Effect of particle size distribution on the rheology of dispersed systems

The rheological properties of aqueous polystyrene latex dispersions from th ree synthetic batches, with nearly the same z-average particle sizes, 400 n m, but varying degrees of polydispersity, 0.085, 0.301, and 0.485, respecti vely, were systematically investigated using steady-state shear and oscilla tory shear measurements. The particles were sized with photon correlation s pectroscopy and transmission electron microscopy and were stabilized steric ally with PEO-PPO-PEO triblock copolymer (Synperonic F127). Results from st eady-state shear measurements show that the viscosities of the systems exhi bit shear-thinning behavior at high solid fractions. However, the degree of shear thinning depends on the breadth of particle size distribution, with the narrowest distribution suspension exhibiting the highest degree of shea r thinning. The Herschel-Bulkley relationship best describes the flow curve s. The relative viscosities as a function of volume fraction data were comp ared, and it was found that the broadest distribution suspension had the lo west viscosity for a given volume fraction. In addition, the data were fitt ed to the Krieger-Dougherty equation for hard spheres. A reasonable agreeme nt of theory with experiment is observed, particularly and surprisingly for the very broad distribution. However, when the contribution to the volume due to the adsorbed polymer layer is considered, the agreement between expe riment and theory becomes closer for all the suspensions, although the agre ement for the broad distribution suspension is now worse. Fitting the Dough erty-Krieger theory to the experimental data based on our experimental maxi mum packing fractions gives very good agreement for all the systems studied . From oscillatory shear measurements, the moduli were obtained as a functi on of frequency at various latex volume fractions. The results show general change of the dispersions from viscous (G " > G') at low volume fractions (0.25-0.30) to moderately elastic (G' > G ") at moderately high volume frac tions (0.41-0.45). The change at this concentration level is likely due to some compression and interpenetration of the stabilizing polymer chain at t he periphery, indicating the dominance of the interparticle forces. Overall , the very broad distribution was found to have the lowest elastic modulus for a given volume fraction. (C) 1999 Academic Press.