Rheology of polyethylene oxide—polypropylene oxide block copolymer stabilized latices and emulsions

Abstract

The effect of three stabilizing polyethylene oxide—polypropylene oxide ABA block copolymers on the steady shear and oscillatory shear rheology of latices and emulsions has been studied. The polymers contained a constant propylene oxide (PO) content but differing amounts of ethylene oxide (EO). The adsorption onto latices was Langmuirian and of a high affinity. With increasing molecular weight, the weight of polymer adsorbed per unit area increased slightly, which corresponded to a respective decrease in the number of moles per unit area. The area occupied per molecule was governed by the radius of gyration, R g, of the ethylene oxide chains (i.e. proportional to the number of EO groups) while in the emulsions the adsorbed amount was some two to three times greater and was relatively insensitive to the length of the EO chains. In the case of the latices, the PO was located at the interface, probably in the form of a tight coil which constrained the EO chains, whereas with the emulsions the PO occupied no area and extended into the oil phase. Large latices (430 nm) showed little deviation in rheology due to the adsorbed polymers, and behaved like hard spheres. A fine latex (67 nm) was much softer in its behaviour and showed significant deviations which increased with increasing copolymer molecular weight. Dynamic measurements showed a rapid transition from a viscous fluid to an elastic gel, which corresponded to an observed phase transition from an opaque, white, disordered suspension to a beige-coloured, translucent, ordered structure or glass with increasing volume fraction, φ. At higher φ values, compression of the adsorbed polymer layers was observed. The emulsions showed similar dynamic behaviour, indicating deformation of the droplets at high φ values and/or compression of the polymer chains. The φ value at which the transition from viscous to elastic behaviour occurred decreased with increasing molecular weight of the adsorbed copolymers. Adsorbed copolymer layer thicknesses were determined from the steady shear and oscillatory shear measurements on the latices. They increased with increasing EO content of the polymers and scaled with R g for the EO chains. The rheological and adsorption measurements indicated that the copolymer's EO chains protruded from the surface of the latex particles in the form of elongated coils, whereas in the case of the emulsions they were more linear and extended.