TRANSIENT SHEAR VISCOSITY OF WEAKLY AGGREGATING POLYSTYRENE LATEX DISPERSIONS

The transient behavior of the viscosity (stress growth) of a weakly ag gregating polystyrene latex dispersion after a step from a high shear rate to a lower shear rate has been measured and modeled. Single parti cles cluster together into spherical fractal aggregates. The steady st ate size of these aggregates is determined by the shear stresses exert ed on the latter by the flow field. The restructuring process taking p lace when going from a starting situation with monodisperse spherical aggregates to larger monodisperse spherical aggregates is described by the capture of primary fractal aggregates by growing aggregates until a new steady state is reached. It is assumed that the aggregation mec hanism is diffusion limited. The model is valid if the radii of primar y aggregates R(prim). are much smaller than the radii of the growing a ggregates. Fitting the model to experimental data at two volume fracti ons and a number of step sizes in shear rate yielded physically reason able values of R(prim) at fractal dimensions 2.1 less-than-or-equal-to d(f) less-than-or-equal-to 2.2. The latter range is in good agreement with the range 2.0 less-than-or-equal-to d(f) less-than-or-equal-to 2 .3 obtained from steady shear results. The experimental data have also been fitted to a numerical solution of the diffusion equation for pri mary aggregates for a cell model with moving boundary, also yielding 2 .1 less-than-or-equal-to d(f) less-than-or-equal-to 2.2. The range for d(f) found from both approaches agrees well with the range d(f) almos t-equal-to 2.1-2.2 determined from computer simulations on diffusion-l imited aggregation including restructuring or thermal breakup after fo rmation of bonds. Thus a simple model has been put forward which may c apture the basic features of the aggregating model dispersion on a mic rostructural level and leads to physically acceptable parameter values .