AN EXPERIMENTAL AND THEORETICAL INVESTIGATION OF THE RHEOLOGICAL PROPERTIES OF DISPERSIONS OF COLLOIDAL PARTICLES IN A POLYMERIC FLUID (FUMED SILICA, POLYDIMETHYLSILOXANE, PHENOMENOLOGICAL, BEAD-SPRING)
Abstract
Steady state, stress relaxation and stress growth rheological properties were measured for dispersions composed of a colloidal sized fumed silica in polydimethylsiloxane fluids. These filled fluids exhibited (i) shear thinning steady state properties, (ii) stress relaxation whose rate increased as the previous shear rate increased, and (iii) stress growth functions which could exhibit large stress growth overshoot. The polydimethylsiloxane molecular weight determined if stress growth maxima were observed; specifically, if the dispersion's polydimethylsiloxane molecular weight was below or greatly exceeded the entanglement molecular weight, significant stress growth maxima were not observed. However, when the dispersion's polydimethylsiloxane molecular weight only slightly exceeded the entanglement molecular weight, large stress growth maxima were observed after sufficient rest in the undeformed state. The fumed silica concentration only affected quantitative but not qualitative changes in the dispersion's rheological properties. The experimentally observed rheological properties are consistent with a molecular deformation mechanism where particles in a dispersion do not directly interact, but rather interact via entanglements of surface adsorbed polymer chains with their surroundings. Purely viscoelastic, purely microstructural and combined viscoelastic/microstructural phenomenological models were formulated to predict the effects of the deformation history on steady state, stress relaxation and stress growth rheological properties for the fumed silica/polydimethylsiloxane dispersions. The purely viscoelastic and purely microstructural models were unable to correctly predict even qualitatively the experimentally observed rheological properties. Combined viscoelastic/microstructural models were able to predict all the qualitative features of the experimental data. However, quantitative descriptions of the experimental data by the combined viscoelastic/microstructural models suffered the following deficiencies (i) the effects of previous deformations disappeared too quickly, and (ii) a single set of model constants could not be used to describe quantitatively the steady state, stress relaxation and stress growth properties. A molecular model was derived to describe interactions of polymer chains adsorbed on adjacent particles. Bead-spring assemblages attached to planar surfaces were used to predict the force required to maintain the motion of one of the surfaces. The interlayer bead interactions were described by a hydrodynamic drag coefficient which depended on the weight fraction of the adjacent layer beads in the vicinity.
Degree
Ph.D.
Subject Area
Chemical engineering
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