Microstructure and rheology of stimuli-responsive microgel systems—effect of cross-linked density

The effect of cross-linked density on the rheological behavior of model pH-responsive microgel systems consisting of methacrylic acid-ethyl acrylate (MAA-EA) cross-linked with di-allyl phthalate (DAP) was examined. Neutralization of acid groups increases the osmotic pressure exerted by counter-ions trapped in the polymeric network against the ions in bulk solution, which is responsible for the swelling and increase in viscosity. The viscosity exhibits a maximum at ∼1 wt.% DAP and it decreases to a steady value at 4 wt.% DAP, which is independent of pH and particle concentrations. Static light scattering results confirmed this optimum density as the critical point where sufficient cross-link points are present to produce permanent junctions that permit optimal swelling of the microgel particles. In addition, the variation of relative swelling with cross-linked densities of our model microgel systems agrees with the theoretical scaling law, Q∝(yαNx)3/2 for cross-linked densities beyond this optimum point (Q is the swelling ratio, y is the acidic MAA content, Nx is the average number of monomer units between two cross-linked points, and α is the degree of neutralization). By combining the results from light scattering and rheological measurements, we are able to correlate the microstructural evolution of the colloidal systems with their bulk rheological behavior.