Colloidal Processing of Silicon Nitride with Poly(acrylic acid): II, Rheological Properties

Abstract

Poly(acrylic acid) (PAA) was the focus of an investigation in the role and behavior of polyelectrolyte dispersants in aqueous‐based colloidal processing of silicon nitride. The steady‐shear flow properties of dense suspensions were examined as a function of pH, particle volume fraction, chain length, and polymer concentration. Suspensions exhibited shear‐thinning behavior, which was analyzed using the constitutive Bingham equation for plastic flow. The data showed that polyelectrolyte charge and configuration were important in the observed flow behavior. In acidic media, PAA behaved similar to a neutral polymer in a poor solvent, whereas, in alkaline media, PAA behaved similar to a rigid asymmetric particle. Although electrostatic forces dominated flow properties in alkaline media, where PAA was predominantly in the free‐polymer form, at lower pH values, PAA interactions with the particle surface resulted in a complex array of suspension effects, including polymer bridging, electrosteric stabilization, and flocculation.