RHEOLOGICAL STUDIES ON BRIDGING FLOCCULATION

The viscosity and viscoelasticity of silica suspensions in polyacrylamide (PAAm) solutions were measured as a function of particle diameter, molecular weight of PAAm, and mixing ratio of glycerin/water as a solvent. The results are interpreted through the structuring mechanism of associating polymers. For suspensions prepared with PAAm solutions in glycerin, the viscosity is lower than that of the medium. The intrinsic viscosity implies that the polymer coil is tightly contracted by bridging. The particles may act as stickers attached to the backbone. The polymer chain is in an extended conformation in a good solvent. Since the radius of gyration increases with water content of the medium, the polymer chains must begin to overlap, even at low concentrations. Therefore, the particles can cross-link two polymer chains in good solvents. The cross-linking rate is accelerated by shear, the suspensions show an irreversible increase in viscosity above some critical shear rate. When prepared using solvents which contain water at high concentrations, the suspensions are elastic at low frequencies, i.e., in the Newtonian range of the medium. The elastic responses are associated with the three-dimensional network of flocs. The relation between the elastic properties and floc concentration is discussed in terms of percolation process. © 1990.