Poly(divinylbenzene) Microspheres as an Intermediate Morphology between Microgel, Macrogel, and Coagulum in Cross-Linking Precipitation Polymerization

Precipitation copolymerizations of mixtures of divinylbenzene-55 (DVB55) and 4-methylstyrene at total monomer loadings of 4 vol % were conducted in solvent mixtures comprised of mixtures of methyl ethyl ketone and heptane. The experimental compositions hence formed a two-dimensional matrix where the actual DVB content varied from 0 to 55% and the solvent composition varied from 0 to 100 vol % MEK. Four distinct polymer morphologies including microspheres, microgels (and soluble polymer), macrogel, and coagulum were observed and are reported in form of a morphology map superimposed on the above compositional map. The structures of these four polymer architectures are described, and the effects of both DVB concentration and solvency on the transitions between morphology domains are discussed. The portion of reaction volume occupied by the polymer formed decreases with both decreasing solvency and increasing DVB concentration. These results indicate that the microspheres are formed, in part, by an internal contraction which is caused by both the marginal solvency of the continuous phase and the cross-linking of the polymer network. This contraction is likely progressive, supporting the presence of a lightly cross-linked corona around the microspheres which acts as an in-situ steric stabilizer layer.