Molecular-Weight-Dependent Adsorption of Amino-Modified Poly(dimethylsiloxane) on TiO ₂ Nanoparticles and Its Effect on Suspension Rheology

Silicone-based amphiphilic polymers are widely used as dispersants for metal oxide particles in oil media. Because these polymers typically exhibit a broad molecular weight (M) distribution, their interfacial adsorption involves a competitive process within a multicomponent system. In this study, we investigated silicone-oil dispersions of titanium dioxide nanoparticles (TiO2NPs) with amino-modified poly(dimethylsiloxane) (AEAPS), focusing on the competitive adsorption behavior of polydisperse AEAPS and its relationship to the macroscopic rheology of the dispersions. AEAPS-25, with a lower weight-averaged M, exhibited selective adsorption onto TiO2NPs biased toward higher-M components. By contrast, AEAPS-120, which has a higher weight-averaged M, initially exhibited selective adsorption biased toward lower-M components, whereas larger components were increasingly adsorbed over time, replacing the smaller ones. Rheological measurements demonstrated that both AEAPS systems exhibited particle flocculation with shear-thinning behavior at low AEAPS concentrations, transitioning to a Newtonian-like state as the concentration increased, which corresponded to the saturation of the total adsorbed mass. The apparent viscosities of the Newtonian-like dispersions varied with AEAPS concentration and over time. This reflects the contribution of free AEAPS molecules to the viscosity of the continuous phase, resulting from the partitioning of AEAPS between the interface and bulk. In addition, the larger components in the AEAPS-120 composition, which remained primarily in the bulk during the initial stage of time evolution, appeared to form weak bridges between the dispersed TiO2NPs with high surface coverage. This was detected as a slight deviation in the dispersion rheology from Newtonian flow, accompanied by an increased amount of free AEAPS molecules.