Well dispersed fractal aggregates as filler in polymer-silica nanocomposites: long range effects in rheology

TL;DR

This study introduces a novel processing method for polystyrene-silica nanocomposites that achieves well-dispersed fractal aggregates, revealing long-range effects on rheology and mechanical properties through detailed structural and mechanical analysis.

Contribution

A new solvent-based processing technique enables well-dispersed silica aggregates in polymer nanocomposites, elucidating long-range rheological effects and structure-property relationships.

Findings

Divergence of modulus at silica volume fraction threshold due to network formation

Longer terminal elastic contribution observed below the threshold

Aggregates are separated by at least 60 nm, indicating effects beyond glassy layers

Abstract

We are presenting a new method of processing polystyrene-silica nanocomposites, which results in a very well-defined dispersion of small primary aggregates (assembly of 15 nanoparticles of 10 nm diameter) in the matrix. The process is based on a high boiling point solvent, in which the nanoparticles are well dispersed, and controlled evaporation. The filler's fine network structure is determined over a wide range of sizes, using a combination of Small Angle Neutron Scattering (SANS) and Transmission Electronic Microscopy (TEM). The mechanical response of the nanocomposite material is investigated both for small (ARES oscillatory shear and Dynamical Mechanical Analysis) and large deformations (uniaxial traction), as a function of the concentration of the particles. We can investigate the structure-property correlations for the two main reinforcement effects: the filler network contribution, and a filler-polymer matrix effect. Above a silica volume fraction threshold, we see a divergence of the modulus correlated to the build up of a connected network. Below the threshold, we obtain a new additional elastic contribution of much longer terminal time than the matrix. Since aggregates are separated by at least 60 nm, this new filler-matrix contribution cannot be described solely with the concept of glassy layer (2nm).