Packing correlations, collective scattering and compressibility of fractal-like aggregates in polymer nanocomposites and suspensions

Abstract

We employ a small scale Monte Carlo method plus microscopic liquid state integral equation theory to study two models of fractal-like aggregates (single and quenched disordered averaged) as a function of concentration. The models employed are relevant to non-compact carbon black or silica aggregates in polymer nanocomposites and suspensions. When the primary nanoparticles interact as hard spheres the resulting center-of-mass level effective pair potential has a distinctive soft repulsion form which is sensitive to whether geometric disorder at the single aggregate level is included or not. The aggregate fluids show a considerable amount of interpenetration and complex non-hard-sphere packing features in their pair correlation functions. The latter are also different, both qualitatively and quantitatively, for the single versus average aggregate systems. Calculations of the collective static structure factor and compressibility (or inverse bulk modulus) are in qualitative agreement with a recent scattering experiment on suspensions of fumed silica hard fractals. Use of the theoretical structural information in a microscopic dynamical theory of kinetic arrest suggests the presence of multiple length scales and disordered local packing frustrates the emergence of glassy dynamics in these aggregate fluids even at high volume fractions.