Viscoelastic properties of microlatex dispersions

The viscoelastic properties of concentrated microlatex dispersions were investigated using oscillatory measurements. The latices were prepared by polymerisation of styrene-in-water microemulsions using UV and azobiisobutyronitrile initiator. The complex modulus, G*, storage modulus, G′ and loss modulus, G″ were measured as a function of strain amplitude (to obtain the linear viscoelastic region) and frequency at various latex volume fractions. Two latices with radii of 3.9 and 15.1 nm were investigated at 20°C. The results showed a change from predominantly viscous to a predominantly elastic response at a critical volume fraction, φc With the smaller latex, the concentration of the free surfactant in bulk solution was relatively low (2.6%) and the dispersions remained stable. φc was found to be 0.161. Assuming random packing of the particles (volume fraction=0.64), an estimate was obtained for the adsorbed layer thickness and this was found to be 1.4 nm, which is small for a surfactant chain with 15 ethylene oxide units. However, since the surfactant layer is a mixture of chains with 4 and 15 EO units, it is likely that the larger PEO chains will undergo interpenetration and/or compression on close approach of the particles. With the larger latex, on the other hand, there was high free surfactant concentration (9.1%) and this led to depletion flocculation. This results in a lower φc than would be the case in the absence of flocculation.