Fractal Behavior and Scaling Law of Hydrophobic Silica in Polyol

This article examines the rheological properties of a system composed of polyol and colloidal silica. Three types of nanosized silicas with hydrophilic and hydrophobic surfaces were studied: A200 with OH surface groups, R974 with CH(3) surface groups, and R805, which is grafted with a C(8)H(17) alkyl chain. Rheometric measurements showed that the dispersions of R805 silicas have a yield stress at low volume fraction, unlike the R974 and A200 silicas. The plastic behavior of the hydrophobic silicas was quantified by a yield stress sigma(0) and an elastic modulus G'. It is observed that these parameters follow scaling laws as a function of the volume fraction of silica introduced, in the form sigma(0) approximately phi(v)(2.9+/-0.2), G' approximately phi(v)(4.1+/-0.3). Static light scattering (SLS) and small angle neutron scattering (SANS) measurements show a fractal arrangement with a fractal dimension D=1.8 ranging from elementary particles of about 32 nm to aggregates measuring about 6 mum. Correlations were established between the theoretical scaling laws and the experimental scaling laws determined by rheometric measurements. The fractal structure observed in this system is explained by the attractive physical interaction of the octyl chains between the silica particles. Contrary to what has been observed in the past by Khan and Zoeller (J. Rheol. 37 (1993) 1225), the lower molecular weight of the polyol studied here, which has a shorter chain length, allows direct bridging of two separate silicates though alkyl chains, giving rise to the formation of a 3D gel network at volume fractions as low as phi(v)=2.2%.