CONTROLLED FLOCCULATION-DEFLOCCULATION BEHAVIOR OF ADSORBED BLOCK-COPOLYMERS IN COLLOIDAL DISPERSIONS BY MODIFYING SEGMENT SURFACE INTERACTIONS - THE USE OF SMALL DISPLACER MOLECULES TO SELECTIVELY CLEAVE INTERPARTICLE BONDS

It is well established that the flocculation-deflocculation behavior o f adsorbed block copolymers in colloidal dispersions is controlled mai nly by the solvency of the extended polymer chain in solution. In this paper, we describe a colloidal dispersion in which it is possible to modify the flocculation-deflocculation behavior by altering specific p olymer segment/surface interactions without affecting the amount of ad sorbed polymer. Infrared spectroscopy has been used to study the adsor ption of three poly(ethylene oxide-styrene) block copolymers (PEO-PS) on silica particles suspended in CCl4. The three block copolymers chos en for this study differ in the relative sizes of the PEO and PS block s. Although CCl4 is a good solvent for both PEO and PS, the PEO segmen ts of the block copolymer preferentially adsorb on the surface hydroxy l groups (silanols) because the segment/surface interaction of PEO is stronger than that of PS. With an excess of polymer in the solution, a ll three copolymers stabilize the silica dispersions. However, at thes e concentrations, it is found that the PEO segments may not adsorb exc lusively because of the geometric constraints imposed by the relative block sizes. At polymer concentrations in solution that are not in exc ess, the relative number of PS segments adsorbed oh the surface sites is higher due to the formation of interparticle bridges. Flocculation occurs because the PS segments extending from the surface from one par ticle form interparticle bonds by attaching to exposed silanol sites o n other silica particles. We show that redispersion can be induced by adding a small displacer molecule (triethylamine) to selectively cleav e PS/silanol interparticle bonds.