The effects of neutralization on the dynamic rheology of polyelectrolyte microgel mucilages

The dynamic rheological properties of a model poly(acrylic acid) microgel mucilage in the close-packed concentrated region (reduced concentration cQ > 1) were evaluated as a function of the degree of neutralization, a = [NaOH]/[COOH]. As the parent microgel is neutralized, up to an order of magnitude increase is observed in the swelling ratio Q, dynamic modulus, and yield properties. Four regions were observed where the dynamic rheological properties exhibited a different power law dependence with increasing a. The dynamic yield strain does not change remarkably with increased neutralization, whereas the dynamic yield stress increases more than 10-fold. This supports a mechanism where the yield strain depends on geometric considerations, whereas the yield stress reflects the stress required to force microgels past adjacent microgels. A linearity observed between modulus and dynamic yield stress with increased neutralization suggests a concerted expansion and affect on the rheological properties with the microgel core and emanating polymer chains. (C) 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012