Non-equilibrium particle morphology development in seeded emulsion polymerization. 1: penetration of monomer and radicals as a function of monomer feed rate during second stage polymerization

Starve feeding of monomers is often used in an attempt to control latex par ticle morphology, especially when non-equilibrium structures are desired. F or the case of a polar seed polymer and a non-polar second stage polymer, w e have analyzed the relative probabilities of reaction and diffusion of pol ymer radicals and monomers as they penetrate the seed particle. The resulta nt penetration ratios (for polymer radicals and monomers) and fractional pe netration values (depth of penetration) correlate well with a number of dif ferent non-equilibrium morphologies obtained from a wide variety of experim ental reaction conditions. We conclude that the lack of polymer radical pen etration is responsible for non-equilibrium core-shell structures for the g lassy PMMA seed/PS system, while the styrene monomer easily penetrates the entire particle, even at very slow monomer feed rates. When the polar, low T-g, PMA is substituted for the PMMA seed, the polymer radicals cannot be e xcluded from the particle center and an inverted core-shell equilibrium str ucture is obtained at all monomer feed rates. (C) 1999 Elsevier Science B.V . All rights reserved.