Ultrasound scattering from silica fume clusters: in-line shear flow dynamics of hydrophilic or partially hydrophobic silica fume fillers in melts of non-polar polymeric systems

Abstract : Shear flow dynamics of fractal aggregates are investigated by ultrasound scattering to study shear induced disruption processes of hydrophilic polydisperse silica fume fillers in melts of non-polar polymeric systems (polypropylene). A rheo-acoustical model in the low frequency scattering regime only involving structural parameters is proposed. Flow-dependent changes of the ultrasound scattering power per unit of volume from hydrophilic silica fume aggregates during extrusion are analyzed in the frame of the proposed rheo-acoustical model. In a second part, equilibrium structures of polymer coated silica fume fillers are investigated to elucidate the coupled intra- and inter-filler interactions. Surface chemistry of silica fume fillers was modified by grafting amphiphilic molecules with the same hydrophobic tail (long alkyl chains) and various hydrophilic polar heads (amine, carboxylic acid or hydroxyl groups) to obtain a range of hydrophobic fumed silica units. The effects of the surface layer of tethered chains and the extent of the coating level in a non-polar liquid dispersion are analyzed within the framework of the mismatch in chemical nature or solubility parameters between grafted chains and continuous polymer phase. Lastly, the ability of the ultrasound scattering technique to give a quantitative estimate of the critical disaggregation shear stress mainly representative of the particle surface adhesive energy in relation to filler surface modification is shown.