Elasticity of weakly aggregating polystyrene latex dispersions

The elastic behavior of a weakly aggregating polystyrene latex dispersion was investigated using a homemade controlled stress rheometer and commercially available dynamic rheometers. Both the creep measurements and the oscillatory experiments show the absence of a yield stress. The low-shear-limiting viscosity obtained with the controlled stress apparatus shows a power-law dependence upon volume fraction with a power of 5.3±0.3. Combining the data of the different rheometers, the storage shear modulus G’ at high frequencies was found to depend upon volume fraction with a power of 4.6±0.3, being in line with earlier data mentioned in the literature. Using an additional exponent, i.e., one describing the critical strain above which the material deforms nonlinearly, the fractal dimension ${\mathit{d}}_{\mathit{f}}$ and the upper and lower bounds of the structural parameter ɛ were obtained. The latter is a measure for the degree of anisotropy of the network skeleton. The values ${\mathit{d}}_{\mathit{f}}$=2.0±0.2 and 0.5≤ɛ≤1 are consistent with a model based on noncentral or bond-bending interactions. It was shown that the structural parameters and hence the microstructure change significantly when going from rest to high shear rates. Thus, in general, one cannot use the results obtained with one type of experiment in another type of experiment.