Rheology of Dispersions of Swollen Gel Particles *

A centrifugal technique wa.'l used to determine the swelling, Q (g swollen gel/g dry polymer), of crosslinked polyacrylate gel particles in water. Both centrifultal force aRd time of centrifugation affected the magnitude of Q but under constant centrifuge conditions it was found that Q-l 'I\'as proportional to 1'0 ' /:, 1'0 being the ionic strength. The viscoelastic properties of water dispersions of these gel particles were measured as functions of concentration and ionic strength. Viscosity data were reducible to a normalized ma."ter curve by plotting ("ap/cr) versus Crt where c, is a reduced concentration defined a." cQ. Values of shear modulus, G, computed from the first normal stress difference measured at various concentrations and ionic strengths also fell on a single curve when plotted against the reduced concentration, cQ. Comparison of these results 'I\·ith those obtained earlier on a similar polyacrylate dispersion, but one in which as much as 20c;( of the polyacrylate is in a soluble, uncrosslinked form, shows that the soluble fraction has little effect on the magnitude of the viscosity and, moreover, that the presence or absence of the soluble fraction does not affect the transient stress oVershoot phenomenon. As in the earlier work, high absolute values of viscosity orcur when eQ ~ 1, that is when the dispersion consists of close-parked deformable gel particles in intimate contact with little or no interstitial solvent present.