CORRELATION OF THE RHEOLOGY OF CONCENTRATED DISPERSIONS WITH INTERPARTICLE INTERACTIONS

The viscoelastic properties of concentrated latex dispersions were investigated as a function of the volume fraction of particles. Both electrostatically and sterically stabilized dispersions were studied. The dispersions showed a change from a predominantly viscous to a predominantly elastic response when the volume fraction of the dispersion exceeded a critical value phi(cr). With electrostatically stabilized dispersions, phi(cr) was determined by the electrolyte concentration. At low electrolyte concentrations (10(-5) mol dm-3 NaCl) elastic response occurred at relatively low volume fraction phi of a latex dispersion of radius 700 nm. This is due to the high effective volume fraction in this case (phi(eff) = 1.5phi), i.e. when phi almost-equal-to 0.4, one approaches the maximum packing fraction at which double layer overlap occurs. At higher electrolyte concentrations (10(-3) mol dm-3 NaCl), phi(eff) is slightly higher than phi (phi(eff) = 1.05phi) and elastic response occurs at phi values approaching 0.6. In other words, the dispersion approaches a hard-sphere system. The high frequency modulus G' was calculated from knowledge of the double layer parameters. Plots of theoretically obtained G(th)' vs phi were compared with the experimentally determined values obtained from rheology. With sterically stabilized dispersions, the change from predominantly viscous to predominantly elastic response is governed by the ratio of the adsorbed layer thickness DELTA to the particle radius R. Using latex particles with three different radii (core radii 78, 303 and 502 nm) with the same grafted poly(ethylene oxide) (PEO) chains of molecular weight M 2000 (DELTA almost-equal-to 8-21 nm depending on R) it was shown that the dispersions behaved like hard spheres when DELTA/R (defined as the compressibility of the chains) was relatively small, and as soft spheres when DELTA/R was significant. The elastic modulus G' vs phi could be fitted to a scaling law of the form G' = kphi(m) and the exponent m was correlated with the compressibility of the chains. Force-distance curves (F-h) were obtained using mica containing an adsorbed graft copolymer of methyl methacrylate and PEO chains with M = 2000. The F-h curves were converted to G'-phi curves, and the results were compared with those obtained from rheological measurements.