Viscoelastic properties of microlatex dispersions

The viscoelastic properties of concentrated microlatex dispersions were inv estigated using oscillatory measurements. The latices were prepared by poly merisation of styrene-in-water microemulsions using UV and azobiisobutyroni trile initiator. The complex modulus, G*, storage modulus, G' and loss modu lus, G" were measured as a function of strain amplitude (to obtain the line ar viscoelastic region) and frequency at various latex volume fractions. Tw o latices with radii of 3.9 and 15.1 nm were investigated at 20 degrees C. The results showed a change from predominantly viscous to a predominantly e lastic response at a critical volume fraction, phi(c), With the smaller lat ex, the concentration of the free surfactant in bulk solution was relativel y low (2.6%) and the dispersions remained stable. phi(c) 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 t o be 1.4 nm, which is small for a surfactant chain with 15 ethylene oxide u nits. 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 interpen etration and/or compression on close approach of the particles. With the la rger latex, on the other hand, there was high free surfactant concentration (9.1%) and this led to depletion flocculation. This results in a lower phi (c) than would be the case in the absence of flocculation. (C) 2000 Elsevie r Science B.V. All rights reserved.