THE VISCOELASTIC PROPERTIES OF POLYSTYRENE LATEX-PARTICLES BEARING AN ADSORBED LAYER OF POLYVINYL-ALCOHOL

The viscoelastic properties of a concentrated polystyrene latex stabilized by polyvinyl alcohol have been measured using oscillatory shear rheometry. For stable systems it was found that the elastic modulus started to increase dramatically when the interparticle separation was such that the polymer layers were touching. The osmotic pressures of these latex systems were also measured and correlated to the elastic moduli measurements and to the simple scaling theory for adsorbed polymers. It was found that the osmotic pressure of the stabilizing polyvinyl alcohol was the predominant factor influencing the osmotic pressure of the latex. In a complementary series of experiments the oscillatory rheological behavior of these systems as they were flocculated (brought about by an increase in temperature) was measured. At the onset of flocculation both the elastic and loss moduli of the dispersion increased by many orders of magnitude. It was noted that the temperature at which flocculation occurred decreased slowly below φ = 0.40 (at ∼ 54°C) but decreased more sharply at higher volume fractions, in accord with simple statistical mechanical theories. Moreover, the change in G′ with φ after flocculation was found to be in agreement with fractal analysis for percolating networks. To a large extent this flocculation was found to be reversible (i.e., decreasing the temperature brought about a reduction in G′ to a value approximately only double its initial response). © 1991.