COLLOIDAL PROCESSING OF SILICON-NITRIDE WITH POLY(ACRYLIC ACID) .1. ADSORPTION AND ELECTROSTATIC INTERACTIONS

The role of poly(acrylic acid) (PAA) in the dispersion of silicon nitr ide suspensions was investigated experimentally, The effects of concen tration, relative molecular mass, and suspension pH were evaluated, Th e ionization of PAA was characterized by potentiometric titration and indicated a pH-dependent conformational transition, The isoelectric po int for silicon nitride decreased from pH 6.3 to pH 3 as the PAA conce ntration increased, roughly independent of relative molecular mass. A broadening of the stability region for silicon nitride was attributed to this effect. Redispersion in alkaline media, subsequent to destabil ization by surface-charge neutralization, improved following preadsorp tion of PAA in acidic media. This effect was attributed to formation o f a protective polymer adlayer on the surface, which prevented primary minimum aggregation; an electrosteric contribution also may be presen t. The influence of free polymer on the suspension properties also is discussed.