Role of Particle Size and Polymer Length in Rheology of Colloid-Polymer Composites

The effect of particle size on the flow properties of composite melts is explored. We investigate a system composed of silica particles with diameter D-c = 127-730 nm in unentangled poly(ethylene glycol) of two molecular weights (400 with degree of polymerization similar to 9 and 2000 with degree of polymerization of 45). At low concentration, silica particles are stabilized by the adsorbed polymer layer as indicated by intrinsic viscosities slightly larger than the Einstein value of 2.5. Huggins coefficients indicate that at low concentration in PEG400 the particles behave very much like hard spheres, while in PEG2000 the particles experience very weak attractions. At high volume fraction, the absorbed polymer layers begin to interact such that hard-sphere scaling no longer applies to linear and nonlinear theological responses. In PEG2000, at elevated volume fractions, the particles with stronger attractions experience kinetic arrest with average particle surface separations much larger than the polymer radius of gyration, R-g. We show that the linear rheology, yielding behavior, and shear thickening response of dense composites are varied with R-g/D-c, with volume exclusion glass formation observed at low R-g/D-c and gelation at high R-g/D-c, and with yielding and shear thickening properties changed by R-g and D-c independently. The polymer layers are found to alter the approach to the jamming transition even for R-g/D-c as small as 6 X 10(-3).