Negatively Charged Ultrafine Black Particles of P(MMA-co-EGDMA) by Dispersion Polymerization for Electrophoretic Displays

Polymeric particles ranging in diameter from 1 to 3.5 μm containing a black dye, Sudan black B, were prepared by dispersion polymerization in a methanol and water mixture. To control their electrophoretic mobility, a varying amount of charge control additives was added after first labeling them with a fluorescent moiety to trace their distribution by confocal laser microscopy. The particle size was found to be quite sensitive to both the change in polarity of the polymerization medium and the amount of polymeric stabilizer employed. At the same time, increasing the amount of charge control additives resulted in an increase in the particle size. On the other hand, the electrophoretic mobility exhibited a maximum or optimum point at ∼0.3 wt % of charge control additive. This is consistent with the fluorescence intensity profile obtained from the confocal laser measurement, which shows a decrease in the intensity of the particles with increasing concentration of charge control additives. The simple peak patterns in the cross-sectional profile obtained from the confocal laser measurement suggests that the charge control additives are mainly distributed inside of the particles.