The effect of anionic polyelectrolytes on the properties of aqueous silicon nitride suspensions

The effect of anionic polyelectrolytes on the electrokinetic and rheological properties of concentrated Si3N4 suspensions was investigated experimentally. We found that polyelectrolyte adsorption and, thus, colloidal stability at pH > pH(iep)(Si3N4) is mainly governed by the surface charge density of the solid phase. Comparing anionic methacrylic acid comb copolymer modifications with grafted poly(ethylene oxide) chains (PMMA-PEO), with poly(acrylic acid) (PAA) showed that the grafted PEO chains have a minor influence on the colloidal stability. The common viscosity minimum for all the anionic polyelectrolytes around pH 7 suggests that the suspensions are electrosterically stabilized. The polyacrylic backbone attains an extended conformation perpendicular to the surface. The effect of excess addition of polyelectrolyte was also discussed; we attribute the significant increase in suspension viscosity to the increased ionic strength caused by the release of associated counterions of the polymer functional groups. (C) 2000 Elsevier Science Ltd. All rights reserved.