Microgel: An idealized polymer molecule

Pure microgels (i.e., those free from any linear or branched admixed polymers) were prepared by an emulsion polymerization process. The products were characterized by ultracentrifuge measurements. The best crosslinking agent for acrylonitrile microgels was found to be N,N ′‐methylene bisacrylamide, while divinylbenzene gave the best styrene and methyl acrylate microgels. A study of the viscosity of microgel solutions vs. concentration showed that there was a sudden transition from a low viscosity to a high viscosity at a characteristic gel point, G , dependent upon the degree of crosslinking. For monodisperse systems, this coincides with the point where hexagonal close packing of the swollen spherical microgel particles occurs. The following equations relating the gel point and intrinsic viscosity of a microgel were found to apply: [η] = (0.025/ρ) S and ( G /ρ) S = π/3√2, where S is a swelling factor which is equal to the ratio of swollen to dry volume and ρ is the density of the dry microgels. The above relationships were developed by considering microgels as swollen Einstein spheres. Evidence is presented to support the equations by some light scattering and viscosity measurements. The Flory equation relating swelling factor, polymer‐solvent interaction parameter, and degree of crosslinking was successfully applied to microgel solutions. A simple empirical equation, found to correlate the intrinsic viscosity [η] with the mole ratio of crosslinking agent, x for the microgels, was [η] = K (1/ x ) + 0.025/ρ where K is a constant.