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Rheological behavior of acid-swellable cationic copolymer latexes.

2-Vinylpyridine (2VP) was copolymerized with four different cross-linker densities ranging from 0.05 to 0.31 wt % divinylbenzene (DVB) via aqueous emulsion polymerization to produce a series of submicrometer-sized, lightly cross-linked P2VP latexes. Protonation of the 2VP residues leads to a latex-to-microgel transition due to interchain electrostatic repulsion, as confirmed by dynamic light scattering. The DVB content of these pH-responsive copolymer particles strongly affects their rheological behavior. The particle size and viscosity of the swollen cationic microgels exhibit a maximum at approximately 0.11 wt % DVB. Static light scattering results confirm this density as the minimum amount of DVB required to ensure that all P2VP chains are cross-linked (i.e. that there is no soluble fraction), thus allowing optimal swelling of the microgels. Viscosity studies shows that the solution viscosity of a P2VP microgel at low pH follows two models, depending on its concentration. For volume fractions below 0.30, the P2VP microgels behave as hard spheres, as predicted by the Batchelor equation. For more concentrated P2VP microgels (volume fractions above 0.30), the rheological behavior can be predicted using the Krieger-Dougherty model for strong particle-particle interactions; thus, this semiempirical approach provides a useful description of the aqueous solution behavior of microgel.