Investigations of Depletion Flocculation of Concentrated Sterically Stabilized Latex Dispersions Using Viscoelastic Measurements and Microscopy

Abstract

The viscoelastic properties of concentrated polystyrene latex particles containing grafted poly(ethylene oxide) chains were investigated using oscillatory shear rheometry. The effects of varying the volume fraction of the polymer, the molecular weight of the added free polymer (PEO), the volume fraction of the latex, and the latex particle size have been systematically investigated. Above a critical volume fraction of polymer, φ + p, a rapid increase in the storage and loss moduli was observed for all the systems studied. This increase corresponds to flocculation of the latex particles by the free nonadsorbing polymer due to the depletion phenomenon. The values of φ + p obtained from these viscoelastic data corresponded closely to the values estimated from Bingham yield stress values reported previously. Increasing the molecular weight of the (nonadsorbing) polymer caused φ + p to decrease, in line with other studies for depletion flocculated systems. Over the range of volume fraction of latex particles studied, however, there was little dependence of the onset of flocculation on the latex volume fraction φ s. It was noted that the smaller particles had considerably larger storage moduli following flocculation than the larger ones. This is essentially a surface area-to-volume effect. It was found that on increasing φ p above φ + p the storage modulus reached a plateau value, whereas the Bingham yield stress values continuously increased. This is a consequence of the yield stress values being due largely to the energy of separation between the particles which is predicted theoretically to increase with φ p, while the storage modulus is a measure of the number of bonds formed in the flocculated structure, which may not increase above a certain φ p. Optical micrographs of the flocculated suspension revealed that over this polymer volume fraction range the floc structures were very similar, confirming the above hypothesis.