Molecular weight dependence of structures and rheological properties for fumed silica suspensions in polystyrene solutions

Fumed silica suspensions in trans-decalin and trans-decalin solutions of polystyrene (PS) have been studied by a combination of rheology and small-angle neutron scattering (SANS) techniques as functions of the concentrations of silica and PS and the molecular weight of PS. When the fumed silica particles are suspended in the solvent, the storage modulus is higher and independent of frequency, indicating the formation of gel-like network structures through hydrogen-bonding of the surface silanol groups. The scattering experiments have demonstrated that the mass fractal dimension corresponding to the power law correlation was 2.0 +/- 0.05. Rheological measurements at constant silica and PS contents reveal that the storage modulus drastically decreases with an increase in PS molecular weight due to partially breaking down the gel Structures by an effective flocculation of the silica particles by absorption of larger PS chains, leading to an increase in the scattering intensity at lower wave vector ranges. At higher PS concentrations the storage modulus increases, suggesting that chain entanglements occur between the PS chains adsorbed on the silica surfaces and free PS chains.