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

Abstract

Shear flow dynamics of fractal aggregates are investigated by ultrasound scattering tostudy shear induced disruption processes of hydrophilic polydisperse silica fume fillers inmelts of non-polar polymeric systems (polypropylene). A rheo-acoustical modelin the low frequency scattering regime only involving structural parameters isproposed. Flow-dependent changes of the ultrasound scattering power per unit ofvolume from hydrophilic silica fume aggregates during extrusion are analyzed inthe frame of the proposed rheo-acoustical model. In a second part, equilibriumstructures of polymer coated silica fume fillers are investigated to elucidate thecoupled intra- and inter-filler interactions. Surface chemistry of silica fume fillerswas modified by grafting amphiphilic molecules with the same hydrophobic tail(long alkyl chains) and various hydrophilic polar heads (amine, carboxylic acid orhydroxyl groups) to obtain a range of hydrophobic fumed silica units. The effectsof the surface layer of tethered chains and the extent of the coating level in anon-polar liquid dispersion are analyzed within the framework of the mismatch inchemical nature or solubility parameters between grafted chains and continuouspolymer phase. Lastly, the ability of the ultrasound scattering technique to give aquantitative estimate of the critical disaggregation shear stress mainly representative ofthe particle surface adhesive energy in relation to filler surface modification isshown.