pyrene. The concentrations of St, BA, SE1025A and APS were 550, 89, 8.2 and 5 mol/m3 water, respectively. Then the fluorescent particles were covered with cross-linked polymer by emulsion copolymerization of St, BA and divinylbenzene (DVB) with SE1025A emulsifier. In the polymerization, a mixture of St, BA, SE1025A and DVB was added dropwise to the reaction system. The average size of cross-linked fluorescent particles ranged from 50 to 70 nm. The fluorescence intensity of the particles prepared in a pyrene concentration range of 8 - 20 mol/m3 water increased linearly with pyrene concentration. Fluorescence intensities of the particles prepared with and without DVB were also measured. The cross-linked polymer formed by DVB addition drastically suppressed the intensity drop caused by dye leakage almost to one-third of the intensity drop measured for particles prepared without DVB addition.