POLYMERIZATION CONDITIONS AND THE DEVELOPMENT OF A CORE-SHELL MORPHOLOGY IN PMMA PS LATEX-PARTICLES .1. INFLUENCE OF INITIATOR PROPERTIES AND MODE OF MONOMER ADDITION

Latex particles containing equal amounts of poly(methyl methacrylate)( PMMA) and polystyrene (PS) were prepared by polymerizing styrene in th e presence of PMMA seed particles using two different initiators, pota ssium persulfate (KPS) and tert-butyl hydroperoxide (t-BHP). Styrene w as added either before polymerization (batch) or continuously during t he polymerization. Particles prepared using batch addition of styrene showed no signs of a core-shell morphology, and the surface concentrat ion of PS was lower than 50%. Particles with well-defined core-shell s tructures were obtained when styrene was added at a low feed rate. Wit h t-BHP as initiator, the shell layer was compact and distinct and the surface concentration of PS high, whereas with KPS as initiator the s hell layer was thicker and contained domains of PMMA. Composite partic les prepared using a low styrene feed rate, but seed particles contain ing a chain-transfer agent, had PS domains distributed throughout the entire particle volume. Hence, the formation of particles with a core- shell structure was due to the suppression of radical transport to the interior of the seed particles, mainly because of the high internal v iscosity of the latter. The structure composition of the shell layer, however, depends on the chemical nature of the initiator