USE OF VISCOELASTIC MEASUREMENTS IN STUDYING INTERACTIONS IN CONCENTRATED DISPERSIONS

The use of viscoelastic measurements in studying interactions in concentrated dispersions has been demonstrated by using various model systems. These include charge-stabilized polystyrene latex dispersions, sterically stabilized dispersions containing grafted poly(ethylene oxide) chains, or physically adsorbed poly(vinyl alcohol). With electrostatically stabilized dispersions, the system became predominantly elastic when double-layer overlap occurred. This was the case at low electrolyte (10-5 mol dm-3 NaCl) concentrations since the effective volume fraction ϕeff is significantly larger than the actual volume fraction ϕ. Increasing the electrolyte concentration to 10-3 mol dm-3 caused compression of the double layer, and the dispersion became relatively more viscous than elastic. With sterically stabilized dispersions, elastic interaction occurred as soon as the volume fraction reached a value whereby the grafted or adsorbed polymer chains start to overlap. Viscoelastic measurements could also be applied for studying flocculated systems. Three main types were investigated: weakly flocculated dispersions produced by addition of free (nonadsorbing) polymer (depletion flocculation), sterically stabilized dispersions flocculated by reduction of solvency to worse than 0-conditions (incipient flocculation), and strongly flocculated (coagulated) dispersions produced by addition of electrolyte to a charge-stabilized system. With weakly flocculated disperions, all rheological parameters show a rapid increase above a critical volume fraction of the added free polymer. In this case, it was possible to calculate the energy of separation of particles in the floe from the yield value. This was compared with theoretical calculation of the free energy of depletion flocculation. With sterically stabilized dispersions, reduction of solvency was produced by addition of electrolyte or increasing temperature at constant electrolyte concentrations. The results showed an initial reduction in modulus due to collapse of the chains, followed by a rapid increase above the flocculation point. Coagulated suspensions obtained by addition of electrolyte to an electrostatically stabilized system were investigated by using strain and oscillatory measurements. The strain sweep experiments indicated a sudden reduction in moduli value above a critical strain. This indicated breakdown of the structure above this value. The results also show that the modulus increases with the volume fraction ϕ through a power law with an exponent in ϕ of the order of 6. From the critical strain and the storage modulus in the linear region, the energy of cohesion in the floe, Ec, was calculated. Ec was also found to increase with ϕ through a power law with an exponent of 9 in 0. © 1990, American Chemical Society. All rights reserved.