Quantitative Correlation between Hierarchical Nanofiller Structure and Rheology of Polymer/Fumed Silica Nanocomposites

Abstract

Understanding the mechanical reinforcement of polymer nanocomposites (PNCs) filled with agglomerated nanofillers has attracted considerable attention in the past century, but a quantitative description of linear rheology over a wide range of time scales is still a challenge at present. In this study, the hierarchical nanofiller structure and its relationship to the rheology of fumed silica-filled poly(methyl methacrylate) nanocomposites are quantitatively investigated using transmission electron microscopy, small-angle X-ray scattering, and rheological measurements. It is suggested that the polymer entanglements dominate the short-time rheological behavior of nanocomposites, and the enhancement in the rubbery modulus is controlled by the interfacial adsorption-induced entanglements, which depend on the effective surface fractal dimension of nanofillers. At a longer time scale, the discrete agglomerates and the particle networks dominate the modulus enhancement. It was found that there is a power-law relationship between the particle network modulus and the inter-agglomerate mesh size with an exponent of -5, corresponding to a chemical dimension of 1 and rigid particle chains. A new two-phase model is suggested for the linear rheology of PNCs, including the contribution of interfacial entanglements, discrete agglomerates, and particle networks. The linear viscoelasticity of PNCs can be quantitatively described using properly determined structural parameters of particle agglomerates.