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

Abstract

Starve feeding of monomers is often used in an attempt to control latex particle morphology, especially when non-equilibrium structures are desired. For the case of a polar seed polymer and a non-polar second stage polymer, we have analyzed the relative probabilities of reaction and diffusion of polymer radicals and monomers as they penetrate the seed particle. The resultant penetration ratios (for polymer radicals and monomers) and fractional penetration values (depth of penetration) correlate well with a number of different non-equilibrium morphologies obtained from a wide variety of experimental reaction conditions. We conclude that the lack of polymer radical penetration is responsible for non-equilibrium core-shell structures for the glassy 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 excluded from the particle center and an inverted core-shell equilibrium structure is obtained at all monomer feed rates.